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El Niño–Southern Oscillation (ENSO) is the leading mode of interannual ocean–atmosphere coupling in the tropical Pacific, greatly influencing the global climate system. Seasonal phase locking, which means that ENSO events usually peak in boreal winter, is a distinctive feature of ENSO. In model future projections, the ENSO sea surface temperature (SST) amplitude in winter shows no significant change with a large intermodel spread. However, whether and how ENSO phase locking will respond to global warming are not fully understood. In this study, using Community Earth System Model Large Ensemble (CESM-LE) projections, we found that the seasonality of ENSO events, especially its peak phase, has advanced under global warming. This phenomenon corresponds to the seasonal difference in the changes in the ENSO SST amplitude with an enhanced (weakened) amplitude from boreal summer to autumn (winter). Mixed layer ocean heat budget analysis revealed that the advanced ENSO seasonality is due to intensified positive meridional advective and thermocline feedback during the ENSO developing phase and intensified negative thermal damping during the ENSO peak phase. Furthermore, the seasonal variation in the mean El Niño–like SST warming in the tropical Pacific favors a weakened zonal advective feedback in boreal autumn–winter and earlier decay of ENSO. The advance of the ENSO peak phase is also found in most CMIP5/6 models that simulate the seasonal phase locking of ENSO well in the present climate. Thus, even though the amplitude response in the winter shows no model consensus, ENSO also significantly changes during different stages under global warming. [ABSTRACT FROM AUTHOR]

This decade‐long study from the INTERCONF programme addresses the data gap on ocean dynamics in the Southern Hemisphere. Focusing on the Brazil‐Malvinas Confluence (BMC), we investigated the effect of temperature differences between the warm Brazil Current (BC) and the cold Malvinas Current (MC) on water vapour in the marine atmospheric boundary layer (MABL). Our results show a clear distinction: warmer BMC waters have 32% more water vapour (2 kg·m−2 on average) compared to the MC. This highlights the direct link between ocean temperatures and atmospheric processes. Analysis of radiosonde data alongside satellite measurements showed better agreement over cooler waters with lower water vapour, leading to lower variability. This suggests less atmospheric turbulence and improved data compatibility, especially for satellite retrievals such as AIRS. Comparisons between reanalysis data (CFSR), satellite sounders (AIRS) and radiosondes (RS) showed consistent air temperature profiles, with average errors within the 10% threshold for satellite measurements. While capturing humidity variations remains a challenge, especially at high concentrations (indicated by higher mean squared error values), our study highlights the reliability of satellite data, particularly over the cold BMC region. This research highlights the importance of studying the interactions between ocean fronts and atmospheric phenomena for a complete picture of Southern Hemisphere ocean dynamics. It offers valuable insights for scientists across disciplines, providing a broad perspective on the results and their significance in different contexts. [ABSTRACT FROM AUTHOR]

While decadal changes in Madden–Julian oscillation (MJO) have received considerable attention, the corresponding changes in Boreal Summer Intraseasonal Oscillation (BSISO) have yet to be well understood. In this study, we show the enhanced northward propagation of BSISO in the Western North Pacific (WNP) during the 2000s compared to the 1980s–1990s. Observational analyses and model experiments suggest this enhancement is partially attributed to the tropical Indian Ocean (TIO) warming. The TIO warming tends to increase the air-sea interaction, enhancing moisture anomalies in the free atmosphere. Consequently, this increase in moisture anomalies strengthens BSISO-scale convection through increased convective anomalies at the north of the BSISO center, thereby enhancing the northward propagation of BSISO. Additionally, vorticity changes resulting from mean-state zonal vertical shear also contribute to the BSISO decadal change. Our findings underscore the importance of considering the interaction between BSISO and changes in the ocean mean state in future assessments. [ABSTRACT FROM AUTHOR]

The North Pacific Oscillation (NPO) is an important intrinsic atmospheric pattern over the North Pacific. Observations have shown that the boreal winter NPO is a crucial precursor to the El Niño-Southern Oscillation (ENSO), with many ENSO events being preceded by the NPO. It is therefore imperative to assess the ability of current coupled climate models to reproduce the relationship between winter NPO and the subsequent winter ENSO. Previous studies have shown that most coupled climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) underestimate the influence of winter NPO on the subsequent winter ENSO. Simulations from CMIP6, representing the latest generation of climate models, are now available. This study shows a remarkable improvement of the CMIP6 models in simulating the influence of winter NPO on the subsequent ENSO development compared to the CMIP5 models. This improvement is due to the improved air-sea interaction over the subtropical North Pacific in CMIP6. The enhanced air-sea interaction over the subtropical North Pacific promotes the equatorward propagation of the NPO-induced wind and SST anomalies, resulting in enhanced surface zonal wind anomalies over the tropical western Pacific, which further exert a stronger influence on the subsequent winter ENSO development by triggering the tropical Bjerknes positive air-sea interaction. The enhanced subtropical air-sea interaction is thought to be related to a southward shift of the North Pacific intertropical convergence zone in CMIP6 compared to CMIP5. [ABSTRACT FROM AUTHOR]

As the primary interannual signal of variability in the tropical ocean-atmosphere interaction, the El Niño-Southern Oscillation has a considerable impact on tropical cyclone (TC) activity over the western North Pacific (WNP). Both 2018 and 2021 were La Niña decay years, but TC activity over the WNP during the two summers (June–August) showed notable differences. In 2018, summer TC activity was unusually high with a total of 18 TCs, and the region of TC genesis was mainly in the central and eastern WNP. In contrast, only 9 TCs were generated in summer 2021, and the region of TC genesis was primarily in the western WNP. By comparing the characteristics of the large-scale environmental conditions over the regions of TC genesis, the thermal factors of the tropical oceans, and the activity of the Madden-Julian Oscillation (MJO), this study revealed the possible causes for the marked differences in TC genesis over the WNP during the two summers, which both had a similar background of La Niña decay. The Indian Ocean Basin Mode (IOBM) transitioned of a cold anomaly in the winter of 2017/2018 and persisted until summer 2018. At the same time, the Pacific Meridional Mode (PMM) maintained a positive phase, leading to eastward and northward displacement of the Western Pacific Subtropical High in summer, and eastward extension of the tropical monsoon trough, which presented conditions conducive to TC genesis over the Northwest Pacific. Moreover, the days when the MJO stagnated in phases 5 and 6 in the summer of 2018 increased by approximately 150% relative to climatological state, providing dynamic conditions favorable for TC formation. In 2021, the IOBM quickly turned to a warm anomaly in March and persisted until summer, whereas the PMM became a negative phase in January and remained so until summer. At the same time, the MJO stagnated in phases 2 and 3 for up to 47 days, with the center of convection located over the western Maritime Continent, producing conditions unconducive to TC genesis over the Northwest Pacific. Thus, despite being under a similar background of La Niña decaying year, the distinct evolutions of the IOBM, PMM, and MJO in spring and summer of 2018 and 2021 were the main causes of the notable differences in TC activity over the WNP during these two summers, and the anomalies in IOBM and MJO contributed more significantly than those of the PMM. [ABSTRACT FROM AUTHOR]

Current climate forecasting has advanced to the stage of investigating mesoscale air-sea interactions. Recent studies have identified significant structural differences between the cores and edges of mesoscale eddies; however, the effects of these structural variations on air-sea fluxes and the Marine Atmospheric Boundary Layer (MABL) remain underexplored. Traditional observations often fail to capture the detailed structures of eddies, necessitating enhanced observations at high spatiotemporal resolution for mesoscale eddies. To address this, efforts have been made to develop multi-agent platforms and expendable air-sea interface observation technologies. A task-oriented observation scheme was developed to monitor the spatial characteristics of mesoscale eddies. The South China Sea (SCS) is rich in mesoscale eddies with rapid motion changes, requiring enhanced observations of the air-sea interface using multi-agent mobile networking. An anticyclonic eddy was observed in the eastern region of the Xisha Islands in the SCS, and we examined variations in air-sea fluxes across different regions within the eddy. [ABSTRACT FROM AUTHOR]

This study explores gulf-breeze circulations (GBCs) and bay-breeze circulations (BBCs) in Houston–Galveston, investigating their characteristics, large-scale weather influences, and impacts on surface properties, boundary layer updrafts, and convective clouds. The results are derived from a combination of datasets, including satellite observations, ground-based measurements, and reanalysis datasets, using machine learning, changepoint detection method, and Lagrangian cell tracking. We find that anticyclonic synoptic patterns during the summer months (June–September) favor GBC/BBC formation and the associated convective cloud development, representing 74% of cases. The main Tracking Aerosol Convection Interactions Experiment (TRACER) site located close to the Galveston Bay is influenced by both GBC and BBC, with nearly half of the cases showing evident BBC features. The site experiences early frontal passages ranging from 1040 to 1630 local time (LT), with 1300 LT being the most frequent. These fronts are stronger than those observed at the ancillary site which is located further inland from the Galveston Bay, including larger changes in surface temperature, moisture, and wind speed. Furthermore, these fronts trigger boundary layer updrafts, likely promoting isolated convective precipitating cores that are short lived (average convective lifetime of 63 min) and slow moving (average propagation speed of 5 m s−1), primarily within 20–40 km from the coast. [ABSTRACT FROM AUTHOR]

