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This study addresses a critical gap in understanding anthropogenic influences on tropical climate dynamics by investigating the impact of aerosol‐cloud interactions on large‐scale circulation. Despite extensive research on greenhouse gas‐induced warming and its effects on tropical circulation, the impact of aerosols, particularly their interactions with clouds, on large‐scale circulation remains understudied. Utilizing large‐domain radiative convective equilibrium cloud‐resolving simulations, this research demonstrates that increasing aerosol concentration intensifies tropical overturning circulation, evaluated at the mid‐troposphere (I) $(\mathcal{I})$, strongly correlating with domain mean cloud and radiative properties. Employing a weak temperature gradient approximation, I attribute variations in I $\mathcal{I}$ to changes in clear‐sky radiative cooling rather than stability. These radiative cooling changes are linked to humidity changes driven by warm rain suppression by aerosols. This study's findings underscore the need to take into account microphysical changes, particularly aerosol concentrations, when studying anthropogenic effects on tropical circulation. Plain Language Summary: Clouds intricately couple with the large‐scale overturning circulation in the tropics. Aerosols, small particles suspended in the atmosphere, interact with clouds. Thus, changes in aerosols concentration (Na) have the potential to affect cloud properties and by that the large‐scale circulation, with important implications for tropical climate and climate change. Despite this crucial relationship, the subject has been insufficiently explored. In this paper, I use a simplified, yet realistic, representation of the tropical atmosphere—radiative convective equilibrium large‐domain cloud‐resolving simulations—to investigate the effect of aerosol‐cloud interaction on overturning tropical circulation. By varying Na under diverse sea surface temperatures, I demonstrate that the strength of the overturning circulation intensifies with increasing Na. Utilizing a semi‐analytical expression for subsidence strength in clear‐sky tropical regions, I show that the intensified overturning circulation with increased Na can be attributed to heightened humidity in the mid‐troposphere, leading to intense radiative cooling. This mid‐tropospheric humidification can be explained by warm rain suppression by aerosols, which fosters stronger humidity mixing between clouds and their surroundings. These findings underscore the need to consider aerosol concentration changes when examining anthropogenic impacts on tropical circulation. Key Points: The overturning tropical circulation intensifies with aerosol concentration in a radiative convective equilibrium modelChanges in circulation strength can be explained by changes in diabatic subsidence driven by radiative changesRadiative changes can be explained by increased humidity at the mid‐troposphere due to warm rain suppression [ABSTRACT FROM AUTHOR]

Previous studies suggested that Atlantic Multidecadal Variability (AMV) modulations on pan‐Pacific sea surface temperature (SST) variability and prediction are model‐dependent. These results were mainly based on SST forcing experiments in which AMV‐related Atlantic SST anomalies were prescribed. However, the AMV itself is also model‐dependent, but its influences on the Pacific remain unclear. Here, we use multi‐model fully coupled experiments from the Coupled Model Intercomparison Project Phase 6 (CMIP6), along with observations, to study the model‐dependent AMV trans‐basin effects. We found that AMV strength is a key factor: Stronger (Weaker) model AMV than observations overestimates (underestimates) SST response and decadal prediction skills, mainly in the North Pacific. The reason is that stronger positive phased AMV, for example, leads to higher sea level pressure anomalies over the North Pacific, which lifts sea surface height and deepens thermocline to warm SST. Our study highlights the necessity to improve simulations of AMV strength. Plain Language Summary: Pacific sea surface temperature (SST) decadal variability and prediction are important for economy and environment. The Atlantic Multidecadal Variability (AMV) was thought to significantly influence Pacific SST variability and prediction. However, the results of previous studies were model‐dependent. In this paper, we use fully coupled experiments of the Coupled Model Intercomparison Project Phase 6 (CMIP6) models to study what contributes to the model dependency. We found that the strength of the AMV is a key factor influencing AMV trans‐basin modulations on SST variability and decadal prediction, mainly in the North Pacific Ocean. The related mechanisms are discussed. Our paper sheds light on the way to improve North Pacific SST decadal prediction. Key Points: Climate models show a strong inter‐model spread of Atlantic Multidecadal Variability (AMV) strengthModels with stronger (weaker) AMV than observations overestimate (underestimate) North Pacific sea surface temperature (SST) response and decadal predictionThe North Pacific SST response is primarily forced by AMV‐induced wind stress curl and ocean dynamics [ABSTRACT FROM AUTHOR]

Sea surface temperatures (SSTs) vary not only due to heat exchange across the air‐sea interface but also due to changes in effective heat capacity as primarily determined by mixed layer depth (MLD). Here, we investigate seasonal and regional characteristics of the contribution of MLD anomalies to the month‐to‐month variability of SST using observational datasets. First, we propose a metric called Flux Divergence Angle, which can quantify the relative contributions of surface heat fluxes and MLD anomalies to SST variability. Using this metric, we find that MLD anomalies tend to amplify SST anomalies in the extra‐tropics, especially in the eastern ocean basins, during spring and summer. In contrast, MLD anomalies tend to suppress SST anomalies in the eastern tropical Pacific during December‐January‐February. This paper provides the first global picture of the observed importance of MLD anomalies to the local SST variability. Plain Language Summary: Sea surface temperature (SST) is one of the important indicators as well as drivers of climate variability over the globe. SST varies not only due to changes in surface heat fluxes but also due to changes in effective heat capacity as mainly determined by mixed layer depth (MLD). However, the observed characteristics of the latter process associated with the MLD anomalies are limited. In this study, we propose a new metric called "Flux Divergence Angle (FDA)," which can quantify the relative importance of MLD and surface heat flux to the SST variability. Using this metric, we find that the MLD anomalies tend to amplify the local SST variability in the extra‐tropics during spring and summer. On the other hand, MLD anomalies tend to suppress the SST variability in the eastern tropical Pacific during December‐January‐February. This paper provides, for the first time, the global picture of relative importance of MLD anomalies to the SST variability based on observations. Key Points: Relative contributions of mixed layer depth (MLD) and surface heat flux anomalies to sea surface temperature (SST) variability are investigated using the Flux Divergence Angle metricMLD anomalies tend to amplify SST anomalies in the extra‐tropics, especially in eastern part of ocean basin, during the spring and summerMLD anomalies tend to suppress SST anomalies in the eastern tropical Pacific during December‐January‐February [ABSTRACT FROM AUTHOR]

