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This manuscript uses idealized AGCM experiments to explore impacts of observed decadal Atlantic sea surface temperature (SST) changes on El Nino Southern Oscillation teleconnections. Significant regional changes are observed in numerous locations (e.g., Indo‐Pacific and Central Africa) due to the addition of the AMV anomalies. In some regions the teleconnection impacts of AMV forcing appeared to scale with the magnitude of the AMV forcing. However, the AMV induced modulation of temperature and precipitation teleconnections is regionally specific with changes in magnitude, statistical strength, and spatial connection dependent on region. These ENSO teleconnection changes were found to be asymmetric between AMV phases, with negative anomalies appearing to have a larger effect. We hypothesize these changes may be underpinned by a AMV induced modulation and migration of ENSO's equatorial precipitation response. Plain Language Summary: The potential of the Atlantic sea surface temperature (SST) mean state to impact the El Nino Southern Oscillation (ENSO) is analyzed over 24 years using global climate models with different forced Atlantic Ocean mean states. Typical teleconnections of ENSO are investigated finding a significant change in numerous areas which are not found to be globally consistent. Greatest impacts are found to be regionally specific with negative Atlantic anomalies suggested to have a greater impact. In some locations influence was found to scale with the magnitude of the introduced anomaly. It is theorised that these changes are underpinned by a spatial migration in the typical ENSO equatorial precipitation response between different mean states of the Atlantic Ocean. Key Points: Atlantic Multidecadal Variability SSTA can produce statistically significant changes of ENSO teleconnections without any changes in ENSOENSO teleconnection modulation by the AMV was regionally specific with variation in statistical strength and sign relative to the controlTeleconnection modulation is likely a result of tropical Pacific precipitation changes during ENSO induced atmospheric stability changes [ABSTRACT FROM AUTHOR]

Ocean observational gridded products are vital for climate monitoring, ocean and climate research, model evaluation, and supporting climate mitigation and adaptation measures. This paper describes the 4th version of the Institute of Atmospheric Physics (IAPv4) ocean temperature and ocean heat content (OHC) objective analysis product. It accounts for recent developments in quality control (QC) procedures, climatology, bias correction, vertical and horizontal interpolation, and mapping and is available for the upper 6000 m (119 levels) since 1940 (more reliable after ∼ 1957) for monthly and 1°×1° temporal and spatial resolutions. IAPv4 is compared with the previous version, IAPv3, and with the other data products, sea surface temperatures (SSTs), and satellite observations. It has a slightly stronger long-term upper 2000 m OHC increase than IAPv3 for 1955–2023, mainly because of newly developed bias corrections. The IAPv4 0–2000 m OHC trend is also higher during 2005–2023 than IAPv3, mainly because of the QC process update. The uppermost level of IAPv4 is consistent with independent SST datasets. The month-to-month OHC variability for IAPv4 is desirably less than IAPv3 and the other OHC products investigated in this study, the trend of ocean warming rate (i.e., warming acceleration) is more consistent with the net energy imbalance at the top of the atmosphere than IAPv3, and the sea level budget can be closed within uncertainty. The gridded product is freely accessible at 10.12157/IOCAS.20240117.002 for temperature data (Cheng et al., 2024a) and at 10.12157/IOCAS.20240117.001 for ocean heat content data (Cheng et al., 2024b). [ABSTRACT FROM AUTHOR]

Internally generated radiative feedback at interannual timescales can influence climate sensitivity estimates. This study has clarified the mechanisms of variability in radiative feedback associated with the El Niño and Southern Oscillation based on preindustrial control simulations for Coupled Model Intercomparison Project Phase 6. Radiative feedback showed large modulation with a comparable magnitude to the intermodel uncertainty associated with internal changes in the zonal gradient of the ocean thermocline and sea surface temperature (SST) in the equatorial Pacific on interdecadal timescales. When the interdecadal SST anomalies in the eastern and western equatorial Pacific were simulated as positive and negative, respectively, the statistical properties of the interannual variations were also modified with diminished Walker circulation changes per unit of global mean surface temperature rise. Meanwhile, the enhanced local Hadley circulation led to a middle‐to upper‐level cloud decrease and a low‐level cloud increase in the subtropics, causing enhanced negative radiative feedback as a global average. Plain Language Summary: Global warming is usually estimated based on radiative feedbacks: how the climate system responds to external forcing. Even though internal variability can give non‐negligible uncertainty in its estimate, radiative feedbacks characterized by internal variability on interannual timescales such as El Niño and Southern Oscillation (ENSO) potentially contribute to its evaluation. This study clarifies the mechanism of variability on internally generated radiative feedbacks. Radiative feedbacks varied on interdecadal timescales associated with the pattern changes for the background sea surface temperature on the equatorial Pacific. Such background shift strengthens ENSO, thereby enhanced local Hadley circulation strengthens negative feedback. Key Points: Internally generated radiative feedbacks can fluctuate on interdecadal timescales in accordance with the background shiftThe background shift on the equatorial Pacific works to amplify the El Niño and Southern Oscillation (ENSO)Enhanced local Hadley circulation on the equatorial Pacific related to modulated ENSO strengthens negative feedback [ABSTRACT FROM AUTHOR]

The tropical Pacific Ocean is a key regulator of Earth's climate, with teleconnections that influence remote locations all around the world. Here we use partially coupled climate model experiments to show that tropical Pacific cooling related to an abrupt Atlantic Meridional Overturning Circulation (AMOC) slowdown can strengthen the AMOC by ∼25%. This tropical‐extratropical teleconnection occurs initially via atmospheric Rossby waves propagating from the tropical Pacific to the North Atlantic which alter surface climate conditions locally. These changes facilitate ocean heat loss from the subpolar gyre, favoring enhanced oceanic convection. The AMOC strengthening is subsequently enhanced by anomalous northward salt advection in the Atlantic, with a potential contribution from oceanic wave adjustment triggered by increased Southern Ocean westerly winds. These results highlight the influence of the tropical Pacific on the AMOC on multidecadal timescales and suggest that cold phases of tropical Pacific decadal variability could drive temporary strengthening of the AMOC. Plain Language Summary: Changes in the tropical Pacific sea surface temperature can exert significant remote weather and climate impacts via physical mechanisms known as teleconnections. In this study, we report unexplored teleconnections to the Atlantic basin which act in a timescale of decades. In particular we found that a mean cooling in the tropical Pacific could act to accelerate a large‐scale ocean circulation in the Atlantic basin known as the Atlantic Meridional Overturning Circulation (AMOC). This occurs via atmospheric and oceanic waves which propagate to the North Atlantic and alter local conditions, favoring the acceleration of the AMOC. Key Points: Tropical Pacific cooling can drive a strengthening of the Atlantic Meridional Overturning Circulation (AMOC)The Pacific–North Atlantic teleconnection occurs via both atmospheric and oceanic planetary waveguidesPacific–AMOC teleconnections can be induced on a multidecadal time‐scale [ABSTRACT FROM AUTHOR]

This study compares the evolution of atmospheric and oceanic anomalies as well as predictions for the two most recent triple‐dip La Niña events in 1998–2001 and 2020–2023. Subsurface cooling in the equatorial Pacific was stronger and more persistent during 1998–2001. In contrast, surface easterly winds were stronger during 2020–2023 as was the east‐west sea surface temperature (SST) contrast along the equator. We argue that in the absence of appreciable equatorial Pacific heat discharge, persistent and strong surface trade winds and a strengthened mean zonal SST contrast across the tropical Pacific contributed to the development of the 2020–2023 triple‐dip La Niña. In terms of the subsurface layer heat budget, the growth and maintenance of unusually cold SSTs during the triple‐dip La Niña in 1998–2001 were mainly the result of ocean vertical entrainment and diffusion, as well as meridional advection, associated with enhanced equatorial upwelling; while for the triple‐dip La Niña in 2020–2023, zonal advection was the largest contributor. The two events were mostly well predicted by multi‐model averages at 1–8 months lead times. We hypothesize that mean state change with enhanced zonal SST contrast and trade winds over the last several decades altered the physical processes associated with the growth and maintenance of the most recent La Niña, affecting its predictability. Successful prediction in real‐time of the 2020–2023 event more than half a year in advance was surprising because there was little memory in oceanic heat content which is often considered a key predictor. Plain Language Summary: El Niño‐Southern Oscillation is the strongest interannual variability on Earth and the main source of global seasonal climate predictability. Here, we examine the evolution of oceanic and atmospheric anomalies in the tropical Pacific during two triple‐dip La Niñas in 1998–2001 and 2020–2023, comparing the physical processes that gave rise to them and the skill in predicting them. Our results show that the processes giving rise to these events were different and likely affected by mean state changes in the tropical Pacific. In particular, the easterly trade winds and zonal SST contrast across the basin have strengthened that played a crucial role in the growth, maintenance, and prediction of the La Niña in 2020–2023. The evolution of the La Niña in 2020–2023 was successfully predicted in real‐time more than half a year in advance, which is surprising because there was little memory in oceanic heat content which is often considered a key predictor. Key Points: The physical processes responsible for the evolution of the 1998–2001 and 2020–2023 triple‐dip La Niñas were differentOcean heat content as a precursor was more important for the predictability of the 1998–2001 La Niña than the 2020–2023 La NiñaStronger surface easterlies and zonal sea surface temperature gradient in the equatorial Pacific contributed to the predictability of the 2020–2023 La Niña [ABSTRACT FROM AUTHOR]

Future changes in the El Niño–Southern Oscillation (ENSO) are uncertain, both because future projections differ between climate models and because the large internal variability of ENSO clouds the diagnosis of forced changes in observations and individual climate model simulations. By leveraging 14 single model initial-condition large ensembles (SMILEs), we robustly isolate the time-evolving response of ENSO sea surface temperature (SST) variability to anthropogenic forcing from internal variability in each SMILE. We find nonlinear changes in time in many models and considerable inter-model differences in projected changes in ENSO and the mean-state tropical Pacific zonal SST gradient. We demonstrate a linear relationship between the change in ENSO SST variability and the tropical Pacific zonal SST gradient, although forced changes in the tropical Pacific SST gradient often occur later in the 21st century than changes in ENSO SST variability, which can lead to departures from the linear relationship. Single-forcing SMILEs show a potential contribution of anthropogenic forcing (aerosols and greenhouse gases) to historical changes in ENSO SST variability, while the observed historical strengthening of the tropical Pacific SST gradient sits on the edge of the model spread for those models for which single-forcing SMILEs are available. Our results highlight the value of SMILEs for investigating time-dependent forced responses and inter-model differences in ENSO projections. The nonlinear changes in ENSO SST variability found in many models demonstrate the importance of characterizing this time-dependent behavior, as it implies that ENSO impacts may vary dramatically throughout the 21st century. [ABSTRACT FROM AUTHOR]

