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The north and central coast of Chile is influenced by El Niño-Southern Oscillation (ENSO) through oceanic and atmospheric teleconnections. However, it also experiences episodic oceanic warmings off central Chile (30°S) lasting a few months that are not necessarily associated with ENSO. These episodes, called "Chile Niño" events, besides their ecological and socio-economical impacts, have also the potential to influence tropical Pacific variability. Here, we investigate how realistically the models in the Coupled Model Intercomparison Project (CMIP, Phases 5 and 6) simulate Chile Niño/Niña (CN) events, and quantify their changes under anthropogenic forcing. Despite limitations of the global models in simulating realistically coastal upwelling dynamics, we show that they simulate reasonably well the observed spatial pattern, amplitude and seasonal evolution of CN events. They however fail to properly represent the positive skewness from observations. The analysis of a sub-group of models (36) that simulate ENSO realistically reveals that CN events increase in amplitude and variance in the future climate with no changes in their frequency of occurence. This is interpreted as resulting from compensating effects amongst changes in remote drivers and local feedbacks. In particular, ENSO variance increases while that of the South Pacific Oscillation decreases. Conversely, we found that while the Wind-Evaporation-SST feedback tends to increase and the coupling between mixed-layer depth and SST weakens, favoring the development of CN events, the thermocline and wind-SST feedbacks decrease. However, only the change in the thermocline feedback is correlated to changes in CN variance amongst the models, suggesting a dominant role of local oceanic stratification changes in constraining the sensitivity of CN to global warming. [ABSTRACT FROM AUTHOR]

Interactions between ocean surface currents, winds and waves at the atmosphere-ocean interface are key controls of lateral and vertical exchanges of water, heat, carbon, gases and nutrients in the global Earth System. The SeaSTAR satellite mission concept proposes to better quantify and understand these important dynamic processes by measuring two-dimensional fields of total surface current and wind vectors with unparalleled spatial and temporal resolution (1 × 1 km2 or finer, 1 day) and unmatched precision over one continuous wide swath (100 km or more). This paper presents a comprehensive numerical analysis of the expected performance of the Earth Explorer 11 (EE11) SeaSTAR mission candidate in the case of idealised and realistic 2D ocean currents and wind fields. A Bayesian framework derived from satellite scatterometry is adapted and applied to SeaSTAR's bespoke inversion scheme that simultaneously retrieves total surface current vectors (TSCV) and ocean surface vector winds (OSVW). The results confirm the excellent performance of the EE11 SeaSTAR concept, with Root Mean Square Errors (RMSE) for TSCV and OSVW at 1 × 1 km2 resolution consistently better than 0.1 m/s and 0.4 m/s, respectively. The analyses highlight some performance degradation in some relative wind directions, particularly marked at near range and low wind speeds. Retrieval uncertainties are also reported for several variations around the SeaSTAR baseline three-azimuth configuration, indicating that RMSEs improve only marginally (by ∼0.01 m/s for TSCV) when including broadside Radial Surface Velocity or broadside dual-polarisation data in the inversion. In contrast, our results underscore (a) the critical need to include broadside Normalised Radar Cross Section data in the inversion; (b) the rapid performance degradation when broadside incidence angles become steeper than 20° from nadir; and (c) the benefits of maintaining ground squint angle separation between fore and aft lines-of-sight close to 90°. The numerical results are consistent with experimental performance estimates from airborne data and confirm that the EE11 SeaSTAR concept satisfies the requirements of the mission objectives. [ABSTRACT FROM AUTHOR]

The atmosphere‐ocean is a highly coupled system with significant diurnal and hourly variations. However, current coupled models usually lack sub‐diurnal scale processes at the air‐sea interface due to the finite vertical resolution for ocean discretization. Previous modeling studies showed that sub‐diurnal scale air‐sea interaction processes are important for ocean mixing. Here, by designing an integrated sub‐diurnal parameterization (ISDP) scheme which combines different temperature profiling functions, we stress sub‐diurnal air‐sea interactions to better represent the local ocean mixing. This scheme has been implemented into two coupled models which contributed to the Climate Model Intercomparison Project (CMIP), referenced by the Intergovernmental Panel on Climate Change—Community Earth System Model and Coupled Model version 2. The results show that the ISDP scheme improves model simulations with better climatology and more realistic spectra, especially in the tropics and North Pacific Ocean. With the scheme, the tropical cold tongue bias is significantly relaxed by reducing the overestimation of ocean upper mixing, and the cold bias of North Pacific Ocean is reduced due to the improvement on currents and net heat fluxes. Our scheme may help better the simulation and prediction skills of coupled models when their horizontal resolution becomes fine but vertical resolution remains relatively coarse as it describes high‐frequency air‐sea interactions more realistically. Plain Language Summary: The atmosphere and ocean interact with each other. In these interactions, changes may occur over different time periods. Some changes take years or months, while others happen in days or hours. The atmosphere changes quickly throughout the day, such as air temperature, wind, and rain. These quick changes in the atmosphere can affect the ocean rapidly. Similarly, there are quick changes in the ocean. These quick changes also affect the atmosphere in return. Understanding these quick changes is important. However, it is difficult to perfectly capture the quick changes in the climate models because they do not always represent the ocean behavior accurately. In this study, we developed a new scheme (named the sub‐diurnal scale parameterization, ISDP) to better represent these quick changes. We added this new scheme into two widely‐used climate models. These models are important tools for studying climate change. Our new scheme represents better the ocean mixing and interaction based on the local weather conditions. When we use ISDP, the climate models get better at simulating climate. They're more accurate in showing ocean temperatures in the tropics. These models make the tropical oceans seem colder than they really are, but our scheme, to some extent, fixes this problem. The results also show the more accurate ocean temperatures in the North Pacific Ocean. Our new scheme has a great potential to make climate models more accurate. Key Points: An integrated sub‐diurnal scale parameterization (ISDP) scheme has a more appropriate representation for local ocean mixingThe ISDP scheme has been implemented into two models which contributed to the Climate Model Intercomparison Project, referenced by IPCC: Community Earth System Model and Coupled Model version 2The model tropical cold tongue bias with ISDP is relaxed by reducing the overestimation of ocean upper mixing [ABSTRACT FROM AUTHOR]

In this study, we explore the impact of oceanic moisture fluxes on atmospheric blocks using the ECMWF IFS. Artificially suppressing surface latent heat flux over the Gulf Stream (GS) region reduces atmospheric blocking frequency across the Northern Hemisphere by up to 30%. Affected blocks show a shorter lifespan (−6%), smaller spatial extent (−10%), and reduced intensity (−0.4%), with an increased number of individual blocking anticyclones (+17%). These findings are robust across various blocking detection thresholds. Analysis reveals a qualitatively consistent response across all resolutions, with Tco639 (∼18 km) showing the largest statistically significant change across all blocking characteristics, although differences between resolutions are not statistically significant. Exploring the broader Rossby wave pattern, we observe that diminished moisture fluxes favor eastward propagation and higher zonal wavenumbers, while air‐sea interactions promote stationary and westward‐propagating waves with zonal wavenumber 3. This study underscores the critical role of the GS in modulating atmospheric blocking. Plain Language Summary: In our study, we investigated how changes in oceanic moisture, specifically from the Gulf Stream (GS), affect atmospheric blocking events using the ECMWF Integrated Forecast System. By artificially reducing the moisture flux from the GS area, we observed a notable decrease in the occurrence of atmospheric blocks across the Northern Hemisphere‐up to 30%. These blocks also showed changes in their characteristics: they had shorter durations by 6%, were 10% smaller in spatial size, and had a slight decrease in intensity, alongside a 17% increase in their detection rates. Our findings were consistent across different methods of identifying blocks. We also discovered that the model's resolution influences the observed changes, with coarser resolutions (larger than approximately 18 km) not showing significant alterations in some blocking traits despite the overall frequency reduction. Further analysis into Rossby wave patterns revealed that reduced moisture flux tends to favor faster eastward‐moving patterns with shorter wavelengths, whereas interactions between the air and sea support more stationary or westward‐moving waves, particularly those with a zonal wavenumber of 3. This highlights the crucial role that moisture from western boundary currents like the GS plays in the development and behavior of atmospheric blocking patterns. Key Points: Gulf Stream (GS) moisture flux suppression reduces atmospheric blocking across the Northern HemisphereGS moisture fluxes generate larger jet stream perturbations, fostering faster westward‐propagating Rossby wavesHigher resolution models enhance signal transport from the boundary layer to the upper troposphere [ABSTRACT FROM AUTHOR]

Turbulence-enhanced mixing of upper ocean heat allows interaction between the tropical atmosphere and cold water masses that impact climate at higher latitudes thereby regulating air-sea coupling and poleward heat transport. Tropical cyclones (TCs) can drastically enhance upper ocean mixing and generate powerful near-inertial internal waves (NIWs) that propagate down into the deep ocean. Globally, downward mixing of heat during TC passage causes warming in the seasonal thermocline and pumps 0.15 to 0.6 PW of heat into the unventilated ocean. The final distribution of excess heat contributed by TCs is needed to understand subsequent consequences for climate; however, it is not well constrained by current observations. Notably, whether or not excess heat supplied by TCs penetrates deep enough to be kept in the ocean beyond the winter season is a matter of debate. Here, we show that NIWs generated by TCs drive thermocline mixing weeks after TC passage and thus greatly deepen the extent of downward heat transfer induced by TCs. Microstructure measurements of the turbulent diffusivity ([Formula: see text]) and turbulent heat flux (J[Formula: see text]) in the Western Pacific before and after the passage of three TCs indicate that mean thermocline values of [Formula: see text] and J[Formula: see text] increased by factors of 2 to 7 and 2 to 4 (95% confidence level), respectively, after TC passage. Excess mixing is shown to be associated with the vertical shear of NIWs, demonstrating that studies of TC-climate interactions ought to represent NIWs and their mixing to accurately capture TC effects on background ocean stratification and climate.

