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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]

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]

Surface wind divergence is largely modulated by the sea surface temperature (SST) gradient through vertical momentum mixing and pressure adjustment. Here, the two mechanisms affecting the coupling strength between SST gradient and surface wind divergence are examined during an atmospheric front passage in the Southern Ocean. This event is also recorded by an uncrewed surface vehicle (USV). The reanalysis product (ERA5) revealed that downward momentum mixing is the dominant mechanism on the daily time scale. The coupling strength during the day when the atmospheric front passed over declined by 75%, compared to the adjacent days. This implies that the atmospheric front can partially attenuate the SST gradient effect on the surface wind divergence. Furthermore, a decade‐long statistic also showed a decreasing trend of SST‐wind coupling when the atmospheric fronts occur more. Additionally, after removing the mesoscale weather variation, the USV observations showed a remarkable SST imprint on the atmospheric boundary layer in the oceanic submesoscale regime, which denotes the scale below the deformation radius (∼16 km). The submesoscale air‐sea interaction processes also displayed decreased air‐sea coupling strength during atmospheric front passage. This is possible as the vertical velocity induced by the atmospheric front can compensate for the daily averaged uprising vertical velocity due to surface wind convergence. This analysis indicates that the atmospheric front can diminish the coupling between the SST gradient and surface wind divergence, which contrasts the existing statistical results showing that atmospheric fronts tend to enhance such coupling. Plain Language Summary: The atmospheric front is seldom directly observed in the Southern Ocean due to its short‐lived life cycle and the remoteness and harsh conditions. Consequently, the air‐sea interface variation is unknown during the atmospheric front passage. An unmanned surface vehicle coincidentally recorded the air‐sea interface parameter variations during the atmospheric front passage. Combined with the reanalysis data, this study indicates that the atmospheric front can weaken the air‐sea coupling strength and modify the processes of air‐sea energy transfer. A decade‐long statistic supports this result. This finding complements existing statistical results indicating that atmospheric fronts tend to enhance this coupling. Key Points: Atmospheric front can diminish the regional coupling strength between surface wind divergence and sea surface temperature (SST) gradientSaildrone observation supports the diminishes of SST‐wind coupling during atmospheric front passage at the submesoscale regimeDecade‐long statistics show a negative correlation between the SST‐wind coupling and regional atmospheric front ratio [ABSTRACT FROM AUTHOR]

We perform quadrupled CO2 climate simulations with the Community Earth System Model version 1 (CESM1) to study how air‐sea coupling affects the response of tropical rainfall under global warming. We use a hierarchy of ocean models to separate the effects of seasonal mixed‐layer entrainment, wind‐driven Ekman flows directed perpendicular to the wind, and the near‐equator frictional flows directed in the same direction as the wind. We show that the Pacific Ocean's enhanced equatorial warming pattern (EEW) and equatorward ITCZ contraction observed in previous climate simulations emerge when the ocean model includes wind‐driven Ekman and frictional flows. Furthermore, the near‐equator frictional flow contributes more than half of the heat convergence in the equatorial Pacific Ocean. Finally, we show that although Ekman flow and near‐equator frictional flow can both result in EEW, their coupled interactions with the Hadley circulation lead to opposite feedbacks on EEW's strength. Plain Language Summary: The ocean is important in modulating the atmospheric response to climate change. Here, we study how air‐sea coupling affects the response of tropical rainfall under global warming. To identify the importance of individual ocean processes, we use a hierarchy of ocean models to separate the effects of seasonal mixed‐layer entrainment, wind‐driven Ekman flows, and frictional flows. We show that including Ekman and frictional flows allows our simulation to produce the Pacific Ocean's enhanced equatorial warming pattern and equatorward ITCZ contraction noted in previous climate simulations. We also show that the frictional flow, which has yet to receive much attention, is as important as the Ekman flow in generating equatorial heat convergence. Key Points: Air‐sea momentum coupling enhances equatorial sea surface temperature (SST) responseThe frictional ocean flow, parallel with the surface wind stress, contributes half of the ocean heat convergence response near the equatorThe SST and vertical wind response to greenhouse gas forcing are modulated by the ocean's temperature and vertical velocity distribution [ABSTRACT FROM AUTHOR]