Since the early 1990s the Pacific Walker circulation shows a multi‐decadal strengthening, which contradicts future model projections. Whether this trend, evident in many climate indices especially before the 2015 El Niño, reflects the coupled ocean‐atmosphere response to global warming or the negative phase of the Pacific Decadal Oscillation (PDO) remains debated. Here we show that sea surface temperature trends during 1980–2020 are dominated by three signals: a spatially uniform warming trend, a negative PDO pattern, and a Northern Hemisphere‐Indo‐West Pacific warming pattern. The latter pattern, which closely resembles the transient ocean thermostat‐like response to global warming emerging in a subset of CMIP6 models, shows cooling in the central‐eastern equatorial Pacific but warming in the western Pacific and tropical Indian Ocean. Together with the PDO, this pattern drives the Walker circulation strengthening in the equatorial band. Historical simulations appear to underestimate this pattern, contributing to the models' inability to replicate the Walker cell strengthening. Plain Language Summary: This paper investigates the observed changes in the tropical Pacific during the satellite era, including the recent decadal strengthening of the atmospheric zonal circulation—the Walker cell. We aim to understand the extent to which these changes represent a forced response to rising CO2 concentrations versus natural variability. We apply an approach in which we decompose the observed sea surface temperature trends into three components—a pattern associated with the Pacific Decadal Oscillation, which is part of natural variability, a uniform warming pattern, and a residual pattern. This residual pattern shows a remarkable resemblance to a forced ocean thermostat‐like transient response generated in some of the climate models, characterized by equatorial Pacific (EP) cooling, and a broad warming of the Northern Hemisphere, and the Indian Ocean and West Pacific. These results challenge studies arguing that the recent strengthening of the Pacific Walker cell can be explained simply by multi‐decadal natural variability in the tropics. Furthermore, the inability of climate models at large to fully capture this forced pattern with historical forcing puts into focus the reliability of future projections of climate change in the tropical Pacific, specifically the timing of emergence of the eastern EP warming. Key Points: A multi‐decadal strengthening of the Pacific Walker cell is observed in a wide range of indices, especially after 1990A Northern Hemisphere ‐ Indo West Pacific warming sea surface temperature pattern, which differs from the Pacific Decadal Oscillation, is evident since 1980This pattern resembles a forced response to abrupt CO2 forcing, emerging in a subset of climate models, and contributes to the Walker circulation strenthening [ABSTRACT FROM AUTHOR]

Separating the climate response to external forcing from internal climate variability is a key challenge. While most previous studies have focused on surface responses, here we examine zonal‐mean patterns of North Pacific subsurface temperature responses. In particular, the changes since 1950 driven by anthropogenic aerosol emissions are found by using a pattern recognition method. Based on the single‐forcing large‐ensemble simulations from two models, we show that aerosol forcing caused a nonmonotonic temporal response and a characteristic zonal‐mean pattern within North Pacific, which is distinct from the pattern associated with internal variability. The aerosol‐forced pattern with the nonmonotonic temporal feature shows a substantial temperature change in subpolar regions and a reversed change on the southern flank of the subtropical gyre. A similar characteristic pattern and nonmonotonic time evolution are extracted from the subsurface observations, which likely reflect the subsurface responses to the aerosol forcing, although differences exist with the simulated responses. Plain Language Summary: Pacific climate has long been of interest as considerable evidence has emerged of a decades‐long change. As the background noise, such as Interdecadal Pacific Oscillation and El Niño‐Southern Oscillation, is very strong in the Pacific, it remains a major challenge to detect and understand the changes due to human activities. In this study, we take advantage of a pattern recognition method to investigate the North Pacific subsurface temperature response to aerosol emissions, a suspension of fine particles in air, due to industrial development. The results are primarily based on two sets of model simulations with different phases of background noise and identical external forcing, which stems from anthropogenic aerosol emissions only. We find that the aerosol emissions from North America and Europe cause a clear nonmonotonic time evolution (increased during 1950–1985 and then decreased afterward) and a characteristic zonal mean pattern of the subsurface ocean temperature within the North Pacific. The extracted temporal and spatial features from ocean observations are broadly consistent with those from models, although there are some differences. Key Points: A pattern recognition method is applied to extract aerosol‐induced subsurface temperature pattern from internal variability in North PacificBased on two large‐ensemble simulations, aerosols govern a nonmonotonic feature in temperature change with a characteristic patternThis spatiotemporal feature can be obtained from observations since 1950, although there are some differences with the modeled results [ABSTRACT FROM AUTHOR]

The interannual sea surface temperature trend of the Kuroshio in the East China Sea (ECS‐Kuroshio) is significantly different from that of the ECS‐shelf region. Based on 33 years (1982–2014) of observational temperature data in the ECS, the upstream ECS‐Kuroshio, as an upwelling region, shows an opposite temperature trend to the downstream ECS‐Kuroshio, creating a horizontal "dipole" structure. Vertical heat transport modulated by upwelling variability in the upstream ECS‐Kuroshio can be the main factor for the distinct temperature trend in the upstream ECS‐Kuroshio. This upstream trend can expand to downstream areas through horizontal advections. As a result, the weakened upwelling triggered a significant warming trend in the entire upper layer of the ECS‐Kuroshio during the warming period (1999–2014). The weakening trend of upwelling and global rewarming conditions may lead to robust warming in the Kuroshio region in the middle of this century. Plain Language Summary: The trend of sea surface temperature (SST) is used as a climate change indicator over global areas. The recent temperature trend in the Kuroshio can be separated into three stages, namely, the cooling period (1982–1998), the warming period (1999–2014) and the rapid warming period (2014‐present). However, the SST trend in the Kuroshio region in the East China Sea (ECS‐Kuroshio) was much weaker or even opposite to that in the ECS‐shelf, which means that different mechanisms may act strongly on this western boundary current area. Based on 33 years (1982–2014) of three‐dimensional observational data, this study investigated an opposite temperature trend in the ECS‐Kuroshio region in comparison with the global temperature trend. Furthermore, a "dipole" structure of temperature trends between the upstream and downstream of the ECS‐Kuroshio occurred during both periods. The upwelling variability in the upstream ECS‐Kuroshio region, which varies with the intensity of the ECS‐Kuroshio, is responsible for the opposite variability and "dipole" structure of temperature trends. A wider area was affected by weakened upwelling than was affected by strong upwelling, indicating that a weakening Kuroshio can be more sensitive to the climate system and may affect robust warming in the future. Key Points: The interannual temperature trend upstream of the Kuroshio in the East China Sea (ECS) is opposite to the trend of the adjacent shelfThe trends upstream and downstream of the Kuroshio also show opposite signs, shaping a horizontal "dipole" structureThe upwelling variability in the upstream Kuroshio dominates the interannual temperature trend of the Kuroshio in the ECS [ABSTRACT FROM AUTHOR]

The East Asian marginal seas gain heat from the warm pool via intrusion of the Kuroshio Current and play an important role in regulating regional climate. Here, we show that the sea surface temperature rise of the East Asian marginal seas, especially in areas where the Kuroshio intrudes, has far exceeded the rate of global ocean warming. We attribute this to warming of the Pacific Warm Pool since the 1990s. Intensified trade winds warmed the Pacific Warm Pool and caused the surface Kuroshio and the regions where it intrudes into marginal seas to warm rapidly in the late 1990s. Plain Language Summary: In the late 1990s, surface warming signal of the Pacific warm pool entered the East Asia marginal seas via intrusion of the Kuroshio, causing these seas to warm rapidly. The warming of the warm pool is due to an increase in equatorial trade winds, which is attributed to the acceleration of the Walker circulation. The weakening of the mid‐latitude westerly winds in the Pacific produced a positive wind stress curl anomaly in the subtropics, and finally weakened the Kuroshio intensity, prompting it to intrude the marginal seas increasingly. Key Points: Rapid warming of the East Asian marginal seas was attributed to the warming of the Pacific warm pool via Kuroshio intrusionPacific warm pool warming is due to the acceleration of the Walker circulationAnomalous atmospheric circulation in the mid‐latitude Pacific enhanced the Kuroshio intrusion into the marginal seas [ABSTRACT FROM AUTHOR]

Changes to the Amazon hydrological cycle have important consequences for world's largest tropical forest, and the biodiversity it contains. However, a scarcity of long‐term climate data in the region makes it hard to contextualize recent observed changes in Amazon hydrology. Here, we explore to what extent tree‐ring oxygen isotope (δ18OTR) chronologies can inform us about hydrological changes in the Amazon over the past two centuries. Two δ18OTR records from northern Bolivia and the Ecuadorian Andes are presented. The Ecuador record spans 1799–2012 (n = 16 trees) and the Bolivia record spans 1860–2014 (n = 32 trees), making them the longest δ18OTR records from the Amazon, and among the most highly‐replicated δ18OTR records from the tropics to date. The two chronologies correlate well at interannual and decadal timescales, despite coming from sites more than 1,500 km apart. Both δ18OTR records are strongly related to interannual variation in Amazon River discharge measured at Óbidos, and accumulated upwind precipitation, suggesting a common climatic driver. In both records a strong increase in δ18OTR was observed up until approximately 1950, consistent with positive trends in the few other existing δ18O proxy records from across the Amazon. Considering all possible drivers of this long‐term increase, a reduction in rainout fraction over the basin driven by rising sea surface temperatures in the North Atlantic is suggested as the most likely cause. The upward trend in δ18OTR reverses over the past 1–2 decades, consistent with the observed strengthening of the Amazon hydrological cycle since approximately 1990. Plain Language Summary: Long‐term climate measurements from the Amazon are few and far between, so information about the climate in the past needs to come from alternative sources. This is essential to provide context for current climate variability such as the strengthening of the Amazon water cycle since the 1990s. One way we can reconstruct historical climate is using stable oxygen isotopes recorded in tree rings. Previous work has shown that tree‐ring oxygen isotopes from the western Amazon can tell you about the amount of rainfall over the entire basin. Here, we present tree‐ring isotope records from Ecuador and Bolivia that span the past two centuries, making them the longest tree‐ring oxygen isotope records from the region to date. We find that the two records are very similar to one another, despite coming from sites that are more than 1,500 km apart, suggesting they are both influenced by the same climatic factors. We explore the climate signals that these records contain, including interpreting a long‐term positive trend in the isotope record until around 1950. We conclude that the most likely cause of this trend is a long‐term reduction in the fraction of incoming water vapor that falls as rainfall over the Amazon. Key Points: We present two well‐replicated tree‐ring oxygen isotope (δ18OTR) records from tropical South America spanning the past two centuriesThese records capture important large‐scale climate signals, including interannual variation in Amazon basin‐wide precipitationA multi‐decadal long‐term increase in δ18OTR is interpreted as being primarily driven by a long‐term reduction in rainout fraction [ABSTRACT FROM AUTHOR]

Near-term projections of climate change are subject to substantial uncertainty from internal climate variability. Here we present an approach to reduce this uncertainty by sub-selecting those ensemble members that more closely resemble observed patterns of ocean temperature variability immediately prior to a certain start date. This constraint aligns the observed and simulated variability phases and is conceptually similar to initialization in seasonal to decadal climate predictions. We apply this variability constraint to large multi-model projection ensembles from the Coupled Model Intercomparison Project phase 6 (CMIP6), consisting of more than 200 ensemble members, and evaluate the skill of the constrained ensemble in predicting the observed near-surface temperature, sea-level pressure, and precipitation on decadal to multi-decadal timescales. We find that the constrained projections show significant skill in predicting the climate of the following 10 to 20 years, and added value over the ensemble of unconstrained projections. For the first decade after applying the constraint, the global patterns of skill are very similar and can even outperform those of the multi-model ensemble mean of initialized decadal hindcasts from the CMIP6 Decadal Climate Prediction Project (DCPP). In particular for temperature, larger areas show added skill in the constrained projections compared to DCPP, mainly in the Pacific and some neighboring land regions. Temperature and sea-level pressure in several regions are predictable multiple decades ahead, and show significant added value over the unconstrained projections for forecasting the first 2 decades and the 20-year averages. We further demonstrate the suitability of regional constraints to attribute predictability to certain ocean regions. On the example of global average temperature changes, we confirm the role of Pacific variability in modulating the reduced rate of global warming in the early 2000s, and demonstrate the predictability of reduced global warming rates over the following 15 years based on the climate conditions leading up to 1998. Our results illustrate that constraining internal variability can significantly improve the accuracy of near-term climate change estimates for the next few decades. [ABSTRACT FROM AUTHOR]