Abstract The atmosphere‐ocean is a highly coupled system with significant diurnal and hourly variations. However, current coupled models usually lack sub‐diurnal scale processes at the air‐sea interface due to the finite vertical resolution for ocean discretization. Previous modeling studies showed that sub‐diurnal scale air‐sea interaction processes are important for ocean mixing. Here, by designing an integrated sub‐diurnal parameterization (ISDP) scheme which combines different temperature profiling functions, we stress sub‐diurnal air‐sea interactions to better represent the local ocean mixing. This scheme has been implemented into two coupled models which contributed to the Climate Model Intercomparison Project (CMIP), referenced by the Intergovernmental Panel on Climate Change—Community Earth System Model and Coupled Model version 2. The results show that the ISDP scheme improves model simulations with better climatology and more realistic spectra, especially in the tropics and North Pacific Ocean. With the scheme, the tropical cold tongue bias is significantly relaxed by reducing the overestimation of ocean upper mixing, and the cold bias of North Pacific Ocean is reduced due to the improvement on currents and net heat fluxes. Our scheme may help better the simulation and prediction skills of coupled models when their horizontal resolution becomes fine but vertical resolution remains relatively coarse as it describes high‐frequency air‐sea interactions more realistically.

Abstract In this study, we explore the impact of oceanic moisture fluxes on atmospheric blocks using the ECMWF IFS. Artificially suppressing surface latent heat flux over the Gulf Stream (GS) region reduces atmospheric blocking frequency across the Northern Hemisphere by up to 30%. Affected blocks show a shorter lifespan (−6%), smaller spatial extent (−10%), and reduced intensity (−0.4%), with an increased number of individual blocking anticyclones (+17%). These findings are robust across various blocking detection thresholds. Analysis reveals a qualitatively consistent response across all resolutions, with Tco639 (∼18 km) showing the largest statistically significant change across all blocking characteristics, although differences between resolutions are not statistically significant. Exploring the broader Rossby wave pattern, we observe that diminished moisture fluxes favor eastward propagation and higher zonal wavenumbers, while air‐sea interactions promote stationary and westward‐propagating waves with zonal wavenumber 3. This study underscores the critical role of the GS in modulating atmospheric blocking.

The Indian Ocean Dipole (IOD) with worldwide socio-economic impacts has been presented to mature either in boreal summer or autumn, leading to the classification of summer IOD and autumn IOD. Investigating the climate dynamics to distinguish between these two types of IOD can improve our understanding and prediction of the surrounding weather and climate. This study demonstrates that the emergence of the summer IOD is mainly attributed to internal air-sea interactions in the western tropical Indian Ocean (WIO), while the autumn IOD is significantly related to ENSO development. For the summer IOD, broad-scaled warm sea surface temperature anomalies in the WIO are conducive to the enhancement of convective perturbations. Then local ocean–atmosphere feedback associated with changes in convection and surface heat flux into the upper ocean plays a key role in triggering the summer IOD. For the autumn IOD, strong easterly wind anomalies in the eastern Indian Ocean initiate oceanic Rossby waves and Bjerknes feedback, leading to the formation of both the western and eastern poles. It is recognized that these intensified easterly wind anomalies mostly benefit from ENSO variability. The distinctive features and air-sea interactions intrinsic to the summer IOD and the autumn IOD revealed in this study can further contribute to more credible predictive models of diverse IOD events. [ABSTRACT FROM AUTHOR]

Based on outputs of 28 coupled models from the Phase 6 of the Coupled Model Intercomparison Project (CMIP6), we show that the response of the Southeast Asian summer monsoon to the El Niño‐Southern Oscillation (ENSO) during post‐ENSO summer will likely strengthen in a warmer climate, which can be attributed to concurrently weakened sea‐surface temperature anomalies (SSTAs) in the western equatorial Pacific (WEP). The weakened WEP SSTAs are primarily caused by enhanced latent heat damping due to increased surface wind speed anomalies, which are associated with the eastward shift of the El Niño‐induced anomalous Walker circulation due to El Niño‐like sea surface temperature change in the tropical Pacific under global warming. Besides, the climatological zonal ocean currents will slow down due to the weakening of climatological Walker circulation, which also acts to weaken the WEP SSTAs via reducing the advection of anomalous temperature by the mean current. Plain Language Summary: As an important component of the Asian monsoon system, the Southeast Asian summer monsoon (SEASM) is crucial for the livelihoods of billions of people in East Asia, and is closely connected to a climate phenomenon called El Niño‐Southern Oscillation (ENSO). Understanding how the relationship between SEASM and ENSO will change in the future is important for enhancing our knowledge of climate change in East Asia. Outputs of 28 climate models from the Phase 6 of the Coupled Model Intercomparison Project show that ENSO will exert enhanced impacts on the SEASM in a warmer climate. The enhanced influences of ENSO on the monsoon will exacerbate the reduction of rainfall over the western North Pacific during the post‐El Niño summer. We find that such projected changes are mainly caused by weakened warm sea surface temperature (SST) anomalies (SSTAs) in the western equatorial Pacific (WEP). Further analyses indicate that the change in WEP SSTAs can be linked to the El Niño‐like change in climatological SSTs in the tropical Pacific. This study depicts detailed physical processes responsible for the projected changes in ENSO's impacts on the SEASM. Key Points: The effects of the El Niño‐Southern Oscillation on the Southeast Asian summer monsoon will strengthen under global warmingThe enhanced El Niño's impacts result from the weakened warm sea‐surface temperature (SST) anomalies in the western equatorial Pacific (WEP)The weakened WEP SST anomalies are related to the eastward shift of anomalous Walker circulation and the slackened mean zonal ocean currents [ABSTRACT FROM AUTHOR]

Abstract Based on outputs of 28 coupled models from the Phase 6 of the Coupled Model Intercomparison Project (CMIP6), we show that the response of the Southeast Asian summer monsoon to the El Niño‐Southern Oscillation (ENSO) during post‐ENSO summer will likely strengthen in a warmer climate, which can be attributed to concurrently weakened sea‐surface temperature anomalies (SSTAs) in the western equatorial Pacific (WEP). The weakened WEP SSTAs are primarily caused by enhanced latent heat damping due to increased surface wind speed anomalies, which are associated with the eastward shift of the El Niño‐induced anomalous Walker circulation due to El Niño‐like sea surface temperature change in the tropical Pacific under global warming. Besides, the climatological zonal ocean currents will slow down due to the weakening of climatological Walker circulation, which also acts to weaken the WEP SSTAs via reducing the advection of anomalous temperature by the mean current.

In the past 40 years, marine heatwaves (MHWs) have experienced a worldwide increase in duration, intensity, frequency and spatial extent. This trend has been particularly evident in the Mediterranean, where exceptional events were observed during the summers of 2022, 2018 and 2003. This study proposes a twofold analysis of MHWs in the Mediterranean, focusing on their statistical characteristics and physical causes. A satellite dataset is utilized to analyze MHWs via an index, called activity, which aggregates the occurrence, duration, intensity and spatial extent of events. Our results show that the trend toward more active summers for MHWs is strongest in the western Mediterranean basin and long-term warming is the main driver in the whole Mediterranean basin. We also show that in the western and Adriatic Mediterranean region, the increase of SST variability contributes about a third to the MHW activity long-term trend whereas in the central, eastern and Aegean basins, the variability of SST mostly acts to diminish this trend. Through principal component analysis (PCA) of MHW activity, we found that the three most severe summer MHW events in the Mediterranean occur at the same location where the overall trend is highest. Interannual variability increased MHW activity in 2022 around the Balearic Sea, in 2018 in the eastern basins and in 2003 in the central basins. A joint PCA revealed that the long-term trend in MHW activity co-varies with a positive geopotential height anomaly over the Mediterranean, which is consistent with the generation of atmospheric-driven MHWs and which, at the North Atlantic scale, resembles the positive phase of the summer East Atlantic. The additional interannual variability contribution to these three severe summers was associated with western warming and projected onto the positive phase of the summer North Atlantic Oscillation. The increase in MHW over the last 40 years is also associated in the western, central and Adriatic regions with increased downward short-wave radiation and in the eastern Mediterranean with decreased upward long-wave radiation. Increased upward latent heat flux partly compensated for the MHW long-term increase over the whole Mediterranean basin. The interannual variability of MHW activity is related in the western, central and Adriatic basins to increased downward sensible and decreased upward latent heat flux possibly due to warm and humid air intrusion.