Bubble plumes play a significant role in the air–sea interface by influencing processes such as air–sea gas exchange, aerosol production, modulation of oceanic carbon and nutrient cycles, and the vertical structure of the upper ocean. Using acoustic Doppler current profiler (ADCP) data collected off the west coast of Ireland, we investigate the dynamics of bubble plumes and their relationship with sea state variables. In particular, we describe the patterns of bubble plume vertical extension, duration, and periodicity. We establish a power-law relationship between the average bubble penetration depth and wind speed, consistent with previous findings. Additionally, the study reveals a significant association between whitecapping coverage and observed acoustic volume backscatter intensity, underscoring the role of wave breaking in bubble plume generation. The shape of the probability distribution of bubble plume depths reveals a transition toward stronger and more organized bubble entrainment events during higher wind speeds. Furthermore, we show that deeper bubble plumes are associated with turbulent Langmuir number Lat ∼ 0.3, highlighting the potential role of Langmuir circulation on the transport and deepening of bubble plumes. These results contribute to a better understanding of the complex interactions between ocean waves, wind, and bubble plumes, providing valuable insights for improving predictive models and enhancing our understanding of air–sea interactions. Significance Statement: This research contributes to understanding bubble plume dynamics in the upper ocean and their relationship with sea state variables. The establishment of a power-law relationship between the bubble penetration depth and wind speed, along with the association between whitecapping coverage and acoustic backscatter intensity, contributes to improved predictive capabilities for air–sea interactions and carbon dioxide exchange. The identification of the potential influence of Langmuir circulation on bubble plume dynamics expands our understanding of the role of coherent circulations in transporting bubble plumes. Additionally, this study presents a clear methodology using commercial sensors such as an ADCP, which can be easily replicated by researchers worldwide, leading to potential advancements in our comprehension of bubble plume dynamics. [ABSTRACT FROM AUTHOR]

The Pacific meridional mode (PMM) is one of the dominant coupled modes in the northeastern tropical Pacific (NETP), characterized by strip-like sea surface temperature (SST) anomalies spanning from Baja California to the central equatorial Pacific. While the majority of the El Niño events follow a positive PMM, only a few La Niña events are preceded by a negative PMM. Such an asymmetric activity of PMM before the onset of El Niño–Southern Oscillation (ENSO) was previously attributed to the inherent nonlinear response of the wind–evaporation–SST (WES) feedback to trade winds in the NETP. Through data analysis and coupled model experiments, we pointed out that PMM is in fact a highly symmetric phenomenon, and the asymmetry of PMM before the ENSO onset thus must be associated with ENSO. On the one hand, the nonlinear response of deep convection over the equator to symmetric ENSO forcing in the central equatorial Pacific permits a stronger Pacific–North America (PNA) pattern in El Niño years than in La Niña years. On the other hand, since the majority of La Niña events are preceded by a sharp decay of an El Niño, the warm equatorial SST anomalies associated with the preceding El Niño provide another source to trigger PNA before the La Niña onset. The two mechanisms modulate the trade winds and heat fluxes in NETP more heavily before the La Niña onset than the El Niño onset and equally contribute to PMM asymmetry before the ENSO onset. [ABSTRACT FROM AUTHOR]

Precipitation in eastern China exhibits large interannual variability during July with the northward movement of the monsoon rain belt. Thus, eastern China usually experiences severe droughts and floods in July. However, the influences of underlying surface thermal drivers, particularly the land factors, remain poorly understood. This study investigates the leading modes of July precipitation in eastern China and their potential influencing factors. The first and second empirical orthogonal function (EOF) modes show meridional dipole and tripolar precipitation anomalies in eastern China, respectively. The EOF1 mode is found to be closely associated with sea surface temperature (SST) anomalies in the tropical Pacific and North Atlantic Oceans in June, while the EOF2 mode is mainly linked to anomalous Indian Ocean SST and Indochina Peninsula soil moisture in June. During years with a strong El Niño–South Oscillation (ENSO) signal, the EOF1 mode is mainly related to the enhanced Walker and Hadley circulations associated with the cold tropical Pacific SST anomalies. In contrast, during years with a weak ENSO signal, the Eurasian midlatitude wave train and the westward zonal overturning circulation associated with tripole-like North Atlantic SST anomalies play a leading role. The EOF2 mode is mainly influenced by Indian Ocean SST anomalies that alter the Walker circulation and by soil moisture anomalies in the Indochina Peninsula that induce an anomalous regional cyclonic circulation. Numerical experiments further demonstrated that the combined effects of soil moisture and SST exert a more substantial impact than their individual effects. These results emphasize the importance of surface thermal factors in understanding regional climate dynamics. [ABSTRACT FROM AUTHOR]

The East/Japan Sea (EJS), a marginal sea of the Northwestern Pacific, is one of the ocean regions showing the most rapid warming and greatest increases in ocean heatwaves over the last several decades. Predictability and skillful prediction of the summer season EJS variability are crucial, given the increasing severity of ocean temperature events impacting fisheries and reinforcing climate conditions like the East Asian rainy season, which in turn affects adjacent high-population density areas over East Asia. We use observations and the Geophysical Fluid Dynamics Laboratory (GFDL) Seamless System for Prediction and Earth System Research (SPEAR) seasonal forecast system to investigate the summertime EJS Sea Surface Temperature (SST) predictability and prediction skill. The observations and seasonal prediction system show that the summer season EJS SST can be closely linked to the previous winter air-sea coupling and predictable 8–9 months in advance. The SPEAR seasonal prediction system demonstrates skillful forecast of EJS SST events from summer to late fall, with added skill for long-lead forecasts initialized in winter. We find that winter large-scale atmospheric circulations linked to Barents Sea variability can induce persistent surface wind anomalies and corresponding northward Ekman heat transport over the East China Sea. The ocean advection anomalies that enter the EJS in prior seasons appear to play a role in developing anomalous SST during summer, along with instantaneous atmospheric forcing, as the source of long-lead predictability. Our findings provide potential applications of large-scale ocean-atmosphere interactions in understanding and predicting seasonal variability of East Asian marginal seas. [ABSTRACT FROM AUTHOR]

The present study uses reanalysis data to investigate how the East Asian cold surges are linked to the initiation of westerly wind anomalies prior to El Niño events and easterly wind anomalies prior to La Niña events in the western equatorial Pacific. Our analysis shows that East Asian cold surge events can be associated with the formation of an anomalous anticyclone that moves eastward across the North Pacific and cools the ocean surface prior to El Niño events. The outflow of the anomalous anticyclonic circulation is responsible for the initiation of westerly wind anomalies in the western equatorial Pacific. We also find that East Asian cold surge events are involved in the development of an anomalous cyclone that moves northeastward along the subpolar North Pacific and reinforces the horizontal temperature gradient prior to La Niña events. The cross-equatorial flow into the anomalous cyclone in the North Pacific contributes to the development of easterly wind anomalies in the western equatorial Pacific. We show that air-sea interactions can play a role in the propagation of anomalous circulations across the North Pacific, eventually initiating zonal wind anomalies in the western equatorial Pacific. [ABSTRACT FROM AUTHOR]

The observed negative response of sea surface temperature (SST) to local summer rainfall peaks, with a lag of 5–10 days over the Indo-Northwest Pacific region, questions the accuracy of atmospheric models in simulating intraseasonal variability. An Atmospheric Model Intercomparison Project simulation forced by daily-varying SSTs (daily-AMIP) is used to evaluate the above question through comparison with its parallel coupled general circulation model (CGCM). In comparison to CGCM, daily-AMIP overestimates intraseasonal rainfall variability, especially over the South China Sea and the Northwest Pacific, consistent with differences in the mean state featuring a cyclonic low-level circulation pattern. Nevertheless, both CGCM and daily-AMIP demonstrate similar northward propagation of rainfall anomalies on the intraseasonal timescale, but daily-AMIP presents a larger amplitude, attributed to the absence of high-frequency ocean-atmosphere coupling and a stronger conversion of kinetic energy due to enhanced monsoon-trade winds confluence. The feedback of the ocean to the atmosphere is realized through surface heat flux exchange, and the near-quadrature relationship between intraseasonal SST and rainfall is considered evidence for ocean-atmosphere interaction. Implications for assessing the ability of AMIP to simulate intraseasonal variability over the Indo-Northwest Pacific region are discussed. [ABSTRACT FROM AUTHOR]

The super long La Niña phenomenon, which has an extremely long duration, like the recent 2020–2023 La Niña event, is less concerned than the super El Niño. In this study, we identify five super long La Niña events after 1950 and investigate roles of the 2–3-year quasi-biennial (QB) and 3–7-year low-frequency (LF) ocean–atmosphere coupled processes of El Niño–Southern Oscillation (ENSO), and the interdecadal background in forming the large-scale prolonged negative sea surface temperature anomalies (SSTAs) in the central to eastern equatorial Pacific during these events. We group the five events into the thermocline-driven type (the 1983–1986 and 1998–2002 events) and the wind-driven type (the 1954–1957, 1973–1976, and 2020–2023 events). The former inherited a sufficiently discharged state of equatorial upper-ocean heat content from the preceding super El Niño and dominated by the thermocline feedback, leading to a LF oceanic dynamical adjustment to support the maintenance of negative ENSO SSTAs. The latter were promoted by the relatively more important zonal advective feedback and Ekman pumping feedback and deeply affected by a strongly negative equatorial zonal wind stress background state that sourced from the strong negative phase of the Interdecadal Pacific Oscillation. Besides, the QB ENSO variability with casual contributions during these events is less important. Results show that both the LF ENSO variability and the interdecadal Pacific background could assist in generating prolonged La Niñas. [ABSTRACT FROM AUTHOR]

Recent research has revealed links between a quasi-decadal mode of climate variability over the North Pacific – the Pacific Decadal Precession (PDP) – and the North Pacific's western boundary current's extension – the Kuroshio Extension (KE). It is suggested that on decadal time scales the PDP both responds to and influences the KE variability. A question yet to be answered is whether it is the large-scale or the mesoscale variations of the KE region that link with the PDP evolution. Using high-resolution sea surface temperature data (1981–2018) from the global ocean Operational Sea Surface Temperature (SST) and Sea Ice Analysis, low-resolution Extended Reconstructed SST (ERSST) version 3b data (1949–2018), geopotential height reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP) we find that it is the large-scale variations in the KE region that correlate best with the PDP-like response in the overlying and downstream atmosphere as compared to the mesoscale variations. In particular, the second mode of the large-scale KE region, which is characterized by the warming (cooling) of the ocean south (north) of the KE, sets up a PDP-like north-south atmospheric pressure dipole over the North Pacific Ocean by altering the large-scale baroclinicity of the atmosphere and zonal intensification of the subtropical jet stream. In turn, there is a reduction in the zonal propagation of stationary wave energy and an enhancement of the climatological zonal wave heights over North America, which results in a downstream response over the North American continent and the formation of a subsequent east-west pressure dipole over the North Pacific and North American continent. As a result, there is a strong correlation between large-scale SST variations in the KE region and the evolution of the PDP over the next three years. [ABSTRACT FROM AUTHOR]