In a future climate, the Hadley cell and associated trade easterlies are projected to expand poleward. This projected change in the atmospheric circulation is expected to impact the ocean through changes in the mean sea surface temperature (SST). We also expect implications for the large‐scale SST variability, because near‐surface wind is directly related to two drivers of the SST, that is, turbulent heat flux and anomalous wind‐driven Ekman heat flux. Previous studies show that in the subtropics, anomalous turbulent and Ekman heat fluxes oppose each other, acting to reduce SST variability, whereas, in the midlatitudes, they reinforce each other and enhance SST variability. Through analysis of reanalysis products and Coupled Model Intercomparison Project simulations, we find that the subtropical regions where the fluxes oppose each other are projected to expand poleward in a future climate, following the poleward expansion of the Hadley cell, with potential implications for the amplitude of subtropical SST variability. Plain Language Summary: Atmospheric surface wind drives upper ocean circulations. Hence, a change in the surface wind pattern causes a change in ocean circulation. There is evidence that climate change alters the spatial pattern and strength of the surface winds. In particular, easterly winds in the subtropics are projected to expand poleward. The poleward shift of the easterly wind belt also has potential implications for the sea surface temperature (SST) variations because two important drivers of the SST are related to the surface wind: air‐sea heat exchange and the advection of ocean temperature by the wind‐driven ocean current. In the subtropical region, these two mechanisms typically oppose each other, acting to reduce the amplitude of SST variations. We show that the projected poleward expansion of the easterly wind belt will subsequently expand the subtropical region where these two mechanisms oppose each other toward the poles, with potential implications for subtropical SST variability. Key Points: Anomalous Ekman heat flux opposes and reinforces the anomalous turbulent heat flux in the subtropics and midlatitudes, respectivelyThe latitudinal boundary where the role of anomalous Ekman heat flux changes is modulated by the Hadley cell edgeA poleward shift of the Hadley boundary in a future climate drives a poleward shift in the latitude where Ekman heat flux changes its role [ABSTRACT FROM AUTHOR]

A robust simulation of tropical Pacific Ocean decadal‐scale zonal wind stress trends can increase the confidence in projections of global surface temperature and regional sea level rise, yet coupled general circulation models simulate weaker trends than observed. When forced with observed sea surface temperatures, the atmospheric component of these models still simulates a weaker zonal wind stress trend during 1992–2008 (and 1992–2011) than observed. Yet, this decadal wind trend bias is not evident at the 850 hPa level. We show that the modelled wind trend bias is related to deficiencies in monthly wind stress variability. Furthermore, the strength of the connectivity between the surface and 850 hPa was also an indicator of the wind stress trend. Models that simulated a more realistic wind stress trend tend to simulate a stronger Pacific zonal sea level pressure gradient intensification than observed and a South Pacific Convergence Zone shift like that observed. Plain Language Summary: From early 1990s to late 2010, the equatorial trade winds strengthened in the Pacific Ocean. This coincided with a rapid rise in sea level in the western tropical Pacific and a slowdown in global surface temperature warming. This long‐term change in the Pacific had a large impact, but ocean‐atmosphere models cannot produce a change of a similar strength as observed. It is difficult to determine whether the ocean or atmosphere components of the model are at fault. This study looks at factors contributing to the inability of models to simulate trade wind strengthening by focusing on the atmosphere component of the model. We find that improving the month‐to‐month response of wind to sea surface temperatures may improve the strength of long‐term wind changes. Improvements in wind changes may also be made if models improve their connection between winds higher in the atmosphere and those at the surface. Models that have stronger wind changes have a better representation of south tropical Pacific rainfall changes, but also rely on a stronger than observed sea level pressure response. Key Points: Atmosphere‐only models, like coupled models, struggle to simulate tropical Pacific wind stress trends of similar magnitude to observedModels with weaker wind trends tend to have a weaker monthly wind response to sea surface temperatures and a weaker link to winds aloftModels with strong trends have a better South Pacific Convergence Zone shift but a too strong sea level pressure gradient [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) variability in the tropical Pacific (TP) has world‐wide climate and economic influences; hence, improving its prediction is of great interest. Recent modeling studies suggested that north tropical Atlantic variability can influence TP SST variability and prediction. However, models are subject to biases in both the tropical Atlantic and Pacific, and it remains an open question whether these biases affect the trans‐basin linkages and the prediction of TP SST. To investigate this issue, we apply linear inverse modeling to observations and climate models. We find that removing north tropical Atlantic forcing indeed strongly lowers TP SST multiyear prediction skills in observations and models. Models show a large spread of the Atlantic trans‐basin effect, which leads to markedly different TP SST multiyear prediction skills. Our study highlights the necessity to narrow the inter‐model spread for more reliable TP SST multiyear prediction. Plain Language Summary: Tropical Pacific (TP) sea surface temperature (SST) variability is important for climate but hard to predict. Many modeling studies argued that the variability and prediction can be influenced by north tropical Atlantic trans‐basin effect, while the reliability of this argument is influenced by strong model biases. To reveal the influence of model biases in simulating the Atlantic trans‐basin modulation on TP SST prediction, we apply a commonly used tool for prediction called linear inverse modeling. We find that prediction skills of TP SST will strongly decrease without the north tropical Atlantic forcing in both observations and models. However, striking inter‐model spread exists in the strength of the Atlantic trans‐basin effect, resulting in strong inter‐model spread in TP SST multiyear prediction skills. Therefore, more efforts are needed to reduce the inter‐model spread of the Atlantic trans‐basin effect in the future. Key Points: Observational evidence supports that north tropical Atlantic forcing substantially improve tropical Pacific (TP) sea surface temperature (SST) multiyear predictionClimate models show strong inter‐model spread in simulating north tropical Atlantic trans‐basin effectThe spread of north tropical Atlantic forcing in climate models strongly affects TP SST multiyear prediction skills [ABSTRACT FROM AUTHOR]

This study applies a quantitative method to analyze variation in Pacific trade winds in 1950–2020. It's found that the southeast and northeast trade winds enter an acceleration period since the mid to late 1990s while the wind are enhanced in each month of the year. The enhancement of southeast trade wind is less intense than that of northeast trade wind. Further analysis shows that after the acceleration, the correlation between sea level pressure/sea surface temperature (SST) and the trade wind speed in the Pacific subtropical high area are obviously strengthened. The negative correlation between sea level pressure and SST in this area is also enhanced, which may be due to the enhancement of trade wind that have caused a decrease in SST. The reduction further enhanced and maintained a higher sea level pressure, thus forming a configuration which is conducive to the intensification of trade wind. Plain Language Summary: Under the background of global climate change in recent decades, the Pacific trade winds have strengthened since the mid to late 1990s. Further analysis shows that after the intensification, the correlation between sea level pressure/sea surface temperature (SST) and the trade wind speed in the Pacific subtropical high area are obviously strengthened. In the Pacific subtropical high area, the reverse changes of SST and sea level pressure are more obvious. This may be due to the fact that when the trade wind is strong, sea temperature decreases and the reduction maintains a higher sea level pressure. This reverse relationship helps to maintain the trade wind at a strong level. The discovery of this relationship is important for future research on global climate change related to trade winds. Key Points: The Pacific trade winds strengthened in the 1990s and the enhancement of southeast trade wind is less than that of northeast trade windAfter acceleration, correlation between sea level pressure/sea surface temperature (SST) and wind speed in Pacific subtropical high area enhanceThe acceleration of Pacific trade wind could possibly be related to the synergy of sea level pressure and SST [ABSTRACT FROM AUTHOR]

Near-term projections of climate change are subject to substantial uncertainty from internal climate variability. Here we present an approach to reduce this uncertainty by sub-selecting those ensemble members that more closely resemble observed patterns of ocean temperature variability immediately prior to a certain start date. This constraint aligns the observed and simulated variability phases and is conceptually similar to initialization in seasonal to decadal climate predictions. We apply this variability constraint to large multi-model projection ensembles from the Coupled Model Intercomparison Project phase 6 (CMIP6), consisting of more than 200 ensemble members, and evaluate the skill of the constrained ensemble in predicting the observed near-surface temperature, sea-level pressure and precipitation on decadal to multi-decadal time scales. We find that the constrained projections show significant skill in predicting the climate of the following ten to twenty years, and added value over the ensemble of unconstrained projections. For the first decade after applying the constraint, the global patterns of skill are very similar and can even outperform those of the multi-model ensemble mean of initialized decadal hindcasts from the CMIP6 Decadal Climate Prediction Project (DCPP). In particular for temperature, larger areas show added skill in the constrained projections compared to DCPP, mainly in the Pacific and some neighboring land regions. Temperature and sea-level pressure in several regions are predictable multiple decades ahead, and show significant added value over the unconstrained projections for forecasting the first two decades and the 20-year averages. We further demonstrate the suitability of regional constraints to attribute predictability to certain ocean regions. On the example of global average temperature changes, we confirm the role of Pacific variability in modulating the reduced rate of global warming in the early 2000s, and demonstrate the predictability of reduced global warming rates over the following 15 years based on the climate conditions leading up to 1998. Our results illustrate that constraining internal variability can significantly improve the accuracy of near-term climate change estimates for the next few decades. [ABSTRACT FROM AUTHOR]

The equatorial Pacific is the largest oceanic source of carbon dioxide to the atmosphere. This outgassing varies depending on the El Niño‐Southern Oscillation (ENSO) and decadal climate variability. New production, the amount of phytoplankton net primary production driven by upwelled nitrate, plays a significant role in modulating air‐sea CO2 fluxes as the biological carbon pump removes carbon from the surface ocean. We aim to understand how the physical drivers of sea surface temperature and wind speed influence interannual and decadal variability of the equatorial Pacific carbon cycle. In the equatorial Pacific, there are three biogeochemical regimes: the upwelling cold tongue east of 140°W and south of the equator (3°N–15°S); the eastern Pacific warm pool north of the equator (3°–15°N); and the 28.5°C western Pacific warm pool, west of 140°W. We find that between 2000 and 2020, air‐sea CO2 flux and ΔpCO2 increased in the cold tongue (45 mmolC m−2 yr−2, 1.5 μatm yr−1, respectively) but decreased elsewhere, while new production decreased everywhere. The western Pacific occasionally became a weak carbon sink, depending on ENSO and this sink was strongest at 165°E during central Pacific "Modoki" El Niño events. We find that changes in wind speed, temperature and ENSO frequency have altered the surface carbon budget. The mean basin‐wide (150°E−90°W and 15°N–15°S) new production for 2000–2020 was 1.2 ± 0.1 PgC yr−1 and air‐sea CO2 flux was 0.5 ± 0.1 PgC yr−1. New production decreased at −7.7 ± 1.6 TgC yr−2, compared to the CO2 flux trend of −1.7 ± 1.4 TgC yr−2. Plain Language Summary: The equatorial Pacific is the largest source of carbon dioxide that outgasses from the ocean into the atmosphere. This outgassing varies depending on the El Niño‐Southern Oscillation (ENSO) and decadal changes in the climate. Winds in the eastern Pacific drive upwelling which supplies nutrients and carbon to the surface. Some of this carbon is outgassed to the atmosphere, some is consumed by phytoplankton and can either move through the food web or sink into the ocean interior. We are interested in how much carbon is removed from the surface equatorial Pacific through outgassing of carbon and biological consumption. We find that changes in wind speeds, surface temperatures, freshening surface water and changing patterns of ENSO have influenced the equatorial Pacific carbon budget. For example, between 2000 and 2020, CO2 release to the atmosphere increased in the upwelling zone but decreased elsewhere, while biological consumption decreased everywhere. The western Pacific occasionally absorbs carbon from the atmosphere during central Pacific "Modoki" El Niño events, a particular type of El Niño that is becoming more frequent. Key Points: Decadal shifts in upwelling have increased CO2 flux in the cold tongue but not biological production, which is likely nutrient limitedIncreasing frequency of central Pacific El Niño events may amplify decadal CO2 flux trends and drive sink conditions in the westIncreasing surface temperatures contribute to stratification and decreasing new production despite increasing dissolved inorganic carbon [ABSTRACT FROM AUTHOR]