This paper expands on work showing that the winter North Atlantic Oscillation (NAO) is predictable on decadal timescales to quantify the skill in capturing the North Atlantic eddy‐driven jet's location and speed. By focusing on decadal predictions made for years 2–9 from the sixth Coupled Model Intercomparison Project over 1960–2005 we find that there is significant skill in jet latitude and, especially, jet speed associated with the skill in the NAO. However, the skill in the NAO, jet latitude and speed indices appears to be sensitive to the period over which it is assessed. In particular, skill drops considerably when evaluating hindcasts up to the present day as models fail to capture the recent observed northern shift and strengthening of the winter eddy‐driven jet, and more thus positive NAO. We suggest that the drop in atmospheric circulation skill is related to reduced skill in North Atlantic sea surface temperatures. Plain Language Summary: Climate models have the capability to predict the evolution of mean atmospheric circulation over long timescales, from annual to decadal and longer. However, models are overestimating the chaotic, unpredictable component of the climate's variability and, although model predictions follow the observed oscillations of the climate, the strength of these oscillations is critically underestimated. While climate models have demonstrated skill in predicting the North Atlantic Oscillation (NAO), this paper assesses model skill in predicting the evolution of the wintertime North Atlantic eddy‐driven jet, with the aim to highlight how much of the predictive skill of the NAO is derived from accurate predictions of the jet's state. We find levels of skill similar to that for the NAO, with slightly higher skill for the jet's strength (speed) over its location (latitude of maximum speed). We also notice a drop in skill over the last decade, as models fail to capture the latest trends in the NAO and jet's evolution, and suggest that it might be related to degradation in skill at predicting surface temperature variability. Key Points: The winter North Atlantic eddy‐driven jet is predictable on decadal time‐scales with skill in jet speed comparable to that for the North Atlantic Oscillation (NAO)Anomalies in the jet are substantially smaller than expected from skill alone and so suffer from noise overestimationSkill drops significantly in the last decade, as hindcasts do not capture the return to positive NAO conditions post‐2010 [ABSTRACT FROM AUTHOR]

Previous studies suggested that tropical sea surface salinity (SSS) can influence tropical Pacific sea surface temperature (SST) through mixing and entrainment and thus it may be a signal for El Niño‐Southern Oscillation (ENSO) prediction. This paper explores the influence of SSS on ENSO spring predictability barrier (SPB) using an empirical dynamic model ‐ Linear Inverse Model (LIM). By coupling and decoupling SSS in the LIM, we find that tropical Pacific SSS plays a significant role in weakening both Central‐Pacific and Eastern‐Pacific ENSO SPB. The evolution of optimal initial structure also shows the importance of SSS dynamics in ENSO. We found an SSS mode that plays the dominant role in SSS impacting ENSO prediction. By the analysis of lead‐lag correlation, we find that this mode can induce easterlies during the spring, which finally leads to a La Niña‐like SST pattern in the winter through zonal advective and thermocline feedbacks. Plain Language Summary: The spring predictability barrier (SPB) is a phenomenon of forecast skill reduction of the El Niño‐Southern Oscillation (ENSO) when it comes to the boreal spring, regardless of the initial month. Sea surface salinity (SSS) can influence sea surface temperature by altering sea surface density and thus it may be a signal for ENSO prediction. Using a linear dynamical model, we find that SSS plays an important role in improving the forecast skill of ENSO and weakening SPB. We further find that SSS can induce the easterlies during the spring, which finally leads to a La Niña‐like SST pattern. Our study suggests that SSS can be used to predict ENSO about 1 year later. Key Points: A linear dynamical model suggests that taking sea surface salinity (SSS) into consideration can strongly weaken Central‐Pacific and Eastern‐Pacific El Niño‐Southern Oscillation (ENSO) spring predictability barrier (SPB)A new SSS mode is found to be important for weakening ENSO SPBThe SSS mode can predict ENSO events about 1 year earlier by inducing sea surface temperature and wind anomalies in the early spring [ABSTRACT FROM AUTHOR]

The annular structure of Antarctic oscillation (AAO) is a research hotpot, but its asymmetry receives less attention. In this paper, the self‐organizing map method is employed to cluster the AAO patterns into symmetric and asymmetric modes in austral autumn. The asymmetry is mainly reflected in the Pacific‐Atlantic sector, and the AAO evolves toward asymmetric positive polarity, with the most pronounced asymmetry in May. Originating from near Australia, the asymmetry indicates a zonal wave train in the Pacific‐Atlantic sector. Both modeled and observed results demonstrate that the sea surface temperature anomaly in the equatorial western Pacific stimulates a local meridional circulation anomaly and induces anomalous Rossby wave sources near Australia subsequently. An anomalous wave train propagating toward the Antarctic Peninsula is formed, and the associated geopotential anomaly enhances the asymmetry of AAO. Asymmetric AAO is conducive to the Antarctic dipole, which modulates the air temperature and sea ice anomalies around Antarctica. Plain Language Summary: The zonal symmetry of the Antarctic oscillation (AAO; also named southern annular mode) is well known to researchers, while the zonal asymmetry has received less attention. We use a clustering method to cluster the AAO modes in austral autumn into symmetric modes and asymmetric modes. The asymmetry of AAO is mainly reflected in the Pacific‐Atlantic sector. The results show that the AAO mode evolves to a positive asymmetric mode, and the asymmetry is most obvious in May. Both simulations and observations suggest that this asymmetry stems from sea surface temperature anomaly in the equatorial western Pacific that stimulates an anomalous local meridional circulation near Australia, and in turn triggers an anomalous wave train propagating toward the Antarctic Peninsula. The associated geopotential anomalies enhance the asymmetry of AAO. The asymmetric AAO favors the Antarctic dipole, regulating the air temperature and sea ice anomalies around Antarctica. Key Points: Via a cluster method, we obtained the zonal asymmetric Antarctic oscillation (AAO) mode, and analyzed its trend and preferred monthThe source of its asymmetry is from the sea surface temperature anomaly in the tropical western PacificThe asymmetric AAO mode has a vital influence on the Antarctic dipole [ABSTRACT FROM AUTHOR]