The southward propagation of cold-air frontal boundaries into the Gulf of Mexico region initiates a cascade of coupled air–sea processes that manifests along the coastlines as an apparent brightness anomaly in the ocean color signals. Our hypothesis is that the color anomaly is largely due to the turbulent resuspension of sedimentary particles. Initially, there is significant wind-driven ocean turbulence as the frontal boundary passes, followed by the potential for sustained convective instability due to significant heat losses from the ocean surface. These cold front events occur during boreal autumn, winter, and into early spring, and the latter episodes occur in the context of the seasonally recurring thermal stratification of shelf waters. Here, we show that as stratification reasserts thermal stability in the waning days of a cold front episode, daily to hourly ocean color patterns are temporally coherent with the air–sea heat flux changes and the resulting impact on water column stability. Concomitant results from a nested, data-assimilative, and two-way coupled ocean-atmosphere numerical modeling system provides both corroboration and insight into how surface air–sea fluxes and in-water turbulent mixing manifest as hourly changes in apparent surface water turbidity due to the potential excitation and settling of reflective particles. A simple model of particle mixing and settling driven by the simulated turbulence mimics patterns seen in the satellite image sequences. This study offers a preview of potential application areas that may emerge following the launch of a dedicated ocean color geostationary sensor. [ABSTRACT FROM AUTHOR]

The thermal air-sea interaction mechanism that modulates the atmospheric mixing due to sea-surface temperature (SST) variability is studied with long-term consistent satellite records. Statistical analyses of daily and instantaneous wind and SST data are performed over the major western boundary currents (WBCs). This wind-SST coupling, which is mediated by atmospheric mixing, is found to be very relevant on daily, and even shorter, time scales. Co-located and simultaneous SST and surface wind fields (from Advanced Very High Resolution Radiometer and Advanced Scatterometer data) reveal that the atmosphere responds instantaneously to the presence of SST structures with a larger coupling coefficient with respect to daily and monthly time-averaged fields. The coupling strength varies seasonally over WBCs in the Northern Hemisphere, with wintertime coupling being the lowest. Reanalysis data show that this behaviour is related to the seasonality of the air-sea temperature difference over the region of interest. Over the Northern Hemisphere WBCs, dry and cold continental air masses drive very unstable conditions, associated with very weak thermal air-sea coupling. [ABSTRACT FROM AUTHOR]

[Display omitted] The topography of the Tibetan Plateau (TP) has shaped the paleoclimatic evolution of the Asian monsoon. However, the influence of the TP on the global climate, beyond the domain of the Asian monsoon, remains unclear. Here we show that the Pacific and Atlantic Oceans act as efficient repeaters that boost the global climatic impact of the TP. The simulations demonstrate that oceanic repeaters enable TP heating to induce a wide-ranging climate response across the globe. A 1 °C TP warming can result in a 0.73 °C temperature increase over North America. Oceanic repeaters exert their influence by enhancing the air-sea interaction-mediated horizontal heat and moisture transport, as well as relevant atmospheric circulation pathways including westerlies, stationary waves, and zonal-vertical cells. Air-sea interactions were further tied to local feedbacks, mainly the decreased air-sea latent heat flux from the weakening air-sea humidity difference and the increased shortwave radiation from sinking motion-induced cloud reduction over the North Pacific and Atlantic Oceans. Our findings highlight the crucial influence of TP heating variation on the current climate under a quasi-fixed topography, in contrast to topography change previously studied in paleoclimate evolution. Therefore, TP heating should be considered in research on global climate change. [ABSTRACT FROM AUTHOR]

With a pair of high‐resolution (HR, 10 km ocean and 25 km atm) and low‐resolution (LR, 100 km ocean and 100 km atm) Community Earth System Model (CESM) simulations, we investigate and compare the long‐term mean Northern Hemisphere (NH) wintertime storm tracks against reanalysis data. Based on several Eulerian measures, the CESM‐HR shows a poleward shift of the NH storm tracks and a reduced equatorward bias in the North Pacific relative to the CESM‐LR. Those may be associated with the refined topography of the Tibetan Plateau as well as the improved subtropical jet, stationary planetary waves, atmospheric baroclinity, and baroclinic energy conversion over the North Pacific. We also find that sharper oceanic fronts along the Kuroshio Extension accompanied by enhanced local atmospheric baroclinity and strengthened eddy heat flux over the northern North Pacific could be related to those improvements. Nevertheless, the North Atlantic storm track in HR is placed too north, leading to a larger negative bias over the subtropical zone than in LR. The simulated insufficient subtropical sensible and latent heat fluxes and northward zonal winds in HR seemingly coincide with the deficits in the North Atlantic storm track. The stronger NH branch of the Hadley cell and its poleward expansion in HR may be linked to the changes in the tropical precipitation and the poleward shift of the NH storm tracks. Further studies of attributing these biases to a specific aspect of the climate model by sensitivity experiments are warranted for further clarification of the mechanisms. Plain Language Summary: Storm tracks refer to the preferred travel paths of extratropical cyclones and anticyclones, which contribute to precipitation and wind extremes in midlatitudes. So, accurately representing storm tracks is crucial for climate models. This study compares and evaluates the long‐term mean Northern Hemisphere (NH) wintertime storm tracks in a pair of high‐resolution (HR) and low‐resolution (LR) climate model simulations against observations. HR shows a poleward shift of the NH storm tracks, leading to some improvements in the North Pacific relative to LR. The refined Tibetan Plateau and sea surface temperature may play a role in the improved North Pacific storm track by altering atmospheric circulations and air‐sea interactions. Nevertheless, the North Atlantic storm track is placed too north in HR, leading to a larger negative bias over the subtropical zone. The weakened upward heat and moisture fluxes and northward zonal winds are related to the deficits in the North Atlantic storm track simulation. The stronger Hadley cell and its poleward expansion in HR also show associations with the poleward shift of the NH storm tracks. This study discusses how model horizontal resolution affects the simulated NH storm tracks, which may provide a reference for the development of future high‐resolution climate models. Key Points: High‐resolution CESM shows a poleward shift of storm tracks and reduced biases in the North Pacific compared to the low‐resolutionRefined Tibetan Plateau topography and improved SST are related to the better simulated North Pacific storm track by altering baroclinityBetter represented sensible and latent heat flux and zonal wind can be crucial for improving the North Atlantic storm track simulation [ABSTRACT FROM AUTHOR]

Surface ocean temperature and velocity anomalies at meso‐ and sub‐meso‐scales induce wind stress anomalies. These wind‐front interactions, referred to as thermal (TFB) and current (CFB) feedbacks, respectively, have been studied in isolation at mesoscale, yet they have rarely been considered in tandem. Here, we assess the combined influence of TFB and CFB and their relative impact on surface wind stress derivatives. Analyses are based on output from two regions of the Southern Ocean in a coupled simulation with local ocean resolution of 2 km. Considering both TFB and CFB shows regimes of interference, which remain mostly linear down to the simulation resolution. The jointly‐generated wind stress curl anomalies approach 10−5 N m−3, ∼20 times stronger than at mesoscale. The synergy of both feedbacks improves the ability to reconstruct wind stress curl magnitude and structure from both surface vorticity and SST gradients by 12%–37% on average, compared with using either feedback alone. Plain Language Summary: Surface ocean temperature and velocity anomalies at 0.1–100 km scales imprint their signatures on the surface wind stress, which in turn supplies the ocean with momentum. This process is called wind‐front interaction and typically referred to as thermal and current feedbacks when generated by temperature gradients and velocity gradients, respectively. Previously, studies using satellite observations and regional numerical models have studied either feedback in isolation; consideration of both feedbacks in tandem remains immature. Here, we present an approach that assesses both feedbacks' combined and relative impact on the surface wind stress, using output from an air‐sea coupled simulation. This approach allows us to identify constructive and destructive patterns of how the two feedbacks interact, which remain mostly linear down to the simulation resolution. The jointly‐generated wind stress derivative anomalies are 20 times stronger than observed previously at larger scales. Considering both feedbacks, reconstructions of wind stress derivatives are viable and have 10%–40% less error on average compared with using either feedback by itself. Contributions from either feedback in modifying wind stress fields vary temporally and can be related to physical properties such as surface wind speed and air‐sea temperature difference across the studied area. Key Points: Surface temperature gradients and vorticity anomalies at scales down to O(10) km impact wind stress curl and divergence jointlyWind‐front interactions at sub‐mesoscales are at least an order of magnitude stronger than at mesoscalesReconstruction of the wind stress curl using both thermal and current feedback is 10%–40% more accurate than relying on a single feedback [ABSTRACT FROM AUTHOR]

Abstract With a pair of high‐resolution (HR, 10 km ocean and 25 km atm) and low‐resolution (LR, 100 km ocean and 100 km atm) Community Earth System Model (CESM) simulations, we investigate and compare the long‐term mean Northern Hemisphere (NH) wintertime storm tracks against reanalysis data. Based on several Eulerian measures, the CESM‐HR shows a poleward shift of the NH storm tracks and a reduced equatorward bias in the North Pacific relative to the CESM‐LR. Those may be associated with the refined topography of the Tibetan Plateau as well as the improved subtropical jet, stationary planetary waves, atmospheric baroclinity, and baroclinic energy conversion over the North Pacific. We also find that sharper oceanic fronts along the Kuroshio Extension accompanied by enhanced local atmospheric baroclinity and strengthened eddy heat flux over the northern North Pacific could be related to those improvements. Nevertheless, the North Atlantic storm track in HR is placed too north, leading to a larger negative bias over the subtropical zone than in LR. The simulated insufficient subtropical sensible and latent heat fluxes and northward zonal winds in HR seemingly coincide with the deficits in the North Atlantic storm track. The stronger NH branch of the Hadley cell and its poleward expansion in HR may be linked to the changes in the tropical precipitation and the poleward shift of the NH storm tracks. Further studies of attributing these biases to a specific aspect of the climate model by sensitivity experiments are warranted for further clarification of the mechanisms.