The Interdecadal Pacific Variability (IPV) has global climate effects but its decadal prediction is a challenge. Here, we use linear inverse models to study the optimal growth of the IPV and reveal important precursor patterns. We find that the optimal growth of the IPV exists in observations due to interference among various sea surface temperature (SST) decadal variations. Small SST anomalies extending from the northeastern subtropical Pacific to the central tropical Pacific, in concert with thermocline dynamics, can develop into a mature phase of the IPV after ∼3 years. Furthermore, the initial SST anomalies lag the Atlantic Multidecadal Variability (AMV) by up to a decade, suggesting the potential importance of the AMV in IPV decadal predictability. As climate models commonly underestimate the AMV trans‐basin effect, our study calls for more efforts on reducing the model bias to improve IPV decadal prediction. Plain Language Summary: The Interdecadal Pacific Variability (IPV), the leading decadal mode in the Pacific Ocean, can modulate global climate system with broad ecological and socioeconomic influences. However, as partly the residual of El Niño‐Southern Oscillation that shows predictability lower than 2 years, the IPV may be hard to be predicted several years ahead. Here, by using a method called optimal growth, we reveal that an ∼3‐year growth of IPV variance is associated with northeastern subtropical Pacific SST variability and ocean dynamics. Specifically, the Atlantic Multidecadal Variability (AMV), a main climate mode in the Atlantic Ocean, can provide potential decadal predictability to the optimal growth of the IPV. This trans‐basin effect is, however, underestimated in general circulation models that are commonly used for climate prediction. Hence, our study implies that reducing the model bias of the AMV trans‐basin effect will lead to more reliable IPV decadal prediction. Key Points: The Interdecadal Pacific Variability (IPV) exhibits ∼3‐year optimal growth, a time length beyond the upper bound of El Niño‐Southern Oscillation predictabilityThe optimal growth of the IPV is associated with northeastern subtropical sea surface temperature variability and pan‐tropical Pacific thermocline dynamicsThe Atlantic Multidecadal Variability leads the optimal growth of the IPV by up to a decade, an important factor for IPV decadal prediction [ABSTRACT FROM AUTHOR]

This study investigates the large-scale factors controlling interannual variability in dust aerosols over South Asia during 2001–2018. We use a parameter DA % , which refers to the frequency of days in a year when high dust activity is experienced over a region, as determined by a combination of satellite aerosol optical depth and the Ångström exponent. While a positive sea surface temperature (SST) anomaly in the central Pacific Ocean was important in controlling DA % over South Asia during 2001–2010; in recent years, the North Atlantic Ocean has assumed a dominant role. Specifically, high DA % is associated with warming in the midlatitude and cooling in the subtropical North Atlantic SSTs: the location of the two southern arms of the North Atlantic SST tripole pattern. This shift towards a dominant role of the North Atlantic SST in controlling DA % over South Asia coincides with a recent shift towards a persistently positive phase of the North Atlantic oscillation (NAO) and a resultant positive phase of the springtime SST tripole pattern. Interestingly, there has also been a shift in the relation between the two southern arms of the SST tripole and NAO, which has resulted in weakening of the southwest monsoon circulation over the northern Indian Ocean and strengthening of the dust-carrying westerlies and northerlies in the lower troposphere and mid-troposphere. Simulations with an Earth system model show that the positive phase of the North Atlantic SST tripole pattern is responsible for a 10 % increase in the dust optical depth over South Asia during May–September; with increases as high as 30 % during the month of June. This increase is mainly due to transport by the westerlies at the 800 hPa pressure level, which increases the dust concentration at this pressure level by 20 % on average during May–September and up to 50 % during June. [ABSTRACT FROM AUTHOR]

In this study, we investigate the interdecadal variation characteristics of the dominant mode of summer precipitation over East Asia‐Northwest Pacific (EA‐NWP) before and after late 1990s, reveal the physical mechanism, and address its possible connection with the regional ocean warming background. Study results show that the empirical orthogonal function mode 1 (EOF1) of summer precipitation anomalies over EA‐NWP during the global ocean warming acceleration period (1979–1998) present a more significant dipole pattern between Yangtze River Basin (YRB) and tropical NWP compared with that during the global ocean warming slowdown period (1999–2012). And then we find that the anomalous dipoles of low‐level winds, geopotential height, convection, and associated wave activity fluxes over NWP significantly shift westward (eastward) and enhance (weaken) in 1979–1998 (1999–2012), which are regarded as the direct cause of the adjustment of dipole pattern between YRB and tropical NWP. Further analysis reveals that the negative sea surface temperature (SST) anomalies in the tropical NWP are prominent (not prominent), resulting in the stronger (weaker) Pacific–Japan Teleconnection over Japan and Tropics in NWP during the period 1979–1998 (1999–2012), and ultimately lead to the change of summer precipitation EOF1 in EA‐NWP. By regressing vertical velocity, outgoing longwave radiation, relative vorticity, wave activity fluxes, and precipitation onto the NWP SST in the two periods, the impacts of the SST in the NWP on the interdecadal variation of summer precipitation EOF1 in EA‐NWP are verified. Finally, the mechanism diagram of interdecadal variation of summer precipitation is given, which can be useful for short‐term precipitation prediction of East Asia. [ABSTRACT FROM AUTHOR]

The global mean surface temperature has experienced different warming stages, but there is less work on discussion of the regional climate changes under the background of different global warming rates. This study investigated the surface temperature (ST)‐related trends in the East Asian summer monsoon (EASM) and underlying mechanisms during three warming stages, with a view to develop a new understanding of the responses of the EASM to global warming. Based on the statistical method of piecewise linear regression (PLR) to ST anomaly time series, the ST variation process from 1958 to 2014 has been divided into three stages with different linear trends: 1958–1975 (hiatus), 1976–1997 (acceleration) and 1998–2014 (slowdown). During the first stage, accompanied by two anomalous lower‐level cyclones, a weakening trend of the southerly winds over eastern China is observed. The enhanced cyclone over the Yellow Sea, with local ST cooling, is associated with the CGT‐like (circumglobal teleconnection) pattern, while the other one over the Philippines can be attributed to the local sea surface temperature (SST) warming. The ST‐related anomalous EASM circulation exhibits an enhanced lower‐level anticyclone over the northern Philippine Sea during the second stage. The warming around Lake Baikal and the phase change of the East Atlantic/Western Russia (EAWR) pattern can weaken upper‐level East Asian jet stream (EAJS) and then provide a favourable condition for the descent related to the lower‐level anticyclonic anomaly. As for the third stage, the anomalous ST and related EASM circulation both display a dipole pattern. Through influencing the meridional thermal contrast and distribution of convergence over East Asia (EA), the interaction between anomalous ST and EASM circulation may be the major mechanism that results in the increased (decreased) ST and enhanced barotropic anticyclone (cyclone) over southern (northern) EA. [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated that North Tropical Atlantic (NTA) warming can be modulated by El Niño events through atmospheric teleconnections. Considering the diversity of El Niño events, this study focuses on different responses of the NTA sea surface temperature (SST) to eastern Pacific (EP), central Pacific type I and type II (CP‐I and CP‐II) El Niño events in their decaying springs. In EP and CP‐II El Niño events, the pattern of NTA warming is well established, while the NTA SST fails to warm in CP‐I El Niño events. In EP and CP‐II El Niño events, anomalous cyclones can be excited in the subtropical Atlantic. Consequently, anomalous southwesterly winds in the NTA weaken the background northeasterly winds and suppress oceanic latent heat loss, leading to NTA warming. However, in CP‐I El Niño events, anomalous anticyclones are excited in the subtropical Atlantic and anomalous northeasterly winds in the NTA strengthen the background northeasterly winds, which facilitate oceanic latent heat loss and lead to NTA cooling. The inconsistent remote atmospheric responses of the NTA are attributed to the different patterns of warm and/or cold SST anomalies in the tropical Pacific in the decaying springs of different types of El Niño events. The physical mechanism linking the SSTs in the tropical Pacific and the NTA has been verified in a slab ocean model. Key Points: Diverse El Niño events produce different impacts on the North Tropical Atlantic (NTA) sea surface temperature (SST) in the decaying springsThe NTA warms in eastern Pacific and central Pacific type II El Niño events but fails to warm as expected in central Pacific type I El Niño eventsThe distinct responses of the NTA to diverse El Niño events are attributed to the different patterns of SSTs in the tropical Pacific [ABSTRACT FROM AUTHOR]

The Interdecadal Pacific Oscillation (IPO) features a basin‐scale horseshoe‐like sea surface temperature (SST) anomaly pattern. Its cold‐phase shift around 1999, implicated as a driver for the early‐2000s global warming slowdown, has been potentially linked to the Atlantic warming during the positive‐phase Atlantic Multidecadal Variability (AMV). However, the key mechanism for the trans‐basin Atlantic‐Pacific teleconnection remains debatable. Here, we show that an AMV‐SST cooling can initiate a pan‐Pacific response. The North Atlantic cooling induces westerly wind anomalies over the central‐western equatorial Pacific as Kelvin‐wave responses and easterly wind anomalies over the far‐eastern equatorial Pacific as Rossby‐wave responses. Additionally, anomalous lows are generated over the extratropical North and South Pacific through the midlatitude Rossby wave propagation. The tropical and midlatitude teleconnections act together to develop into a warm‐phase IPO‐like pattern through the wind‐induced latent heat. Our results suggest a potential of advancing the predictability of IPO through a skillful simulation of AMV. Plain Language Summary: The Interdecadal Pacific Oscillation (IPO) is the most prominent mode of decadal climate variability over the whole Pacific basin, characterized by a horseshoe‐shaped sea surface temperature (SST) anomaly pattern. Its Atlantic counterpart, the Atlantic Multidecadal Variability (AMV) is characterized by a basin‐wide SST warming or cooling pattern across the North Atlantic. In recent decades, the Atlantic warming strongly tied to the positive phase of AMV has significantly strengthened the tropical Pacific cooling, contributing to the recent global warming hiatus. Yet, the root cause of the trans‐basin Atlantic‐Pacific teleconnection on decadal timescales remains unclear. In this study, using the idealized pacemaker simulations with an imposed AMV‐SST anomaly pattern in a fully coupled model, we rigorously investigate how an AMV‐SST cooling impacts the Pacific‐basin decadal‐scale climate change. We found that an AMV cooling initiates a tropical Gill‐type response and midlatitude Rossby wave trains. The tropical and midlatitude teleconnections work together to form a warm‐phase IPO‐like SST anomaly pattern. Our results highlight that realistic simulation of the AMV may improve the predictability of the IPO state and predictions of global warming rates. Key Points: The Atlantic Multidecadal Variability cooling can drive an Interdecadal Pacific Oscillation‐like warm‐phase patternThe North Atlantic cooling excites a tropical Gill‐type response and midlatitude Rossby wave train responsesBoth the tropical and midlatitude dynamical pathways are central to the decadal‐scale trans‐basin Atlantic‐Pacific connection [ABSTRACT FROM AUTHOR]