Measurements collected by a Remote Environmental Monitoring Units (REMUS) 600 autonomous underwater vehicle (AUV) off the coast of southern California demonstrate large-scale coherent wave-driven vortices, consistent with Langmuir turbulence (LT), and played a dominant role in structuring turbulent dissipation within the oceanic surface boundary layer. During a 10-h period with sustained wind speeds of 10 m s−1, Langmuir circulations were limited to the upper third of the surface mixed layer by persistent stratification within the water column. The ensemble-averaged circulation, calculated using conditional averaging of acoustic Doppler dual current profile (AD2CP) velocity profiles using elevated backscattering intensity associated with subsurface bubble clouds, indicates that LT vortex pairs were characterized by an energetic downwelling zone flanked by broader, weaker upwelling regions with vertical velocity magnitudes similar to previous numerical studies of LT. Horizontally distributed microstructure estimates of turbulent kinetic energy dissipation rates were lognormally distributed near the surface in the wave mixing layer with the majority of values falling between wall layer scaling and wave transport layer scaling. Partitioning dissipation rates between downwelling centers and ambient conditions suggests that LT may play a dominant role in elevating dissipation rates in the ocean surface boundary layer (OSBL) by redistributing wave-breaking turbulence. [ABSTRACT FROM AUTHOR]

Oceanic suspended particulate matter (SPM) plays important roles in the coupling of climate and biogeochemical cycles via ocean–atmosphere interactions. However, methods for quantifying the properties of SPM in seawater have not yet been well established. Here we present the application of the recently developed complex amplitude sensor (CAS) for analyzing the complex forward-scattering amplitude of individual SPM (0.2–5.0 µm in diameter) obtained at depths of 0–100 m during a research cruise in the western North Pacific. The measured distribution of the complex amplitude indicated that the CAS-derived SPM data could be roughly classified into five major types. Comparison with reference sample's complex amplitude data and scanning electron microscopy analysis suggested that these types could be attributed mainly to diatom fragments, carbonaceous materials (likely organic matter), mineral dusts, iron oxides, or black carbon. Depth profiles revealed that relatively high concentrations of SPM, presumably dominated by diatom fragments and carbonaceous materials with peak diameters of 0.7–1.0 µm, were typically associated with elevated turbidities and chlorophyll a concentrations. Based on this case study, we discuss the practical advantages and limitations of using the CAS to measure size-resolved concentrations of SPM in seawater and to characterize its composition. [ABSTRACT FROM AUTHOR]

From 1979 to 2022, the summer monsoon precipitation has increased by a substantial 40% over Northwest India compared to the 1980s. This wetting trend aligns with the future projections of the Coupled Model Intercomparison Project 6 (CMIP6). The observationally constrained reanalysis data indicates that significant sea surface warming in the western equatorial Indian Ocean and the Arabian Sea is likely driving this increase in rainfall by enhancing the cross‐equatorial monsoonal flow and associated evaporation. We demonstrate that the strengthening of the cross‐equatorial monsoon winds is due to the rapid warming of the Indian Ocean and the enhanced Pacific Ocean trade winds, which result from the poleward shift and expansion of the Hadley cell. These strengthened winds boost the latent heat flux (evaporation), leading to increased moisture transport to Northwest India. Plain Language Summary: From 1979 to 2022, the summer monsoon precipitation has increased by a substantial 40% over Northwest India compared to the 1980s. The analysis suggests that a noticeable warming of the sea surface in the western equatorial Indian Ocean and the Arabian Sea could be causing this increase in rainfall. This warming strengthens the winds crossing the equator in the Indian Ocean and increases evaporation. The study also shows that the monsoon winds are strengthened due to the rapid warming of the Indian Ocean and the enhanced Pacific Ocean trade winds. These stronger winds cause more evaporation, which means more moisture is carried from the ocean to the land, leading to increased monsoon rainfall. Key Points: Large increase in summer monsoon precipitation over Northwest IndiaThe strengthening of the monsoon winds and the Indian Ocean warming drives increasing evaporationPoleward shift and expansion of high‐pressure belts and the Indian Ocean warming are responsible for the strengthening of winds [ABSTRACT FROM AUTHOR]

Guanabara Bay, located in the Metropolitan Region of Rio de Janeiro, one of the largest urban coastal areas in the Southern Hemisphere, is subject to intense maritime traffic due to the presence of several ports. These facilities are affected by sea level fluctuations, influenced by atmospheric and astronomical forces, which motivates synoptic and climatological analysis, including meteorological and astronomical tidal phenomena. This study aimed to assess the evolution of tidal components and the relative mean sea level (MSL) between 1990 and 2021, as well as the atmospheric influence on extreme meteorological tide events, in which the MSL exceeded ±2 and ±3 (±29.98 and ±44.97 cm) standard deviations. The results have shown that, albeit small, the main tidal components (M2$$ {M}_2 $$, S2$$ {S}_2 $$, O1$$ {O}_1 $$, M4$$ {M}_4 $$, Q1$$ {Q}_1 $$, K1$$ {K}_1 $$, K2$$ {K}_2 $$ and N2$$ {N}_2 $$) indicated positive trends in the increase of amplitude and phase, despite the small quantities. For the relative MSL, an increase of 0.30 cm·year−1 was observed in agreement with estimates from global series from satellite altimeters and climate model predictions. Pressure and wind field analyses led to the identification of seasonal variability and the pattern of evolution of atmospheric systems associated with extreme events. In the case of positive surges, very intense winds from the SW, extending along the S/SE Brazilian coast, are caused by the presence of a high‐pressure centre over the continent and low‐pressure centres over the ocean, leading to water piling up. In turn, negative surges are caused by the presence of an anticyclone over the ocean, generating NE winds parallel to the coast of Rio de Janeiro, inducing Ekman transport and lowering the sea level. Therefore, the extreme sea level events in the Guanabara Bay are not triggered by local forces, but rather depend on the temporal persistence and direction of winds along the S/SE Brazilian coast. [ABSTRACT FROM AUTHOR]

Deep-water formation in the eastern Subpolar North Atlantic Ocean (eSPNA) and Nordic Seas is crucial for maintaining the lower limb of the Atlantic Meridional Overturning Circulation (AMOC), of consequence for global climate. However, it is still uncertain which processes determine the deep-water formation and how much Atlantic and Arctic waters respectively contribute to the lower limb. To address this, here we used Lagrangian trajectories to diagnose a global eddy-resolving ocean model that agrees well with recent observations highlighting the eSPNA as a primary source of the AMOC lower limb. Comprised of 72% Atlantic waters and 28% Arctic waters, the density and depth of the AMOC lower limb is critically dependent on Atlantic-Arctic mixing, primarily in the vicinity of Denmark Strait. In contrast, Atlantic waters gaining density through air-sea interaction along the eastern periphery of Nordic Seas and not entering the Arctic Ocean make a negligible contribution to the lower limb. The authors use a global eddy-resolving ocean model and show that the Atlantic-Arctic mixing is necessary for determining the density and depth of the Atlantic Meridional Overturning Circulation return flow. [ABSTRACT FROM AUTHOR]

This study explores the added value (AV) of a regional Earth system model (ESM) compared to an atmosphere-only regional climate model (RCM) in simulating West African monsoon (WAM) rainfall. The primary goals are to foster discussions on the suitability of coupled RCMs for WAM projections and deepen our understanding of ocean–atmosphere coupling's influence on the WAM system. The study employs results from dynamical downscaling of the ERA-Interim reanalysis and Max Plank Institute ESM, low resolution (MPI-ESM-LR), by two RCMs, atmosphere only (REMO) and REMO coupled with Max Planck Institute Ocean Model (MPIOM) (ROM), at ∼25-km horizontal resolution. Results show that in regions distant from coupling domain boundaries such as West Africa (WA), constraint conditions from ERA-Interim are more beneficial than coupling effects. REMO, reliant on oceanic sea surface temperatures (SSTs) from observations and influenced by ERA-Interim, is biased under coupling conditions, although coupling offers potential advantages in representing heat and mass fluxes. Contrastingly, as intended, coupling improves SSTs and monsoon fluxes' relationships under ESM-forced conditions. In this latter case, the coupling features a dipole-like spatial structure of AV, improving precipitation over the Guinea Coast but degrading precipitation over half of the Sahel. Our extensive examination of physical processes and mechanisms underpinning the WAM system supports the plausibility of AV. Additionally, we found that the monsoonal dynamics over the ocean respond to convective activity, with the Sahara–Sahel surface temperature gradient serving as the maintenance mechanism. While further efforts are needed to enhance the coupled RCM, we advocate for its use in the context of WAM rainfall forecasts and projections. [ABSTRACT FROM AUTHOR]

We used daily sea surface temperature (SST) data and hourly drifter data to investigate ocean responses to tropical cyclone (TC) intensity and outer size (wind radius of 34 kt, or R34) in the Northwest Pacific. Results showed that SST cooling is more sensitive to TC R34 than to TC intensity; namely, TCs with a larger R34 cause stronger SST cooling regardless of their intensity. TCs with an R34 ≥125 nmi could cool SST 0.9°C more than TCs with an R34<125 nmi. Drifter data indicated that TCs generate a large current with near-inertial periods. The filtered near-inertial currents were used to calculate the time series of near-inertial kinetic energy Ef, and found that TCs with a larger R34 will trigger stronger Ef. Further analysis revealed that the non-dimensional storm speed S, which is defined as the ratio of the local near-inertial period to the residence time of the TC, is correlated closely with the amplitude of SST cooling when R34 is used to quantify the scale of the TC. Most TCs have a residence time smaller than the local near-inertial period, and therefore, TCs with a large R34 have longer residence times and are closer to the local near-inertial period, which is favorable for stronger SST and current responses. This impact of TC outer size on the surface ocean response implies the critical role of TC outer size in ocean processes under a TC background. [ABSTRACT FROM AUTHOR]