In this study, we obtained the first leading mode and principal component 1 (PC1) of North Pacific sea surface temperature (SST). The PC1‐related SST anomalies, located relatively north/south, are referred to as North/South PC1 events. Model outputs, observations, reanalysis datasets and sensitivity experiments show that during the North PC1 events, an enhanced Aleutian low occurs and is located relatively north, favoring anomalous southerlies over western North America. The anomalous southerlies induce a strong warming anomaly over North America via warm advection of the anomalous southerlies and temperature advection of the climatological westerlies over North America. However, the Aleutian low anomaly and corresponding southerly anomaly associated with South PC1 events shift southward, favoring weakened effects of South PC1 events on atmospheric circulations and surface temperatures over North America. The meridional position of PC1 events deserves to be considered in the studies of the PC1 and its related climate changes. Plain Language Summary: Understanding of surface temperature variations is important since it largely affects human and society. This paper categorizes the North Pacific sea surface temperature anomaly events, which are associated with the first leading mode of North Pacific sea surface temperature, into North and South events according to their meridional positions. Using model outputs, observations, reanalysis datasets and sensitivity experiments, we found that the surface temperature anomalies over North America during the North events are quite different from those during the South events. During the North events, an enhanced Aleutian low occurs over high‐latitude North Pacific. The enhanced Aleutian low tends to induce anomalous southerlies over western North America. The anomalous southerlies favor warming anomalies over North America via the warm advection of anomalous southerlies and temperature advection of climatological westerly. However, the Aleutian low anomaly and meridional wind anomaly during the South events shift southward, leading to weakened effects of the South events on atmospheric circulations and surface temperatures over North America. Key Points: North American warming anomaly occurs during North Pacific sea surface temperature anomaly events located northThe warming anomaly is related to an enhanced Aleutian low anomaly located north and climatological westerlyDuring the events located south, North American surface temperature responses are weak due to a southward‐shift Aleutian low anomaly [ABSTRACT FROM AUTHOR]

Blooms of various types of seaweeds have been reported worldwide, with recent expansions in surface waters. While most of the expansions have been attributed to eutrophication due mainly to human activities, any potential role of climate change is unclear. Here we show that, in the East China Sea and Yellow Sea, increased biomass of Sargassum horneri (S. horneri, brown seaweed) from 2000 to 2021 appears to be caused primarily by ocean warming, as S. horneri prefers a certain temperature range to grow. In contrast, while increases of Ulva prolifera (U. prolifera, green seaweed) in the same regions might also be related to ocean warming, during the same period, human activities such as coastal aquaculture or seaweed mitigation may muddle such effects. With the projected ocean warming in the next decades, we hypothesize that S. horneri blooms may occur earlier during the year and may continue to expand in the future. Plain Language Summary: Seaweed macroalgae blooms have been reported around the world in the last two decades, yet the exact reasons are often unclear. This paper attempts to solve this puzzle for the Sargassum and Ulva seaweeds in the East China Sea and Yellow Sea using long‐term observations and laboratory‐based seaweed physiology measurements. The paper shows not only how seaweeds have expanded in the two marginal seas in the past four decades, but also how a warming ocean (due to climate variability) and nutrient enrichment (due mainly to human activities) controlled such expansions. Among 63 large marine ecosystems (LMEs) in the world, the East China Sea (where seaweeds expanded) is one of the three LMEs that experienced "super‐fast" warming. Given the IPCC‐projected seawater temperature rise of 1.6°C by 2050, will these seaweeds continue to expand? Key Points: Multi‐sensor satellite data show significant and near‐synchronized expansions of Sargassum horneri and Ulva prolifera in the past 38 yearsOcean warming appears to be a main factor behind the recent expansions of S. horneri because of its physiological requirementOcean warming may also play a role in U. prolifera expansions, but the effect appears confounded by human mitigation efforts [ABSTRACT FROM AUTHOR]

This paper investigates the impacts of tropical Atlantic on El Niño‐Southern Oscillation (ENSO) predictability through an empirical dynamical model‐ Linear Inverse Model (LIM). By selectively including or excluding the coupling between tropical Atlantic and tropical Pacific in LIM, we find that the tropical Atlantic dynamics significantly improve the Eastern Pacific (EP)‐ENSO prediction and weaken the EP‐ENSO predictability barrier (PB), with the Equatorial Atlantic (EA) mode playing a more important role than the Tropical North Atlantic (TNA) mode. The tropical Atlantic impacts on Central Pacific (CP)‐ENSO predictability and PB are relatively smaller. Consistent with observations, the tropical Atlantic can weaken PB in most CMIP6 models. The evolution of the tropical Atlantic optimum initial structures confirms the important influence of the EA mode on the eastern tropical Pacific. Therefore, the tropical Atlantic dynamics, especially the EA mode, should be appropriately considered to improve the prediction of EP‐ENSO and weaken the EP‐ENSO PB. Plain Language Summary: El Niño–Southern Oscillation (ENSO) is one of the dominant climate modes in the tropical Pacific on interannual timescales, with significant global impacts. The tropical Atlantic sea surface temperature (SST) variability, especially in the tropical North Atlantic (TNA) and equatorial Atlantic (EA), is shown to have important impacts on ENSO variability. Using a linear dynamical model, we find that including the dynamics of the tropical Atlantic, especially the EA SST mode, can significantly improve the forecast skill of Eastern Pacific ENSO and weaken the EP‐ENSO spring predictability barrier (SPB), which is a sudden reduction of ENSO prediction skill in the boreal spring. However, Central Pacific (CP)‐ENSO's forecast skill is less influenced by the tropical Atlantic dynamic. Consistent with the observations, the topical Atlantic can weaken ENSO SPB in most CMIP6 climate models. Our results suggest that the tropical Atlantic dynamics should be appropriately considered to improve the forecast skill of EP‐ENSO. Key Points: A linear dynamical model is used to study the impact of the tropical Atlantic on El Niño‐Southern Oscillation (ENSO) predictabilityTropical Atlantic dynamics can improve Eastern Pacific (EP)‐ENSO predictability and weaken EP‐ENSO spring predictability barrier while the impact on Central Pacific‐ENSO is limitedThe Equatorial Atlantic mode plays an important role in the tropical Atlantic influence of EP‐ENSO prediction [ABSTRACT FROM AUTHOR]