Abstract Surface ocean temperature and velocity anomalies at meso‐ and sub‐meso‐scales induce wind stress anomalies. These wind‐front interactions, referred to as thermal (TFB) and current (CFB) feedbacks, respectively, have been studied in isolation at mesoscale, yet they have rarely been considered in tandem. Here, we assess the combined influence of TFB and CFB and their relative impact on surface wind stress derivatives. Analyses are based on output from two regions of the Southern Ocean in a coupled simulation with local ocean resolution of 2 km. Considering both TFB and CFB shows regimes of interference, which remain mostly linear down to the simulation resolution. The jointly‐generated wind stress curl anomalies approach 10−5 N m−3, ∼20 times stronger than at mesoscale. The synergy of both feedbacks improves the ability to reconstruct wind stress curl magnitude and structure from both surface vorticity and SST gradients by 12%–37% on average, compared with using either feedback alone.

Mesoscale eddies are the important phenomenon of world oceans that play a prominent role in influencing the near‐surface atmosphere and transport of heat. Ocean and atmosphere responses due to the presence of oceanic mesoscale eddies are not well studied in the Northern Indian Ocean (NIO) region. The present study analysed the ocean and atmospheric response due to the presence of mesoscale eddies using a composite analysis. Generally, it is assumed that the anti‐cyclonic eddies are associated with the positive anomalies of sea surface temperature (SST), and cyclonic eddies are accompanied by negative anomalies. However, for the two marginal seas in the NIO, that is, the Bay of Bengal (BoB) and Arabian Sea (AS), the composite picture of SST anomalies associated with mesoscale eddies showed an abnormal response of SST. This unusual behaviour of cold SST near anti‐cyclonic eddies and warm SST near cyclonic eddies in BoB and AS is analysed with the mean composites of precipitation, outgoing longwave radiation (OLR), and turbulent fluxes. The mean composites over all anti‐cyclonic eddies reveal the presence of enhanced precipitation and convection (low OLR), whereas over cyclonic eddies, suppressed convection and negative precipitation anomalies are evident. The composites of latent heat flux (LHF), precipitation and OLR near surface cold anti‐cyclonic eddies indicate the loss of heat from the ocean to the atmosphere, intense precipitation along with cloudy conditions are responsible for the cooling of the ocean. Along with this, stronger winds are also evident during cloudy conditions could play a role in anomalous cooling of SST observed near anti‐cyclonic through intense surface mixing. Similarly, vice versa is observed near the surface warm cyclonic eddies. The overlying atmosphere and abnormal response of SST near oceanic eddies are essential for understanding the significant role of oceanic mesoscale eddies in influencing the large‐scale circulation of the atmosphere and climate. [ABSTRACT FROM AUTHOR]

Recent results using wind and sea surface temperature data from satellites and high-resolution coupled models suggest that mesoscale ocean–atmosphere interactions affect the locations and evolution of storms and seasonal precipitation over continental regions such as the western US and Europe. The processes responsible for this coupling are difficult to verify due to the paucity of accurate air–sea turbulent heat and moisture flux data. These fluxes are currently derived by combining satellite measurements that are not coincident and have differing and relatively low spatial resolutions, introducing sampling errors that are largest in regions with high spatial and temporal variability. Observational errors related to sensor design also contribute to increased uncertainty. Leveraging recent advances in sensor technology, we here describe a satellite mission concept, FluxSat, that aims to simultaneously measure all variables necessary for accurate estimation of ocean–atmosphere turbulent heat and moisture fluxes and capture the effect of oceanic mesoscale forcing. Sensor design is expected to reduce observational errors of the latent and sensible heat fluxes by almost 50%. FluxSat will improve the accuracy of the fluxes at spatial scales critical to understanding the coupled ocean–atmosphere boundary layer system, providing measurements needed to improve weather forecasts and climate model simulations.

Wind‐driven mixing and Ekman pumping driven by slow‐moving tropical cyclones (TCs) can bring nutrients to the euphotic zone, promoting phytoplankton blooms (TC‐PBs) observable by satellite remote sensing. We examine an exceptional (z‐score = 18–48) TC‐PB induced by category‐1 Cyclone Oma near the South Pacific island of Vanuatu in February 2019, the most extreme event in the observed satellite record of South Pacific surface Chlorophyll‐a (Chl‐a). Examining all 156 South Pacific TC since 1997, we identify a "hover" parameter derivable from storm track data correlated with post‐TC surface Chl‐a (r = 0.84). Using a data set of synthetic storm tracks, we show revisit times for South Pacific TC‐PBs are O(250) years and O(1,500) years for Oma‐scale TC‐PBs. The episodic, extreme, but consistent nature of such events means they may imprint on sediment records. If so, we show their signature could be used to reconstruct past TC variability assuming near‐stationarity of TC statistics. Plain Language Summary: We demonstrate that a relatively weak Tropical Cyclone, Oma, produced the most extreme primary production event observed in the South Pacific satellite record of surface Chlorophyll‐a. We examined all 156 South Pacific tropical cyclones since 1997—finding 15 storms had blooms in their wake, although none compared to the response in the wake of Oma. Phytoplankton blooms require specific conditions for formation, namely that they "hover" in place over a long enough period of time, so that wind‐driven upwelling of nutrient‐rich deep water can enter the sunlit surface layers of the ocean and promote photosynthesis. We can characterize the "hovering" of cyclones using a simple parameter derived from location data alone, and so we examine a 10,000‐year synthetic data set of 96,000 South Pacific tropical cyclone tracks to explore the recurrence period of these blooms in current climate. We find that Oma‐type blooms recur only once in 1,500–2,000 years. If biological material from these extreme events reaches the sea floor, the rare but consistent nature of such blooms may be visible in the sedimentary record and help constrain past variability in tropical cyclones. Key Points: An extreme (σ > 18) surface Chlorophyll‐a event was observed following Tropical Cyclone Oma passing near Vanuatu in 2019South Pacific chlorophyll response is correlated to a "hover" parameter derivable from storm track data aloneOma‐sized blooms recur on millennial and longer timescales, and may imprint on the sedimentary record [ABSTRACT FROM AUTHOR]

The formation and intensification of typhoons is a complex process where energy and mass exchanges happen between the ocean and the atmosphere. In most typhoon numerical studies, a static ocean and a dynamic atmosphere are used to reduce the complexity of modeling. Using the COAWST model, we studied the air-sea interactions of Typhoon Mujigae in 2015, Typhoon Merbok in 2017, and Typhoon Hato in 2017 over the South China Sea. With different translation speeds, track shapes, and intensities between these cyclones, they act as an excellent case study to analyze the air-sea coupling in the models. The inclusion of coupling between the ocean and atmosphere is found to improve the typhoon track simulation significantly. The bias in the cyclone tracks is reduced by 10%–40% in the coupled model. The upper ocean response to the typhoon was also analyzed using the coupled model output. The coupled simulations show that the major energy extraction occurs to the right of the track, which is consistent with satellite observation and latent heat release analysis. The coupling process shows the air-sea interactions and exchanges in the upper ocean along with the energy released during the passage of typhoons. The heat budget analysis shows that the cooling of the upper ocean is mainly attributed to the advection associated with the typhoon forcing. This study shows that it is necessary to include ocean feedback while analyzing a typhoon, and the application of coupled models can improve our understanding as well as the forecasting capability of typhoons.

This study examines the performance of 52 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) in capturing the effects of the Indian summer monsoon on the evolution of El Niño–Southern Oscillation (ENSO). The ISM’s impacts on ENSO show a substantial diversity among the models. While some models simulate the strength of the impacts comparable to observations, others represent much weaker influences. Results indicate that the diversity is highly related to inter-model spread in interannual variability of ISM rainfall (ISMR) among the models. Models with a larger ISMR variability simulate stronger ISM-induced anomalies in precipitation and atmospheric circulation over the western North Pacific during the monsoon season. As a result, these models exhibit larger wind anomalies induced by monsoon on the south flank of the anomalous circulation in the western Pacific, thereby influencing subsequent ENSO evolution more significantly by causing stronger air-sea coupling processes over the tropical Pacific.

Abstract Wind‐driven mixing and Ekman pumping driven by slow‐moving tropical cyclones (TCs) can bring nutrients to the euphotic zone, promoting phytoplankton blooms (TC‐PBs) observable by satellite remote sensing. We examine an exceptional (z‐score = 18–48) TC‐PB induced by category‐1 Cyclone Oma near the South Pacific island of Vanuatu in February 2019, the most extreme event in the observed satellite record of South Pacific surface Chlorophyll‐a (Chl‐a). Examining all 156 South Pacific TC since 1997, we identify a “hover” parameter derivable from storm track data correlated with post‐TC surface Chl‐a (r = 0.84). Using a data set of synthetic storm tracks, we show revisit times for South Pacific TC‐PBs are O(250) years and O(1,500) years for Oma‐scale TC‐PBs. The episodic, extreme, but consistent nature of such events means they may imprint on sediment records. If so, we show their signature could be used to reconstruct past TC variability assuming near‐stationarity of TC statistics.