Knowledge of spring warm pool in the Bay of Bengal (BoBWP) is key for further understanding the climate variability in this region and beyond, but little is known about the BoBWP climatological state and the related mechanisms. In this study, we investigate the spatial structure and evolution of the BoBWP using daily Optimum Interpolation SST data from 1982-2022 in combination with multi-source data. Our analysis shows that the BoBWP is located in the central bay (6°-13°N) with a thickness around 20m~ 40 m. Composite analysis indicates that the BoBWP emerges in early April, peaks in early May and dissipates in early June. During the developing period, the net heat flux dominates the formation of spring warm pool through significant air-sea coupling processes, and induces the warming rate of 0.27°C/10d in the mixed layer, which is far larger than the contribution of oceanic dynamical processes (0.01°C/10d). During the decaying period, the net heat flux also plays a dominant role, with a cooling rate of -0.21°C/10d, meanwhile ocean dynamical processes contribute to the cooling of the warm pool with a rate of -0.01°C/10d. Additionally, the SST and the area of the BoBWP are significantly correlated with ENSO (r=0.66 and 0.73, p=0.05). During El Niño decaying year, the BoBWP primarily expands in a southward direction, with a 75% increase in area. Conversely, during La Niña decaying year, the BoBWP almost disappears, with a 52% decrease. [ABSTRACT FROM AUTHOR]

This study introduces an innovative analytical methodology for examining the interconnections among the atmosphere, ocean, and society. The primary area of interest pertains to the North Atlantic Oscillation (NAO), a notable phenomenon characterised by daily to decadal fluctuations in atmospheric conditions over the Northern Hemisphere. The NAO has a prominent impact on winter weather patterns in North America, Europe, and to some extent, Asia. This impact has significant ramifications for civilization, as well as for marine, freshwater, and terrestrial ecosystems, and food chains. Accurate predictions of the surface NAO hold significant importance for society in terms of energy consumption planning and adaptation to severe winter conditions, such as winter wind and snowstorms, which can result in property damage and disruptions to transportation networks. Moreover, it is crucial to improve climate forecasts in order to bolster the resilience of food systems. This would enable producers to quickly respond to expected changes and make the required modifications, such as adjusting their food output or expanding their product range, in order to reduce potential hazards. The forecast centres prioritise and actively research the predictability and variability of the NAO. Nevertheless, it is increasingly evident that conventional analytical methods and prediction models that rely solely on scientific methodologies are inadequate in comprehensively addressing the transdisciplinary dimension of NAO variability. This includes a comprehensive view of research, forecasting, and social ramifications. This study introduces a new framework that combines sophisticated Big Data analytic techniques and forecasting tools using a generalised additive model to investigate the fluctuations of the NAO and the interplay between the ocean and atmosphere. Additionally, it explores innovative approaches to analyze the socio-economic response associated with these phenomena using text mining tools, specifically modern deep learning techniques. The analysis is conducted on an extensive corpora of free text information sourced from media outlets, public companies, government reports, and newspapers. Overall, the result shows that the NAO index has been reproduced well by the Deep-NAO model with a correlation coefficient of 0.74. [ABSTRACT FROM AUTHOR]

Coastal areas play a crucial role in regional development and the environment, but they are increasingly vulnerable to extreme weather events due to global climate change. Sea surface temperature (SST) is an important parameter in the ocean-atmosphere system and can help monitor and understand climate change. Changes in SST have significant impacts on extreme weather events, such as typhoons, and can threaten the safety of coastal cities. Advances in satellite remote sensing technology have improved the accuracy and resolution of SST data, providing new opportunities for studying the impact of SST on urban development in coastal cities. Future research should focus on the impact of SST on the ocean-atmosphere CO2 flux, climate change, typhoons, hazardous weather, the energy balance of coastal megacities, and sea-land breeze circulation. [Extracted from the article]

Reconstructing the spatial patterns in thermocline depth is critical for understanding ocean‐atmosphere interactions. Previous foraminiferal proxies of thermocline depth focus on gradients between planktonic foraminifera living in the surface and subsurface ocean. However, both thermocline depth changes and stratification changes will impact this measure. In this study, we outline a method for reconstructing the tropical upper ocean vertical water column profile, enabling the separate assessment of thermocline depth and stratification changes. This method uses oxygen isotope data from surface and sub‐surface calcifying planktonic foraminifera (Globigerinoides ruber albus, Globorotalia tumida, Neogloboquadrina dutertrei, and Pulleniatina obliquiloculata) as well as data from benthic foraminifera from a core site below the thermocline. Using newly generated and compiled oxygen isotope data from Holocene‐aged marine sediments, we construct vertical profiles at 20 core sites in the Tropical Pacific Ocean. Quantitative estimates of thermocline depth along with error ranges from Monte Carlo simulations are extracted from the reconstructed profiles. There is a strong correlation between reconstructed Holocene and climatological thermocline depth, but the East‐West contrast in the depth of the thermocline is underestimated by 30%. Incorporating benthic information in thermocline estimates results in a dramatic improvement in the reconstruction of spatial gradients in thermocline depth compared to a simpler proxy, the difference in oxygen isotope ratio between a deeper calcifying planktonic species and the surface species, G. ruber. Plain Language Summary: The thermocline is a layer of the ocean where temperature changes rapidly, and the depth of this layer is related to many climatic phenomena. Understanding where and when this layer was deeper and shallower in the past is important to our overall understanding of the climate system. We outline a novel method to reconstruct the thermocline with microscopic shells collected from the sea floor. Our method can reconstruct the changes we see across the Pacific today and also recreate the changes found between the last ice age and today. Key Points: We show a novel method for reconstructing the upper water column using oxygen isotope measurements on multiple species of foraminiferaThis method can be used to quantitatively reconstruct spatial and temporal changes in thermocline depthIf a simple differencing method is preferred, thermocline depth can be most accurately reconstructed using subsurface species only [ABSTRACT FROM AUTHOR]

This study reveals the remarkable interdecadal changes in the influence of boreal winter Arctic sea ice concentration (SIC) anomalies (ASICAs) in the Greenland–Barents Seas on the subsequent El Niño–Southern Oscillation (ENSO) development. Winter ASICA is strongly associated with the subsequent winter ENSO before the late 1980s and after the late 2000s, but their connection is weak during the 1990s and the 2000s. The interdecadal variations in the influence of ASICA on ENSO are associated with changes in the spatial structure of the ASICA-induced North Pacific atmospheric anomalies. During high-correlation periods, winter SIC increases in the Greenland–Barents Seas lead to tropospheric cooling via the suppression of upward surface heat fluxes, which further trigger an atmospheric teleconnection from the Arctic to the North Pacific. The accompanying North Pacific Oscillation–like atmospheric anomalies result in sea surface temperature (SST) warming in the subtropical North Pacific, which extends southward into the tropical Pacific via wind–evaporation–SST feedback and leads to surface westerly anomalies over the tropical western Pacific in the following summer. The tropical western Pacific westerly wind anomalies impact the subsequent ENSO development via triggering positive Bjerknes air–sea interaction. During low-correlation periods, atmospheric anomalies over the North Pacific generated by the winter ASICA are located more northward and cannot induce marked subtropical North Pacific SST anomalies and thus have a weak impact on the following ENSO development. Numerical experiments suggest that the interdecadal variation in the spatial structure of the North Pacific atmospheric anomalies induced by the winter ASICA is partly attributed to change in the atmospheric mean flow. Significance Statement: Arctic sea ice is an important component of the global climate system. Studies have shown that Arctic sea ice has decreased significantly in recent decades, which is considered to be one of the most important responses of Earth's climate system to global climate change. A number of studies have shown that Arctic sea ice anomalies have significant impacts on weather and climate over mid–high latitudes through tropospheric and stratospheric processes. Recent studies have suggested that the effects of Arctic sea ice anomalies could extend to the tropics via air–sea interactions. El Niño–Southern Oscillation (ENSO) is the strongest air–sea coupled system in the tropics and can have a significant impact on climate over the globe. ENSO is also considered to be one of the most important sources of subseasonal–seasonal climate predictability over many parts of the globe. It is therefore important to study the factors for the ENSO occurrence. A recent study showed that Arctic sea ice anomalies during boreal winter in the Greenland–Barents Seas have a significant impact on the following winter ENSO. In this study, we further reveal that the influence of winter Arctic sea ice anomalies in the Greenland–Barents Seas on the subsequent ENSO is unstable and has undergone significant interdecadal variation. We then investigate the mechanisms underlying the interdecadal variation via observational analysis and numerical simulations. The results of this study not only have implications for improving the prediction of ENSO but also could improve our understanding of the physical link between high-latitude climate systems and tropical air–sea coupling systems. [ABSTRACT FROM AUTHOR]

Introduction. Today, the study of climate is one of the most important tasks of modern science. Particular attention is paid to issues of climate variability on different time scales. The El Niño – Southern Oscillation (ENSO) is the leading signal of interannual climate variability that influences the weather and climate of the globe. It has been revealed that during the onset of its different phases, weather and climate anomalies (up to extreme ones) are observed not only over the tropical zone of the Pacific Ocean, but also in extratropical latitudes, which are transmitted through teleconnections in the ocean-atmosphere system. Numerous studies have shown that ENSO modulates tropical cyclone activity in the northwest Pacific Ocean. The purpose of the study is to determine the relationship between sea surface temperature (SST) changes in the equatorial Pacific Ocean and the frequency of tropical cyclones of varying intensity in the northwestern Pacific Ocean under current climatic conditions. Materials and methods of research. The work uses data of the number of cases and intensity of tropical cyclones (TCs) for the period 1991-2020, and series of indices ONI and SOI. Using these data, statistics on the frequency of TCs in the study area were calculated and Pearson correlation coefficients were calculated. The scientific novelty lies in obtaining quantitative characteristics of the relationship between SST and the number of TCs of varying intensity in modern climatic conditions. The practical significance of the work lies in the fact that TCs are associated with the occurrence of severe weather conditions, which, when a TC reaches the densely populated coast of Southeast Asia, can pretend to be significant losses. Therefore, information that can be used in forecasting TCs and their trajectories is very high importance. Research results. Analysis of the frequency of TCs of different intensity in the northwestern Pacific Ocean during the 1991-2020 showed that there was a decrease in the number of tropical depressions and typhoons. However, there has been a slight increase in the occurrence of tropical storms. The results of the analysis of the influence of SST variability in the equatorial Pacific Ocean on the frequency of TCs, showed that there is a statistically significant relationship only with the frequency of typhoons. The establishment of positive SST in the center of the equatorial Pacific Ocean, associated with the El Niño episode, is associated with an increase in typhoons in the northwestern Pacific Ocean.

