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The El Niño and Southern Oscillation (ENSO), a phenomenon of air–sea coupling in the tropical Pacific, has strong response to global climate change. In this study, the primary region where ENSO occurred during the period 1955–2020 was selected as the key ENSO region, and the changes in air–sea coupling in this region were explored. The New Southern Oscillation Index (NSOI), modified from the previous Southern Oscillation Index, represents atmospheric changes, and the Niño-3.4 index represents oceanic changes. The absolute value of the running correlation coefficient between the Niño-3.4 index and NSOI in the 121-month time window was defined as the Intensity of Air–Sea Coupling (IASC) in the key ENSO region. The results showed that the IASC has significantly increased, with a confidence level of 95%, during the period 1955–2020, and the range where the correlation coefficient between the Niño-3.4 index and the sea level pressure anomaly over the key ENSO region was greater than 0.6 has evidently expanded in the context of global warming, which corresponded to the increase in IASC. Moreover, the coupling positions of sea surface temperature and wind anomalies changed, tending to the east of the equatorial Pacific during 1977–1998, and to the west during 1999–2020.

Les modèles de climat constituent un laboratoire numérique pour mieux comprendre le système climatique et en proposer des projections. La modélisation du climat nécessite de représenter toutes les composantes qui interagissent à ces échelles de temps : atmosphère, océan, biosphère et cryosphère. Le couplage de toutes ces composantes est nécessaire pour représenter les flux de masse et d'énergie qui régissent ces interactions. Les modèles sont imparfaits mais nous poursuivons constamment leur développement en vue de les rendre le plus réaliste possible. Au fur et à mesure que nos connaissances progressent et que les capacités des supercalculateurs s’accroissent, les modèles se font plus précis et représentent plus de processus.Mes activités de recherche accompagnent l’évolution du modèle de climat CNRM-CM, développé conjointement par le CNRM et le Cerfacs. Elles illustrent ce processus de développement d’un modèle de climat sur deux décennies. D’une part, mes travaux ont contribué à l’amélioration de la modélisation du couplage entre les composantes du système climatique. D’autre part, je mène des études ciblées sur la représentation de processus couplés océan-atmosphère. Ces études ont un double objectif, mieux comprendre le laboratoire numérique pour guider les voies de développement futures et ainsi concourir à son amélioration, mais aussi faire avancer nos connaissances sur les processus couplés qui régissent le système climatique. Dans ce mémoire, je retrace une partie de mes travaux en suivant ces deux axes : je présente dans un premier chapitre certains de mes travaux qui ont contribué à l’amélioration du modèle et dans un deuxième chapitre, des études plus spécifiques qui ont permis de faire avancer notre compréhension des processus couplés qui régissent le système climatique. Dans cette deuxième partie, je mets plus particulièrement en avant les travaux réalisés via l’encadrement d’étudiants (stages, thèses et post-doctorats).La communauté des modélisateurs du climat fait le constat qu’il est de plus en plus difficile d’accroître les performances des modèles, d’une part parce que l’accroissement de la complexité ne les rend pas forcément plus réalistes, mais aussi parce que notre marge de progression semble plus ténue. D’ailleurs, que veut dire améliorer leurs performances ? Cherche-t-on à développer un modèle universel qui permette d’étudier tous les processus climatiques ou bien devons-nous développer autant de modèles que d’applications ? Pour progresser, notre communauté se retrouve face à de nouveaux défis qu’elle tente de relever. Pour ma part, je propose de revenir sur la méthode de calibration des modèles. En effet, la calibration a été, jusque là, réalisée de façon heuristique et est peu documentée. Améliorer cette étape en développant une méthode objective permettra de renforcer notre confiance dans les modèles, de mieux qualifier leurs incertitudes paramétriques mais également d’affiner notre mise en œuvre de modèles à plus haute résolution.

An atmosphere-only model system for making seasonal prediction and projecting future intensities of landfalling tropical cyclones (TCs) along the South China coast is upgraded by including ocean and wave models. A total of 642 TCs have been re-simulated using the new system to produce a climatology of TC intensity in the South China Sea. Detailed comparisons of the simulations from the atmosphere-only and the fully coupled systems reveal that the inclusion of the additional ocean and wave models enable differential sea surface temperature responses to various TC characteristics such as translational speed and size. In particular, interaction with the ocean does not necessarily imply a weakening of the TC, with the coastal bathymetry possibly playing a role in causing a near-shore intensification of the TC. These results suggest that to simulate the evolution of TC structure more accurately, it is essential to use an air-sea coupled model instead of an atmosphere-only model.