In this study, ocean and atmosphere satellite observations, an atmospheric reanalysis and a set of regional numerical simulations of the lower atmosphere are used to assess the coupling between the sea-surface temperature (SST) and the marine atmospheric boundary layer (MABL) as well as the latent heat flux (LHF) sensitivity to SST in the north-west tropical Atlantic Ocean. The results suggest that the SST-MABL coupling depends on the spatial scale of interest. At scales larger than the ocean mesoscale (larger than 150 km), negative correlations are observed between near-surface wind speed (U10m) and SST and positive correlations between near-surface specific humidity (q2m) and SST. However, when smaller scales (1 – 150 km, i.e., encompassing the ocean mesoscale and a portion of the submesoscale) are considered, U10m-SST correlate inversely and the q2m-SST relation significantly differs from what is expected using the Clausius-Clapeyron equation. This is interpreted in terms of an active ocean modifying the near-surface atmospheric state, driving convection, mixing and entrainment of air from the free troposphere into the MABL. The estimated values of the ocean-atmosphere coupling at the ocean small-scale are then used to develop a linear and SST-based downscaling method aiming to include and further investigate the impact of these fine-scale SST features into an available low-resolution latent heat flux (LHF) data set. The results show that they induce a significant increase of LHF (30% to 40% per °C of SST). We identify two mechanisms causing such a large increase of LHF: (1) the thermodynamic contribution that only includes the increase in LHF with larger SSTs associated with the Clausius-Clapeyron dependence of saturating water vapor pressure on SST and (2) the dynamical contribution related to the change in vertical stratification of the MABL as a consequence of SST anomalies. Using different downscaling setups, we conclude that largest contribution comes from the dynamic mode (28% against 5% for the thermodynamic mode). To validate our approach and results, we have implemented a set of high-resolution WRF numerical simulations forced by high-resolution satellite SST that we have analyzed in terms of LHF using the same algorithm. The LHF estimate biases are reduced by a factor of 2 when the downscaling is applied, providing confidence in our results.

This study uses the 30 General Circulation Models (GCMs) from the Coupled Model Intercomparison Project phase-6 (CMIP6) to examine the simulations of the surplus/deficit Indian summer monsoon rainfall (ISMR) and its associated air-sea interactions on intraseasonal to interannual timescales. The majority of the CMIP6 models simulate the seasonal mean state of ISMR over the Indian mainland with systematic biases. Best performing models (BPM; AWI-ESM-1-1-LR, BCC-CSM2-MR, BCC-ESM1, CNRM-CM6-1, CNRM-ESM2-1, GFDL-CM4, INM-CM5-0, MIROC-ES2L, MIROC6, TaiESM1) well simulated the seasonal mean precipitation with Taylor skill score > 0.75 and normalized root mean square error (NRMSE) is < 0.7. However, the models are failed to simulate precipitation over the orographic regions (Western Ghats). Improving the simulations of low-level winds and sea surface temperature (SST) with high spatial resolutions would provide better precipitation simulations. B-MME (multimodel ensemble mean of BPM) can capture the negative IOD-like (Indian Ocean Dipole) pattern during deficit monsoon years and fail to capture the positive IOD-like pattern during surplus monsoon years. Models overestimate the moisture transport from the West Indian Ocean to the sub-continent of India during deficit monsoons, which plays a crucial role in modulating the precipitation and its associated intraseasonal variability. The present analysis identified that during deficit monsoon years, the faster moving 20–100 days oscillations are evident; however, these oscillations are sluggish during surplus monsoon years, which affects the duration of convection activity and causes dry conditions over the regions. During surplus monsoon years, the Bay of Bengal (Arabian Sea) responds strongly (slowly) to the atmosphere than the deficit monsoon years. However, models are fail to represent the ocean's response to the atmosphere over the Bay of Bengal. The freshwater forcing improvement in the models simulates the ocean to atmosphere response over the Indian region. The present study further suggests that the improved simulation of the Indian summer monsoon (ISM) variability by the GCMs is possible by improving the ocean and atmosphere feedback mechanisms, sensitivities of the models among internal variables, and orographic features necessary for the accurate simulation of intraseasonal variability. [ABSTRACT FROM AUTHOR]

The intensity simulation of tropical cyclones (TCs) has been a long‐standing challenge for numerical models, and an accurate sea surface roughness (z0 ${z}_{0}$) parameterization scheme is the key to enhance the intensity prediction. In our study, a new z0 ${z}_{0}$ parameterization scheme (SD21) is proposed and applied in the Coupled Ocean‐Atmosphere‐Wave‐Sediment Transport model to simulate two super typhoons. The SD21 takes into account both the wave state and sea foam, and it is suitable for low to extreme wind conditions. The results of the eight numerical experiments show that the TC intensity and structure are sensitive to the choice of z0 ${z}_{0}$ parameterization schemes. Compared with the widely used z0 ${z}_{0}$ parameterization schemes, the SD21 scheme presents much better results in the simulation of the intensity and intensification speed of strong TCs. Notably, the simulation of the wind speeds generated by the SD21 is more compatible with the best track data and significantly better than that of the other schemes. Furthermore, we find that the wave state and sea foam remarkably affect the magnitude and spatial distribution of z0 ${z}_{0}$, the following two conclusions are obtained: (a) The z0 ${z}_{0}$ parameterization that takes into account the wave state can reduce the excessive roughness at the TC periphery and restrict the high‐value area of the roughness to the TC‐core region. (b) The sea foam significantly decreases the roughness value in areas with 10 m wind speeds above 40 m/s. Plain Language Summary: In nature, tropical cyclones are a destructive weather phenomenon. The strong winds that occur during these cyclones lead to the intense exchange of the air‐sea momentum flux. The momentum flux will directly affect the intensity and structure of a TC. Additionally, the momentum flux exchange depends on the sea surface roughness; thus, proposing a more accurate parameterization scheme of the sea surface roughness is significant for accurately predicting TCs with numerical models. This study proposed a new roughness parameterization scheme that considers the wave age, wave steepness and sea foam. The roughness that is calculated under high wind speeds by this scheme can also be very consistent with the experimental and field observation data. In this study, the parameterization scheme is applied to the Coupled Ocean‐Atmosphere‐Wave‐Sediment Transport model to simulate the two super typhoons, Mangkhut and Yutu. Compared with the existing parametric schemes, this scheme significantly improves the simulation results of TC intensity and structure. Key Points: A new sea surface roughness parameterization scheme that considers the wave state and sea foam is proposed and applied to a coupled modelThe wave state and sea foam can affect the transmission of the air‐sea momentum flux by changing the sea surface roughnessThe sea foam reduces the values of the sea surface roughness in the core region of TC and significantly optimizes the simulation of a TC [ABSTRACT FROM AUTHOR]

The southward propagation of cold-air frontal boundaries into the Gulf of Mexico region initiates a cascade of coupled air–sea processes that manifests along the coastlines as an apparent brightness anomaly in the ocean color signals. Our hypothesis is that the color anomaly is largely due to the turbulent resuspension of sedimentary particles. Initially, there is significant wind-driven ocean turbulence as the frontal boundary passes, followed by the potential for sustained convective instability due to significant heat losses from the ocean surface. These cold front events occur during boreal autumn, winter, and into early spring, and the latter episodes occur in the context of the seasonally recurring thermal stratification of shelf waters. Here, we show that as stratification reasserts thermal stability in the waning days of a cold front episode, daily to hourly ocean color patterns are temporally coherent with the air–sea heat flux changes and the resulting impact on water column stability. Concomitant results from a nested, data-assimilative, and two-way coupled ocean-atmosphere numerical modeling system provides both corroboration and insight into how surface air–sea fluxes and in-water turbulent mixing manifest as hourly changes in apparent surface water turbidity due to the potential excitation and settling of reflective particles. A simple model of particle mixing and settling driven by the simulated turbulence mimics patterns seen in the satellite image sequences. This study offers a preview of potential application areas that may emerge following the launch of a dedicated ocean color geostationary sensor.

Low‐level clouds over the tropical and sub‐tropical oceans play a crucial role in the planetary radiative energy budget. However, they are challenging to model in climate simulations because they are affected by local processes that are still partially unknown. The control that mesoscale sea surface temperature (SST) structures have on the dynamics of the lower atmosphere on daily scales is emerging to be non‐negligible and calls for more effort to be understood. During the EUREC4A field campaign, two of the research vessels (R/Vs) involved in the experiment sampled the edge of a cold mesoscale SST patch in the north‐western tropical Atlantic, crossing a gradient of roughly 0.75°C/100 km. The comprehensive set of instruments carried by the R/Vs allows an unprecedented characterization of the atmospheric response to the cold water forcing. The cold ocean patch weakens the vertical atmospheric mixing, reducing the boundary layer depth of roughly 200 m and the horizontal wind intensity of approximately 3 m s−1. At the same time, the humidity content in the sub‐cloud layer increases and these conditions decrease the latent heat flux (by roughly 80 W m−2) and reduce vertical velocity fluctuations, making it less likely that moisture exceeds the lifting condensation level. As a consequence, fewer and thinner low‐level clouds form over cold water. Independent satellite measurements are found to agree with the in situ observations. The observed link between sea temperature and low‐level clouds highlights its importance in the puzzle of modeling the sea‐air‐cloud interactions. Plain Language Summary: Puffy clouds typically visible at sea strongly challenge climate models that struggle to represent their interaction with the sea surface and solar radiation. And as a consequence, these climate models cannot precisely estimate how much the Earth's temperature will increase 100 years from now and its uncertainty. We went into the western Atlantic ocean for a month in January–February 2020 to measure sea surface temperature and cloud properties and observe how these changes occur. We saw clouds grow deeper over the warm water patches, holding more water and eventually raining. Weaker winds and more humid air occur on cold patches. Satellite observations seem to record the same behavior over a larger area in the same region. We detected a clear difference in cloud properties and amounts over warm and cold patches from all sensors, recording essential evidence of a feature that is hard to predict. We hope these observations will help to properly simulate the intensity and signs of low‐cloud feedback over warm and cold oceanic patches of water to improve climate models. Key Points: Ship‐based observations are used to characterize the lower atmospheric response to a cold patch in the north‐western subtropical AtlanticSignatures in dynamical and thermodynamical atmospheric properties agree with a reduced vertical mixing over the cold patchSuch a weaker vertical mixing is linked to a reduced shallow cloud cover because less moisture reaches the level of saturation [ABSTRACT FROM AUTHOR]