The Pacific/North American (PNA) pattern is a major low‐frequency variability in boreal winter. A recent modeling study suggested that PNA variability increases through extratropical atmosphere‐ocean coupling, but the effect was not fully extracted due to a particular experimental design. By comparing coupled and two sets of uncoupled large‐ensemble global model simulations, here we show that the PNA‐induced horseshoe‐shaped sea‐surface temperature (SST) anomaly in the North Pacific returns a non‐negligible influence on the PNA itself. Its magnitude depends on the presence or absence of atmosphere‐ocean coupling. The coupling accounts for ∼16% of the PNA variance, while the horseshoe‐shaped SST anomaly explains only 5% under the uncoupled condition. The coupling reduces the damping of available potential energy by modulating turbulent heat fluxes and precipitation, magnifying the PNA variance. Precipitation processes in the extratropics as well as tropics are therefore important for realistically representing PNA variability and thereby regional weather and climate. Plain Language Summary: Atmospheric flow is not entirely random; patterns of circulation variability appear recurrently in the same regions, known as teleconnection patterns. A major wintertime teleconnection pattern over the North Pacific‐North American sector is called the Pacific/North American (PNA) pattern. It causes strong fluctuations in precipitation, air temperature, and pressure over North America through persistent strengthening or meandering of the jet stream. While the influence of tropical ocean variability, such as El Niño/La Niña, on the formation and persistence of the PNA has been known, the role of the extratropical ocean remains unclear. Here we perform a vast number of numerical model simulations to detect the influence of the extratropical ocean on PNA. We show that the atmosphere‐ocean coupling (two‐way interaction between the ocean and atmosphere) enhances the PNA variability (i.e., the magnitude of meandering and strengthening of the westerlies) compared to the uncoupled condition. Furthermore, we propose possible mechanisms behind this enhancement. The findings of this study are expected to contribute to improving the accuracy of long‐term forecasts, such as one‐month predictions, and reducing uncertainty in future climate change projections through the improvement of numerical models. Key Points: Extratropical air‐sea coupling enhances the variance of the Pacific/North American (PNA) pattern, which explains 16% of the total varianceThe enhancement is due to the reduced damping of available potential energy through modulations of turbulent heat fluxes and precipitationAtmosphere‐only simulation is likely to underestimate the impact of extratropical sea surface temperature anomalies on the PNA variability [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]

This study reports a moored array dataset (MASCS 1.0) consisting of five buoys and four moorings in the northern South China Sea during 2014–2015. The dataset includes measurements of sea surface meteorological data using two sets of instruments: sea surface waves recorded using a wave recorder, temperature and salinity from the surface to a depth of 400 m, and at 10 m and 50 m above the ocean bottom using conductivity, temperature, and depth (CTD) recorders. It also includes currents from the surface to a depth of 850 m measured using acoustic Doppler current profilers (ADCPs) and at 10 m, 50 m, and 100 m above the surface measured using current meters. Additional measurements were taken for sea surface radiation, air visibility, chlorophyll, turbidity and chromophoric dissolved organic matter at Buoy 3, which was located at the center of the moored array. The dataset captures the effects of tropical cyclones Rammasun, Kalmaegi, Fung-wong, and Hagupit on the moored array, as well as the transition from summer to winter monsoon in the northern South China Sea. The data reveal oceanic processes in the upper and bottom ocean and are valuable for further studies on air-sea interactions and ocean dynamics. All the data described here are made publicly available from https://zenodo.org/records/12635331 (Zhang et al. 2024). [ABSTRACT FROM AUTHOR]

In mid-June to July 2023, North China witnessed extreme heatwaves, marked by intense near-surface warming with an advanced seasonal cycle of local air temperature. An unconventional upper-tropospheric cold vortex in early June, deviating from conventional "heat dome" patterns, preceded the heatwave extremes. The zonal SSTA gradient in Indo-Pacific warm pool initially suppressed Indian summer monsoon convection, which stimulated the cold vortex around North China via a tropical-extratropical teleconnection. This anomaly intensified the air-land thermal contrast, leading to increased sensible heating and reduced soil moisture in situ. The drier soil conditions maintained and further augmented sensible heating, escalating surface air temperature, and culminating in extraordinary heatwaves. The air column was then destabilized to mitigate the upper-level cold vortex. Historical records corroborate the extremity of the air-sea interactions in 2023. The ECMWF real-time subseasonal-to-seasonal (S2S) forecasts successfully capture the air-land feedback in both cold vortex and heatwave stages, albeit with an underestimation of heatwave intensity due to biases in soil moisture anomalies. Consequently, the initial cold vortex condition and air-land-sea interactions yield S2S predictability to the historic 2023 heatwaves in North China. [ABSTRACT FROM AUTHOR]

The ocean-surface downward longwave radiation (Rl) is one of the most fundamental components of the radiative energy balance, and it has a remarkable influence on air–sea interactions. Because of various shortcomings and limits, a lot of empirical models were established for ocean-surface Rl estimation for practical applications. In this paper, based on comprehensive measurements collected from 65 moored buoys distributed across global seas from 1988 to 2019, a new model for estimating the all-sky ocean-surface Rl at both hourly and daily scales was built. The ocean-surface Rl was formulated as a nonlinear function of the screen-level air temperature, relative humidity, cloud fraction, total column cloud liquid, and ice water. A comprehensive evaluation of this new model relative to eight existing models was conducted under clear-sky and all-sky conditions at daytime/nighttime hourly and daily scales. The validation results showed that the accuracy of the newly constructed model is superior to other models, yielding overall RMSE values of 14.82 and 10.76 W/m2 under clear-sky conditions, and 15.95 and 10.27 W/m2 under all-sky conditions, at hourly and daily scales, respectively. Our analysis indicates that the effects of the total column cloud liquid and ice water on the ocean-surface Rl also need to be considered besides cloud cover. Overall, the newly developed model has strong potential to be widely used. [ABSTRACT FROM AUTHOR]

The zonal wind stress () over the equatorial Pacific plays a vital role in the air-sea interaction associated with the El Niño-Southern Oscillation (ENSO). The equatorial westerly wind stress (w ) anomalies during the El Niño (EN) decay phase propagate eastward and decrease rapidly, while the equatorial easterly wind stress (e ) anomalies during the La Niña (LN) decay phase do not display obvious zonal movement and remain near the dateline. The cause of the asymmetry in anomalies and its contribution to ENSO asymmetric decays are investigated in this study. The difference in anomalies is due to the interaction between the atmospheric response to sea surface temperature anomalies (SSTAs) and the forcing of the wind field on subsurface temperature. The results of numerical experiments demonstrate that the e anomalies during the LN decay phase have a strong and prolonged suppressive effect on the zonal advective feedback and thermocline feedback, resulting in the weakness of the delayed negative feedback. However, due to the eastward propagation and rapid decay of w anomalies, the suppressive effect during the EN decay phase is weak and favors its rapid decay. Previous studies have extensively analyzed the asymmetric response of the atmosphere to SSTAs. The difference in zonal displacement of air-sea couplings during ENSO decay phases leads to the decay asymmetry between EN and LN through weakening the negative air-sea feedback during LN, in which our results highlight that the zonal propagation asymmetry of anomalies plays a crucial role. [ABSTRACT FROM AUTHOR]

El Niño-Southern Oscillation (ENSO) is the strongest interannual climate mode influencing the coupled ocean-atmosphere system in the tropical Pacific, and numerous dynamical and statistical models have been developed to simulate and predict it. In some simplified coupled ocean-atmosphere models, the relationship between sea surface temperature (SST) anomalies and wind stress (τ) anomalies can be constructed by statistical methods, such as singular value decomposition (SVD). In recent years, the applications of artificial intelligence (AI) to climate modeling have shown promising prospects, and the integrations of AI-based models with dynamical models are active areas of research. This study constructs U-Net models for representing the relationship between SSTAs and τ anomalies in the tropical Pacific; the UNet-derived τ model, denoted as τUNet, is then used to replace the original SVD-based τ model of an intermediate coupled model (ICM), forming a newly AI-integrated ICM, referred to as ICM-UNet. The simulation results obtained from ICM-UNet demonstrate their ability to represent the spatiotemporal variability of oceanic and atmospheric anomaly fields in the equatorial Pacific. In the ocean-only case study, the τUNet-derived wind stress anomaly fields are used to force the ocean component of the ICM, the results of which also indicate reasonable simulations of typical ENSO events. These results demonstrate the feasibility of integrating an AI-derived model with a physics-based dynamical model for ENSO modeling studies. Furthermore, the successful integration of the dynamical ocean models with the AI-based atmospheric wind model provides a novel approach to ocean-atmosphere interaction modeling studies. [ABSTRACT FROM AUTHOR]