Tropical cyclones tend to result in distinctive spatial and temporal characteristics in the upper ocean, which suggests that traditional, parametrisation‐based background‐error covariances in oceanic data assimilation (DA) may not be suitable. Using the case study of Cyclone Titli, which affected the Bay of Bengal in October 2018, we explore hybrid methods that combine the traditional covariance modelling strategy used in variational methods with flow‐dependent estimates of the ocean's error covariance structures based on a short‐range ensemble forecast. This hybrid approach is investigated in the UK Met Office's state‐of‐the‐art system. Single‐observation experiments in the ocean reveal that the hybrid approach is capable of producing analysis increments that are time‐varying, more anisotropic and vertically less uniform. When the hybrid oceanic covariances are incorporated into a weakly coupled DA system, the sea‐surface temperature (SST) in the path of the cyclone is changed, not only through the different specifications of background‐error covariances used in the SST assimilation, but also through the propagation of subsurface temperature differences to the surface as a result of vertical mixing associated with the cyclone's strong winds. The coupling with the atmosphere then leads to a discrepancy in the cyclone's central pressure, which brings forth further SST differences due to the different representations of the cyclone's emerging cold wake. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) anomalies over the tropical Pacific, such as El Niño‐Southern Oscillation (ENSO) events, can cause intensified air‐sea couplings and different climate impacts. Although there are quite a few indices describing the event magnitude, little attention is paid to directly measuring air‐sea coupling strengths. Using Dynamical Systems Metrics, we introduce an index to quantify air‐sea coupling strengths over the tropical Pacific. This coupling index explains the relatively weak impact of the super El Niño in 2015/2016, and characterizes the ENSO diversity and asymmetry. Particularly, this index can identify several unusual neutral days with prolonged strong air‐sea couplings, where SST anomaly distributions are not as homogeneous as the ENSO events, but which have caused intense climate impacts. Our results demonstrate the close relationship among air‐sea coupling strengths, SST anomalies, and coupled feedbacks, which may promote better understandings on the events diversity related to SST anomalies over the tropical Pacific. Plain Language Summary: It is widely known that each El Niño event has its unique climate impacts. Noting that El Niño‐Southern Oscillation (ENSO) is essentially an atmosphere‐ocean coupling phenomenon, the different ENSO impacts may indicate different air‐sea coupling strengths. Taking both spatial and temporal features of the observational data sets into account, we capture the co‐variation patterns between atmospheric and oceanic meteorological fields, and present a coupling index to infer the instantaneous air‐sea coupling strength over tropical Pacific. Analyses show that the sea surface temperature (SST) anomalies exhibit diverse spatial patterns, which accompanies with different air‐sea coupling strengths and climate impacts. These differences exactly characterize the diversity of tropical Pacific SST anomalies, which exist not only in El Niño and La Niña events, but also in neutral days. Compared to the existing SST‐based Niño indices, it may be a more effective way to infer the potential SST impacts through measuring the air‐sea coupling strengths directly. Furthermore, this coupling metric may also serve as a useful tool in the study of atmosphere‐ocean coupling dynamics and extreme impacts. Key Points: Instantaneous air‐sea coupling strengths over tropical Pacific are estimated by Dynamical Systems MetricsThe air‐sea coupling strengths are linked to the El Niño‐Southern Oscillation (ENSO) diversity and climate impactsIn addition to ENSO events, the sea suface temperature anomaly patterns with strong air‐sea interactions also exist in some unusual neutral days [ABSTRACT FROM AUTHOR]