The North Atlantic Oscillation (NAO) is a major climatic phenomenon in the Northern Hemisphere, but the underlying air–sea interaction and physical mechanisms remain elusive. Despite successful short-term forecasts using physics-based numerical models, longer-term forecasts of NAO continue to pose a challenge. In this study, we employ advanced data-driven causal discovery techniques to explore the causality between multiple ocean–atmosphere processes and NAO. We identify the best NAO predictors based on this causality analysis and develop NAO-MCD, a multivariate air–sea coupled model that incorporates causal discovery to provide 1–6 month lead seasonal forecasts of NAO. Our results demonstrate that the selected predictors are strongly associated with NAO development, enabling accurate forecasts of NAO. NAO-MCD significantly outperforms conventional numerical models and provides reliable seasonal forecasts of NAO, particularly for winter events. Moreover, our model extends the range of accurate forecasts, surpassing state-of-the-art performance at 2- to 6-month lead-time NAO forecasts, substantially outperforming conventional numerical models.

The tropical Atlantic Ocean receives an important freshwater supply from river runoff and from precipitation in the intertropical convergence zone. It results in a strong salinity stratification that may influence vertical mixing, and thus sea surface temperature (SST) and air–sea fluxes. The aim of this study is to assess the impact of salinity stratification on the tropical Atlantic surface variables. This is achieved through comparison among regional 1/4 $$^{\circ }$$ coupled ocean–atmosphere simulations for which the contribution of salinity stratification in the vertical mixing scheme is included or discarded. The analysis reveals that the strong salinity stratification in the northwestern tropical Atlantic induces a significant increase of SST (0.2 $$^{\circ }$$ C–0.5 $$^{\circ }$$ C) and rainfall (+ 19%) in summer, hereby intensifying the ocean–atmosphere water cycle, despite a negative atmospheric feedback. Indeed, the atmosphere dampens the oceanic response through an increase in latent heat loss and a reduction of shortwave radiation reaching the ocean surface. In winter, the impacts of salinity stratification are much weaker, most probably because of a deeper mixed layer at this time. In the equatorial region, we found that salinity stratification induces a year-round shoaling of the thermocline, reinforcing the cold tongue cool anomaly in summer. The concept of barrier layer has not been identified as relevant to explain the SST response to salinity stratification in our region of interest.

International audience; The Indian Ocean Dipole (IOD) is one of the dominant modes of variability of the tropical Indian Ocean and it has been suggested to have a crucial role in the teleconnection between the Indian summer monsoon and El Niño Southern Oscillation (ENSO). The main ideas at the base of the influence of the IOD on the ENSO-monsoon teleconnection include the possibility that it may strengthen summer rainfall over India, as well as the opposite, and also that it may produce a remote forcing on ENSO itself. In the future, the IOD is projected to increase in frequency and amplitude with mean conditions mimicking the characteristics of its positive phase. Still, state-of-the-art global climate models have large biases in representing mean state and variability of both IOD and ISM, with potential consequences for their future projections. However, the characteristics of the IOD and ENSO are likely to continue in a future warmer world, with a persistence of their linkage.

Using the observational data and the Coupled Model Intercomparison Project Phase 5 (CMIP5) models to show the influence of the North Pacific Victoria mode (VM) on the Madden–Julian Oscillation (MJO) are examined in this analysis. The results show that the February–April (FMA) VM had a significant influence on the development and propagation of the MJO over the equatorial central–western Pacific (ECWP) during spring (March–May) between 1979 and 2017. Specifically, MJO development was favored more by positive VM events than negative VM events. One probably description for these complicated connections is that the SST gradient anomalies associated with positive VM events enhance the convergence of low-level over the ECWP, which, combined with the warm SST anomalies (SSTAs) in the equatorial central Pacific that lead to a boost in the Kelvin wave anomalies, results in the enhanced MJO activity over the ECWP. These conclusions indicate that the VM is an important factor in MJO diversity.

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.

International audience; 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.

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.

The effects of air–sea coupling over the tropical Indian Ocean (TIO) on the eastward propagating boreal winter intraseasonal oscillation (MJO) are investigated by comparing a fully coupled and a partially decoupled Indian Ocean experiment using the SINTEX-F coupled model. Air–sea coupling over the TIO significantly enhances the intensity of the eastward propagations of the MJO along the 5°–10°S zonal areas. The zonal asymmetry of the SST anomaly (SSTA) is responsible for the enhanced eastward propagation. A positive SSTA appears to the east of the MJO convection, which results in the boundary layer moisture convergence and positively feeds back to the MJO convection. In addition, the air–sea interaction effect on the eastward propagation of the MJO is related to the interannual variations of the TIO. Air–sea coupling enhances (reduces) the eastward-propagating spectrum during the negative Indian Ocean dipole mode and positive Indian Ocean basin mode. Such phase dependence is attributed to the role of the background mean westerly in affecting the wind–evaporation–SST feedback. Air–sea coupling (decoupling) enhances (reduces) the zonal asymmetry of the low-level specific humidity, and thus the eastward propagation spectrum of the MJO.

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.