Operational data assimilation systems for coupled atmosphere–ocean prediction are usually "weakly coupled", in which there is no explicit interaction between the atmosphere and ocean within the data assimilation step. Explicitly allowing for cross‐correlations between the ocean and the atmosphere may have potential benefits in improving the consistency of atmosphere and ocean analyses, as well as allowing a better use of observations at the interface. To understand whether such correlations are significant on the time‐scales of numerical weather prediction, we investigate the atmosphere–ocean cross‐correlations of short‐term forecast errors from the Met Office coupled prediction system, considering their temporal and spatial variability. We find that significant correlations exist between atmosphere and ocean forecast errors on these time‐scales, and that these vary diurnally, from day to day, spatially and synoptically. For correlations between errors in the atmospheric wind and ocean temperature, positive correlations in the North Atlantic region are found to be synoptically dependent, with correlation structures extending into the ocean throughout the deep mixed layer, beyond a depth of 100 m. In contrast, negative correlations over the Indian Ocean are very shallow and are associated with the diurnal cycle of solar radiation. The significance and variability of cross‐correlations indicates that there should be a benefit from including them in data assimilation systems, but it will be important to allow for some flow‐dependence in the correlations. Furthermore, the differing vertical extents of the cross‐correlations in different regions implies the need for situation‐dependent localisation of ensemble correlations when including them in coupled data assimilation systems. [ABSTRACT FROM AUTHOR]

Data from an air–sea interaction tower are used to close the turbulent kinetic energy (TKE) budget in the wave-affected surface layer of the upper ocean. Under energetic wind forcing with active wave breaking, the dominant balance is between the dissipation rate of TKE and the downward convergence in vertical energy flux. The downward energy flux is driven by pressure work, and the TKE transport is upward, opposite to the downgradient assumption in most turbulence closure models. The sign and the relative magnitude of these energy fluxes are hypothesized to be driven by an interaction between the vertical velocity of Langmuir circulation (LC) and the kinetic energy and pressure of wave groups, which is the result of small-scale wave–current interaction. Consistent with previous modeling studies, the data suggest that the horizontal velocity anomaly associated with LC refracts wave energy away from downwelling regions and into upwelling regions, resulting in negative covariance between the vertical velocity of LC and the pressure anomaly associated with the wave groups. The asymmetry between downward pressure work and upward TKE flux is explained by the Bernoulli response of the sea surface, which results in groups of waves having a larger pressure anomaly than the corresponding kinetic energy anomaly, consistent with group-bound long-wave theory. [ABSTRACT FROM AUTHOR]

In order to understand the active mesoscale processes in the Kuroshio extension (KE) region, and reveal the three-dimensional structure of mesoscale eddies in this region, as well as the contributions of mesoscale eddies to water mass redistribution and ocean-atmosphere interactions, based on shipboard observations and satellite remote sensing data, this study investigates the three-dimensional structural characteristics of a typical eddy dipole composed of cyclonic eddy (CE) and an anticyclonic eddy (AE) distributed on the north and south sides of the KE axis, respectively, during June-July 2022. Results show that the CE has a southward cross-frontal motion dominated by shear forcing, while the AE remains relatively stable. The "cold tongue" structure of the CE gradually diminishes due to sea surface warming and heat diffusion from the KE axis, with the AE exhibiting a "reverse eddy sea surface temperature anomaly" structure due to the entrainment of cold water carried by the CE. The vertical multi-cores of internal temperature and salinity anomalies in the eddy dipole are related to the vertical redistribution of water masses with different characteristics. The AE's internal flow field is strongly baroclinic, with around 50% of it having a Richardson number less than the critical value of 0.25, indicating a tendency for turbulent mixing. The upper layer of the CE is barotropic, hindering substantial diapycnal mixing. This study helps to enhance understanding of the eddy dipole characteristics and internal structure of eddies in the KE region, providing information for further research on eddy dynamics. [ABSTRACT FROM AUTHOR]

Upper ocean fronts are dynamically active features of the global ocean playing a key role in the air‐sea exchanges of properties and their transport in the ocean interior. With scales ranging from the submesoscale (0.1–10 km) to the mesoscale (10–100s km) and a temporal variability from hours to months, collecting in situ observations of these structures is challenging and this has limited our understanding of their associated processes and impacts. During the EUREC4A‐OA/ATOMIC field experiment, which took place in the northwest tropical Atlantic in January–February 2020, a large number of uncrewed platforms, including five Saildrones, were deployed to provide a detailed picture of the upper‐ocean fine‐scale variability. This region is strongly influenced by the outflow of the Amazon River, even in winter, which is the minimum outflow season. Here, the generation of fine‐scale horizontal thermohaline gradients is driven by the stirring of this freshwater river input by large anticyclonic eddies, the so‐called North Brazil Current Rings. Vertical shear estimates using the Saildrones ADCP show that partial temperature compensation occurs along restratifying submesoscale salinity‐dominated fronts. The distribution of surface along‐track gradients, as sampled by different horizontal length‐scales, reveals the prevalence of submesoscale fronts. This is supported by a flattening of the spectral slopes of surface density at the submesoscale. This study emphasizes the need to resolve the upper ocean at high spatial resolution to understand its impact on the broader circulation and to properly represent air‐sea interactions. Plain Language Summary: Oceanic eddies and filaments that range between 10 and 100 km in size can be identified in the study region of the northwestern tropical Atlantic using ocean color as viewed from space (a proxy for chlorophyll‐a). The ocean color maps show that these eddies and filaments are associated with the detachment from the shelf of a freshwater Amazon plume and its interaction with the larger oceanic motions O $\mathcal{O}$(100 km). Field observations from different measurements acquired from research vessels and five uncrewed surface vehicles (USVs) reveal the prevalence and scale of upper ocean fronts, whose magnitude results from the combined effect of temperature and salinity. The Amazon freshwater plume is key to the formation of strong salinity‐driven density fronts. However, when looking at O $\mathcal{O}$(1 km) scales, we detect horizontal temperature variations along slumping fronts that partially counteract the effect of salinity. This leads to a damping of the lateral density fronts. This study contributes to the development of a detailed picture of the ocean fine scales, which is necessary to improve our understanding of air‐sea interactions over frontal regions. Key Points: Vertical shear sections from the Saildrone ADCP data reveal that temperature compensation develops along restratifying submesoscale frontsTemperature and salinity spectra show an increased temperature contribution to salinity‐dominated fronts at the submesoscaleA flattening of the density spectra from ~−3 to ~−2 at the submesoscale (1–10 km) is observed in the Boulevard des Tourbillons [ABSTRACT FROM AUTHOR]

Submesoscale process is an important part in the kinetic energy cascade from large‐scale circulation to turbulent dissipation, and a key component of the global heat budget. Its dynamic response to weather event is an important process in forecasting marine bio‐chemical matter transport. So how will submesoscale instabilities response to tropical cyclones (TCs) is worth studying. Based on underwater glider observations and 1‐km high resolution numerical modeling, we investigated two TCs (Roke and Haitang)‐induced submesoscale baroclinic instabilities and their dynamic mechanisms in the Northern South China Sea. The TC Haitang induced significant surface cooling, mixed layer deepening, front sharpening, and enhanced the mixed layer baroclinic and symmetric instabilities. The submesoscale kinetic energy also enhanced sharply after TC Haitang, which was higher correlated with increased mesoscale strain rates. The submesoscale energetics analysis revealed that the enhanced frontal submesoscale kinetic energy after TC Haitang was mainly from potential energy release via baroclinic energy conversion. Four groups of sensitivity numerical experiments revealed that the turbulent heat buoyancy flux and the Ekman buoyancy flux contributed equally to the positive baroclinic energy conversion during the TC Haitang. This study helps us to understand the multiscale oceanic energy transfers and submesoscale air‐sea interaction processes. Plain Language Summary: The oceanic submesoscale process has scale ranges with 0.1–10 km in the horizontal, 0.01–1 km in the vertical, and hours‐days in time. The submesoscale process is a key component of the global energy balance and the heat budget, and can be generated by the atmosphere forcing, for example, cold weather, tropical cyclones (TCs). Based on 2‐km high resolution underwater glider and our set‐up 1‐km numerical modeling data, we found TC Haitang could trigger submesoscale symmetric instability and mixed layer baroclinic instabilities. The former instability gains kinetic energy from geostrophic currents, and the latter draw energy from the mesoscale flow potential energy in the northern South China Sea. The frontal submesoscale kinetic energies (KEs) enhanced sharply after TC Haitang. Based on the submesoscale kinetic energy budget analysis, we suggested that the enhanced frontal KEs after TC Haitang was transferred from potential energy via mixed layer baroclinic energy conversion. After conducting four groups of sensitivity numerical experiments, we found that the turbulent heat buoyancy flux and the Ekman buoyancy flux contributed to the strong vertical buoyancy fluxes during the TC Haitang. This study improves our understanding in submesoscale air‐sea interaction. Key Points: Both the submesoscale frontogenetic function and kinetic energy got enhanced after tropical cyclone (TC) HaitangThe enhanced submesoscale kinetic energy after TC Haitang was primarily dominated by the mixed layer baroclinic instabilityThe vertical buoyancy fluxes were mainly induced by turbulent heat and Ekman buoyancy fluxes during TC Haitang [ABSTRACT FROM AUTHOR]