The Walker circulation (WC) responds to CO2 forcing at both short and long timescales. In climate models, the fast response accounts for a substantial portion of the total responses, but its mechanisms, particularly those pertaining to air‐sea interactions, remain unclear. We find contrasting fast WC responses in the first 2 years of abrupt CO2 forcing, determined by the models' air‐sea coupling strength in the equatorial Pacific. In models with a strong coupling, wind anomalies induced by the instantaneous land‐sea thermal contrast trigger a Bjerknes feedback, leading to cooling in the equatorial Pacific and WC strengthening. The WC weakens gradually as the Bjerknes feedback wanes and the subsurface warm pool water migrates eastward as Kelvin waves. In models with a weaker coupling, the WC weakens monotonically. Our results suggest that the inter‐model discrepancy in WC changes is associated with the uncertainty in the fast component. Plain Language Summary: The Walker circulation (WC), a large‐scale tropical air flow in zonal and vertical directions, is a product of the atmosphere‐ocean interaction. A strong WC is associated with a more positive air‐sea coupling: as the westward trade winds accumulate warm water in the western Pacific and draw up cold water in the eastern Pacific, the zonal sea surface temperature (SST) gradient and atmospheric convection above warm regions are enhanced. The zonal SST and sea level pressure gradient reinforce the trade winds. We use multiple climate models to study the response of the WC to CO2 at different time scales. Two different WC responses are found and the difference is determined by simulated air‐sea coupling strength among models. In the first class of models, a strong air‐sea coupling causes a strengthening of WC right after imposed forcing—opposite to the longterm response. In the second class of models, the WC weakens from the beginning of the simulation. Key Points: Climate models disagree on the fast response of the Pacific Walker circulation to CO2The disagreement is determined by models' air‐sea coupling strengthModels simulating stronger air‐sea coupling establish a strengthening of the Walker circulation during fast response [ABSTRACT FROM AUTHOR]

Cyclones with tropical characteristics called medicanes ("Mediterranean Hurricanes") eventually develop in the Mediterranean Sea. They have large harmful potential and a correct simulation of their evolution in climate projections is important for an adequate adaptation to climate change. Different studies suggest that ocean–atmosphere coupled models provide a better representation of medicanes, especially in terms of intensity and frequency. In this work, we use the regionally‐coupled model ROM to study how air‐sea interactions affect the evolution of medicanes in future climate projections. We find that under the RCP8.5 scenario our climate simulations show an overall frequency decrease which is more pronounced in the coupled than in the uncoupled configuration, whereas the intensity displays a different behaviour depending on the coupling. In the coupled run, the relative frequency of higher‐intensity medicanes increases, but this is not found in the uncoupled simulation. Also, this study indicates that the coupled model simulates better the summer minimum in the occurrence of medicanes, avoiding the reproduction of unrealistically intense events that can be found in summer in the uncoupled model. [ABSTRACT FROM AUTHOR]

Satellite remote sensing data revealed a widespread sea fog event over the central/southern Yellow Sea (YS), following Typhoon Muifa passage in August 2011. Despite the importance of sea fog prediction for coastal safety, improving accuracy remains a challenging issue. By analyzing results from air‐sea coupled and uncoupled simulations, this study aims to investigate how the air‐sea coupling improves the sea fog formation and duration and examine atmospheric responses to spatiotemporally varying sea surface temperature (SST) over the YS. Unlike the uncoupled model, the SST simulated by the coupled model dramatically decreased and maintained its low temperature for more than a week after the typhoon passed over the YS, showing better agreement with the observations. The sharp SST decrease over the YS cools the air temperature at low‐level atmosphere and enhances horizontal convergence in the moisture flux over the cooler ocean, which provides favorable conditions for sea fog formation. The long‐lasting oceanic cooling stabilizes the atmospheric boundary layer and suppresses atmospheric vertical mixing, delaying the dissipation of sea fog for more than a week. This study highlights that air‐sea coupling can improve the sea fog simulation by providing more realistic oceanic conditions. Plain Language Summary: Because sea fog poses a severe risk to many human activities, such as shipping and aviation in coastal regions, accurately predicting the sea fog is of extreme importance for coastal and marine safety. Despite its importance, improving the sea fog simulation still remains a challenging issue. In this study, we investigate how air‐sea coupling improves the accuracy of sea fog simulation and analyze atmospheric responses to spatial and temporally varying sea surface temperature (SST) over the Yellow Sea (YS). To do so, we used a fully coupled ocean–atmosphere modeling system and computed comparative experiments of coupled and uncoupled simulation for Typhoon Muifa that passed over the central YS in August 2011. The comparison shows that typhoon‐induced SST cooling enhances horizontal convergence in moisture flux and vertical stability, providing favorable conditions for the formation and duration of sea fog. This study demonstrates that air‐sea coupling is instrumental in producing oceanic and atmospheric conditions and simulating associated extreme weather events in the YS. Key Points: Abrupt and widespread sea surface temperature cooling occurred with decreasing surface air temperature due to typhoon‐induced strong vertical mixing in the Yellow SeaOceanic cooling enhances horizontal convergence in moisture flux and vertical stability, providing favorable conditions for sea fog formationAir‐sea coupling improves sea fog simulation by providing more rapidly altered oceanic conditions [ABSTRACT FROM AUTHOR]