Observations from two autonomous Wave Gliders and six Lagrangian Surface Wave Instrument Float with Tracking drifters in the northwestern tropical Atlantic during the January–February 2020 NOAA Atlantic Tradewind Ocean‐atmosphere Mesoscale Interaction Campaign (ATOMIC) are used to evaluate the spatial variability of bulk air‐sea heat, moisture, and buoyancy fluxes. Sea surface temperature (SST) gradients up to 0.7°C across 10–100 km frequently persisted for several days. SST gradients were a leading cause of systematic spatial air‐sea sensible heat flux gradients, as variations over 5 Wm−2 across under 20 km were observed. Wind speed gradients played no significant role and air temperature adjustments to SST gradients sometimes acted to reduce spatial flux gradients. Wind speed, air temperature, and air humidity caused high‐frequency spatial and temporal flux variations on both sides of SST gradients. A synthesis of observations demonstrated that fluxes were usually enhanced on the warm SST side of gradients compared to the cold SST side, with variations up to 10 Wm−2 in sensible heat and upward buoyancy fluxes and 50 Wm−2 in latent heat flux. Persistent SST gradients and high‐frequency air temperature variations each contributed up to 5 Wm−2 variability in sensible heat flux. Latent heat flux was instead mostly driven by air humidity variability. Atmospheric gradients may result from convective structures or high‐frequency turbulent fluctuations. Comparisons with 0.05°‐resolution daily satellite SST observations demonstrate that remote sensing observations or lower‐resolution models may not capture the small‐scale spatial ocean variability present in the Atlantic trade wind region. Plain Language Summary: Two autonomously piloted Wave Gliders and six surface current‐following drifters were deployed in the northwestern tropical Atlantic during the January–February 2020 NOAA Atlantic Tradewind Ocean‐atmosphere Mesoscale Interaction Campaign to study how air‐sea interaction varies on small spatial scales. The observations show that sea surface temperature (SST) can vary by up to 0.7°C over tens of kilometers of distance and that these SST fronts often persisted for several days. Larger fluxes of heat, moisture, and buoyancy typically occurred between the atmosphere and the ocean on the warm sides of SST fronts. The spatial variations of air‐sea fluxes across tens of kilometers are important because these small scales are not resolved in many prediction models or satellite‐based products. Thus, this work emphasizes the physics of, and importance of considering, persistent small‐scale gradients in SST and air‐sea fluxes in regional and global analyses. Key Points: Autonomous piloted and Lagrangian drifting platforms measured air‐sea properties and fluxes in the northwestern tropical AtlanticSensible heat, latent heat, and upward buoyancy flux varied by as much as 10, 50, and 10 Wm−2 across 10–100 km sea surface temperature (SST) gradientsSpatial SST gradients were a leading cause of sensible heat flux variability, but humidity fluctuations largely modulated latent heat flux [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs) dominate moisture transport globally; however, it is unknown what impact ARs have on surface ocean buoyancy. This study explores the surface buoyancy gained by ARs using high‐resolution surface observations from a Wave Glider deployed in the subpolar Southern Ocean (54°S, 0°E) between 19 December 2018 and 12 February 2019 (55 days). When ARs combine with storms, the associated precipitation is significantly enhanced (189%). In addition, the daily accumulation of AR‐induced precipitation provides a buoyancy gain to the surface ocean equivalent to warming by surface heat fluxes. Over the 55 days, ARs accounted for 47% of the total precipitation equating to 10% of the summer surface ocean buoyancy gain. This study indicates that ARs play an important role in the summer precipitation over the subpolar Southern Ocean and that they can alter the upper‐ocean buoyancy budget from synoptic to seasonal timescales. Plain Language Summary: Atmospheric rivers (ARs) are river‐like features in the sky that carry enormous amounts of water vapor from the tropics toward Antarctica and across the Southern Ocean. However, it is not well documented how this freshwater carried by ARs in the atmosphere impacts rainfall over the ocean. This study uses an uncrewed surface vehicle, called a Wave Glider, to measure the meteorology and surface ocean temperature and salinity. We show that ARs almost double rainfall, which directly impact the surface ocean buoyancy (i.e., rainfall adds freshwater and lightens the surface layer waters). These results provide evidence and highlight the importance of ARs in the Atlantic Southern Ocean when it comes to lightening the surface waters. Key Points: Atmospheric rivers (ARs) significantly increase precipitation (189%) in midlatitude cyclones over the Atlantic Southern OceanDuring intense AR‐driven rainfall events, precipitation contributes to daily buoyancy forcing equivalent to the surface heat fluxPrecipitation associated with ARs contributes cumulatively to 10% of the surface ocean buoyancy gain in summer [ABSTRACT FROM AUTHOR]

Abstract Atmospheric rivers (ARs) dominate moisture transport globally; however, it is unknown what impact ARs have on surface ocean buoyancy. This study explores the surface buoyancy gained by ARs using high‐resolution surface observations from a Wave Glider deployed in the subpolar Southern Ocean (54°S, 0°E) between 19 December 2018 and 12 February 2019 (55 days). When ARs combine with storms, the associated precipitation is significantly enhanced (189%). In addition, the daily accumulation of AR‐induced precipitation provides a buoyancy gain to the surface ocean equivalent to warming by surface heat fluxes. Over the 55 days, ARs accounted for 47% of the total precipitation equating to 10% of the summer surface ocean buoyancy gain. This study indicates that ARs play an important role in the summer precipitation over the subpolar Southern Ocean and that they can alter the upper‐ocean buoyancy budget from synoptic to seasonal timescales.

Concentrations and the stable isotopic composition of bulk aerosol nitrate (NO3−) were quantified from two GEOTRACES cruises: (a) Alaska–Tahiti (GP15; n = 22) and (b) Peru–Tahiti (GP16; n = 17) to explore the hypothesis that a marine source influences aerosol NO3− in the equatorial Pacific. The δ15N‐NO3− ranged from −14.5‰–0.5‰, with lowest values furthest from the coast, primarily reflecting a shift in sources. The δ18O‐ and Δ17O‐NO3− were both relatively high (65.2‰–85.4‰ and 21.4‰–30.7‰, respectively) and decreased away from continental regions, reflecting a shift in the oxidants that influence the formation of NO3−. Transport modeling and co‐occurrence of low δ15N, δ18O and Δ17O provided evidence for an important influence of marine‐derived alkyl nitrates (RONO2) on aerosol NO3− formation. Based on the Δ17O, we quantified that the contribution of RONO2 to aerosol NO3− can be as high as 47.5% (range 7.5%–47.5%). We also estimate an average δ15N‐RONO2 of −27.8‰ ± 23.3‰. Plain Language Summary: The availability of nutrients in surface waters, including inorganic nitrogen (i.e., nitrate, nitrite, and ammonium), regulates oceanic primary production. In the Pacific, the amount of nitrogen entering the ocean from the atmosphere (via precipitation and aerosols) has increased since the industrial revolution and is predicted to continue to grow with anthropogenic nitrogen emissions. The isotopic composition of atmospheric nitrate provides a novel tool for exploring the formation of nitrate in different environments. Here, we detail whether nitrate is primarily derived from external anthropogenic sources or internal oceanic sources. The nitrogen and oxygen isotopic compositions allow us to quantify that an important portion of the nitrogen entering the ocean from the atmosphere is recycled oceanic emissions, which means that excess nitrogen detected in the North Pacific may not be the result of external processes. Key Points: Low δ15N, δ18O, and Δ17O in aerosol nitrate from the Equatorial Pacific are consistent with alkyl nitrates being a sourceWe quantify that alkyl nitrates contribute between 7.5% and 47.5% to aerosol nitrate in the Equatorial PacificWe estimate that alkyl nitrates contribute a δ15N signature of −27.8‰ ± 23.3‰, with a latitudinal trend [ABSTRACT FROM AUTHOR]

Thermal structures at the sea surface are known to affect the overlying atmospheric dynamics over various spatio‐temporal scales, from hourly and sub‐kilometric to annual and O(1,000 km). The relevant mechanisms at play are generally identified by means of correlation coefficients (in space or time) or by linear regression analysis using appropriate couples of variables. For fine spatial scales, where sea surface temperature (SST) gradients get stronger, the advection might disrupt these correlations and, thus, mask the action of such mechanisms, just because of the chosen metrics. For example, at the oceanic sub‐mesoscale, around 1–10 km and hourly time scales, the standard metrics used to identify the pressure adjustment mechanism (that involves the Laplacian of sea surface temperature, SST, and the wind divergence) may suffer from this issue, even for weak wind conditions. By exploiting high‐resolution realistic numerical simulations with ad hoc SST forcing fields, we introduce some new metrics to evaluate the action of the pressure adjustment atmospheric response to the surface oceanic thermal structures. It is found that the most skillful metrics is based on the wind divergence and the SST second spatial derivative evaluated in the across direction of a locally defined background wind field. Plain Language Summary: The ocean surface is characterized by a range of warm and cold structures that are known to influence the overlying atmospheric flow through different mechanisms. One of these mechanisms involves the variation of sea level pressure that can drive secondary wind circulations according to how the sea surface temperature is distributed in space. To assess whether this mechanism is in action, the co‐location of sea temperature maxima (or minima) with zones of wind convergence (divergence) is generally considered. However, the presence of the wind itself has been shown to displace and delay the wind response so that there are cases where the pressure field responds to the sea temperature forcing but this is not detected by the standard metrics. Since pressure variability is generated in all directions, we propose to measure this kind of wind response in the direction perpendicular to the background wind in order to avoid the masking effect of the background wind. Key Points: The standard metrics for the pressure adjustment mechanism is adversely affected by advectionThree new metrics are introduced and testedThe pressure adjustment is detectable in the direction perpendicular to the background wind [ABSTRACT FROM AUTHOR]