Standing meanders along the Antarctic Circumpolar Current (ACC) have been shown to be regions of elevated eddy variability, meridional heat transport, and vertical exchange. In this study, we investigate the influence of a standing meander south of Australia on air‐sea heat fluxes, upper ocean structure, and subduction in the 1/10° ACCESS‐OM2 ocean‐sea ice model forced by the JRA55 atmospheric reanalysis. We track the model's Subantarctic and Polar Fronts based on their jet and water mass structure, and produce composites of thermodynamical and dynamical properties of the meander in relaxed and flexed states. The standing meander induces trough‐to‐crest variations in surface heat flux, mixed layer depth (MLD), wind stress curl, vertical velocity, and subduction. At the crests, the ocean loses heat and the mixed layer is deeper; at the troughs, the ocean gains heat and the mixed layer is shallower. Wind stress curl, vertical velocity, and subduction change sign on entering and exiting crests and troughs. Vertical velocity due to the curvature of the meander is an order of magnitude larger than Ekman vertical velocity. The poleward excursion of Polar Front meander crests extends subduction to Antarctic Intermediate Water density classes. Finally, flexing of the meander enhances both air‐sea exchange and vertical velocity. The results show that standing meanders of the ACC influence the distribution and magnitude of air‐sea fluxes of heat and momentum and exchange between the surface and interior ocean. Plain Language Summary: Meanders along the Antarctic Circumpolar Current (ACC) funnel heat toward Antarctica. The research clarifies how meandering of ocean currents shapes air‐sea interaction and the transfer of surface ocean properties into the ocean interior in the ACC south of Australia. Meanders refer to curved patterns in the ACC's flow. We track specific ACC features and look at what is different inside and outside the meander in a computer model. We find that ocean meanders shape where the Southern Ocean gains and loses heat. These changes in heat exchange shape the mixed layer depth (MLD) in the ocean. The MLD is important for climate, biology productivity, and absorption of heat and carbon by the ocean. Meandering produces stronger vertical motion and increases sinking of water from the surface into the ocean interior, compared to regions where meanders are not present. When the meander flexes and becomes more curved, the vertical motion and atmosphere‐ocean exchange become even stronger. The results show that small‐scale patterns in ocean flow can have a strong influence on the atmosphere, with implications for climate and ocean circulation. These patterns are not well represented in climate models and our study is a step toward accounting for their absence. Key Points: Meanders in the Antarctic Circumpolar Current are locations of enhanced vertical velocity and air‐sea exchange of momentum and heatFlexing of the meander further enhances air‐sea exchange and vertical velocities due to increased curvaturePoleward excursion of meander crests extends subduction to Antarctic Intermediate Water densities along the Polar Front [ABSTRACT FROM AUTHOR]

The relationship between minimum sea level pressure (MSLP) and maximum wind speed, is a commonly employed metric for assessing tropical cyclone (TC) forecast skill. However, accurately reproducing this relationship in TC forecasts is challenging. By introducing a new air‐sea momentum flux scheme considering both wave state and saturation (decrease) effect at high winds into a fully coupled atmosphere‐ocean‐wave model, our numerical results reveal that the maximum wind speed and MSLP respond oppositely to the air‐sea momentum flux change. The simulated wind‐pressure relationship aligns well with observations when the new flux scheme is used. This study highlights the large sensitivity of wind‐pressure relationship to the air‐sea momentum flux parameterization at high winds. Furthermore, the air‐sea momentum flux has a significant effect on ocean wave characteristics and sea surface temperature (SST) cooling in TC simulations. Plain Language Summary: Although great progresses have been made in TC track forecasting in recent years, the improvement of TC intensity lags far behind that of TC track. One notable challenge is to represent the observed wind‐pressure relationship. Utilizing a fully coupled atmosphere‐ocean‐wave model, this study suggests that the simulated wind‐pressure relationship is tied to air‐sea momentum flux configuration in the model. Our numerical results reveal that the maximum wind speed and minimum sea level pressure varies differently with the air‐sea momentum flux. The proposed new air‐sea momentum flux scheme successfully reproduced this relationship compared with observations. Wave properties and sea surface temperature response to momentum flux are also evaluated in this work. The findings emphasize the importance of understanding the air‐sea interaction at high wind speeds for improving TC prediction. Key Points: The maximum wind speed and minimum sea level pressure (MSLP) respond oppositely to the air‐sea momentum flux changes, considering both wave state effect and drag coefficient saturation (decrease) at high windsThe maximum surface wind of tropical cyclone (TC) is more sensitive to the air‐sea momentum flux than the MSLPThe introduction of new air‐sea momentum scheme at high winds enhances the representation of TC wind‐pressure relationship and significant wave heights [ABSTRACT FROM AUTHOR]

Recent studies show that climate change signals in the long‐term trend of the tropical ecosystems have emerged earlier than projected by climate models. However, it remains unclear whether tropical ocean surface chlorophyll (SChl) shows a robust trend in the available satellite data era, and what possible physical mechanisms can be responsible for this trend. Here, using combined data from observations, hindcast biogeochemical simulations, and climate model outputs, we document consistently decreasing trends of SChl in the three ocean basins (Indian Ocean, Pacific Ocean, and Atlantic Ocean) with varying magnitude from −1.6% to −10.0% per decade during 1998–2020, with tropical ocean SChl showing a decreasing trend of −7.1% per decade. In the Indo‐Pacific Ocean, mechanisms for the two hotspots with significantly decreasing SChl trends are identified. (a) In the northern tropical Pacific, under the anthropogenic forcing, enhanced stratification associated with frequent interannual surface warming overwhelms the Ekman pumping effect due to positive wind stress curl, leading to a decrease in SChl. (b) In the southern tropical Indian Ocean, the downwelling process dominates the decreasing SChl trend due to the Ekman pumping associated with the negative wind stress curl prevailing in the Indian Ocean, while the contribution from the stratification change is negligible. This study identifies two hotspots with consistently decreasing SChl trends in the tropical ocean which are influenced by the complex physical processes under a warmer climate and calls for more comprehensive understanding of the interactions between physical processes and biogeochemical cycles in the tropical ocean ecosystems. Plain Language Summary: Predicting the response of phytoplankton to climate change is vital to the global biogeochemical cycle and marine economic activities. Previous studies indicated that more than 30 years of continuous data are needed to detect the tropical biogeochemical trends driven by climate change. However, recent studies reveal that climate change signals have emerged in the tropical marine ecosystem, earlier than climate models projected. Nonetheless, it remains unclear whether tropical ocean surface chlorophyll (SChl) shows a robust trend (i.e., hotspots) in the available satellite data era, and what physical mechanism controls this trend. Here, we document that tropical ocean SChl exhibits a decreasing trend of −7.1% per decade from 1998 to 2020. Two emerging hotspots (the northern Tropical Pacific and the southern Tropical Indian Ocean) are identified with sharply decreasing SChl trends, which can be attributed to the enhanced ocean stratification due to external forcing (i.e., a greenhouse warming climate scenario) and downwelling process suppressing nutrients supply due to internal variability (i.e., the atmosphere‐ocean interaction), respectively. These diverse but consistent SChl trends suggest that the contribution from a greenhouse warming to the SChl trend during available satellite data (September 1997 to present) needs to be clarified. Key Points: Two hotspots with remarkably decreasing surface chlorophyll (SChl) trends are identified in the northern tropical Pacific and the southern tropical Indian OceanTropical SChl exhibits a decreasing trend by −7.1% (−4.4%) per decade when extreme ENSO effects are included (excluded) for periods up to 2020Increased upper ocean stratification and downwelling are major factors driving the SChl trend in the two hotspots, respectively [ABSTRACT FROM AUTHOR]

Interannual sea surface temperature (SST) variations in the subtropical‐midlatitude Southern Hemisphere are often associated with a circumpolar wavenumber‐4 (W4) pattern. This study is the first attempt to successfully simulate the SST‐W4 pattern using a state‐of‐the‐art coupled model called SINTEX‐F2 and clarify the underlying physical processes. It is found that the SST variability in the southwestern subtropical Pacific (SWSP) plays a key role in triggering atmospheric variability and generating the SST‐W4 pattern during austral summer (December‐February). In contrast, the tropical SST variability has a very limited effect. The anomalous convection and associated divergence over the SWSP induce atmospheric Rossby waves confined in the westerly jet. Then, the synoptic disturbances circumnavigate the subtropical Southern Hemisphere, establishing an atmospheric W4 pattern. The atmospheric W4 pattern has an equivalent barotropic structure in the troposphere, and it interacts with the upper ocean, causing variations in mixed layer depth due to latent heat flux (LHF) anomalies. As incoming climatological solar radiation goes into a thinner (thicker) mixed layer, the shallower (deeper) mixed layer promotes surface warming (cooling). This leads to positive (negative) SST anomalies, developing the SST‐W4 pattern during austral summer. Subsequently, anomalous entrainment due to the temperature difference between the mixed layer and the water below the mixed layer, anomalous LHF, and disappearance of the overlying atmospheric W4 pattern cause the decay of the SST‐W4 pattern during austral autumn. These results indicate that accurate simulation of the atmospheric forcing and the associated atmosphere‐ocean interaction is essential to capture the SST‐W4 pattern in coupled models. Plain Language Summary: In the subtropical‐midlatitude Southern Hemisphere, we often observe year‐to‐year fluctuations in sea surface temperature (SST) linked to a specific pattern known as wavenumber‐4 (W4). This study represents the first successful attempt to simulate this temperature pattern using a climate emulator called SINTEX‐F2, allowing us to uncover its physical processes. Our research reveals that SST variations in the southwestern subtropical Pacific (SWSP) play a pivotal role in generating the W4 pattern in the atmosphere, subsequently influencing SST during austral summer. Interestingly, this pattern is almost independent of tropical SST variability. The process starts with heating in the SWSP, causing atmospheric disturbances. This leads to an undulation in mid‐latitude atmospheric flow, evolving into a well‐established global Rossby wave with four positive (negative) loading centers, forming a W4 pattern. This atmospheric wave interacts with the ocean's surface, leading to heat exchange between the atmosphere and the upper ocean. In turn, it influences the depth of the mixed layer in the upper ocean, which receives solar energy. When solar energy penetrates into a shallower (deeper) mixed layer, it warms (cools) the mixed layer effectively, resulting in higher (lower) SSTs. Afterwards, the energy exchange between the mixed layer and the deep ocean contributes to the decay of the SST pattern. Key Points: First attempt to successfully simulate the wavenumber‐4 (W4) pattern of Southern Ocean sea surface temperature (SST) using a coupled model, uncovering the underlying physical processesSouthwestern subtropical Pacific SST plays a crucial role in generating SST W4 pattern via circumpolar atmospheric variabilityThe ocean mixed layer and upper ocean processes are found to be important for the growth and decay of the SST pattern [ABSTRACT FROM AUTHOR]