This paper assesses the impact of air–sea coupling on Indian Ocean tropical cyclones (TCs) by comparing a 20-year long simulation of a ¼° regional coupled ocean–atmosphere model with a twin experiment, where the atmospheric component is forced by sea surface temperature from the coupled simulation. The coupled simulation reproduces the observed spatio-temporal TCs distribution and TC-induced surface cooling reasonably well, but overestimates the number of TCs. Air–sea coupling does not affect the cyclogenesis spatial distribution but reduces the number of TCs by ~ 20% and yields a better-resolved bimodal seasonal distribution in the northern hemisphere. Coupling also affects intensity distribution, inducing a four-fold decrease in the proportion of intense TCs (Cat-2 and stronger). Air–sea coupling damps TCs growth through a reduction of inner-core upward enthalpy fluxes due to the TC-induced cooling. This reduction is particularly large for the most intense TCs of the northern Indian Ocean (up to 250 W m−2), due to higher ambient surface temperatures and larger TC-induced cooling there. The negative feedback of air–sea coupling on strongest TCs is mainly associated with slow-moving storms, which spend more time over the cold wake they induce. Sensitivity experiments using a different convective parameterization yield qualitatively similar results, with a larger (~ 65%) reduction in the number of TCs. Because of their relatively coarse resolution (¼°), both set of experiments however fail to reproduce the most intense observed TCs. Further studies with finer resolution models in the Bay of Bengal will be needed to assess the expectedly large impact of air–sea coupling on those intense and deadly TCs.

Simulations of a Regional Climate Model (RCM) driven by identical lateral boundary conditions but initialized at different times exhibit the phenomenon of so-called internal model variability (or in short, Internal Variability&mdash, IV), which is defined as the inter-member spread between members in an ensemble of simulations. Our study investigates the effects of air-sea coupling on IV of the regional atmospheric model COSMO-CLM (CCLM) of the new regional coupled system model GCOAST-AHOI (Geesthacht Coupled cOAstal model SysTem: Atmosphere, Hydrology, Ocean and Sea Ice). We specifically address physical processes parameterized in CCLM, which may cause a large IV during an extreme event, and where this IV is affected by the air-sea coupling. Two six-member ensemble simulations were conducted with GCOAST-AHOI and the stand-alone CCLM (CCLM_ctr) for a period of 1 September&ndash, 31 December 2013 over Europe. IV is expressed by spreads within the two sets of ensembles. Analyses focus on specific events during this period, especially on the storm Christian occurring from 27 to 29 October 2013 in northern Europe. Results show that simulations of CCLM_ctr vary largely amongst ensemble members during the storm. By analyzing two members of CCLM_ctr with opposite behaviors, we found that the large uncertainty in CCLM_ctr is caused by a combination of two factors (1) uncertainty in parameterization of cloud-radiation interaction in the atmospheric model. and (2) lack of an active two-way air-sea interaction. When CCLM is two-way coupled with the ocean model, the ensemble means of GCOAST-AHOI and CCLM_ctr are relatively similar, but the spread is reduced remarkably in GCOAST-AHOI, not only over the ocean where the coupling is done but also over land due to the land-sea interactions.

International audience; This work assesses the sensitivity of the offshore wind energy density and production to SST biases and air-sea feedbacks along the French and Spanish coast in the North-Western Mediterranean. It makes use of a set of three 20-years simulations from atmosphere stand-alone and atmosphere/ocean coupled models. In this numerical experiment, the effects of SST bias and air-sea feedbacks are isolated without any interference with other sources of error and uncertainty propagation, meaning that the same model and hence the same physics are used, all other things being equal. This study shows that the effects of SST bias or air-sea feedbacks on wind energy density and production estimation can reach up to 10% and 5%, respectively. Especially, accounting for air-sea coupled processes on sub-monthly time scales weakens systematically the energy density and production by 6.5% and 2.4% with respect to the configuration where these effects are neglected. The relative variability over the 20 years of simulation does not exceed 20% so the impact of air-sea feedbacks is very robust in time in terms of wind energy density and production assessment. Uncertainties up to 6.5 and 2.5% in the evaluation of the potential in terms of wind energy density and production potential can have severe consequences on the whole industry by lowering the projects profitability. This study shows that the effects of SST bias and air-sea feedbacks usually extend vertically up to the hub-height but their magnitude depends on the stability of the atmosphere. This study concludes that reducing the SST bias at the lower boundary of numerical atmospheric models and accounting for rapid interactions and feedbacks between the ocean and the atmosphere are key to improve the reliability of offshore wind energy density and production assessment.