Daily initialized coupled and uncoupled numerical weather prediction (NWP) forecasts from the global Met Office Unified Model (MetUM) are compared for the 2016 Indian summer monsoon. Three MetUM configurations are used: atmosphere only (ATM), coupled to a mixed‐layer ocean model (KPP), and coupled to a dynamical ocean model (NEMO). The analysis focuses on the impact of air–sea coupling, particularly in the Bay of Bengal (BoB), on NWP for monsoon rainfall. Seasonal‐mean biases in all three configurations are highly consistent and driven by errors in atmospheric processes. Rainfall is initially overestimated over India, but underestimated over the BoB, the latter associated with too much short‐wave radiation and too little cloud cover in MetUM. The excess short‐wave radiation (>40 W·m−2 over the northwest BoB) is partially compensated by additional latent cooling, primarily due to overestimated surface wind speeds. In NEMO and KPP, coupling improves the timing of intraseasonal active and break phases over India, primarily the end of these phases, which are systematically too late in ATM. NEMO and KPP show a more realistic intraseasonal local phase relationship between sea surface temperature (SST) and rainfall throughout the BoB, but no configuration reproduces the observed significant lagged relationship between BoB SST and Indian rainfall. The lack of this relationship may be partly attributed to weak heat flux feedbacks to northern BoB SST, with the forecast short‐wave feedback having systematically the wrong sign (positive) compared to satellite radiation, and thus contributing to SST warming at all lead times. Based on these MetUM forecasts, there is a limited impact of coupling on NWP for monsoon rainfall, both for the mean rainfall and intraseasonal variability. Further research to improve NWP for monsoon rainfall should focus on reducing MetUM atmospheric systematical biases. [ABSTRACT FROM AUTHOR]