Growth-increment widths of Pacific geoduck (Panopea generosa), a long-lived bivalve, are used to develop the first marine-based, multicentennial, annually resolved, and exactly dated archive of Northeast Pacific sea surface temperatures (SST). The chronology is sampled from the Tree Nob Islands, British Columbia, Canada, continuously covers 1725-2008, and also contains nine older radiocarbondated segments, which together span 58% of the last 1,500 years. Age-related growth declines were removed by aligning all increments relative to age of increment formation and fitting with a single detrending curve to preserve low-frequency signals. The geoduck chronology was used to reconstruct local SST variability across the seasonal window of April through November. The chronology at both the concurrent (lag-0) and following (lag+1) year are both highly significant predictors of SST in a stepwise multiple linear regression, explaining 54% of the variance in the period of instrumental overlap (1940-2001), passing strict tests of calibration-verification. Reconstructed SSTs contained significant spectral power at periods from 3 to 64 years, suggesting that 20th century variability in these periodicities is not unusual in the longer-term context. The period of lowest growth coincided with the Dalton minimum, an episode of reduced solar irradiance from 1790-1830, as well as the 1809 Unknown eruption, suggesting that solar and volcanic signals are present in the SST history. The most conspicuous aspect of the reconstruction is the steady and unprecedented warming trend that began in the mid-1800s and continues through present. The post-1976 interval includes the two warmest decades of the reconstruction. [ABSTRACT FROM AUTHOR]

Climate variability on multidecadal timescales appears to be organized in pronounced patterns with clear expressions in sea surface temperature, such as the Atlantic Multidecadal Variability and the Pacific Decadal Oscillation. These patterns are now well studied both in observations and global climate models and are important in the attribution of climate change. Results from CMIP5 models have indicated large biases in these patterns with consequences for ocean heat storage variability and the global mean surface temperature. In this paper, we use two multi-century Community Earth System Model simulations at coarse (1 ∘) and fine (0.1 ∘) ocean model horizontal grid spacing to study the effects of the representation of mesoscale ocean flows on major patterns of multidecadal variability. We find that resolving mesoscale ocean flows both improves the characteristics of the modes of variability with respect to observations and increases the amplitude of the heat content variability in the individual ocean basins. In the strongly eddying model, multidecadal variability increases compared to sub-decadal variability. This shift of spectral power is seen in sea surface temperature indices, basin-scale surface heat fluxes, and the global mean surface temperature. This implies that the current CMIP6 model generation, which predominantly does not resolve the ocean mesoscale, may systematically underestimate multidecadal variability. [ABSTRACT FROM AUTHOR]

The Institut Pierre‐Simon Laplace Climate Modeling Center has produced an ensemble of extended historical simulations using the IPSL‐CM6A‐LR climate model. This ensemble (referred to as IPSL‐EHS) is composed of 32 members over the 1850–2059 period that share the same external forcings but differ in their initial conditions. In this study, we assess the simulated decadal to multidecadal climate variability in the IPSL‐EHS. In particular, we examine the global temperature evolution and recent warming trends, and their consistency with ocean heat content and sea ice cover. The model exhibits a large low‐frequency internal climate variability. In particular, a quasi‐bicentennial mode of internal climate variability is present in the model and is associated with the Atlantic Meridional Overturning Circulation. Such variability modulates the global mean surface air temperature changes over the historical period by about ∼ 0.1K. This modulation is found to be linked to the phase present in the initial condition state of each member. This variability appears to decrease during the 1850–2018 period in response to external forcings. The analysis of the ocean heat content reveals furthermore an overestimation of the ocean stratification, which likely leads to an overestimation of the recent warming rate on average. Plain Language Summary: The Institut Pierre‐Simon Laplace (IPSL) developed an ensemble of 32 simulations over the 1850–2059 period using the IPSL‐CM6A‐LR climate model. Such a large ensemble allows a better sampling of the internally generated variability. Moreover, the ensemble averaging provides an estimation of the forced variability induced by the greenhouse gases and the aerosol concentration used as boundary conditions. In this study, we assess the simulated decadal to multidecadal climate variability in the IPSL ensemble. Relative to the large variability of the model, the evolution of observed surface temperature and sea ice cover is within the range of possibilities of the ensemble. The oceanic circulation and sea surface temperature over the North Atlantic are key players in the low‐frequency internal variability of the model. Key Points: A large part of the spread of temperature and sea ice trends in the IPSL ensemble is related to a large multicentennial internal variabilitySome members of the IPSL ensemble are consistent with the observed surface temperature, sea ice variations, and ocean heat content evolutionThe low‐frequency internal climate variability of IPSL‐CM6A‐LR decreases since the 2000s in response to external forcing [ABSTRACT FROM AUTHOR]

Tropical precipitation anomalies show very different patterns over the central‐eastern equatorial Pacific between the developing and decay phases of extreme El Niño despite similar patterns of sea surface temperature (SST) anomalies. Using observational data and atmospheric simulations for 1979–2018, we identified a meridional dipole mode of tropical precipitation variability, called the Pacific precipitation dipole mode (PPDM), which is critical for seasonally varying rainfall patterns related to extreme El Niño. We examined the mechanism of PPDM using a numerical model and found that the seasonal shift of the warm SST band in the eastern equatorial Pacific, when coupled with extreme SST anomalies, causes phase reversal of the PPDM. The PPDM can be regarded as a different manifestation of the El Niño/Southern Oscillation combination mode, as revealed by the surface wind anomalies, which also explains the distinct impact of extreme El Niño events over East Asia. Plain Language Summary: The rainfall patterns over the tropical Pacific are altered by the underling sea surface temperatures (SSTs). The patterns of rainfall anomalies, that is, deviation from climatological mean states, vary between the developing and decay seasons of extreme El Niño, despite similarity in patterns of SST anomalies. Using observational data and atmospheric simulations driven by observed SST for 1979–2018, we identified a meridional dipole mode of tropical rainfall variability that was asymmetric about the equator, called the Pacific precipitation dipole mode (PPDM). PPDM changes the polarity between autumn and spring, before and after the peak of extreme El Niño, and therefore is critical for seasonally varying rainfall patterns related to extreme El Niño. We examined factors that control the polarity of the PPDM by using a mechanistic model of the atmosphere and found that a coupling between SST anomalies associated with extreme El Niño and the seasonal cycle of the warm SST band in the eastern Pacific, migrating between the two hemispheres in a year, causes PPDM to change polarity between autumn and spring. The excitation of PPDM explains a distinct climatic impact over East Asia during extreme El Niño and also a feedback to extreme El Niño. Key Points: The second leading mode of tropical Pacific precipitation variability shows a meridional dipole pattern called the Pacific precipitation dipole mode (PPDM)PPDM amplifies and reverses its sign between developing and decay phases of extreme El Niño, indicating a nonlinear feature of ENSOCoupling between sea surface temperature anomalies and seasonal cycle in the eastern Pacific excites PPDM, which is a manifestation of ENSO combination mode [ABSTRACT FROM AUTHOR]

Observational and modeling evidence suggest a recent acceleration of the stratospheric Brewer-Dobson circulation (BDC), driven by climate change and stratospheric ozone depletion. However, slowly varying natural variability can compromise our ability to detect such forced changes over the relatively short observational record. Using observations and chemistry-climate model simulations, we demonstrate a link between multi-decadal variability in the strength of the BDC and the Interdecadal Pacific Oscillation (IPO), with knock-on impacts for composition in the stratosphere. After accounting for the IPO-like variability in the BDC, the modeled trend is approximately 7%-10% dec-1 over 1979-2010. Furthermore, we find that sea surface temperatures explain up to 50% of the simulated decadal variability in tropical mid-stratospheric ozone. Our findings demonstrate strong links between low-frequency variability in the oceans, troposphere and stratosphere, as well as their potential importance in detecting structural changes in the BDC and future ozone recovery. Plain Language Summary Natural variability is a key element of the climate system and can mask human-induced changes. Here, we are interested in the naturally varying strength of the stratospheric global circulation and how this impacts the composition of the stratosphere. Using observations and model simulations, we show that slowly changing (multi-decadal) natural variability in the Pacific Ocean is reflected in the stratospheric circulation. This link helps us to better understand structural changes in the stratospheric circulation arising due to human interferences. In turn, slow transport variability reconciles recent low levels of ozone in the middle tropical stratosphere, which otherwise are in disagreement with the expected ozone recovery. These results have implications for both reconciling theory and observed changes in the stratospheric circulation, as well as better understanding the rate of stratospheric ozone recovery. [ABSTRACT FROM AUTHOR]

Change in global mean surface temperature (ΔGMST), based on a blend of land air and ocean water temperatures, is a widely cited climate change indicator that informs the Paris Agreement goal to limit global warming since preindustrial to "well below" 2°C. Assessment of current ΔGMST enables determination of remaining target‐consistent warming and therefore a relevant remaining carbon budget. In recent IPCC reports, ΔGMST was estimated via linear regression or differences between decade‐plus period means. We propose nonlinear continuous local regression (LOESS) using ±20 year windows to derive ΔGMST across all periods of interest. Using the three observational GMST data sets with almost complete interpolated spatial coverage since the 1950s, we evaluate 1850–1900 to 2019 ΔGMST as 1.14°C with a likely (17%–83%) range of 1.05°C–1.25°C, based on combined statistical and observational uncertainty, compared with linear regression of 1.05°C over 1880–2019. Performance tests in observational data sets and two model large ensembles demonstrate that LOESS, like period mean differences, is unbiased. However, LOESS also provides a statistical uncertainty estimate and gives warming through 2019, rather than the 1850–1900 to 2010–2019 period mean difference centered at the end of 2014. We derive historical global near‐surface air temperature change (ΔGSAT), using a subset of CMIP6 climate models to estimate the adjustment required to account for the difference between ocean water and ocean air temperatures. We find ΔGSAT of 1.21°C (1.11°C–1.32°C) and calculate remaining carbon budgets. We argue that continuous nonlinear trend estimation offers substantial advantages for assessment of long‐term observational ΔGMST. Key Points: Continuous local regression is an alternative to traditional IPCC temperature change estimation methodsGlobal warming, estimated from combined land and sea‐surface temperature observational series with enhanced surface coverage, reached 1.14°C in 2019 relative to 1850–1900 (likely range 1.05°C–1.25°C)Global surface air temperature anomalies reached 1.21°C in 2019 relative to 1850–1900 (1.11°C–1.32°C), implying a remaining carbon budget of ∼220 GtCO2 to limit warming to 1.5°C [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) could significantly affect the dynamic and thermodynamic conditions of the atmospheric circulation and consequently the cloud variations. Here we use several different satellite records to extract the spatial‐time modes of total cloud cover (TCC) by employing a pairwise rotation of Empirical Orthogonal Function analysis. The results show that the first two principal oscillation modes of TCC are closely associated with the Central Pacific El Niño Southern Oscillation (CP ENSO) and Eastern Pacific (EP) ENSO during the 1980s–2000s, while the ENSO‐like mode of TCC can provide an evident contribution to the TCC change during the 2000s–2010s. In CP El Niño, the cloud vertical structure decomposed from CloudSat observations shows an increase of cloud occurrence frequency near the equator and around 40°, and a decrease around 10° in both hemispheres, suggesting a symmetric tightening of Hadley cell (HC). In addition, cloud occurrence frequency increases around 180°, which is accompanied by an eastward shift of Walker circulation (WC). In EP El Niño, TCC increases (decreases) over the Equatorial Eastern Pacific (Western Pacific warm pool), and decreases asymmetrically over the subtropical Pacific Ocean, indicating a weakening of WC and an asymmetric tightening of HC, respectively. The different responses of circulation and clouds to CP and EP El Niño highlight the nonlinearity of El Niño SST forcing. We also construct a trend mode of TCC to investigate cloud long‐term responses to SST warming by transferring the linear trends of the rest modes to a specific mode. The principal components (PCs) of TCC trend modes are strongly correlated with global‐mean SST (GSST) with correlation a coefficient of about 0.60 during the 1980s–2000s and 0.45 during the 2000s–2010s, suggesting a continued influence of global SST warming on TCC. The global TCC change is mainly influenced by the combined effects of Atlantic Multi‐decadal Oscillation (AMO), Pacific Decadal Oscillation (PDO) and Indian Ocean Dipole (IOD). The variation about the trend mode of TCC is closely associated with PDO and IOD. [ABSTRACT FROM AUTHOR]