temporary reference; This PhD thesis studies the air/sea interactions at the oceanic mesoscale (10-300km) in the South-East Pacific and their consequences on the oceanic mesoscale activity.Satellite observations and a high-resolution regional ocean-atmosphere coupled model areused to characterize the mesoscale Sea Surface Temperature (SST)-wind stress (WS) in-teractions. The intensity of the WS response to SST displays similar spatial and seasonalvariability both in the model and the observations. The simulation is further analyzed tostudy this variations and to understand the boundary layer adjustment mechanisms. A mo-mentum balance evidenced that the near surface wind anomalies are created by anomaliesof the turbulent mixing term.The consequences on the ocean dynamics of the modulation of air-sea momentum, heatand fresh water fluxes by mesoscale SST and surface current are investigated using a setof sensitivity experiments. On one hand, near the coast, the WS response to the upwellingSST front decreases both the upwelling and the eddy kinetic energy (EKE) generationthrough baroclinic conversion. A negative feedback of the atmospheric response on theSST anomalies amplitude is also evidenced. On the other hand, the WS modulation byoceanic surface currents decreases the EKE generation by the mesoscale wind work. It alsocreates an Ekman pumping attenuating the sea surface height anomalies associated to thecoherent eddies.; Cette thèse s’intéresse aux interactions entre l’océan et l’atmosphère dans lePacifique Sud-Est, à des échelles comprises entre 10 et 300 km ("meso-échelle" océanique).Des observations satellites et un modèle couplé à haute résolution (1/12°) sont utiliséspour caractériser la relation entre la meso-échelle de température de surface de la mer(SST) et celle de l’intensité de la tension de vent (TV). L’intensité de la réponse de laTV aux anomalies de SST présente des variations spatiales et saisonnière marquées. Uneanalyse de l’ajustement de la couche limite atmosphérique aux anomalies de de SST dansles simulations numériques permet d’expliquer l’origine et l’intensité des variations de TVet de vitesse du vent. L’étude du bilan de quantité de mouvement montre que les anomaliesde la vitesse du vent près de la surface sont créées par les anomalies du terme de mélangeturbulent.Le modèle couplé a également permis d’étudier la réponse océanique à la modulation desflux à l’interface air-mer par la meso-échelle de courant de surface et de SST. D’une part,près de la côte, la réponse de la TV à la présence du front de SST diminue l’intensité del’upwelling et la génération d’énergie cinétique turbulente (EKE) par instabilité barocline.La réponse de l’atmosphère à la meso-échelle de SST a également une rétroaction négativesur les anomalies de SST. D’autre part, la modulation de la TV par les courants de surfacediminue la génération d’EKE due au travail des anomalies de TV, et créé un pompaged’Ekman atténuant les anomalies de niveau de la mer associées aux tourbillons meso-échelle.

temporary reference; This PhD thesis studies the air/sea interactions at the oceanic mesoscale (10-300km) in the South-East Pacific and their consequences on the oceanic mesoscale activity.Satellite observations and a high-resolution regional ocean-atmosphere coupled model areused to characterize the mesoscale Sea Surface Temperature (SST)-wind stress (WS) in-teractions. The intensity of the WS response to SST displays similar spatial and seasonalvariability both in the model and the observations. The simulation is further analyzed tostudy this variations and to understand the boundary layer adjustment mechanisms. A mo-mentum balance evidenced that the near surface wind anomalies are created by anomaliesof the turbulent mixing term.The consequences on the ocean dynamics of the modulation of air-sea momentum, heatand fresh water fluxes by mesoscale SST and surface current are investigated using a setof sensitivity experiments. On one hand, near the coast, the WS response to the upwellingSST front decreases both the upwelling and the eddy kinetic energy (EKE) generationthrough baroclinic conversion. A negative feedback of the atmospheric response on theSST anomalies amplitude is also evidenced. On the other hand, the WS modulation byoceanic surface currents decreases the EKE generation by the mesoscale wind work. It alsocreates an Ekman pumping attenuating the sea surface height anomalies associated to thecoherent eddies.; Cette thèse s’intéresse aux interactions entre l’océan et l’atmosphère dans lePacifique Sud-Est, à des échelles comprises entre 10 et 300 km ("meso-échelle" océanique).Des observations satellites et un modèle couplé à haute résolution (1/12°) sont utiliséspour caractériser la relation entre la meso-échelle de température de surface de la mer(SST) et celle de l’intensité de la tension de vent (TV). L’intensité de la réponse de laTV aux anomalies de SST présente des variations spatiales et saisonnière marquées. Uneanalyse de l’ajustement de la couche limite atmosphérique aux anomalies de de SST dansles simulations numériques permet d’expliquer l’origine et l’intensité des variations de TVet de vitesse du vent. L’étude du bilan de quantité de mouvement montre que les anomaliesde la vitesse du vent près de la surface sont créées par les anomalies du terme de mélangeturbulent.Le modèle couplé a également permis d’étudier la réponse océanique à la modulation desflux à l’interface air-mer par la meso-échelle de courant de surface et de SST. D’une part,près de la côte, la réponse de la TV à la présence du front de SST diminue l’intensité del’upwelling et la génération d’énergie cinétique turbulente (EKE) par instabilité barocline.La réponse de l’atmosphère à la meso-échelle de SST a également une rétroaction négativesur les anomalies de SST. D’autre part, la modulation de la TV par les courants de surfacediminue la génération d’EKE due au travail des anomalies de TV, et créé un pompaged’Ekman atténuant les anomalies de niveau de la mer associées aux tourbillons meso-échelle.