The importance of air‐sea coupling in the simulation and prediction of the Madden‐Julian Oscillation (MJO) has been well established. However, it remains unclear how air‐sea coupling modulates the convection and related oceanic features on the subdaily scale. Based on a regional cloud‐permitting coupled model, we evaluated the impact of the air‐sea coupling on the convection during the convectively active phase of the MJO by varying the coupling frequency. The model successfully reproduced the atmospheric and oceanic variations observed by satellite and in situ measurements but with some quantitative biases. According to the sensitivity experiments, we found that stronger convection was mainly caused by the higher sea surface temperatures (SSTs) generated in high‐frequency coupled experiments, especially when the coupling frequency was 1 hr or shorter. A lower coupling frequency would generate the phase lags in the diurnal cycle of SST and related turbulent heat fluxes. Our analyses further demonstrated that the phase‐lagged diurnal cycle of SST suppressed deep convection through a decrease in daytime moistening in the lower troposphere. Meanwhile, in the upper ocean, the high‐frequency air‐sea coupling helped maintain the shallower mixed and isothermal layers by diurnal heating and cooling at the sea surface, which led to a higher mean SST. In contrast, the low‐frequency coupled experiments underestimated the SST and therefore convective activities. Overall, our results demonstrated that high‐frequency air‐sea coupling (1 hr or shorter) could improve the reproducibility of the intensity and temporal variation in both diurnal convection and upper ocean processes. Plain Language Summary: The Madden‐Julian Oscillation (MJO) is one of the most important sources of atmospheric variability in tropical regions; however, even the modern numerical models could not well reproduce the MJO. We believe that the underestimation of air‐sea coupling may cause some parts of such biases in simulating/predicting the MJO. Therefore, our study is aimed to uncover the impact of the air‐sea coupling on convection and the related oceanic feature during the MJO. By varying the air‐sea coupling frequency, our results showed that the 1‐hr or higher‐frequency coupled experiments had better performance due to the well‐reproduced sea surface temperature, while suppressed convection was found in low‐frequency coupled experiments. In general, our study suggested that the high‐frequency air‐sea coupling could improve the reproducibility of both convection and upper ocean features during the MJO. Key Points: High‐frequency air‐sea coupling improves the reproducibility of both convection and upper ocean processesLow‐frequency coupling causes phase lagging in diurnal cycleThe warmer daily mean SST generated by high‐frequency coupling is more responsible for convection than the maximum daytime SST [ABSTRACT FROM AUTHOR]

This study is to quantify the effects of mesoscale eddies on air‐sea heat fluxes and related air‐sea variables in the South China Sea. Using satellite observations of sea surface temperature (SST) and sea surface height anomaly and a high‐resolution air‐sea heat flux product for the 16‐year period from 2000 to 2015, we conducted the composite patterns of air‐sea fluxes and variables associated with anticyclonic eddies (AEs) and cyclonic eddies (CEs). It is found that the SST‐sea surface height correlations over eddies are not always positive. Only 56% of AEs are corresponded with positive SST anomalies (SSTA), that is, SST+ AEs, and 58% of CEs with negative SSTA, that is, SST− CEs. The percentage of these eddies increases with eddy amplitude and shows slight seasonal variations, higher in winter and lower in summer. Composites of SSTA, air‐sea variables, and fluxes are constructed over all eddies, including both SST+ eddies and SST− eddies. All composites show asymmetric patterns, showing that the centers (where the extrema are located) of the fluxes and variables shift westward and poleward (equatorward) relative to the AEs (CEs) cores. Besides, composites of latent heat flux (LHF), sensible heat flux (SHF), and air temperature show monopole patterns, while composites of wind speed and specific humidity show dipole patterns. For SST+ AEs, the coupling strength is 39.6 ± 6.5 W/m2 (7.2 ± 1.7 W/m2) per degree increase of SSTA for LHF (SHF). For SST− CEs, the coupling strength is 39.0 ± 2.0 W/m2 (9.0 ± 0.96 W/m2) per degree decrease of SSTA for LHF (SHF). Key Points: SST‐SSH correlations over eddies are not always positiveComposite heat flux pattern over eddies is asymmetric and similar to SSTCoupling strength between SST and heat flux is 46.8 (48.0) W/m2 per degree over AEs (CEs) [ABSTRACT FROM AUTHOR]

