Interactions couplées océan-atmosphère à méso-échelle dans le Pacifique Sud-Est

temporary referenceThis 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

Disentangling the mesoscale ocean‐atmosphere interactions

International audienceIn the decades, the use of scatterometer data allowed to demonstrate the global ubiquity of the ocean mesoscale thermal feedback (TFB) and current feedback (CFB) effects on surface winds and stress. Understanding these air-sea interactions is of uttermost importance as the induced atmospheric anomalies partly control the ocean circulation and thus can influence the Earth climate. Whether the TFB and CFB effects can be disentangled, and whether satellite scatterometers can properly reveal them, remain rather unclear. Here, using satellite observations and ocean-atmosphere coupled mesoscale simulations over 45 degrees S to 45 degrees N, we show that the CFB effect can be properly characterized and unraveled from that due to the TFB. We demonstrate that the TFB can be unambiguously characterized by its effect on the stress (and wind) divergence and magnitude. However, its effect on the wind and stress curl is contaminated by the CFB and thus cannot be estimated from scatterometer data. Finally, because scatterometers provide equivalent neutral stability winds relative to the oceanic currents, they cannot characterize adequately the CFB wind response and overestimate the TFB wind response by approximate to 25%. Surface stress appears to be the more appropriate variable to consider from scatterometer data

Impacts of the Mesoscale Ocean-Atmosphere Coupling on the Peru-Chile Ocean Dynamics: The Current-Induced Wind Stress Modulation

International audienceThe ocean dynamical responses to the surface current-wind stress interaction at mesoscale are investigated in the South-East Pacific using a high-resolution regional ocean–atmosphere coupled model. Two simulations are compared: one includes the surface current in the wind stress computation while the other does not. In the coastal region, absolute wind velocities are different between the two simulations but the wind stress remains very similar. As a consequence, the mean regional oceanic circulation is almost unchanged. On the contrary, the mesoscale activity is strongly reduced when taking into account the effect of the surface current on the wind stress. This is caused by a weakening of the eddy kinetic energy generation near the coast by the wind work and to intensified offshore eddy damping. We show that, above coherent eddies, the current-stress interaction generates eddy damping through Ekman pumping and eddy kinetic energy dissipation through wind work. This alters significantly the coherent eddy vertical structures, weakening the temperature and vorticity anomalies and increasing strongly the vertical velocity anomalies associated to eddies

A new coupled tropical channel WRF-OASIS-NEMO: Sensitivity to the horizontal resolution

International audienceDespite tremendous increase in computing power, current climate models resolution (~100km) is still too low to explicitly represent several key processes in the atmosphere and ocean, which must therefore be parameterized and constitute one of the main causes of systematic errors in global ocean-atmosphere coupled models. Small-scale processes can play a major role in the variability of the climate at global scale through the intrinsic non-linearity of the system and the positive feedback associated with the ocean-atmosphere interactions. In this study high-resolution simulations of an extended tropical channel are performed in order to allow feedback of regional processes on the tropical climate mean state. In this perspective the atmospheric WRF model is coupled with the oceanic NEMO model via the OASIS3-MCT coupler. We analyze 20-years of coupled simulations with two different horizontal resolution, namely 0.75 and 0.25 degree. Comparisons between those simulations allow to determine the impacts of horizontal resolution on the climate mean state and its variability. The analysis focuses on the following key climate components: 1) the different monsoon systems and the associated regional circulations in the Indian, West African and North American regions, 2) the spatial and temporal distributions of the large-scale El-Niño Southern Oscillation, and 3) the upwelling systems in coastal and equatorial regions in the Pacific and Atlantic basins

Modeling studies of ocean-atmosphere coupling at the oceanic mesoscale

International audienceSeveral recent studies, mainly based on satellite observations, have shown significant coupled interactions between the ocean and the atmosphere at oceanic mesoscales. There are unambiguous relations between sea surface temperature gradient and surface winds and clear imprints of oceanic eddies in the lower atmosphere. We use high resolution regional ocean-atmosphere coupled model configurations to study these interactions in eastern boundary upwelling systems (California and Peru-Chile). Different model solutions are analyzed to understand the combined effects of sea surface temperature and oceanic surface currents on the ocean-atmosphere interaction. A particular attention is given to the impact of the coupling processes on coherent anticyclonic and cyclonic mesoscale eddies. We show significant modification of the eddies characteristics (e.g. intensity, horizontal and vertical structures) and of the above atmospheric conditions (e.g. near-surface wind, boundary layer). Geographical and temporal variations of the mesoscale coupling intensity are also discussed. Perspectives are given by presenting preliminary results from an extended tropical channel coupled ocean-atmosphere model at high-resolution (1/12°)

Modulation of the Oceanic Mesoscale Activity by the Mesoscale Thermal Feedback to the Atmosphere