International audience; The SAMM (Sea Atmosphere Mediterranean Model) has been developed to study the climate evolution of the Mediterranean and European regions for the 21st Century. SAMM is a new concept of AORCM (Atmosphere-Ocean Regional Climate Model), where a global atmosphere model is locally coupled with a regional ocean circulation model. It consists of the global spectral AGCM ARPEGE-Climate model, whose variable resolution is maximum in the Mediterranean region (50 km), which has been coupled to the Mediterranean Sea limited area OGCM OPAMED (10 km). A 140-year numerical experiment starting in 1960 was run with the AORCM. Up to year 2000, forcing was prescribed from observed values, whereas forcing following a SRES-A2 scenario was applied beyond 2000. In order to ensure the model stability, a simple monthly heat flux correction on air-sea exchanges was applied. The present-climate validation proves that the AORCM is comparable to the state-of-the-art European Atmosphere Regional Climate Models (ARCM) at the same resolution. At first order, the climate change impact over Europe simulated by the AORCM is comparable with ARCM simulations. However the AORCM significantly amplifies the climate change signal over large parts of Europe with respect to the corresponding ARCM: the warming is higher in all seasons and in many areas of Europe (up to 25% of the signal), winters are wetter over northern Europe and summers drier over southern and eastern Europe (up to 50% of the signal). These differences are highly significant and the choice between coupled and non-coupled regional models could be an additional source of uncertainty when evaluating the climate change response over Europe. The factors responsible for these differences are discussed. Among them, the response of the Mediterranean SST, better simulated by the high resolution Mediterranean Sea model of the AORCM, seems to be preponderant. Further mechanism studies and model inter-comparisons are however required to legitimate the present results.

International audience; 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 among the regions hit by heavy rain during HyMeX 1st 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 to 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 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 with changes in surface heat fluxes. Based on the calculation of submonthly variations in these regions during HyMeX, HPEs on the 28/09/2012 and 12/10/2012 in Valencia and on 25-26/10/2012 in the Cévennes have most probably been affected by submonthly air-sea coupling.

International audience; The Mediterranean Sea water budget (MWB) is a key parameter, as it controls the Mediterranean Sea water loss and thus the Atlantic Water inflow and the Mediterranean general circulation. More accurately, the MWB controls the net flow through the Strait of Gibraltar, which implies both inflow and outflow. Generally considered at the basin scale and over long-term periods, the MWB is in fact characterized by a large variability in space and time, induced by the complex topography of the region, mesoscale processes and (short) intense events in the ocean and atmosphere compartments. In this study, we use an ocean-atmosphere coupled system at mesoscale able to represent such phenomena, to evaluate the MWB atmospheric components: Evaporation (E) and Precipitation (P). We compare two companion regional simulations: an uncoupled atmospheric run using the ERA-interim Sea Surface Temperature (SST) reanalysis and a coupled run using the MORCE system with the two-way coupling between the NEMO-MED12 eddy-resolving ocean model and the non-hydrostatic Weather Research and Forecasting atmospheric model. We first evaluate the SST validity against satellite data and evidence the coupled system ability in representing SST mesoscale structures, characteristics of the Mediterranean circulation and of small-scale ocean processes, despite a colder mean value and a lower amplitude of the annual cycle. Then, the comparison aims to examine the coupled processes effects (meaning the impacts of the interactive high-resolution and high-frequency SST) on E and P and on their variability. The comparison highlights that the SST is the controlling factor for E and P budgets, with reduction by 6 and 3 % in the coupled run compared to the uncoupled run, respectively. The modifications propagate until 750 km inland far from the Mediterranean coast, as towards the Atlantic Ocean and the Black Sea. This indicates that coupling plays a major role in distributing water at mesoscale. The coupling directly modifies the seasonal variability. It also significantly decreases extreme evaporation and precipitation occurences and intensities. This extreme events mitigation in the coupled run contributes for ~50 % to the decrease in the E and P budgets.

International audience; This study presents the first multidecadal and coupled regional simulation of cyclonic activity in the South Pacific. The long-term integration of state-of the art models provides reliable statistics, missing in usual event studies, of air-sea coupling processes controlling tropical cyclone (TC) intensity. The coupling effect is analyzed through comparison of the coupled model with a companion forced experiment. Cyclogenesis patterns in the coupled model are closer to observations with reduced cyclogenesis in the Coral Sea. This provides novel evidence of air-sea coupling impacting not only intensity but also spatial cyclogenesis distribution. Storm-induced cooling and consequent negative feedback is stronger for regions of shallow mixed layers and thin or absent barrier layers as in the Coral Sea. The statistical effect of oceanic mesoscale eddies on TC intensity (crossing over them 20 % of the time) is also evidenced. Anticyclonic eddies provide an insulating effect against storm-induced upwelling and mixing and appear to reduce sea surface temperature (SST) cooling. Cyclonic eddies on the contrary tend to promote strong cooling, particularly through storm-induced upwelling. Air-sea coupling is shown to have a significant role on the intensification process but the sensitivity of TCs to SST cooling is nonlinear and generally lower than predicted by thermodynamic theories: about 15 rather than over 30 hPa °C−1 and only for strong cooling. The reason is that the cooling effect is not instantaneous but accumulated over time within the TC inner-core. These results thus contradict the classical evaporation-wind feedback process as being essential to intensification and rather emphasize the role of macro-scale dynamics.