This paper evaluates sea surface temperature (SST) biases of coupled models participating in Coupled Model Intercomparison Project Phase 5 (CMIP5) and CMIP6. Overall, CMIP6 models perform better than CMIP5 ones in reproducing SST climatology, with lower multi‐model ensemble mean (MME) globally averaged absolute bias (1.17 vs. 1.31 K). MME bias in global mean annual SST shifts from cooling (−0.09 ± 0.52 K) to warming (0.23 ± 0.60 K). Regionally, in CMIP6 cooling biases over the Northwest Pacific and North Atlantic are reduced by 20% and 18%, while warming biases over the Northeast Pacific, Southeast Atlantic and Southern Ocean are increased by 25%, 16% and 107% respectively. These changes are mainly attributed to the combined effects from aggravated positive (or alleviated negative) bias in clear‐sky surface downward longwave radiation, and alleviated negative bias in cloud radiative effect, partially reduced by enhanced cooling bias in clear‐sky surface downward shortwave radiation. Plain Language Summary: As the primary approach to projecting future climate change, state‐of‐the‐art climate models still suffer pronounced biases in climatological annual mean sea surface temperature (SST), such as cold biases over the Northwest Pacific and North Atlantic, and warm biases over the Northeast Pacific, Southeast Pacific, Southeast Atlantic and Southern Ocean. We have evaluated the changes in mean‐state SST biases between the Coupled Model Intercomparison Project Phase 5 (CMIP5) and CMIP6. CMIP6 models perform better in reproducing SST climatology with lower absolute bias, which is attributed to the process‐level improvement. Overall, annual global mean SST bias shifts from cold (−0.09 ± 0.52 K) to warm (0.23 ± 0.60 K), which is mainly due to the regionally alleviated cooling biases or aggravated warming biases. This warmer shift is contributed by the increased positive (or decreased negative) bias in clear‐sky surface downward longwave radiation and decreased negative bias in cloud radiative effect. Key Points: Coupled Model Intercomparison Project Phase 6 (CMIP6) models perform better than CMIP5 ones, with significantly lower global‐mean absolute bias in annual sea surface temperature (SST)Global‐mean SST bias is with a warmer shift (+0.32 K) in CMIP6, with salient regional cold biases alleviated and warm biases aggravatedReduced cold bias in cloud radiative effect and positive change in bias in clear‐sky surface downward longwave together account for the shift [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]

The strong sea‐surface temperature (SST) gradient associated with the Gulf Stream (GS) is widely acknowledged to play an important role in shaping mid‐latitude weather and climate. Despite this, an index for the GS SST gradient has not yet been standardized in the literature. This paper introduces a monthly index for the large‐scale SST gradient across the separated GS based on the time‐varying GS position detected from sea‐surface height. Analysis suggests that the variations in the monthly average SST gradient throughout the year result primarily from SST variability to the north of the GS, with little contribution from SST to the south. The index exhibits a weak periodicity at ∼2 years. Sea level pressure and turbulent heat flux patterns suggest that variability in the large‐scale SST gradient is related to atmospheric (rather than oceanic) forcing. Ocean‐to‐atmosphere feedback does not persist throughout the year, but there is some evidence of wintertime feedback. Plain Language Summary: The Gulf Stream is a narrow and fast‐flowing oceanic boundary current that transports warm tropical water north along the coast of Florida and out into the North Atlantic past Cape Hatteras. It is a region where sea‐surface temperatures (SSTs) change dramatically over very short distances with much cooler temperatures on one side of the Gulf Stream (GS) (to the north) and warmer temperatures on the other side (to the south). This strong change in SST across the GS is important for weather systems that propagate in the Euro‐Atlantic sector. Here, an index that measures how much the SSTs change with distance from month‐to‐month across the GS once it separates past Cape Hatteras is developed for the first time. It is found that variability in the index primarily results from SST variations to the north of the separated GS, which themselves are influenced by the atmosphere. There is evidence however that the SST variability represented by the index can also exert an influence on the atmosphere in wintertime. Additionally, the value of the index at any given time seemingly provides some information about the state of the index several years into the future. Key Points: A monthly index for the large‐scale sea‐surface temperature gradient of the separated Gulf Stream is presentedVariations in the index throughout the year are related to atmospheric forcingEvidence of ocean‐atmosphere feedback in the index is only apparent in wintertime [ABSTRACT FROM AUTHOR]

The temperature of the subsurface water entrained into the surface mixed layer plays a key role in controlling the sea surface temperature (SST) and its interannual variability in the equatorial Pacific. In this paper, we combine a hyperbolic tangent function bounded by the warm pool SST and centered at the thermocline depth with a variable sharpness parameter to describe the time‐space evolutions of the subsurface temperature. Under simple approximations of the sharpness parameter, this concise expression becomes remarkably efficient in capturing the observed and climate‐model simulated subsurface temperature variability in terms of anomalies of the thermocline depth and SST of the El Niño‐Southern Oscillation (ENSO) phenomenon. The formulations for the subsurface temperature and thermocline sharpness developed in this work should be useful tools for evaluating and understanding the role of the thermocline feedback in ENSO behaviors in both theoretical and comprehensive climate models. Plain Language Summary: While significant advances have been made in our ability to understand, simulate, and predict the El Niño‐Southern Oscillation (ENSO), its remarkable complexity is still not well understood. The subsurface ocean temperature in the equatorial Pacific is known to play a key role in controlling sea surface temperature and its interannual variations. An accurate description of its relationship with thermocline depth variability is of essential importance to understanding ENSO's complex dynamics in mechanistic studies. In this paper, we develop a concise expression which captures the observed relationship between anomalies of the subsurface temperature and thermocline depth remarkably well. This expression should be useful for ENSO mechanistic model studies and diagnostic investigations into climate models' performances in their ENSO simulations. Key Points: The equatorial Pacific subsurface temperature is expressed as a hyperbolic tangent function of thermocline depth and sharpnessEffective parameterizations for the subsurface temperature are derived by simple approximations of the thermocline sharpnessThese parameterizations can be useful tools for evaluating the impacts of thermocline feedback on ENSO [ABSTRACT FROM AUTHOR]