Statistical models are an alternative to numerical models for reconstructing storm surges at a low computational cost. These models directly link surges to metocean variables, i.e., predictors such as atmospheric pressure, wind and waves. Such reconstructions usually underestimate extreme surges. Here, we explore how to reduce biases on extremes using two methods—multiple linear regressions and neural networks—for surge reconstructions. Models with different configurations are tested at 14 long-term tide gauges in the North-East Atlantic. We found that (1) using the wind stress rather than the wind speed as predictor reduces the bias on extremes. (2) Adding the significant wave height as a predictor can reduce biases on extremes at a few locations tested. (3) Building on these statistical models, we show that atmospheric reanalyses likely underestimate extremes over the 19th century. Finally, it is demonstrated that neural networks can effectively predict extreme surges without wind information, but considering the atmospheric pressure input extracted over a sufficiently large area around a given station. This last point may offer new insights into air-sea interaction studies and wind stress parametrization. [ABSTRACT FROM AUTHOR]

Extreme El Niño events have outsized global impacts and control the El Niño Southern Oscillation (ENSO) warm/cold phases asymmetries. Yet, a consensus regarding the relative contributions of atmospheric and oceanic nonlinearities to their genesis remains elusive. Here, we isolate the contribution of oceanic nonlinearities by conducting paired experiments forced with opposite wind stress anomalies in an oceanic general circulation model, which realistically simulates extreme El Niño events and oceanic nonlinearities thought to contribute to ENSO skewness (Tropical Instability Waves (TIWs), Nonlinear Dynamical Heating (NDH)). Our findings indicate a weak contribution of oceanic nonlinearities to extreme El Niño events in the eastern Pacific, owing to compensatory effects between lateral (NDH and TIWs) and vertical processes. These results hold across different vertical mixing schemes and modifications of the upper‐ocean heat budget mixed layer criterion. Our study reinforces previous research underscoring the pivotal role of atmospheric nonlinearities in shaping extreme El Niño events. Plain Language Summary: The El Niño‐Southern Oscillation (ENSO) is the primary driver of year‐to‐year climate variations in the tropics and beyond. Originating from air‐sea interactions in the tropical Pacific, ENSO oscillates between warm (El Niño) and cold (La Niña) phases, modulating sea surface temperature in the central and eastern equatorial Pacific. Occasionally, El Niño events intensify into "super" El Niño events, causing widespread impacts globally. Utilizing a state‐of‐the‐art oceanic model, our research challenges previous results suggesting a strong oceanic contribution to the amplitude difference between "normal" and "super" El Niño events. Instead, our findings reveal that potential oceanic influences on "super" El Niño events tend to offset each other. This is consistent with recent research highlighting the crucial role of atmospheric processes in the transformation from a "normal" to a "super" El Niño. Key Points: A state‐of‐the‐art ocean model reproduces extreme El Niño events and the corresponding nonlinear oceanic processes realisticallyContributions from oceanic nonlinearities are isolated using paired simulations forced by opposite wind stress anomaliesEffects of oceanic nonlinearities on extreme El Niño events are small, due to compensation between lateral and vertical processes [ABSTRACT FROM AUTHOR]

Air‐sea interaction in late boreal winter is studied over the extratropical North Atlantic (NA) during 1960–2020 by examining the relationship between sea‐surface temperature (SST) and total turbulent heat flux (THF). The two quantities are positively correlated on interannual timescales over the central‐midlatitude and subpolar NA, suggesting the atmosphere on average drives SST and THF variability is independent of SST. On decadal timescales and over the central‐midlatitude NA the correlation is negative, suggesting ocean processes on average drive SST and THF variability is sensitive to SST. The correlation is positive over the subpolar NA. There, interannual and decadal THF variability is governed by the North Atlantic Oscillation (NAO). During the major late 20th and early 21st century SST increase in the subpolar NA diminishing oceanic heat loss associated with a weakening NAO was observed. This study suggests that the atmosphere is more sensitive to SST over the central‐midlatitude than subpolar NA. Plain Language Summary: The relationship between the sea‐surface temperature (SST) and air‐sea heat exchange, is studied over the extratropical North Atlantic (NA) for late boreal winter during 1960–2020. This relationship provides information about the roles of atmospheric and oceanic processes in driving SST variations. We consider two regions, the central‐midlatitude (35°N–50°N) and subpolar (50°N–60°N) NA. Over both regions, the atmosphere tends to drive the variations on the short interannual timescales. On the longer decadal timescales and over the central‐midlatitude NA, oceanic processes tend to drive the SST anomalies that in turn influence the air‐sea heat exchange. Air‐sea heat exchange over the subpolar NA is mostly driven by the North Atlantic Oscillation on interannual and decadal timescales, which is the leading mode of internal atmospheric variability in winter. This study suggests that the atmosphere is more sensitive to SST over the central‐midlatitude than subpolar NA. Key Points: Regional variation in the nature of air‐sea interaction over the extratropical North Atlantic (NA) north of 35°NTimescale dependence in relationship between sea‐surface temperature (SST) and turbulent heat flux over the central‐midlatitude NAThe atmosphere is more sensitive to SST variability over the central‐midlatitude than subpolar NA [ABSTRACT FROM AUTHOR]

The Planetary Boundary Layer height (PBLH) is essential for studying PBL and ocean‐atmosphere interactions. Marine PBL is usually defined to include a mixed layer (ML) and a capping inversion layer. The ML height (MLH) estimated from the measurements of aerosol backscatter by a lidar was usually compared with PBLH determined from radiosondes/dropsondes in the past, as the PBLH is usually similar to MLH in nature. However, PBLH can be much greater than MLH for decoupled PBL. Here we evaluate the retrieved MLH from an airborne lidar (HSRL‐2) by utilizing 506 co‐located dropsondes during the ACTIVATE field campaign over the Northwest Atlantic from 2020 to 2022. First, we define and determine the MLH and PBLH from the temperature and humidity profiles of each dropsonde, and find that the MLH values from HSRL‐2 and dropsondes agree well with each other, with a coefficient of determination of 0.66 and median difference of 18 m. In contrast, the HSRL‐2 MLH data do not correspond to dropsonde‐derived PBLH, with a median difference of −47 m. Therefore, we modify the current operational and automated HSRL‐2 wavelet‐based algorithm for PBLH retrieval, decreasing the median difference significantly to −8 m. Further data analysis indicates that these conclusions remain the same for cases with higher or lower cloud fractions, and for decoupled PBLs. These results demonstrate the potential of using HSRL‐2 aerosol backscatter data to estimate both marine MLH and PBLH and suggest that lidar‐derived MLH should be compared with radiosonde/dropsonde‐determined MLH (not PBLH) in general. Plain Language Summary: The Planetary Boundary Layer Height (PBLH) is essential for studying the lower atmosphere and its interaction with the surface. Usually, it contains a mixed layer (ML) with vertically well‐mixed (i.e., nearly constant) specific humidity and potential temperature. Over the ocean, the PBL is usually coupled (vertically well‐mixed) and the ML height (MLH) is usually close to PBLH, hence the MLH estimated from the measurements of aerosol backscatter by a lidar is traditionally compared with PBLH determined from radiosondes/dropsondes. However, when the PBL is decoupled (not vertically well mixed), the MLH differs from the PBLH. Here we used dropsondes' thermodynamic profile to evaluate the airborne High‐Spectral‐Resolution Lidar—Generation 2 (HSRL‐2) estimation of MLH and PBLH in airborne field campaign over the northwestern Atlantic (ACTIVATE) from 2020 to 2022. We show that the HSRL‐2 has excellent MLH estimation compared to the dropsondes. We also improved the HSRL‐2 estimation of PBLH. Further data analysis indicates that these conclusions remain the same for cases with different cloud fractions, and for decoupled PBLs. These results demonstrate the potential of using HSRL‐2 aerosol backscatter data to estimate both marine MLH and PBLH and suggest that lidar‐derived MLH should be compared with radiosonde/dropsonde‐determined MLH (not PBLH) in general. Key Points: Dropsondes over the northwest Atlantic are used to determine mixed layer height (MLH) and boundary layer height (PBLH)HSRL‐2 lidar MLH product compares well with dropsonde‐derived MLH but does not correspond to PBLH for decoupled PBLThe current operational HSRL‐2 algorithm is modified to include retrieval of the PBLH for decoupled PBL [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST), with its complex and dynamic behavior, is a major driver of ocean–atmosphere interactions. The purpose of this study is to investigate the behavior of SST and its prediction using a chaotic approach. Average mutual information (AMI) and Cao methods were used to reconstruct the phase space. The Lyapunov exponent and correlation dimension were used to investigate chaos. The Lyapunov exponent index was used to predict SST with a 5-year average prediction horizon using the local prediction method between 2023 and 2027. The results showed a 3-month delay time for the Pacific and Antarctic Oceans, and a 2-month delay time for the Atlantic, Indian, and Arctic Oceans. The optimal embedding dimension for all oceans is between 6 and 7. Our analysis reveals that the dynamics of SST in all oceans exhibit varying degrees of chaos, as indicated by the correlation dimension. The local prediction method achieves relatively accurate short-term SST predictions due to the clustering of SST points around specific attractors in the phase space. However, in the long term, the accuracy of this method decreases as the points in the phase space of SST can spread randomly. The model performance ranking with a Percent Mean Relative Absolute Error shows that the Indian Ocean has the best performance compared to other oceans, while the Atlantic, Pacific, and Antarctic and Arctic Oceans are in the next ranks. This study contributes to understanding the dynamics of SST and has practical value for use in the development of climate models. [ABSTRACT FROM AUTHOR]