This study examines the respective influences of Pacific and Indian Ocean couplings on tropical intraseasonal oscillation (ISO) in boreal winter (November–April). Three basin-coupling experiments are performed with a coupled atmosphere-ocean general circulation model (CGCM), in which air-sea coupling is limited respectively to the Indian Ocean, the Pacific Ocean, and both the oceans. The modelling results show that zonal ISO propagations can be found in the Indo-Pacific region with or without the ocean coupling; however, the propagation signals are enhanced by the coupling. In this particular model the Pacific Ocean coupling has a stronger influence on the ISO propagation than the Indian Ocean coupling. Without the Pacific coupling, the ISO propagation signal is weakened significantly when it enters the Pacific Ocean sector. Without the Indian Ocean coupling, the simulated ISO intensity is weakened less in the Indain Ocean, and significant zonal propagations of ISO can still be simulated in the sector. The relative importance of the ocean coupling is likely related to the smaller ISO-related sea surface temperature (SST) anomalies in the Indian Ocean than in the Pacific Ocean. In this particular CGCM, the ocean coupling affects the ISO propagation mainly through a wind-evaporation-SST feedback rather than a cloud-radiation-SST feedback, while in observations both processes are equally important. To further confirm the importance of the ocean coupling, forced atmospheric GCM experiments are performed with SSTs prescribed from the climatologies produced in the basin-coupling experiments. It is found that the eastward propagating feature seen in the CGCM experiments is weakened and dominated by the strong standing feature in the forced AGCM runs. The difference demonstrates the contribution of the ocean coupling to the ISO propagation, and also confirm that biases in the mean SST and low-level wind caused by the ocean coupling are responsible for the spurious standing ISO oscillation feature in the central Pacific and Indian Oceans.

International audience; The near-sea surface meteorological conditions associated with the Mediterranean heavy precipitation events constitute, on a short time scale, a strong forcing on the ocean mixed layer. This study addresses the question of the optimal time frequency of the atmospheric forcing to drive an ocean model in order to make it able to capture the fine scale ocean mixed layer response to severe meteorological conditions. The coupling time frequency should allow the ocean model to reproduce the formation of internal low-salty boundary layers due to sudden input of intense precipitation, as well as the cooling and deepening of the ocean mixed layer through large latent heat fluxes and stress under the intense low-level jet associated with these events. In this study, the one-dimensional ocean model is driven by 2.4-km atmospheric simulated fields on a case of Mediterranean heavy precipitation, varying the time resolution of the atmospheric forcing. The results show that using a finer temporal resolution than 1 h for the atmospheric forcing is not necessary, but a coarser temporal resolution (3 or 6 h) modifies the event course and intensity perceived by the ocean. Consequently, when using a too coarse temporal resolution forcing, typically 6 h, the ocean model fails to reproduce the ocean mixed layer fine scale response under the heavy rainfall pulses and the strong wind gusts.

International audience; South-eastern France is prone to heavy rain events during the autumn. For these extreme precipitation events, the Mediterranean Sea fuels the atmospheric boundary layer in heat and moisture and sometimes contributes to flooding, owing to the large swell and waves produced in these situations. The aim of this study is to examine how the severe atmospheric conditions over the sea associated with these events alter the ocean mixed layer, and what feedback the ocean contributes to the precipitation events. To address these questions, a high-resolution air-sea coupled system is developed between the atmospheric MESO-NH model and a one-dimensional ocean model. It is applied for short range (24 h) and high-resolution (2-3 km) simulations of three representative torrential rainfall events: 12-13 November 1999 (Aude case), 8-9 September 2002 (Gard case) and 3 December 2003 (Hérault case). In those meteorological situations characterized by moderate to intense low-level winds, the Mediterranean Sea globally loses energy, to the benefit of the convective precipitating systems. The result is an overall decrease of the thermal content all along the simulation of the events. Significant cooling and deepening of the ocean mixed layer are found in the areas of intense low-level winds. A notable result of the study concerns the impact of the torrential rainfall on the ocean mixed layer. The most important disturbances of the ocean mixed layer are indeed found underneath the heavy precipitation. The salt content is decreased all along the ocean mixed layer depth, but more significantly in the first ten metres near the air-sea interface, with the formation of a salt barrier. By performing both two-way and one-way coupled simulations, it is found that the interactive coupling tends to moderate both the atmospheric and ocean responses compared with the one-way mode. The differences between the two-way and one-way coupled simulations are, however, found to be relatively small considering the atmospheric short-range forecast.