Atmosphere‐ocean feedbacks often improve the Madden‐Julian oscillation (MJO) in climate models, but these improvements are balanced by mean state biases that can degrade the MJO through changing the basic state on which the MJO operates. The Super‐Parameterized Community Atmospheric Model (SPCAM3) produces perhaps the best representation of the MJO among contemporary models, which improves further in a coupled configuration (SPCCSM3) despite considerable mean state biases in tropical sea surface temperatures and rainfall. We implement an atmosphere‐ocean‐mixed‐layer configuration of SPCAM3 (SPCAM3‐KPP) and use a flux‐correction technique to isolate the effects of coupling and mean state biases on the MJO. When constrained to the observed ocean mean state, air‐sea coupling does not substantially alter the MJO in SPCAM3, in contrast to previous studies. When constrained to the SPCCSM ocean mean state, SPCAM3‐KPP fails to produce an MJO, in stark contrast to the strong MJO in SPCCSM3. Further KPP simulations demonstrate that the MJO in SPCCSM3 arises from an overly strong sensitivity to El Niño–Southern Oscillation events. Our results show that simulated interannual variability and coupled‐model mean state biases affect the perceived response of the MJO to coupling. This is particularly concerning in the context of internal variability in coupled models, as many previous MJO sensitivity studies in coupled models used relatively short (20‐ to 50‐year) simulations that undersample interannual‐decadal variability. Diagnosing the effects of coupling on the MJO requires simulations that carefully control for mean state biases and interannual variability. Plain Language Summary: Many studies suggest that feedbacks between the atmosphere and ocean are essential for simulating the Madden‐Julian oscillation, which controls weekly to monthly tropical rainfall. However, model experiments that diagnose the effect of these feedbacks are often not performed in a controlled manner, because introducing atmosphere‐ocean feedbacks changes other aspects of the simulation, including the background climate and phenomena such as the El Niño–Southern Oscillation. These changes may affect the Madden‐Julian oscillation as well, separately from direct atmosphere‐ocean feedbacks, but these changes are often erroneously considered part of the sensitivity of the Madden‐Julian oscillation to atmosphere‐ocean feedbacks. We demonstrate that in one model that simulates the Madden‐Julian oscillation well, the Super‐Parameterized Community Atmospheric Model, intraseasonal atmosphere‐ocean feedbacks alone do not substantially alter the Madden‐Julian oscillation. Rather, the previously perceived improvement to the Madden‐Julian oscillation from atmosphere‐ocean feedbacks arises from a too strong response of the Madden‐Julian oscillation to El Niño events. Future studies must carefully control for how introducing atmosphere‐ocean feedbacks affects other aspects of the simulation, to better isolate the effect of those feedbacks on the Madden‐Julian oscillation. Key Points: The response of the Madden‐Julian oscillation (MJO) to coupling is sensitive to the representation of interannual variabilityDiagnosing intraseasonal coupled feedbacks to the MJO requires controling for interannual variability and mean state biasesAir‐sea coupling is not essential to simulate the MJO in the Super‐Parameterized Community Atmospheric Model [ABSTRACT FROM AUTHOR]

This study introduces a new flow‐dependent distribution sampling (FDDS) scheme for air–sea coupling. The FDDS scheme is implemented in a climate model and used to improve the simulated mean and variability of atmospheric and oceanic surface fields and thus air–sea fluxes. Most coupled circulation models use higher resolutions in the sea ice and ocean compared to the atmospheric model component, thereby explicitly simulating the atmospheric subgrid‐scale at the interface. However, the commonly applied averaging of surface fields and air–sea fluxes tends to smooth fine‐scale structures, such as oceanic fronts. The stochastic FDDS scheme samples the resolved spatial ocean (and sea ice) subgrid distribution that is usually not visible to a coarser‐resolution atmospheric model. Randomly drawn nodal ocean values are passed to the corresponding atmospheric boxes for the calculation of surface fluxes, aiming to enhance surface flux variability. The resulting surface field perturbations of the FDDS scheme are based on resolved dynamics, displaying pronounced seasonality with realistic magnitude. The AWI Climate Model is used to test the scheme on interannual time‐scales. Our set‐up features a high ocean‐to‐atmosphere resolution ratio in the Tropics, with grid‐point ratios of about 60:1. Compared to the default deterministic averaging, changes are largest in the Tropics leading to an improved spatial distribution of precipitation with bias reductions of up to 50%. Enhanced sea‐surface temperature variability in boreal winter further improves the seasonal phase locking of temperature anomalies associated with the El Niño–Southern Oscillation. Mean 2m temperature, sea ice thickness and concentration react with a contrasting dipole pattern between hemispheres but a joint increase of monthly and interannual variability. This first approach to implement a flow‐dependent stochastic coupling scheme shows considerable benefits for simulations of global climate, and various extensions and modifications of the scheme are possible. [ABSTRACT FROM AUTHOR]