The global mean surface temperature does not increase monotonously over the past decades, with accelerated warming from the late 1970s to 1998 and followed by the warming slowdown during 1998–2013 (the so‐called warming hiatus). This study assessed and compared the temporal evolutions of sea surface temperature (SST) trends in the East China Seas (ECS, including Bohai, Yellow and East China Sea) and other key ocean regions. The results revealed an amplified response of ECS SST to both global warming acceleration and hiatus. This amplification was manifested as remarkably enhanced winter warming rates in the ECS of over 1.1 and 0.8°C⋅decade–1 for the 1980–1989 and 1990–1999, respectively, and as an obvious cooling trend of nearly −0.5°C⋅decade‑1 during 1998–2013. The enhanced response can be explained well by the combined effects of the Kuroshio Current (KC) and East Asian Winter Monsoon (EAWM). The weakened EAWM during the 1980s, accompanied by lower wind speeds and higher air temperatures, impeded the releases of latent and sensible heat fluxes from the ocean to the atmosphere, resulting in rapid ECS warming. During 1998–2013, the strengthened EAWM led to the enhanced cooling trend in the ECS. In addition, the volume transport of the KC east of Taiwan strengthened (weakened) during the 1990s (2000s), which might also have contributed to the enhanced warming (cooling) in the ECS. Our research suggests their sensitivity makes the ECS potentially vulnerable to the effects of global climate change, which has important implications regarding regional impact assessments. [ABSTRACT FROM AUTHOR]

It has been reported that the Southern Hemisphere oceans experienced rapid warming during the decade‐long global surface warming slowdown (2003–2012) and the earlier period of the Argo record (2006–2013). In this study, we analyze updated observations to show that this rapid warming has slowed down, leading to less contribution of the Southern Hemisphere oceans to the global ocean heat storage (∼65% over the available Argo period 2006–2019). Two warming hotspot regions, the southeast Indian Ocean and South Pacific Ocean, have experienced cooling over 2013–2019. This decadal shift is related to variations in the Southern Annular Mode and Interdecadal Pacific Oscillation. The isopycnal deepening (shoaling) forced by changing winds dominated the regional ocean temperature changes over the earlier warming (later cooling) period. Our finding demonstrates how decadal variability modulates long‐term climate change and provides important observational information for the ongoing calibration of decadal prediction systems. Plain Language Summary: The global ocean has been warming up, as it absorbs most of the extra heat in the Earth system due to anthropogenic climate change. Regional patterns and magnitudes of ocean warming vary with time due to the natural fluctuations of the coupled air‐sea system. Since 2006, the international Argo program has been providing continuous real‐time ocean observations in the upper 2,000 m from a near‐global array of autonomous profiling floats. Previous studies based on the earlier Argo record (2006–2013) have shown rapid warming in the Southern Hemisphere Oceans and most of the global ocean heat storage occurred in the Southern Hemisphere. However, updated observations show that this rapid warming has slowed down and was even changed to cooling in certain regions since 2013. This shift is mainly driven by changing winds through the downwelling of the upper‐layer warmer waters over the earlier period and the upward pumping of subsurface cooler waters over the later period. Overall, the findings of this study highlight the importance of maintaining the current ocean observing system in order to obtain extended and reliable ocean records, which are necessary for revealing the long‐term climate change signals of anthropogenic warming. Key Points: The rapid warming in the Southern Hemisphere extratropical ocean has slowed down since around 2013The observed shift is related to decadal variations in the Southern Annular Mode and Interdecadal Pacific OscillationThe regional ocean temperature changes are primarily induced by the vertical heave of isopycnals in response to changing winds [ABSTRACT FROM AUTHOR]

Fifteen years of satellite observations of sea surface temperature (SST) are used to identify frontal activities in the eastern Pacific Ocean. The frontogenesis and variability of frontal activities are mainly determined by wind forces. Strong frontal activities are distributed mainly near the coast and decrease with increasing offshore distance. The seasonal cycle dominates the variability in the SST frontal probability (FP) in coastal regions, and large interannual variability is identified in the tropics. Coastal frontal activities are driven by the local wind such that the equatorward wind stress forces coastal upwelling, which brings subsurface cold water to the surface and induces fronts. Frontal activities are generally enhanced during local summer, when an upwelling-favorable wind prevails. A prominent offshore migration pattern is identified for mid-latitude regions, and the associated propagation speed is consistent with the wind-induced offshore Ekman transport and mesoscale eddies. The coastal tropical Pacific is also characterized by a strong FP with large seasonal variability. The corresponding fronts result from the confluence between the boundary current and wind-driven cold water as the wind jet passes through mountain gaps. The frontal activities near the equator are driven by wind-induced tropical upwelling associated with a westward propagation at speeds close to the speed of Rossby waves. In the tropics, strong interannual variability in FP is identified that is, modulated by the El Niño-Southern Oscillation. Frontal activities are suppressed during El Niño years because trade winds are characterized by strong convergence, which induces downwelling and subsequently reduces FP. Plain Language Summary Oceanic fronts delineate the boundaries of different water masses and frontal zones are associated with intensive dynamics and high levels of nutrients. The eastern Pacific Ocean is characterized by abundant fronts, which are important for supplying fishery resources. In this study, 15 years of satellite observations of sea surface temperature are used to identify the frequency of frontal occurrence, namely, frontal probability (FP), and its associated variability. High FP distributes generally near the coast and decreases with offshore distance. In coastal regions, high FP is driven mainly by the local wind such that the equatorward wind stress forces coastal upwelling, which generates fronts. Frontal activities are generally enhanced during local summer when upwelling-favorable wind prevails. In mid-latitude regions, the fronts generated nearshore migrate offshore with propagation speeds close to the speed of Rossby waves. In the tropical Pacific Ocean, trade winds drive strong divergence, which induces upwelling that subsequently drives fronts. Large interannual variability is also modulated by the El Niño-Southern Oscillation. Frontal activities are suppressed during El Niño years and elevated during La Niña years because of the corresponding wind dynamics. [ABSTRACT FROM AUTHOR]

The El Niño–Southern Oscillation (ENSO) influences the most extensive tropospheric circulation cells on our planet, known as Hadley and Walker circulations. Previous studies have largely focused on the effect of ENSO on the strength of these cells. However, what has remained uncertain is whether interannual sea surface temperature anomalies can also cause synchronized spatial shifts of these circulations. Here, by examining the spatiotemporal relationship between Hadley and Walker cells in observations and climate model experiments, we demonstrate that the seasonally evolving warm-pool sea surface temperature (SST) anomalies in the decay phase of an El Niño event generate a meridionally asymmetric Walker circulation response, which couples the zonal and meridional atmospheric overturning circulations. This process, which can be characterized as a phase-synchronized spatial shift in Walker and Hadley cells, is accompanied by cross-equatorial northwesterly low-level flow that diverges from an area of anomalous drying in the western North Pacific and converges towards a region with anomalous moistening in the southern central Pacific. Our results show that the SST-induced concurrent spatial shifts of the two circulations are climatically relevant as they can further amplify extratropical precipitation variability on interannual timescales. [ABSTRACT FROM AUTHOR]

The Great Barrier Reef (GBR) is an internationally recognized and widely studied ecosystem, yet little is known about its sea surface temperature (SST) evolution since the Last Glacial Maximum (LGM) (~20 kyr BP). Here, we present the first paleo‐application of Isopora coral‐derived SST calibrations to a suite of 25 previously published fossil Isopora from the central GBR spanning ~25–11 kyr BP. The resultant multicoral Sr/Ca‐ and δ18O‐derived SST anomaly (SSTA) histories are placed within the context of published relative sea level, reef sequence, and coralgal reef assemblage evolution. Our new calculations indicate SSTs were cooler on average by ~5–5.5°C at Noggin Pass (~17°S) and ~7–8°C at Hydrographer's Passage (~20°S) (Sr/Ca‐derived) during the LGM, in line with previous estimates (Felis et al., 2014, https://doi.org/10.1038/ncomms5102). We focus on contextualizing the Younger Dryas Chronozone (YDC, ~12.9–11.7 kyr BP), whose Southern Hemisphere expression, in particular in Australia, is elusive and poorly constrained. Our record does not indicate cooling during the YDC with near‐modern temperatures reached during this interval on the GBR, supporting an asymmetric hemispheric presentation of this climate event. Building on a previous study (Felis et al., 2014, https://doi.org10.1038/ncomms5102), these fossil Isopora SSTA data from the GBR provide new insights into the deglacial reef response, with near‐modern warming during the YDC, since the LGM. Key Points: Near‐modern sea surface temperatures were reached during the Younger Dryas Chronozone on the Great Barrier ReefSea surface temperature, reef assemblage, and sea level data provide an overview of the evolution of the Great Barrier Reef since the LGMIsopora‐based sea surface temperature calibrations can be applied to fossil Isopora to understand environmental change on coral reefs [ABSTRACT FROM AUTHOR]