Large-amplitude seiches (up to 40 cm) were recorded at a tide-gauge inside Bakar Bay from 19–21 August 2006. They were analysed using available tide-gauge and meteorological (air pressure and wind, vertical sounding) data. Sea level and air pressure spectra showed simultaneous increases of energy in the periods in which the seiches of both Bakar Bay (about 20 min) and Rijeka Bay (ca. 2 h) occurred. The propagation speed and direction as well as the horizontal span of the four pronounced pressure disturbances, favourable for coupling with Rijeka Bay waters via the Proudman resonance, were determined. It is argued that only short atmospheric pressure disturbances may generate significant barotropic sea waves in a small basin such as Rijeka Bay; once generated they are amplified when hitting Bakar Bay due to the harbour resonance. Furthermore, examination of the synoptic situation and upper-air soundings revealed the presence of dynamically unstable layer at a height of around 2000 m which was also the height of temperature inversion that developed as a consequence of an advection of warm African air from the south-east. During the afternoon hours of 19 August 2006 wind speed in this layer was close to the estimated speed of propagation of the air pressure disturbances which, together with dynamical instability, made conditions favourable for long distance propagation and trapping of atmospheric gravity waves., Seši iznimne amplitude (do 40 cm) zabilježeni su u nekoliko navrata na mareografskoj postaji u Bakarskom zaljevu između 19. i 21. kolovoza 2006. Analizirani su dostupni mareografski (razina mora) i meteorološki (tlak zraka, brzina i smjer vjetra, radio-sondažna mjerenja) podaci. Spektralna analiza vremenskih nizova razine mora i tlaka zraka ukazala je na istovremeni porast energije na periodima koji uključuju periode seša Bakarskog (oko 20 min) i Riječkog zaljeva (oko 2 h). Izračunati su brzina i smjer rasprostiranja te horizontalna širina četiri poremećaja u polju tlaka zraka koji su Proudmanovom rezonancom mogli djelovati na Riječki zaljev. Konačno, zaključeno je da su samo kratki poremećaji u polju tlaka zraka mogli generirati značajne barotropne valove u zaljevu ograničenih dimenzija kakav je Riječki zaljev, te da su se ti valovi dodatno pojačali u Bakarskom zaljevu mehanizmom lučke rezonance. Uz to, analiza sinoptičke situacije i radio-sondažnih mjerenja ukazala je da su na visini od oko 2000 m postojali dinamički nestabilan sloj te temperaturna inverzija koja se razvila zbog advekcije toplog afričkog zraka. Tijekom popodnevnih sati 19. kolovoza brzina vjetra u ovom sloju je bila bliska izračunatoj brzini rasprostiranja poremećaja u polju tlaka zraka. Ovakvo podudaranje brzina uz postojanje dinamički nestabilnog sloja pogoduje dugotrajnom rasprostiranju i zarobljavanju atmosferskih težinskih valova.

Large-amplitude seiches (up to 40 cm) were recorded at a tide-gauge inside Bakar Bay during 19 – 21 August 2006. They were analysed using available tide-gauge and meteorological (air pressure and wind, vertical sounding) data. Sea level and air pressure spectra showed simultaneous increase of energy at the periods containing those of both Bakar Bay (about 20 min) and Rijeka Bay (ca. 2 h) seiches. The propagation speed and direction as well as the horizontal span of the four pronounced pressure disturbances, favourable for the coupling with the Rijeka Bay waters via the Proudman resonance, were determined. It is argued that only short atmospheric pressure disturbances may generate significant barotropic sea waves in a small basin such as Rijeka Bay ; once generated they are amplified when hitting Bakar Bay due to the harbour resonance. Furthermore, examination of synoptic situation and upper-air soundings revealed a presence of dynamically unstable layer at the height of around 2000 m, being also the height of temperature inversion that developed as a consequence of an advection of warm African air from the south-east. During afternoon hours of 19 August 2006 wind speed in this layer was close to the estimated speed of propagation of the air pressure disturbances, which together with dynamical instability made conditions favourable for long distance propagation and trapping of atmospheric gravity waves.

[1] CTD data were collected weekly during 20 cruises from May to September 2003 over a 19 station grid in a coastal region of the northeastern Adriatic Sea as part of the Adriatic Sea Integrated Coastal Areas and River Basin Management System Pilot Project (ADRICOSM). Relative geostrophic velocities indicated fine scale circulation patterns in the area consisting of two pronounced circulation cells, of cyclonic and anticyclonic character, and that were present in the area for most of the investigated period. These motions were induced primarily by spatial variations in the temperature field and may be strengthened during episodes of very strong northeasterly bora wind, when the investigated region is not heated (cooled) uniformly.