The 2020 'Elucidating the role of clouds-circulation coupling in climate-Ocean-Atmosphere' (EUREC4A-OA) and the 'Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign' (ATOMIC) campaigns focused on improving our understanding of the interaction between clouds, convection and circulation and their function in our changing climate. The campaign utilized many data collection technologies, some of which are relatively new. In this study, we used saildrone uncrewed surface vehicles, one of the newer cutting edge technologies available for marine data collection, to validate Level 2 and Level 3 Soil Moisture Active Passive (SMAP) satellite and Hybrid Coordinate Ocean Model (HYCOM) sea surface salinity (SSS) products in the Western Tropical Atlantic. The saildrones observed fine-scale salinity variability not present in the lower-spatial resolution satellite and model products. In regions that lacked significant small-scale salinity variability, the satellite and model salinities performed well. However, SMAP Remote Sensing Systems (RSS) 70 km generally outperformed its counterparts outside of areas with submesoscale SSS variation, whereas RSS 40 km performed better within freshening events such as a fresh tongue. HYCOM failed to detect the fresh tongue. These results will allow researchers to make informed decisions regarding the most ideal product and its drawbacks for their applications in this region and aid in the improvement of mesoscale and submesoscale SSS products, which can lead to the refinement of numerical weather prediction (NWP) and climate models. [ABSTRACT FROM AUTHOR]

This study investigates the influence of oceanic and atmospheric processes in extratropical thermodynamic air‐sea interactions resolved by satellite observations (OBS) and by two climate model simulations run with eddy‐resolving high‐resolution (HR) and eddy‐parameterized low‐resolution (LR) ocean components. Here, spectral methods are used to characterize the sea surface temperature (SST) and turbulent heat flux (THF) variability and co‐variability over scales between 50 and 10,000 km and 60 days to 80 years in the Pacific Ocean. The relative roles of the ocean and atmosphere are interpreted using a stochastic upper‐ocean temperature evolution model forced by noise terms representing intrinsic variability in each medium, defined using climate model data to produce realistic rather than white spectral power density distributions. The analysis of all datasets shows that the atmosphere dominates the SST and THF variability over zonal wavelengths larger than ∼2,000–2,500 km. In HR and OBS, ocean processes dominate the variability of both quantities at scales smaller than the atmospheric first internal Rossby radius of deformation (R1, ∼600–2,000 km) due to a substantial ocean forcing coinciding with a weaker atmospheric modulation of THF (and consequently of SST) than at larger scales. The ocean forcing also induces oscillations in SST and THF with periods ranging from intraseasonal to multidecadal, reflecting a red spectrum response to ocean forcing similar to that driven by atmospheric forcing. Such features are virtually absent in LR due to a weaker ocean forcing relative to HR. Plain Language Summary: This study investigates the importance of atmospheric processes (weather) and ocean currents in driving variations in sea surface temperature (SST) and the air‐sea heat exchange at mid‐latitudes. Our analysis uses satellite observations, a high‐resolution (HR) climate model that resolves ocean currents with dimensions of tens of km, and a low‐resolution model (LR) that can only simulate ocean currents with hundreds of km in size. We specifically examine the SST and heat exchange variability resolved by these datasets at horizontal scales between 50 and 10,000 km and time scales from 2 months to 80 years in the Pacific Ocean. Using a simple mathematical model to interpret the results, we find that variability at scales larger than 2,000 km is driven predominantly by weather. At smaller scales, SST and heat exchange are more variable in HR than in LR and agree better with satellite observations. We also find that ocean processes drive variability in SST with time scales ranging from 2 months to several decades, similar to those caused by weather, which in turn induces slow variations in the air‐sea heat exchange. Key Points: We use spectral methods to examine the role of oceanic and atmospheric processes in Pacific sea surface temperature (SST) and turbulent heat flux variabilityAt mid‐latitudes, the atmosphere controls variability with scales larger than 2,000 km while ocean processes dominate at smaller scalesOcean phenomena drive a red spectrum SST response similar to that induced by the atmosphere, which is mirrored by the turbulent fluxes [ABSTRACT FROM AUTHOR]

We present results from a new, global, high‐resolution (∼3‐km for ocean and ∼6‐km for atmosphere) realistic earth system simulation. This simulation allows us to examine aspects of small‐scale air‐sea interaction beyond what previous studies have reported. Our study focuses on recurring intermittent wind events in the Gulf Stream region. These events induce local air‐sea heat fluxes above Sea Surface Temperature (SST) anomalies with horizontal scales smaller than 500‐km. In particular, strong latent heat bursts above warm SST anomalies are observed during these wind events. We show that such wind events are associated with a secondary circulation that acts to fuel the latent heat bursts by transferring dry air and momentum down to the surface. The intensity of this secondary circulation is related to the strength of small‐scale SST fronts that border SST anomalies. The study of such phenomena requires high‐resolution in both the atmospheric and oceanic components of the model. Plain Language Summary: We explore the atmospheric circulation above Sea Surface Temperature (SST) anomalies of less than 500 km‐scale using a new, global, coupled ocean‐atmosphere simulation performed at high horizontal resolution and integrated for 3 months. Our study focuses on intermittent wind events in the Gulf Stream region and the resulting local air‐sea heat fluxes above warm SST anomalies: A strong latent heat burst above these SST anomalies is observed during the intermittent wind events. Furthermore, during these events, a secondary circulation develops up to altitudes of 2,000 m above warm SST anomalies, which results in sinking of warm and dry air and air momentum from upper levels down to the sea‐surface. Such secondary circulation is triggered by the strong wind stress divergences that develop above small‐scale SST fronts bordering the SST anomalies. The consequence is an increase of latent heat fluxes above SST anomalies. Key Points: Strong turbulent flux discontinuities observed at ocean fronts suggest the importance of small scales for air‐sea interactionsIntermittent large‐scale winds together with mesoscale Sea Surface Temperature variations trigger secondary circulations in the atmospheric boundary layerAir‐sea interactions are explored under a wider range of periods and wind speeds than previously examined [ABSTRACT FROM AUTHOR]

The mesoscale enhancement of surface turbulent fluxes at the air–sea interface is driven by the mesoscale surface wind‐speed variability, especially the gustiness velocity and the mesoscale wind‐speed standard variation. This study proposes a parametrization of these two variables. A large dataset based on the operational 2.5‐km AROME convection‐permitting model is used in a coarse‐graining framework, to quantify various quantities that are subgrid at the scale of a 100‐km resolution global circulation model grid cell. This provides a learning dataset to help build the parametrization. The analysis of two case studies of intense wind‐speed mesoscale variability, combined with a literature review, provides a physically based set of 12 potential predictors, accounting for the convection activity and the large‐scale dynamics. The least absolute shrinkage and selection operator then frames a penalized multivariate linear regression approach to identify the most relevant predictors objectively. Five predictors are selected for predicting the gustiness velocity: the updraft mass flux at the lifting condensation level, the density‐current spreading velocity, the large‐scale horizontal shear and divergence, and the large‐scale wind speed. The parametrization of the mesoscale wind‐speed standard deviation requires an additional predictor, namely the cold‐pool object aggregation index. The proposed parametrization performs significantly better than the previously published parametrizations and is able to capture 80, 99, and 93%$$ \% $$ of the mesoscale enhancement of the momentum, sensible heat, and latent heat fluxes, respectively. From the perspective of a global circulation model implementation, in which some predictors may be unavailable, simpler versions of the parametrization, that is, involving fewer predictors, are also discussed. [ABSTRACT FROM AUTHOR]

In this study, the northward-moving tropical cyclones (TCs) that influence Northeast China (NEC) in July–September are identified based on the distance between the TC center and the boundaries of NEC. Then, based on a machine learning algorithm named hierarchical agglomerative clustering, the tracks of northward-moving TCs that influence NEC are classified into the eastern-track type and the turning type. In NEC, the precipitation induced by eastern-track type TCs gradually decreases from east to west, and the precipitation induced by turning-type TCs gradually decreases from south to north. For eastern-track type TCs, negative Niño3 sea surface temperature (SST) anomalies in preceding January–March can induce cyclonic circulation anomalies and positive vorticity anomalies over the Philippine Sea during subsequent July–September, which favors the genesis of TCs. Moreover, the westerly anomaly in the subtropical western north Pacific and the strengthening of cyclonic steering flow over the Philippines jointly steer the TCs to move northward along the northerly airflow on the west of the western Pacific subtropical high, which favors the genesis of eastern-track type TCs. For turning-type TCs, the positive SST anomalies in the West Wind Drift area during preceding May–July cause positive vorticity anomalies from the northern Philippines to Taiwan from July to September. The cyclonic steering flow over the Philippines and the anticyclonic steering flow over the Sea of Japan lead the TCs to move northwestwards and then turn to northeast, which is conducive to the genesis of turning type northward-moving TCs. Finally, the results of numerical experiments have confirmed these findings.