Due to climate change, global oceanic dissolved oxygen (DO) has been decreasing, and oxygen minimum zones (OMZs) have been expanding. Here, we estimate the annual global and regional OMZ areas using geostatistical regression combined with Monte Carlo. From 1960 to 2019, annual global OMZ20 (DO < 20 μmol/kg) and OMZ60 (DO < 60 μmol/kg) areas cover 5%–14% and 15%–32% of the global ocean, respectively. The global and most regional OMZ areas after the late 2000s were all significantly larger than those in previous years. Most oceanic regions likely experienced significant expansions in OMZ areas, especially the North Pacific. However, the equatorial Pacific with the largest OMZ area showed insignificant horizontal expansion. The expanding OMZ areas of most oceans responded more quickly and effectively to the surrounding sea temperatures than to the sea surface temperatures. Our research highlights the need for marine resource management to account for the challenges from OMZs. Plain Language Summary: Expanding oxygen minimum zones (OMZs, low dissolved oxygen [DO] zones) in the open ocean have been the topic of intense interest. However, annual OMZ areas, trends in OMZ areas, and causes of OMZ expansions remain unclear. Our paper explores the locations and annual areas of global and regional OMZ areas based on observations from 1960 to 2019. Over the last six decades, annual global OMZ20 (DO < 20 μmol/kg) and OMZ60 (DO < 60 μmol/kg) areas cover 5%–14% and 15%–32% of the global ocean, respectively. The global OMZ areas show great interannual variability but remain stably large after the late 2000s. Most oceanic regions have experienced significant OMZ expansion, especially the North Pacific. The equatorial Pacific Ocean has the largest OMZ area, but shows insignificant expansion. Subsurface sea temperatures are more associated with the expanding OMZ areas than sea surface temperatures. Increasing sea temperature would substantially increase OMZ areas by the end of the century for the future societal development pathway (SSP585) scenario. Understanding how OMZs will change over the long‐term due to climate change is beneficial for the protection of marine resources. Key Points: From 1960 to 2019, most oceanic regions experienced significant horizontal expansions in oxygen minimum zone (OMZ) areas, especially the North PacificThe equatorial Pacific has the largest OMZ area but shows an insignificant increasing trendThe OMZ areas of most oceans respond more quickly and effectively to surrounding sea temperatures than to sea surface temperatures [ABSTRACT FROM AUTHOR]

We present preliminary results from the field program Organization of Tropical East Pacific Convection (OTREC), with measurements during August and September of 2019 using the NSF/NCAR Gulfstream V over the tropical East Pacific and Southwest Caribbean. We found that active convection in this region has predominantly bottom‐heavy vertical mass fluxes, while decaying systems exhibit top‐heavy fluxes characteristic of stratiform rain regions. As in other regions that have been studied, a strong anti‐correlation exists between the low to mid‐level moist convective instability and the column relative humidity or saturation fraction. Finally, the characteristics of convection as a function of latitude differ greatly between the Southwest Caribbean and Colombian Pacific coast on one hand, and the intertropical convergence zone to the west. In particular, the strongest convection in the former is to the south, while it is to the north in the latter, in spite of similar latitudinal sea surface temperature distributions. Plain Language Summary: Understanding and modeling the physics of tropical atmospheric convection still poses a big challenge. Things are complicated over the tropical oceans because there is no easy way to gather data. Field projects such as the one presented in this paper, Organization of Tropical East Pacific Convection (OTREC2019) are of great importance to our scientific community for this reason. During OTREC, 22 research flights were performed over East Pacific and Southwest Caribbean gathering data from 13 km to surface. This paper presents early results that will help us understand the physics of convection and improve our weather and climate models. Key Points: Thermodynamic parameters instability index and saturation fraction show anti‐correlation, as explained by moisture quasi‐equilibrium theoryVertical mass flux profiles are generally bottom heavy for developing convection and top heavy for decaying convectionStrong differences exist between convection in the East Pacific ITCZ and in the Caribbean and Pacific coast of Colombia [ABSTRACT FROM AUTHOR]

Downstream of Cape Hatteras, the Gulf Stream (GS) is bounded to the north by a sharp temperature front known as the North Wall (NW). Previous studies have generally assumed that variations of the NW and GS are equivalent. Using satellite sea surface height to identify the GS and the 15 °C isotherm at 200‐m depth to represent the NW, this paper examines their similarities and differences during 1993–2016. The NW and GS are geographically close and vary similarly only to the west of 71°W. Downstream of that, they rapidly diverge—and the variances of their latitudes increase by more than a factor of 2—as the GS flows past the New England Seamounts. Evidence is presented to show that the difference in properties of the NW and the GS is related to the presence of mesoscale eddies in the region separating them. Plain‐Language Summary: To the east of Cape Hatteras where the Gulf Stream leaves the continental shelf, a sharp gradient in ocean temperature, known as the North Wall, is seen just north of the current. Fluctuations in the position of the North Wall have large impacts in the distribution of heat in the North Atlantic and on plankton populations. There is a widespread assumption in the literature that variations of the North Wall and the Gulf Stream are similar. Results presented in this paper show that variations of these two features become increasingly different as the current proceeds eastward and the divergence in their properties is due to the presence of turbulent eddies, which arise after the Gulf Stream interacts with the New England Seamounts. These results suggest that recognition of distinct behaviors of the Gulf Stream and the North Wall is needed for a better understanding of their dynamics and their impact on climate. Key Points: The Gulf Stream and its North Wall are close to each other west of 71°W but depart sharply in their properties farther downstreamDifferences in the behaviors of the Gulf Stream and the North Wall are related to mesoscale eddies between themRecognition of distinct properties of Gulf Stream and the North Wall is necessary for understanding their dynamics and impact on climate [ABSTRACT FROM AUTHOR]

Using 25 years of ERA5 reanalysis data, this study shows that wind divergence is partially driven by small‐scale sea surface temperature (SST) patterns via their effect on the boundary layer stability. Moreover, strong warm‐to‐cold fronts (the upper quartile) are associated with a mean increase of cloud cover of 10% ± 5% and a mean increase in the probability of a rain event of 15% ± 6%, with respect to the average values. The cloud and rainfall dependence on SST fronts is more pronounced in Fall, probably due to the stronger SST gradients present at the end of the summer season. Plain Language Summary: Oceanic thermal structures impact surface winds, low‐level clouds, and rainfall. Using reanalysis data over the Mediterranean Sea, this paper shows that the reduced stability over warm patches increases the coupling of surface winds with winds aloft. This process affects the entire marine atmospheric boundary layer. This results in a cloud cover and rainfall response: when the wind blows from warm to cold (from cold to warm) water, a converging (diverging) cell is enhanced, increasing (decreasing) low‐cloud cover and favoring by about 15% a rain event. Key Points: Oceanic thermal feedback acts on the marine atmospheric boundary layer dynamics at daily time‐scaleThe downward momentum mixing mechanism imprints cloud cover and rainSST gradients partially drive cloud structures and rainfall over the Mediterranean Sea [ABSTRACT FROM AUTHOR]