Air-sea fluxes, open-sea deep convection and cyclogenesis are studied in the Mediterraneanwith the development of a regional coupled model (AORCM). It accurately simulatesthese processes and their climate variabilities are quantified and studied. The regionalcoupling shows a significant impact on the number of winter intense cyclogenesis as wellas on associated air-sea fluxes and precipitation. A lower interannual variability than innon-coupled models is simulated for fluxes and deep convection. The feedbacks drivingthis variability are understood. The climate change response is then analysed for the 21stcentury with the non-coupled models : cyclogenesis decreases, associated precipitationincreases in spring and autumn and decreases in summer. Moreover, a warming and saltingof the Mediterranean as well as a strong weakening of its thermohaline circulation occur.This study also concludes with the necessity of using AORCMs to assess climate changeimpacts on the Mediterranean.; Les flux air-mer, la convection profonde et la cyclogénèse sont étudiés en Méditerranéegrâce au développement d'un modèle régional couplé (AORCM). Il reproduit correctementces processus et permet de quantifier et d'étudier leur variabilité climatique. Le couplagerégional a un impact significatif sur le nombre de cyclogénèses intenses en hiver et surles flux et précipitations associés. Il simule une variabilité interannuelle plus faible qu'enmode forcé pour les flux et la convection et permet de comprendre les rétroactionsqui la pilotent. L'impact régional d'un scénario climatique est analysé avec les modèlesnon-couplés : le nombre de cyclogénèses diminue, les pluies associées augmentent auprintemps et en automne et diminuent en été. En outre, la Méditerranée se réchauffe,se sale et sa circulation thermohaline s'affaiblit fortement. Cette thèse conclut de plus àla nécessité des AORCMs pour étudier l'impact du changement climatique en Méditerranée.

International audience; A simplified coupled ocean–atmosphere model, consisting of a one-layer bidimensional ocean model and a one-layer unidimensional energy balance atmospheric model [J. Clim. 13 (2000) 232] is used to study the unstable interactions between zonal winds and ocean gyres. In a specific range of parameters, decadal variability is found. Anomalies, quite homogeneous zonally, show small-scale wavelength in latitude: perturbations emerge and grow at the southern limb of the intergyre boundary and propagate southward before decaying. The wind stress anomalies are proportional to the meridional gradient of the atmospheric temperature anomalies: this ratio acts as a positive amplification factor, as confirmed by a parameter sensitivity analysis. Assuming zonally-averaged anomalies harmonic in the meridional direction, a very simple analytical model for the perturbations is derived, based on forced Rossby wave adjustment of the western boundary current and its associated anomalous heat transport: it accounts for the scale selection, the growth and the southward propagation of sea surface temperature anomalies in the subtropical gyre. The latter is not only due to the slow advection by the mean current, but to a prevailing mechanism of self-advecting coupled oceanic and atmospheric waves, out of phase in latitude. Relevance to the observational record is discussed.

14 pages, 14 figures.-- Full-text version available Open Access at: http://www.iim.csic.es/~barton/html/pdfs.html, Cape Finisterre is the most northwest point in the Galician region and separates the meridional west and zonal north coasts of Galicia. The wind field there has high spatial and temporal variability throughout the year. No clear seasonal signal is evident; upwelling and nonupwelling patterns alternate in all seasons. Two outstanding features of the Galician region are persistent upwelling near Cape Finisterre even when not present farther south and alternation of strong upwelling on north and west coasts. Up to now, explanations have relied upon particular dynamics of oceanic flow past Finisterre. We find that major features of upwelling around Finisterre are related to strong spatial structure in the wind field. Analysis of QuikScat wind data for July 1999 to May 2001 shows strong repeatable patterns in the synoptic wind field. These wind patterns emerge as the combination of the two dominant modes in a complex empirical orthogonal function (CEOF) analysis representing over 85% of the variance. Summer wind patterns give rise to characteristic distributions of upwelling along the coast and favor development of filaments in particular locations. The wind measured at Finisterre itself, often used as a general indicator of upwelling conditions around the Galician coast, is not always representative of the overall wind field. The relevant wind fields allowed a qualitative explanation of temperature structure seen in sea surface temperature images, and of differences in both the upwelling and downwelling regimes between the two years., This work was supported by the European Union under MAST 3 programme, contract number MAS3-CT97-0076 OMEX. Dr. E.D. Barton was partially funded by Mexican CONACYT, Catedra Patrimonial EX-010009.

The paper presents the results of the experiment CAPMOS'05 performed at an offshore oceanographic platform in the Black Sea. The experiment was aimed at air-sea coupling investigations by means of direct and remote measurements. A specialized research platform managed by the Marine Hydrophysical Institute is located on the shelf slope approximately 600m to the south of Crimea coast, Ukraine. Spectral parameters of wind and waves were estimated from direct and remote measurements. The peak frequency of the wind retrieved from radar measurements varied from 0.002 to 0.007 Hz, corresponding to wavelengths between 500 and 7000 m. The spectral peak frequency of gravity waves varied in the range from 0.2 to 0.7 Hz that corresponded to dominating wave lengths from 50 down to several meters. Comparison of two different techniques of wave spectrum retrieval from radar data, and also comparison of radar and radiometric data shows satisfactory agreement.