We examine the merit of atmosphere–ocean coupled models for tropical cyclone (TC) predictions in the western North Pacific (WNP), where accurate TC predictions remain challenging. The UK Met Office operational atmospheric global numerical weather prediction (NWP) model is compared with two trial coupled configurations, in which the operational atmospheric model is coupled to a one‐dimensional mixed‐layer ocean model and a three‐dimensional dynamical ocean model. Reforecasts for the 2016 TC season show that the coupled models outperform the NWP model for TC location predictions, with a systematic improvement of 50–100 km over the seven‐day forecasts, but the coupled models amplify the underestimation of TC intensity in the NWP model. Nearly identical TC predictions (for both location and intensity) are found in the two coupled models, indicating the dominance of thermodynamic processes at the air–sea interface for TC predictions on these timescales. The improved prediction of the TC position in the coupled models is associated with an enhanced Western North Pacific Subtropical High (WNPSH), which introduces an anticyclonic steering flow anomaly that shifts TC tracks further west in the southern part of the region and further east in the northern part. Based on sensitivity experiments, we show that these improvements in the coupled models are due mainly to colder initial sea‐surface temperatures (SSTs). Air–sea feedbacks do not change the WNPSH or TC tracks noticeably. Apart from the effect of the initial SSTs, tropical ocean warming due to air–sea interaction in the coupled forecasts can also reduce the predicted TC intensity, presumably due to a stronger regional Hadley circulation with increased subtropical subsidence. [ABSTRACT FROM AUTHOR]

The densely populated Bay of Bengal (BoB) rim witnesses the deadliest tropical cyclones (TCs) globally, before and after the summer monsoon. Previous studies indicated that enhanced salinity and reduced thermal stratification reduce cooling under BoB TCs after the monsoon, suggesting that air‐sea coupling may favor stronger TCs during that season. Using observations and simulations from a one fourth degree regional ocean‐atmosphere model, we show that BoB TCs are stronger before the monsoon due to a more favorable large‐scale background state (less vertical wind shear and higher sea surface temperature). Air‐sea coupling however alleviates this background state influence, by reducing the number of premonsoon intense TCs, because of larger cooling and reduced upward enthalpy fluxes below TCs during that season. As the impact of air‐sea interactions on BoB TCs is largest for intense TCs, it should be further investigated for Category 3 and above TCs, which are not reproduced at one fourth degree resolution. Plain language summary: Tropical cyclones that develop in the Bay of Bengal are amongst the most lethal globally, owing to the dense and vulnerable coastal population living along its rim. These cyclones mostly occur in April–May and October–December, that is, before and after the Indian summer monsoon. In this study, we show that there are more intense cyclones before than after the monsoon, because of more favorable large‐scale background conditions. We however also show that more intense interactions between the cyclone and the ocean tend to reduce risks for intense cyclones before the monsoon, hence opposing the effect of background conditions. Since air‐sea coupling clearly contributes to the Bay of Bengal cyclone intensity, in particular before the monsoon, it should be accounted for in operational forecasts. Key Points: Bay of Bengal tropical cyclones are stronger before than after the monsoon, due to more favorable large‐scale background conditionsAir‐sea coupling alleviates the effect of large‐scale background conditions, by inhibiting premonsoon tropical cyclonesThe air‐sea coupling negative feedback on tropical cyclones is weak after the monsoon, due to a fresher, less thermally stratified BoB [ABSTRACT FROM AUTHOR]