In early August 2018, a record‐setting marine heatwave (MHW) occurred along the coast of Southern and Baja California. Water temperature at Scripps Pier rose to 26.4°C, the highest in 102 years of measurements. This paper investigates the large‐scale ocean‐atmospheric conditions for this Southern California MHW event. This intense and sustained event is the result of the superposition of "weather" timescale warming by a coastal wind relaxation and intraseasonal warming by coastally trapped waves. For weather‐scale warming, climatological upwelling was weakened by a wind relaxation, due to anomalous offshore low pressure associated with passing tropical cyclones John and Kristy. For intraseasonal warming, the upwelling was inhibited and the thermocline was deepened by poleward‐propagating coastally trapped waves, which provided a slow warming background. The mixed layer heat budget analysis indicates that the reduced upwelling and deepened thermocline are the major cause of the MHW. Plain Language Summary: The Southern California marine heatwave in August 2018, which set a record for warmest sea surface temperature ever recorded at San Diego, encompasses two timescales. "Slow" warming began due to an earlier coastal wind relaxation, which reduced the upwelling of cool water; and the propagation of coastally trapped waves. "Fast" warming occurred in August due to a wind relaxation associated with an offshore tropical cyclone. Key Points: A record‐setting marine heatwave (MHW) occurred along the coast of Southern and Baja California in early August 2018This MHW is the result of a coastal wind relaxation on the weather timescale and intraseasonal coastally trapped wavesThe ocean upwelling and vertical mixing are the primary cause of the MHW event [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) observations in the North Atlantic since 1870 reveal a region of enhanced warming off the northeastern coast of North America, and a region of cooling to the south of Greenland. It has been hypothesized that these adjacent SST trends are a result of long‐term changes in the buoyancy‐driven ocean circulation—a slowdown of the Atlantic Meridional Overturning Circulation. The impacts of these historical SST trends on the atmosphere are estimated using idealized atmospheric general circulation model experiments in which the global atmosphere is exposed to modern climatological forcing minus the aforementioned regional SST trends. The local response includes a negative North Atlantic Oscillation tendency and southward shift of the wind forcing for the subtropical gyre. Due to planetary wave propagation, the regional SST trends also induce a northward shift of the intertropical convergence zone over the Indian Ocean. Implications for climate feedbacks and projections are discussed. Plain Language Summary: The ocean surface is warming over most of the planet, with a couple of notable exceptions. One is over the North Atlantic Ocean, just south of Greenland, where the temperature of the ocean surface has actually been cooling by about 1°C since 1870. Nearby, extending off the northeastern coastline of North America is a region where the ocean surface has been warming much faster than the global ocean on average—by about 1.5°C since 1870. This side‐by‐side pair of accelerated warming and cooling trends is likely due to a slowdown of the ocean's overturning circulation, which carries large amounts of heat energy from the tropics toward the poles. This paper reveals the effects of those unusual ocean temperature trends in the North Atlantic on the atmospheric circulation using a computer model of the global atmosphere. The model simulations indicate that these ocean temperature trends are causing shifts in the jet stream, changing the way the winds propel the upper ocean currents, and even have impacts quite far away by moving the tropical rain belt northward in the Indian Ocean. Key Points: The impact of historical sea surface temperature trends in the North Atlantic since 1870 is simulated with a global atmospheric modelAdjacent warming and cooling trends in the North Atlantic Ocean induce a negative NAO‐like response and southward‐shifted gyre forcingPropagation of planetary waves away from the North Atlantic induces a northward‐shifted intertropical convergence zone in the Indian Ocean [ABSTRACT FROM AUTHOR]

Long‐term droughts, closely linked to oceanic climate variability, are a great threat to the global economy, as well as food and water security. Skillful predictions will provide enormous benefit to the human societies. By taking advantages of the newly available decadal hindcasts and target experiments from the Coupled Model Intercomparison Project phase 6 (CMIP6), this paper evaluates potential predictability and decadal prediction skill of droughts over major river basins across the globe. The results show that the CMIP6 drought prediction is skillful within 3–6 years, and the skill score is 11% higher than CMIP5 over 47% river basins. Prediction skill is enhanced by 15% (16%) over 47% (49%) basins, forced by the actual sea surface temperature over tropical eastern Pacific (North Atlantic). Our findings imply the possibility of drought adaptation based on skillful decadal prediction with sufficient lead time. Plain Language Summary: A reliable drought prediction is helpful to reduce adverse impact and economic loss by increasing the preparedness. Although operational seasonal drought prediction has been applied over the world, decadal prediction is still a grand challenge due to limited knowledge about the predictability and mechanism of long‐term drought. Here we apply the decadal hindcasts from CMIP6 to evaluate the probabilistic decadal prediction skill of drought and analyze the impacts of sea surface temperature on the prediction through CMIP6 pacemaker experiments. Our results show the skillful prediction can be found over half of the major river basins at 1–4 years lead. In addition, incorporating the actual sea surface temperature in tropical eastern Pacific or North Atlantic can efficiently enhance the drought prediction skill over 48% basins as well. This study suggests that it is possible to provide useful long‐term drought information for water and agricultural management through using coupled models, but the skill can be improved further by optimization of initialization and ensemble methods. Key Points: Probabilistic predictions of basin‐scale droughts by dynamical models are skillful within 3–6 years across the globeThere is an 11% improvement from CMIP6 models compared to CMIP5 over 47% of the largest 177 global river basinsSSTs in the tropical eastern Pacific and the North Atlantic have critical importance for the success over about 48% basins [ABSTRACT FROM AUTHOR]