International audienceAbstract Ocean mesoscale thermal feedback (TFB) is the influence of mesoscale sea surface temperature (SST) anomalies on the overlying atmosphere and its feedback to the ocean. Over the past few decades, TFB has been shown to affect the atmosphere by inducing low-level wind and surface stress anomalies and modulating ocean–atmosphere heat fluxes ubiquitously over the global oceans. These anomalies can alter the climate variability. However, it is not clear yet to what extent heat and momentum flux anomalies modulate the mesoscale ocean activity. Here, using coupled ocean–atmosphere mesoscale simulations over a realistic subtropical channel centered on the equator in which the TFB can be turned off by spatially smoothing the SST as seen by the atmosphere, we show that TFB can damp the mesoscale activity, with a more pronounced effect near the surface. This damping appears to be sensitive to the cutoff filter used: on average, the surface mesoscale activity is attenuated by 9% when smoothing the SST using an ∼1000-km cutoff but by only 2% when using an ∼350-km cutoff. We demonstrate that the mesoscale activity damping is primarily caused by a sink of available eddy potential energy that is controlled by the induced-anomalous heat fluxes, the surface stress anomalies having a negligible role. When TFB is neglected, the absence of sink of potential energy is partly compensated by a more negative eddy wind work. We illustrate that TFB filtering in a coupled model must be done carefully because it can also impact the large-scale meridional SST gradients and subsequently the mean large-scale wind stress curl and ocean dynamics

Mesoscale ocean-atmosphere coupling in the Peru-Chile upwelling system

International audienceUnderstanding the dynamics of Eastern boundary upwelling systems such as the Peru-Chile region is of major interest as these regions host an intense biological activity and productive fisheries. Moreover, they are generally poorly represented in global climate models due to biases in low cloud cover and a misrepresentation of mesoscale processes. The mesoscale activity of these systems has been studied quite extensively with observations and with ocean models which generally do not take into account the feedback of the ocean mesoscale (eddies, filaments and fronts) on the atmospheric forcing. However it has been evidenced that oceanic thermal gradients associated with oceanic eddies induce an atmospheric response, in particular in the surface wind, which is the main forcing in upwelling systems. As a consequence, the Humboldt current system appears to be a fully coupled system in which mesoscale air-sea interactions are to be taken into account. Using a regional coupled model (WRF-NEMO) at ~9 km resolution, we study the interactions between wind stress and sea surface temperature (SST) mesoscale structures in the Peru region. The relevant coupling scales are isolated and the spatial and temporal variations of these interactions are characterized. Correlations between mesoscale wind stress and SST compare well with results from satellite data when model fields are smoothed on the observations grid (~50 km), while model small-scale structures (at ~10km resolution) are more strongly coupled. The large-scale wind intensity and steadiness modulate the coupling intensity spatially and at seasonal time scales. The physical mechanisms of the atmosphere response to an SST gradient are also investigated, including the role of the sea surface oceanic current

Impact of ocean-atmosphere current feedback on ocean mesoscale activity : regional variations and sensitivity to model resolution

International audienceOcean mesoscale eddies are characterized by rotating-like and meandering currents that imprint the low-levelatmosphere. Such a current feedback (CFB) has been shown to induce a sink of energy from the ocean to theatmosphere, and consequently to damp the eddy kinetic energy (EKE), with an apparent regional disparity.In a context of increasing model resolution, the importance of this feedback and its dependence on oceanicand atmospheric model resolution arise. Using a hierarchy of quasi-global coupled models with spatial res-olutions varying from 1/48to 1/128, the present study shows that the CFB induces a negative wind work atscales ranging from 100 to 1000 km, and a subsequent damping of the mesoscale activity by ~ 30% on average,independently of the model resolution. Regional variations of this damping range from ~ 20% in very richeddying regions to;40% in poor eddying regions. This regional modulation is associated with a differentbalance between the sink of energy by eddy wind work and the source of EKE by ocean intrinsic instabilities.The efficiency of the CFB is also shown to be a function of the surface wind magnitude: the larger the wind, thelarger the sink of energy. The CFB impact is thus related to both wind and EKE. Its correct representationrequires both an ocean model that resolves the mesoscale field adequately and an atmospheric model reso-lution that matches the ocean effective resolution and allows a realistic representation of wind patterns. Theseresults are crucial for including adequately mesoscale ocean–atmosphere interactions in coupled generalcirculation models and have strong implications in climate research

Peru-Chile upwelling dynamics under climate change

International audienceThe consequences of global warming on the Peru-Chile Current System (PCCS) ocean circulation are examined with a high-resolution, eddy-resolving regional oceanic model. We performed a dynami-cal downscaling of climate scenarios from the IPSL-CM4 Coupled General Circulation Model (CGCM), corresponding to various levels of CO2 concentrations in the atmosphere. High-resolution atmospheric forcing for the regional ocean model are obtained from the IPSL atmospheric model run on a stretched grid with increased horizontal resolution in the PCCS region. When comparing future scenarios to preindustrial (PI) conditions, the circulation along the Peru and Chile coasts is strongly modified by changes in surface winds and increased stratification caused by the regional warming. While the coastal poleward undercurrent is intensified, the surface equatorial coastal jet shoals and the nearshore mesoscale activity are reinforced. Reduction in alongshore wind stress and nearshore wind stress curl drive a year-round reduction in upwelling intensity off Peru. Modifications in geostrophic circulation mitigate this upwelling decrease in late austral summer. The depth of the upwelling source waters becomes shallower in warmer conditions, which may have a major impact on the system's biological productivity

Interactions océan-atmosphère à mésoéchelle : sensibilité au couplage, à la paramétrisation de couche limite et à la résolution

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