Ocean variability is a dominant source of remote rainfall predictability, but in many cases the physical mechanisms driving this predictability are not fully understood. This study examines how ocean mesoscales (i.e., the Gulf Stream SST front) affect decadal Southeast US (SEUS) rainfall, arguing that the local imprint of large‐scale teleconnections is sensitive to resolved mesoscale features. Based on global coupled model experiments with eddying and eddy‐parameterizing ocean, we find that a resolved Gulf Stream improves localized rainfall and remote circulation response in the SEUS. The eddying model generally improves the air‐sea interactions in the Gulf Stream and the North Atlantic Subtropical High that modulate SEUS rainfall over decadal timescales. The eddy‐parameterizing simulation fails to capture the sharp SST gradient associated with the Gulf Stream and overestimates the role of tropical Pacific SST anomalies in the SEUS rainfall. Plain Language Summary: Current global climate models (GCMs) typically fail to fully resolve mesoscale ocean features (with length scales on the order of 10 km) such as western boundary currents, which potentially limit rainfall predictability over decadal timescales. Improvements in high‐performance climate modeling enable us to incorporate high‐resolution ocean models (0.1°) that capture these important mesoscale features with increased fidelity. Here we show that the inclusion of mesoscale ocean processes produces a more realistic Gulf Stream and improves both localized rainfall patterns and large‐scale teleconnections. The high‐resolution model shows a better representation of the air‐sea interactions between the sea surface temperature and low‐level atmosphere over the Gulf Stream, thus improving low‐frequency rainfall variations over the Southeast US. The results further imply that high‐resolution GCMs with increased ocean model resolution may be needed in future climate prediction systems. Key Points: Decadal rainfall pattern and associated mechanism in the Southeast US are examined from an eddying global coupled modelEddying CCSM4 improves the air‐sea interactions in the Gulf Stream and the North Atlantic Subtropical High, modulating Southeast US rainfallEddy‐parameterizing CCSM4 and CMIP5 models may overestimate the role of tropical sea surface temperature in decadal Southeast US rainfall [ABSTRACT FROM AUTHOR]

We analyse a tropical cyclone simulated for a realistic ocean‐eddy field using the global, nonhydrostatic, fully coupled atmosphere–ocean ICOsahedral Nonhydrostatic (ICON) model. After intensifying rapidly, the tropical cyclone decays following its interaction with a cold wake and subsequently reintensifies as it encounters a subsurface, warm‐core eddy. To understand the change in the azimuthal‐mean structure and intensity of the tropical cyclone, we invoke a conceptual framework, which recognises the importance of both boundary‐layer dynamics and air–sea interactions. Crucially, the framework recognises that the change in the mean radius of updraught at the boundary‐layer top is regulated by the expanding outer tangential wind field through boundary‐layer dynamics. The decrease in the average equivalent potential temperature of the boundary‐layer updraught during the early decay phase is related to an increase in the mean radius of the updraught rather than air–sea interactions. However, later in the decay phase, air–sea interactions contribute to the decrease, which is accompanied by a decrease in the vertical mass flux in the eyewall updraught and, ultimately, a more pronounced spin‐down of the tropical cyclone. Air–sea interactions are also important during reintensification, where the tendencies are reversed, that is, the mean radius of the boundary‐layer updraught decreases along with an increase in its average equivalent potential temperature and vertical mass flux. The importance of boundary‐layer dynamics to the change in the azimuthal‐mean structure is underscored by the ability of a steady‐state slab boundary‐layer model to predict an increasing and, to a lesser extent, decreasing radius of forced ascent for periods of decay and reintensification, respectively. Finally, our simulation highlights the importance of the ocean‐eddy field for tropical cyclone intensity forecasts, since the simulated warm‐core eddy does not display any sea‐surface temperature (SST) signal until it is encountered by the tropical cyclone. [ABSTRACT FROM AUTHOR]

The Nordic Seas have a significant impact on the climate system. Here 23-day air-sea heat fluxes were analyzed from an in situ air-sea coupled buoy deployed in the Lofoten Basin from 5 August 2012 to 27 August 2012. The buoy captured two stages of strong south and north winds. The observations indicate that warm and wet air transported by the south wind can lead to decreased sensible and latent heat fluxes and net longwave radiation. The total oceanic heat loss was 50–60 W m−2. Thus, this stage was called the heat insulation process. By contrast, the heat dissipation process occurred with the north wind condition during advection of the cold and dry air. During this process, sensible and latent heat fluxes and net longwave radiation notably increased. The total oceanic heat loss during the heat dissipation process reached 240 W m−2, which was four-fold greater than that in the heat insulation process. Given that the heat insulation process is dominant in summertime, the ocean lost minimal heat but absorbed strong solar energy. Thus, a large quantity of energy was stored in the ocean. Heat was transported to the Arctic Ocean and accelerated Arctic warming. The heat dissipation process is dominant in autumn and winter when the ocean releases considerably more energy. The two processes revealed in this paper can be applied to warm-water areas in high latitudes. [ABSTRACT FROM AUTHOR]

The boreal summer intraseasonal oscillations (BSISO) are the prominent features of South Asian summer monsoon and mainly governed by the internal atmospheric dynamics and air-sea interactions. The present study aims to understand and evaluate the relationship between the convection and the associated air-sea interactions during the BSISO over the Indian monsoon region. To accomplish this, the present study utilizes observations and the 22 general circulation model (GCM) simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Representation of Indian monsoon season rainfall, sea surface temperature (SST) and latent heat fluxes in CMIP5 models such as climatological and intraseasonal are assessed using Global Precipitation Climatology Project rainfall by Taylor diagram metric. Results suggest that the majority of CMIP5 models simulated the northward propagation of precipitation and zonal wind at 850 hPa. However, models bias of BSISO variance shows significant spatial heterogeneity over the regions of the Arabian Sea (AS), Sub-Continent of India (SCI) and Bay of Bengal (BoB). The CMIP5 model which shows large biases in the mean state is degrading the northward propagation of BSISO. The phase relationship of ocean (land) and atmospheric interactions are diagnosed with lead-lag regression analysis. On ISO timescales over north Indian Ocean (NIO) convection leads the turbulent fluxes and westerly winds by a week. However, the majority of the models shows large uncertainty to represent this prominent feature over AS and SCI. Further, improper representation of the lead-lag relationship of SST and precipitation on ISO scales over the AS, BoB, and NIO in the CMIP5 models are attributing for significant bias variances. The present study advocates that BSISO propagation in CMIP5 models is mainly attributing from the internal atmospheric dynamics and air-sea interactions. However, for the realistic amplitude simulation of BSISO, proper representation of air-sea feedback mechanisms is crucial in CMIP5 models. The present study further suggests that the oceanic feedback processes of the CMIP5 models need to be improved for the accurate prediction of the intraseasonal variations. [ABSTRACT FROM AUTHOR]

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The Western North Atlantic Ocean (WNAO) is a complex land‐ocean‐atmosphere system that experiences a broad range of atmospheric phenomena, which in turn drive unique aerosol transport pathways, cloud morphologies, and boundary layer variability. This work, Part 2 of a 2‐part paper series, provides an overview of the atmospheric circulation, boundary layer variability, three‐dimensional cloud structure, and precipitation over the WNAO; the companion paper (Part 1) focused on chemical characterization of aerosols, gases, and wet deposition. Seasonal changes in atmospheric circulation and sea surface temperature explain a clear transition in cloud morphologies from small shallow cumulus clouds, convective clouds, and tropical storms in summer, to stratus/stratocumulus and multilayer cloud systems associated with winter storms. Synoptic variability in cloud fields is estimated using satellite‐based weather states, and the role of postfrontal conditions (cold‐air outbreaks) in the development of stratiform clouds is further analyzed. Precipitation is persistent over the ocean, with a regional peak over the Gulf Stream path, where offshore sea surface temperature gradients are large and surface fluxes reach a regional peak. Satellite data show a clear annual cycle in cloud droplet number concentration with maxima (minima) along the coast in winter (summer), suggesting a marked annual cycle in aerosol‐cloud interactions. Compared with satellite cloud retrievals, four climate models qualitatively reproduce the annual cycle in cloud cover and liquid water path, but with large discrepancies across models, especially in the extratropics. The paper concludes with a summary of outstanding issues and recommendations for future work. Key Points: Atmospheric circulation and sea surface temperature drive large seasonal changes in precipitation, surface fluxes, and cloud typesSynoptic activity in winter yields the highest seasonal rain rates, low‐cloud occurrence, and cloud droplet number concentrationsClimate models simulate a wide range of low‐cloud properties, with improved results for models with more sophisticated turbulence schemes [ABSTRACT FROM AUTHOR]

The Southern Annular Mode (SAM) plays an important role in regulating Southern Hemisphere extratropical circulation. State-of-the-art models exhibit intermodel spread in simulating long-term changes in the SAM. Results from Atmospheric Model Intercomparison Project (AMIP) experiments from 28 models archived in CMIP5 show that the intermodel spread in the linear trend in the austral winter (June–July–August) SAM is significant, with an intermodel standard deviation of 0.28 (10 yr)−1, larger than the multimodel ensemble mean of 0.18 (10 yr)−1. This study explores potential factors underlying the model difference from the aspect of extratropical sea surface temperature (SST). Extratropical SST anomalies related to the SAM exhibit a dipole-like structure between middle and high latitudes, referred to as the Southern Ocean Dipole (SOD). The role of SOD-like SST anomalies in influencing the SAM is found in the AMIP simulations. Model performance in simulating the SAM trend is linked with model skill in reflecting the SOD-SAM relationship. Models with stronger linkage between the SOD and the SAM tend to simulate a stronger SAM trend. The explained variance is about 40% in the AMIP runs. These results suggest improved simulation of the SOD-SAM relationship may help reproduce long-term changes in the SAM. [ABSTRACT FROM AUTHOR]