Fire activity in Equatorial Asia shows large interannual variability. Teleconnections by El Niño‐Southern Oscillation and Indian Ocean Dipole are linked to drought and fire events; however, we found here that significant role of local Sea Surface Temperature (SST) over the Banda Sea in interannual variability of Equatorial Asian burned area in October even after removing the linear effects of teleconnections. October is the transient period from dry to wet season and strengthened seasonal circulation in October leads to a negative SST anomaly through Wind‐Evaporation‐SST mechanism. This anomalous local air‐sea interaction sustains the dry season into October and stronger fire activity. Moreover, we found that the sensitivity of precipitation to SST is higher in October than in other months, hence fires in Equatorial Asia can be sensitively driven by local SST changes. Identification of this sensitivity will underpin better predictions of fire activity and air quality in Equatorial Asia. Plain Language Summary: Previous studies have emphasized that the remote forcing associated with El Niño‐Southern Oscillation and Indian Ocean Dipole are the most important climate factors in controlling fire activity in Equatorial Asia. However, we found here that there is a huge diversity of fire activity among El Niño events. In this paper, we newly found that the local air‐sea interaction significantly influences on fire activity in Equatorial Asia. When the seasonal wind gets stronger, the Banda Sea, local sea located between Kalimantan and New Guinea, is cooler and it leads to the drought and strong fire activity. We also found that the role of local air‐sea interaction strongly depends on the seasonality, which is related to seasonal variation of sea surface temperature climatology. Our findings will be utilized for better predictions of fire activity and air quality in Equatorial Asia. Key Points: Strong diversity of fire activity in Equatorial Asia among El Niño eventsImportance of local air‐sea interaction on fire activity; cooler Banda Sea leads drought and stronger fire activityFire activity is sensitively driven by local SST changes due to strong local air‐sea interaction in October [ABSTRACT FROM AUTHOR]

Climate models often parameterize isopycnal mixing by mesoscale eddies using a diffusion operator that acts in the isopycnal direction, multiplied by an isopycnal‐mixing coefficient. The magnitude of this coefficient is uncertain, with observational estimates ranging from 10 to 10,000 m2/s. In an idealized‐geometry ocean model, the isopycnal‐mixing coefficient is varied across a similar range without allowing the circulation to change: This leads to large changes in the ventilation of the deep ocean. Passive tracers are used to assess the impact of varying the isopycnal‐mixing coefficient on water mass distributions. Increasing the isopycnal‐mixing coefficient from 50 to 5,000 m2/s leads to a 63% reduction in the amount of North Atlantic Deep Water and a doubling in the amount of Antarctic Bottom Water in the deep Pacific ocean. This change is associated with a 700‐yr reduction in the ideal age of water in the deep Pacific ocean. Plain Language Summary: The ocean is full of eddies at the 100‐km scale that stir tracers along surfaces of constant density. Climate models generally have coarse grids that are not able to resolve these ocean eddies. Instead, climate models represent this stirring using a diffusion operator along surfaces of constant density: This diffusion operator smooths out gradients in ocean tracers like temperature, salinity, oxygen, and nutrients. Based on currently available observations, it is unclear how large this diffusion operator should be in climate models. The results shown in this paper suggest that the amount of diffusion along surfaces of constant density influences the pathway along which water reaches the deep Pacific. Improving estimates of the size of this diffusion operator would probably increase the accuracy of climate models. Key Points: Passive tracers are used to quantify the amount of deep water that originated in each region of the surface oceanIncreasing the isopycnal‐mixing coefficient reduces the fraction of NADW in the deep PacificIncreasing the isopycnal‐mixing coefficient reduces the ideal age of water in the deep Pacific [ABSTRACT FROM AUTHOR]

Ocean heat transport through the Barents Sea Opening (BSO) has strong impacts on the Barents Sea ice extent and the climate. In this paper we quantified the contributions from different atmospheric forcing components to the trend and interannual variability of the BSO heat transport. Ocean‐ice model simulations were conducted in which the interannual variation of atmospheric forcing was maintained only in or outside the Arctic in two different simulations. The sum of their BSO heat transport anomalies reasonably replicated the trend and variability from a hindcast simulation. The upward trend of the BSO heat transport mainly stems from the increasing ocean temperature in the subpolar North Atlantic. For the interannual variability, the local wind and upstream forcing are similarly important. The location of the Atlantic Water boundary current in the Nordic Seas, influenced by the cyclonic atmospheric circulation, is crucial in determining part of the BSO inflow variability. Key Points: Forcing responsible for the trend and interannual variability of ocean heat transported into the Barents Sea is disentangledThe upward trend of the heat inflow stems from warming in the subpolar North Atlantic, while local wind accounts for half of its varianceThe location of the Atlantic Water boundary current relative to the Barents Sea entrance is crucial in determining the heat inflow [ABSTRACT FROM AUTHOR]

The asymmetry of mixed layer salinity (MLS) anomalies in response to El Niño and La Niña events in the tropical Pacific is examined for the first time based on Argo observations and Estimating the Circulation and Climate of the Ocean simulation. The difference of MLS anomalies between El Niño and La Niña shows a dipole structure, with El Niño featuring strong negative salinity anomalies east of 160°E while La Niña shows remarkable positive salinity anomalies west of 160°E. A salinity budget analysis suggests that nonlinear zonal advection plays a dominant role in generating the asymmetric MLS structure. This dipole MLS structure acts to generate a dipole structure of barrier layer thickness and thus likely plays an important role in the development of El Niño–Southern Oscillation asymmetries in sea surface temperature. Plain Language Summary: The El Niño–Southern Oscillation (ENSO) is well known for its great impacts on global climate. Previous studies on the differences between El Niño and La Niña, defined as ENSO asymmetry, focused primarily on temperature, but the differences in salinity between the two ENSO phases and their potential effects on the ENSO asymmetry are relatively unexplored. In this paper, we have for the first time documented salinity differences associated with El Niño‐La Niña asymmetry and diagnosed the air‐sea processes in controlling these differences based on observations and model simulations. We find a dipole structure exists in mixed layer salinity (MLS) variations between El Niño and La Niña in the western‐central equatorial Pacific, with stronger negative MLS anomalies to the east of 160°E during El Niño and stronger positive MLS anomalies to the west of 160°E during La Niña. Based on a salinity budget analysis, the MLS dipole is mostly attributed to the nonlinear zonal advection. This dipole MLS structure in relation to a dipole structure in barrier layer thickness, and their potential to feedback on El Niño and La Niña sea surface temperature asymmetry, is also discussed. Key Points: A dipole structure of mixed layer salinity in the western‐central equatorial Pacific is a characteristic of El Niño‐La Niña asymmetryNonlinear zonal advection plays a dominant role in generating the dipole salinity structureThe dipole salinity structure has potential effects on the asymmetry of ENSO SST [ABSTRACT FROM AUTHOR]