Rapid increases in upper 700‐m Indian Ocean heat content (IOHC) since the 2000s have focused attention on its role during the recent global surface warming hiatus. Here, we use ocean model simulations to assess distinct multidecadal IOHC variations since the 1960s and explore the relative contributions from wind stress and buoyancy forcing regionally and with depth. Multidecadal wind forcing counteracted IOHC increases due to buoyancy forcing from the 1960s to the 1990s. Wind and buoyancy forcing contribute positively since the mid‐2000s, accounting for the drastic IOHC change. Distinct timing and structure of upper ocean temperature changes in the eastern and western Indian Ocean are linked to the pathway how multidecadal wind forcing associated with the Interdecadal Pacific Oscillation is transmitted and affects IOHC through local and remote winds. Progressive shoaling of the equatorial thermocline—of importance for low‐frequency variations in Indian Ocean Dipole occurrence—appears to be dominated by multidecadal variations in wind forcing. Plain Language Summary: Indian Ocean surface waters have warmed more than the tropical Atlantic or Pacific over the last 60 years. In contrast, the amount of heat stored in the upper 700 m of the Indian Ocean did not exhibit strong increases between 1960 and 2000, which is counter to temperature trends in other ocean regions across the globe. Only since the 2000s was rapid warming down to 700 m observed for the Indian Ocean. Using ocean model simulations, our study demonstrates that the unusual behavior of Indian Ocean temperatures over the last 60 years was mainly due to wind conditions: Two different pathways how winds can impact the upper ocean temperature structure in the Indian Ocean, either through the atmosphere or via an oceanic connection from the Pacific, are highlighted. In fact, wind trends during 1960–2000 counteracted additional heat input into the Indian Ocean due to an overall warming climate. These long‐term changes in the Indian Ocean temperature structure affect regional climate in surrounding countries; they are also of importance for predicting how the Indian Ocean will respond in the near future to a warming climate. Key Points: Multidecadal variations in Indian Ocean heat content exhibit distinct spatiotemporal signals regionally and with depthWinds counteracted (western) Indian Ocean heat content increase due to buoyancy forcing prior to the 2000s but contributed to the rapid rise sinceProgressive shoaling of the Indian Ocean thermocline appears to be dominated by multidecadal variation in wind forcing [ABSTRACT FROM AUTHOR]

A profound change in the variations of warm water volume (WWV) of the tropical Pacific occurred around 1999/2000, coincided with the interdecadal shift of the El Niño–Southern Oscillation (ENSO) properties to a higher frequency and smaller amplitude regime. The persistence of WWV is decreased for all calendar months since 2000, associated with a decrease in its variance and an increase in its frequency. Moreover, the WWV‐sea surface temperature relationship associated with the ENSO cycle is weakened during 2000–2019, implying that the recharge/discharge process is less efficient in driving the phase transition of an ENSO cycle and ENSO became less periodical compared with that during 1979–1999. Corresponding to the weaker precursory connection of WWV to ENSO, the geographical origin of precedent thermocline anomalies has shown a northwestward migration since 2000. This is consistent with the suppression of the equatorial air‐sea feedback and the systematical northwestward shift of the atmosphere‐ocean coupling center in the tropical Pacific. Changed annual cycle and enhanced zonal contrast of sea surface temperature in the equatorial Pacific may be connected with the interdecadal variations of the ENSO and WWV features. Key Points: Amplitude and persistence decreases and northwestward shift of WWV since 2000Less efficient of the recharge/discharge process in driving the phase transition of an ENSO cycle since 2000Less periodical and more irregular ENSO cycle since 2000 [ABSTRACT FROM AUTHOR]

The Southern Ocean plays a fundamental role in the global energy and carbon budgets via the uptake of large quantities of heat and carbon in subantarctic mode water (SAMW). Even though SAMW variability is closely tied to variations in mixed layer depth (MLD) and properties, little is known about what controls interannual to multidecadal MLD variations. Here we use atmospheric observations combined with a global 1/10° ocean‐sea ice model to examine the mechanisms controlling Southern Ocean mixed layer variability. We find that tropical teleconnections from the Indo‐Pacific to the Amundsen Sea Low via El Niño–Southern Oscillation and the Indian Ocean Dipole control mixed layer variability along the path of a Rossby wave train across the South Pacific during early winter. This suggests recent multidecadal variations in mixed layer properties are likely to have been significantly impacted by transitions in the Interdecadal Pacific Oscillation. Plain Language Summary: In the Southern Ocean, subantarctic mode water and mixed layers undergo substantial interannual and multidecadal variability, with a wave‐like pattern of anomalies appearing across the Southern Ocean. This zonally asymmetric pattern is thought to be important for regulating the uptake of anthropogenic carbon and heat. Yet the mechanism for the variations in Southern Ocean mixed layer properties and how this relates to tropical sea surface temperature variability remain unclear. This study demonstrates that tropical teleconnections control mixed layer variability, particularly along the pathway of a Rossby wave train propagating from the tropical Indo‐Pacific toward the Amundsen‐Bellingshausen Seas. Key Points: Winter deep mixed layers exhibit substantial spatial and temporal variability across the Southern OceanTropical teleconnections from ENSO and the Indian Ocean Dipole control mixed layer variability along the path of a Rossby wave trainA propagating pattern of anomalous meridional airflow, causing net air‐sea and Ekman heat flux variations, drives the mixed layer anomalies [ABSTRACT FROM AUTHOR]

Long‐term predictability of the North Atlantic sea surface temperature (SST) is commonly attributed to buoyancy‐forced changes of the Atlantic Meridional Overturning Circulation. Here we investigate the role of surface wind stress forcing in decadal hindcasts as another source of extratropical North Atlantic SST predictability. For this purpose, a global climate model is forced by reanalysis (ERA‐interim) wind stress anomalies over the period 1979–2017. The simulated climate states serve as initial conditions for decadal hindcasts. Significant skill in predicting detrended observed annual SST anomalies is observed over the extratropical central North Atlantic with anomaly correlation coefficients exceeding 0.6 at lead times of 4 to 7 yrs. The skill is insensitive to the calendar month of initialization and primarily linked to upper ocean heat content anomalies that lead anomalous SSTs by several years. Plain Language Summary: Certain aspects of the climate can be predicted several years in advance. This is primarily due to the slow components of the climate system, especially the ocean, which can influence the atmosphere on time scales of years to decades. It is thus essential to start long‐term climate forecasts from a realistic ocean state. However, ocean observations, especially subsurface data, are limited. Hence, predictions are often only based on surface observations. Here we demonstrate that the use of observed surface wind stress yields skillful predictions of the sea surface temperature (SST) over the extratropical North Atlantic at lead times of up to about 7 yrs. This is because the ocean circulation responds to the wind stress changes, which generate subsurface temperature anomalies that affect the surface with a time delay. These results may not only be of scientific but also of public and economic interest, because extratropical North Atlantic SST is thought to influence mean climate and extreme weather events over the adjacent continents. Key Points: Wind stress initialization yields skillful multiyear hindcasts of annual extratropical North Atlantic sea surface temperature anomaliesThe skill is essentially insensitive to the initialization calendar monthThe skill is linked to an upper ocean heat content anomaly that leads anomalous sea surface temperatures by several years [ABSTRACT FROM AUTHOR]

Ocean energetics is a useful framework for understanding El Niño–Southern Oscillation; however, its key element, available potential energy (APE), requires accurate ocean subsurface data that are hard to measure. Here, we describe a sea surface height‐based index, SSHI, that accurately captures APE variations and can be easily computed from satellite observations. Using SSHI, we obtain an observation‐based estimate of the APE damping timescale of approximately 1.7 years, slightly longer than previous ocean reanalysis‐based estimates. Furthermore, SSHI serves as an indicator for El Niño "flavors" while recording the relative strength of the thermocline feedback. SSHI captures a decadal shift in El Niño–Southern Oscillation properties in early 2000s, with a more tilted mean thermocline and weaker thermocline slope variations indicative of the "Central Pacific" El Niño type dominating the past two decades, whereas SSH averaged over the tropical Pacific (a proxy for upper‐ocean heat content) shows a significant rising trend over this time. Plain Language Summary: El Niño is the warm phase of a climate oscillation originating in the tropical Pacific typically referred to as the El Niño–Southern Oscillation. The maximum sea surface temperature anomaly of canonical El Niño, called Eastern Pacific (EP) El Niño, is located over the equatorial EP. In recent years, another type of El Niño has become more prominent with its maximum sea surface temperature anomaly over the equatorial Central Pacific (CP), termed CP El Niño. Scientists often use variations in the available potential energy (APE) of the tropical ocean to describe El Niño–Southern Oscillation. However, APE computation requires accurate ocean subsurface data. Here, we describe a sea surface height‐based index, SSHI, that accurately captures APE changes but can be easily computed from satellite observations. Negative SSHI indicates a flatter thermocline and an El Niño state. We show that larger negative SSHI values typically indicate EP events, while small negative values tend to coincide with CP events. Moreover, SSHI indicates a more tilted mean thermocline and weaker thermocline slope variations after the early 2000s, indicative of the dominant "Central Pacific" El Niño. Using SSHI, we obtain an observation‐based estimate of APE damping timescale at approximately 1.7 years. Key Points: A sea surface height index (SSHI) is introduced as a linear component of anomalous SSH squared and averaged over the tropical PacificSSHI records thermocline variations, approximates ocean energetics in the tropical Pacific, and helps distinguish different El Niño typesSSHI gives an estimate of the damping timescale of available potential energy in the tropical Pacific ocean of approximately 1.7 years [ABSTRACT FROM AUTHOR]

Climate variations can have profound impacts on marine ecosystems and the socioeconomic systems that may depend upon them. Temperature, pH, oxygen (O2) and net primary production (NPP) are commonly considered to be important marine ecosystem drivers, but the potential predictability of these drivers is largely unknown. Here, we use a comprehensive Earth system model within a perfect modeling framework to show that all four ecosystem drivers are potentially predictable on global scales and at the surface up to 3 years in advance. However, there are distinct regional differences in the potential predictability of these drivers. Maximum potential predictability (>10 years) is found at the surface for temperature and O2 in the Southern Ocean and for temperature, O2 and pH in the North Atlantic. This is tied to ocean overturning structures with "memory" or inertia with enhanced predictability in winter. Additionally, these four drivers are highly potentially predictable in the Arctic Ocean at the surface. In contrast, minimum predictability is simulated for NPP (<1 years) in the Southern Ocean. Potential predictability for temperature, O2 and pH increases with depth below the thermocline to more than 10 years, except in the tropical Pacific and Indian oceans, where predictability is also 3 to 5 years in the thermocline. This study indicating multi-year (at surface) and decadal (subsurface) potential predictability for multiple ecosystem drivers is intended as a foundation to foster broader community efforts in developing new predictions of marine ecosystem drivers. [ABSTRACT FROM AUTHOR]

We present an alternative method of calculating the historical effective radiative forcing using the observed temperature record and a kernel based on the 5th Climate Model Intercomparison Project (CMIP5) temperature response. This estimate is the effective radiative forcing time series that the average climate model would need to simulate the observed global mean surface temperature anomalies. We further infer the anthropogenic aerosols radiative forcing as a residual using the better‐known greenhouse gas radiative forcing. This allows an independent estimate of anthropogenic aerosol radiative forcing, which suggests a cooling influence due to aerosols in the early part of the twentieth century. The temporal kernels are also used to calculate decadal contributions from the dominant forcing agents to present‐day temperature, ocean heat content, and thermosteric sea level rise. The current global mean temperature anomaly is dominated by emissions in the past two decades, while current ocean heat content is more strongly affected by earlier decades. Key Points: We calculate effective radiative forcing by applying model‐derived kernels to observational estimates of global mean surface temperatureOur independent estimate of anthropogenic aerosol forcing suggests substantial cooling due to aerosols in the early part of the twentieth centuryWe attribute current temperature change and sea level rise to past decadal emissions using temporal kernels [ABSTRACT FROM AUTHOR]