The Mediterranean Sea is an important source of heat and moisture for heavy precipitation events (HPEs). Moreover, the ocean mixed layer (OML) evolves rapidly under such intense events. Whereas short-term numerical weather prediction systems generally use low-resolution non-evolving sea surface temperature (SST), the development of high-resolution high-frequency coupled system allows us to fully take into account the fine-scale interactions between the low-level atmosphere and the OML which occur during HPEs. The aim of this study is to investigate the impact of fine-scale air-sea interactions and coupled processes involved during the HPEs which occurred during 12-15 October 2012 (IOP13) and 26-28 October 2012 (IOP16a/b) of the HyMeX first field campaign. For that purpose, the high-resolution coupled system AROME-NEMO WMED was developed. This system is based on the 2.5 km-resolution non-hydrostatic convection-permitting atmospheric model AROME-WMED and the 1/36°-resolution NEMO-WMED36 ocean model. The coupling frequency is 1 h. To distinguish the effects due to the change in the initial SST field from that due to the interactive 3D ocean, the coupled run is compared to two AROME-WMED atmosphere-only experiments with no SST evolution during the 48 h forecast cycles-one using the AROME-WMED SST analysis, the second using the SST field of the coupled experiment each day at 0000 UTC. The results of the three experiments re-assert that the SST initial condition strongly influences the HPE forecast, in terms of intensity and location. With water budget analyses, the significant impact of the ocean interactive evolution on the surface evaporation water supply for HPEs is also highlighted. In cases of strong and intense air-sea exchanges, as in the mistral event of IOP16b, the coupling reproduces the intense and rapid surface cooling and demonstrates the importance of representing the ocean turbulent mixing with entrainment at the OML base. [ABSTRACT FROM AUTHOR]

The north-western Mediterranean Sea is a key location for the thermohaline circulation of the basin. The area is characterized by intense air-sea exchanges favored by the succession of strong northerly and north-westerly wind situations (mistral and tramontane) in autumn and winter. Such meteorological conditions lead to significant evaporation and ocean heat loss that are well known as the main triggering factor for the Dense Water Formation (DWF) and winter deep convection episodes. During the HyMeX second field campaign (SOP2, 1 February to 15 March 2013), several platforms were deployed in the area in order to document the DWF and the ocean deep convection, as the air-sea interface conditions. This study investigates the role of the ocean-atmosphere coupling on DWF during winter 2012-2013. The coupled system, based on the NEMO-WMED36 ocean model (1/36° resolution) and the AROME-WMED atmospheric model (2.5 km resolution), was run during 2 months covering the SOP2 and is compared to an ocean-only simulation forced by AROME-WMED real-time forecasts and to observations collected in the north-western Mediterranean area during the HyMeX SOP2. The comparison shows small differences in terms of net heat, water, and momentum fluxes. On average, DWF is slightly sensitive to air-sea coupling. However, fine-scale ocean processes, such as shelf DWF and export or eddies and fronts at the rim of the convective patch, are significantly modified. The wind-current interactions constitute an efficient coupled process at fine scale, acting as a turbulence propagating vectors, producing large mixing and convection at the rim of the convective patch. [ABSTRACT FROM AUTHOR]

Heavy precipitation events (HPEs) can be a major cause of damage and casualties in the Mediterranean basin. With the use of atmosphere-ocean coupled regional climate models (AORCMs) and the advantage of 24 years of simulation (1989-2012), we identified regions with potential impact of submonthly air-sea coupling on HPEs from among the regions hit by heavy rain during the Hydrological Cycle in the Mediterranean Experiment (HyMeX) first special observation period (SOP1): Valencia in Spain, the Cévennes in Southern France, Liguria in Northwestern Italy, Calabria in Southern Italy and Northeastern Italy. A first evaluation of the two AORCMs (MORCE and CNRM-RCSM4) against gridded precipitation datasets showed that 70-90% of the 30 most intense HPEs simulated were observed HPEs for most regions. The Cévennes, Valencia and Calabria were the only three regions to show a statistical relationship between rain differences and sea-surface temperature (SST) differences where the low-level jets that feed the events most frequently blow. This sensitivity of precipitation to SST changes is due to low-level wind changes related to changes in surface heat fluxes. Based on the calculation of submonthly variations in these regions during HyMeX, HPEs on 28 September 2012 and 12 October 2012 in Valencia and on 25-26 October 2012 in the Cévennes have most probably been affected by submonthly air-sea coupling. [ABSTRACT FROM AUTHOR]