We demonstrate in this paper that conventional time‐averaged decadal hindcast skill estimates can overestimate or underestimate the credibility of an individual decadal climate forecast. We show that hindcast skill in a long period can be higher or lower than skill in its subperiods. Instead of using time‐averaged hindcast skill measures, we propose to use the physical state of the climate system at the beginning of the forecast to judge its credibility. We analyze hindcasts of North Atlantic sea surface temperature (SST) in an initialized prediction system based on the MPI‐ESM‐LR for the period 1901–2010. Subpolar North Atlantic Ocean heat transport (OHT) strength at hindcast initialization largely determines the skill of these hindcasts: We find high skill after anomalously strong or weak OHT, but low skill after average OHT. This knowledge can be used to constrain conventional hindcast skill estimates to improve the assessment of credibility for a decadal forecast. Plain Language Summary: Credible predictions of climate up to 10 years into the future, so‐called decadal climate predictions, can be a potent tool for decision makers. However, previous work indicated that such predictions are sometimes credible and sometimes not. This study illustrates that knowing the physical state of the climate system at the start of a decadal climate prediction helps assessing the credibility of that prediction. Analyzing sea surface temperature in the North Atlantic as a case study, we show that northward heat transport in the ocean provides a good indicator of the credibility of decadal predictions in the North Atlantic. Unlike previous studies, we do not only analyze time‐averaged prediction credibility, but look at individual predictions in the past. This makes our findings particularly relevant for individual forecasts and decision makers. Key Points: Decadal hindcast skill estimates can misrepresent forecast skill as skill depends on the length of the time window used for calculationNorth Atlantic SST hindcast skill is high when subpolar ocean heat transport is strong or weak at initialization and low after average OHTAnalyzing individual hindcasts shows that accounting for ocean heat transport helps to better constrain credibility of individual forecasts [ABSTRACT FROM AUTHOR]

The climate over Europe is significantly affected by the state of the atmospheric circulation over the North Atlantic, which in turn is influenced by various slowly varying components of the climate system. Linear models are employed to investigate how much of the European winter air temperature variability can directly be predicted from the sea surface temperature. By comparing relations from a statistical model to those from an adjoint to a linearized climate model we evaluate the ability of the lagged maximum covariance analysis to reconstruct the dynamical relation between sea surface temperature (SST) and atmosphere temperature. Applied to specific SST conditions, both approaches allow prediction of a similar and significant fraction of the air temperature variability from SST. For a 1‐month lag, extreme air temperature responses are significant and found to be associated with an omega‐blocking pattern over Scandinavia. Plain Language Summary: A connection between sea surface temperature (SST) and atmospheric circulation anomalies has been established for decades. Certain SST anomalies force changes in the atmospheric circulation and cause over Europe typical weather patterns (dry and cold or wet and warm) after some lag. A relationship between the SST pattern and the associated response can be derived from numerical models or statistical relations and used for forecasting the atmospheric response to SST anomalies. In this paper, we validate this forecasting method with observational data. Key Points: Linear models show that sea surface temperature anomalies may provide significant skill for predicting northern European air temperaturesStatistical methods can provide sufficiently precise estimates of the lagged relation between sea surface and air temperature [ABSTRACT FROM AUTHOR]

Decadal climate predictability has received considerable scientific interest in recent years, yet the limits and mechanisms for decadal predictability are currently not well known. It is widely accepted that noise due to internal atmospheric dynamics at the air‐sea interface influences predictability. The purpose of this paper is to use the interactive ensemble (IE) coupling strategy to quantify how internal atmospheric noise at the air‐sea interface impacts decadal predictability. The IE technique can significantly reduce internal atmospheric noise and has proven useful in assessing seasonal‐to‐interannual variability and predictability. Here we focus on decadal timescales and apply the nonlinear local Lyapunov exponent method to the Community Climate System Model comparing control simulations with IE simulations. This is the first time the nonlinear local Lyapunov exponent has been applied to the state‐of‐the‐art coupled models. The global patterns of decadal predictability are discussed from the perspective of internal atmospheric noise and ocean dynamics. Key Points: For the first time, the nonlinear local Lyapunov exponent method has been applied to the state‐of‐the‐art coupled climate modelsDepending on region reducing internal atmospheric noise reduces decadal variability but either increases or decreases decadal predictabilityDecadal‐scale subsurface predictability is examined for the North Atlantic region [ABSTRACT FROM AUTHOR]

Oceanic intraseasonal variability (ISV) plays a vital role in the initiation and development of El Niño events. Mooring observations reveal contrasting characteristics of upper‐ocean zonal current ISV during different El Niño events. Compared to the 1997–1998 event, ISV of the 2015–2016 event was significantly weaker (by around 30–50%) over the equatorial Pacific basin, and the variability maximum was shifted to the east (~160°E). These differences can be largely explained by wind forcing features. ISV of surface winds in 2015 is overall weaker than in 1997. Intraseasonal westerly wind bursts in 1997 occurred near the western boundary and originated from the Indian Ocean, whereas in 2015, perturbations were mainly propagated from the northeast Pacific and achieved the peak power at ~160°E of the equator. These changes are likely associated with the decadal warming of the northeast subtropical Pacific and weakening of the Indian Ocean intraseasonal oscillation. Plain Language Summary: Sea surface warmings of strong El Niño events are usually triggered by intraseasonal oscillations prior to the events. Particularly, intraseasonal eastward ocean currents generated by westerly wind bursts carry the warm water of the western Pacific warm pool to the central Pacific and contribute to the surface warming. In this paper, we compare the intraseasonal zonal current variations during two strong El Niño events, namely, the 1997–1998 and 2015–2016 events. Although the two El Niño events were of comparable strengths, intraseasonal variations in 1997 were stronger than those in 2015 by 3050%. These differences can be largely explained by the contrasting features of wind forcing. Intraseasonal oscillations were originated from the Indian Ocean in 1997, whereas those in 2015 were mainly from the northeast subtropical Pacific. The varying origin is likely associated with the warming of the northeast subtropical Pacific and weakened Indian Ocean intraseasonal oscillation during recent years. Key Points: Oceanic ISV during the 2015–2016 El Niño was significantly weaker than that in the 1997–1998 eventWeaker ocean current ISV has smaller effect on SST anomalies through advectionDifferences in ocean ISV are induced by contrasting westerly wind events that have different origins during the two events [ABSTRACT FROM AUTHOR]