The variability of the sea surface temperature (SST) in the northwest Pacific has been studied on seasonal, annual and interannual scales based on the monthly datasets of extended reconstructed sea surface temperature (ERSST) 3b (1854–2017, 164 years) and optimum interpolation sea surface temperature version 2 (OISST V2 (1988–2017, 30 years). The overall trends, spatial–temporal distribution characteristics, regional differences in seasonal trends and seasonal differences of SST in the northwest Pacific have been calculated over the past 164 years based on these datasets. In the past 164 years, the SST in the northwest Pacific has been increasing linearly year by year, with a trend of 0.033 ∘ C/10 years. The SST during the period from 1870 to 1910 is slowly decreasing and staying in the range between 25.2 and 26.0 ∘ C. During the period of 1910–1930, the SST as a whole maintained a low value, which is at the minimum of 164 years. After 1930, SST continued to increase until now. The increasing trend in the past 30 years has reached 0.132 ∘ C/10 years, and the increasing trend in the past 10 years is 0.306 ∘ C/10 years, which is around 10 times that of the past 164 years. The SST in most regions of the northwest Pacific showed a linear increasing trend year by year, and the increasing trend in the offshore region was stronger than that in the ocean and deep-sea region. The change in trend of the SST in the northwest Pacific shows a large seasonal difference, and the increasing trend in autumn and winter is larger than that in spring and summer. There are some correlations between the SST and some climate indices and atmospheric parameters; the correlations between the SST and some atmospheric parameters have been discussed, such as those of the North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO), Southern Oscillation Index (SOI) anomaly, total column water (TCW), NINO3.4 index, sea level pressure (SLP), precipitation, temperature at 2 m (T2) and wind speed. The lowest SST in China offshore basically occurred in February and the highest in August. The SST fluctuation in the Bohai Sea and Yellow Sea (BYS) is the largest, with a range from 5 to 22 ∘ C; the SST in the East China Sea (ECS) is from 18 to 27 ∘ C; the smallest fluctuations occur in the South China Sea (SCS), maintained at range of 26 to 29 ∘ C. There are large differences between the mean and standard deviation in different sea regions. [ABSTRACT FROM AUTHOR]

Satellite altimetry reveals substantial decadal variability in sea level ζ across the tropical Pacific during 1993–2015. An ocean state estimate that faithfully reproduces the observations is used to elucidate the origin of these low‐frequency tropical Pacific ζ variations. Analysis of the hydrostatic equation reveals that recent decadal ζ changes in the tropical Pacific are mainly thermosteric in nature, related to changes in upper‐ocean heat content. A forcing experiment performed with the numerical model suggests that anomalous wind stress was an important driver of the relevant heat storage and thermosteric variation. Closed budget diagnostics further clarify that the wind‐stress‐related thermosteric ζ variation resulted from the joint actions of large‐scale ocean advection and local surface heat flux, such that advection controlled the budget over shorter, intraseasonal to interannual time scales, and local surface heat flux became increasingly influential at longer decadal periods. In particular, local surface heat flux was important in contributing to a recent reversal of decadal ζ trends in the tropical Pacific. Contributions from local surface heat flux partly reflect damping latent heat flux tied to wind‐stress‐driven sea‐surface‐temperature variations. Key Points: Sea level in the western tropical North Pacific and eastern and central equatorial Pacific underwent decadal variation over 1993–2015This decadal variation reflects ocean heat storage related to ocean heat transport and local surface heat flux due to anomalous wind stressContributions from air‐sea heat exchange arise from latent heat flux associated with wind‐stress‐related sea‐surface temperature changes [ABSTRACT FROM AUTHOR]

A greater warming trend of sea surface temperature in the tropical Indian Ocean than in the tropical Pacific is a robust feature found in various observational data sets. Yet this interbasin warming contrast is not present in climate models. Here we investigate the impact of tropical Indian Ocean warming on the tropical Pacific response to anthropogenic greenhouse gas warming by analyzing results from coupled model pacemaker experiments. We find that warming in the Indian Ocean induces local negative sea level pressure anomalies, which extend to the western tropical Pacific, strengthening the zonal sea level pressure gradient and easterly trades in the tropical Pacific. The enhanced trade winds reduce sea surface temperature in the eastern tropical Pacific by increasing equatorial upwelling and evaporative cooling, which offset the greenhouse gas warming. This result suggests an interbasin thermostat mechanism, through which the Indian Ocean exerts its influence on the Pacific response to anthropogenic greenhouse gas warming. Key Points: The interbasin warming contrast is a robust observational feature, which is not present in climate modelsThe Indian Ocean warming effectively reduces the Pacific warming response to anthropogenic greenhouse gasesThe reason that climate models fail to capture the interbasin warming contrast is likely due to model bias [ABSTRACT FROM AUTHOR]

The shallow subtropical cells (STCs) in the Pacific Ocean are thought to modulate the background state that the El Niño‐Southern Oscillation (ENSO) operates in. This modulation is proposed to impact the frequency and intensity of ENSO events and their teleconnections. We use a high‐resolution ocean model to investigate the volume transports associated with the STC branches along 5° N and 5° S. We find three prominent differences between the Southern hemisphere (SH) STC and the Northern hemisphere (NH) STC: (i) the NH STC varies 26% stronger than the SH STC; (ii) the NH STC appears to lead the SH STC by 3 months which causes the NH and SH STCs to play different roles during the course of El Niño and La Niña events; and (iii) in spite of the relative symmetry of the wind stress trends, the STCs have differing decadal trends, with the SH STC clearly dominating the changes in the post‐1993 period. To investigate the mechanisms driving the STC variability, we identify winds that are linearly and nonlinearly related to ENSO to force the ocean model. The hemispheric difference in interannual variance as well as the phase difference between the STCs can be explained with ENSO forcing. Our results suggest ENSO to be an important factor in modulating its own background state, with a prominent role for the winds that are nonlinearly related to ENSO. The decadal trends and their interhemispheric disparity, however, cannot be reproduced by our targeted ENSO experiments. Plain Language Summary: The tropical Pacific Ocean plays a key role in modulating the climate worldwide. It is home to the El Niño‐Southern Oscillation (ENSO) which lets the ocean temperature rise and fall on time scales of a few years. The tropical Pacific Ocean also varies on time scales of a decade or longer, which is thought to influence the frequency, intensity, and potential predictability of ENSO. It is therefore important to understand the dynamics behind the decadal variability. The Pacific Ocean shallow circulation in the upper 200 m is assumed to be an important driver of the decadal changes in the tropical Pacific. Our study focuses on understanding the drivers of the Pacific Ocean shallow circulation. We conduct various experiments with an Ocean General Circulation Model using different wind forcings to investigate the role of specific wind patterns for the ocean circulation. We find prominent hemispheric differences of the shallow circulation that can be largely explained by wind forcing associated with ENSO. Most interestingly, we also find that changes associated with a phase change of the Pacific Decadal Oscillation in the Year 1993 appear to be largely induced by the Southern hemispheric circulation highlighting the importance of understanding the dynamics there. Key Points: The STC interannual variability is largely driven by ENSO‐related windsWe can attribute a crucial role to the winds nonlinearly related to ENSO which are not considered by the classic linear view of ENSOThe decadal changes in the tropical Pacific after the Year 1993 appear to be mainly influenced by the Southern hemisphere STC [ABSTRACT FROM AUTHOR]

The ocean's response to direct atmospheric effects of increased carbon dioxide's (CO2) radiative forcing is examined. These direct effects are defined as the climate changes that result from forcing on a fast time scale of about a year, independent of the slower surface warming that the forcing also provokes. To evaluate how these direct effects impact ocean heat uptake and circulation, output of atmospheric general circulation model (GCM) simulations are used to force an ocean GCM with comprehensive boundary conditions. Perturbation simulations with the prescribed response to a quadrupling of atmospheric CO2 include altered surface winds, freshwater fluxes, downwelling shortwave radiation, and downwelling longwave cloud radiative effect. The perturbation simulations show that the intensification and poleward shift of surface winds, particularly in the Southern Ocean, strengthen the shallow overturning circulation in the tropical Pacific and deep overturning in the Atlantic. This, in turn, has a cooling effect on the global ocean at shallow depths. A two‐layer energy balance model, designed to capture transient global mean climate change, is adapted to account for the altered ocean heat uptake from direct effects. The direct change in global mean ocean heat uptake is a decrease of about 0.3 W/m2 for quadrupling of CO2, offsetting about 5% of the surface longwave forcing. Key Points: The direct effects of CO2 on the surface atmospheric fields that interact with the ocean are analyzedThe influence of direct effects on ocean heat content and circulation in ocean general circulation model (GCM) simulations is assessedGCM results can be emulated by adapting two‐layer energy balance model of ocean heat uptake to include direct effects [ABSTRACT FROM AUTHOR]

The spring persistence barrier remains a critical challenge for the prediction of El Niño–Southern Oscillation (ENSO) because it severely limits the potential ability of ENSO forecast across the boreal spring. Our observational analysis shows that the spring persistence barrier exhibited a significant decadal modulation in the last half century. Further analysis shows that this decadal modulation is caused by the decadal variation in spring ENSO stability, which, in turn, is dominated by the thermal damping feedback of the sea surface temperature (SST) through latent heat flux and shortwave radiation flux. The change of the thermal damping feedback is associated with the background mean SST, with a cooler SST corresponding to a greater ENSO instability and a severer spring persistence barrier and vice versa. Plain Language Summary: Obtaining effective and correct El Niño–Southern Oscillation (ENSO) forecasts is always an inspiring and challenging task but requires additional research for optimization. One fundamental obstacle that must be faced is the so‐called boreal spring persistence barrier, which relates to the significant decrease in ENSO's prediction skill that always occurs during the boreal spring. Through investigating the spring persistence barrier issues by the observational analyses, our study proposes a mechanism for controlling its variability on the decadal time scale and highlights the significant effects of the interdecadal climate variability in the tropical Pacific. As the mean state of the tropical Pacific warms, more moisture is released into the atmosphere for the same magnitude of sea surface temperature warming, which further leads to the thermal damping over the tropical Pacific and induces more stable ENSO during the boreal spring. The more stable ENSO then weakens the spring persistence barrier, making ENSO easier to predict. This study will draw the scientific community's attention on understanding about ENSO and ENSO predictability; the decadal variations in spring persistence barrier intensity and its related physical processes should be adequately represented in climate models used for seasonal predictions and studies of climate variability, change, and projection. Key Points: The ENSO spring persistence barrier exhibited a significant decadal variability in the last half centuryThe decadal modulation of the barrier is caused by the decadal variation in spring ENSO stabilityThe ENSO's stability is further dominated by the nonlinear heat flux‐SST damping feedback [ABSTRACT FROM AUTHOR]

A framework for analyzing and benchmarking climate model outputs is built upon δ‐MAPS, a recently developed complex network analysis method. The framework allows for the possibility of highlighting quantifiable topological differences across data sets, capturing the magnitude of interactions including lagged relationships and quantifying the modeled internal variability, changes in domains properties and in their connections over space and time. A set of four metrics is proposed to assess and compare the modeled domains shapes, strengths, and connectivity patterns. δ‐MAPS is applied to investigate the topological properties of sea surface temperature from observational data sets and in a subset of the Community Earth System Model (CESM) Large Ensemble focusing on the past 35 years and over the 20th and 21st centuries. Model ensemble members are mapped in a reduced metric space to quantify internal variability and average model error. It is found that network properties are on average robust whenever individual member or ensemble trends are removed. The assessment identifies biases in the CESM representation of the connectivity patterns that stem from too strong autocorrelations of domains signals and from the overestimation of the El Niño–Southern Oscillation amplitude and its thermodynamic feedback onto the tropical band in most members. Key Points: A new framework to validate climate models and compare reanalyses products is presentedWe identify biases in the CESM Large Ensemble in relation to tropical teleconnections in SSTWe investigate the time evolution of ENSO variability in observational data sets and CESM Large Ensemble [ABSTRACT FROM AUTHOR]