Air-Sea Interactions and Variability: the effects of SST gradients and high resolution on atmospheric variability – ASIV

We will concentrate on different sub-problems in order to better understand the key mechanisms of the air-sea coupling : the impact of SST anomalies of meso and sub-meso scales on the storm track ; the role of the diurnal cycle of the oceanic mixed layer on the low-frequency variability of the atmosphere ; the feedbacks of the atmosphere on the ocean, in particular on the oceanic eddies and on the oceanic mixed layer.
These studies will rely on very high resolution simulations with idealized or realistic frameworks and on the analysis of observations.

We have validated a mechanism of the response of atmospheric boundary layer to the oceanic fine scales and we have shown that fronts such as the Gulf Stream have an impact on the storm tracks, which has been validated by an analysis of observations. The different modes of variability of the tropospheric jet stream in relation with SST anomalies have been identified.

We are presently developing new simulation set-ups that will allow to better highlight the ocean-atmosphere coupling. We wish to analyze more precisely the response of the storm track, the tropospheric jet and the low-frequency modes of variability of the atmosphere to the oceanic fine scales.

J. Lambaerts, G. Lapeyre, R. Plougonven and P. Klein, 2013, Atmospheric response to sea surface temperature mesoscale structures, J. Geophys. Res. in revision.
C. Michel et G. Rivière, 2013, Sensitivity of the position and variability of the eddy driven jet to different SST profiles in an aquaplanet general circulation model, J. Atmos. Sci. in revision.

The mechanisms of air-sea interactions have been studied since several decades for their impact on large-scale climate. Until ten years ago, the influence of midlatitude Sea Surface Temperature (SST) anomalies on the atmosphere was thought to be moderate. Only tropical SST anomalies were recognized to play a role in the variability of the mid-latitudes (through planetary teleconnections). Since then, a new insight on the problem was brought and there is compelling evidence that sharp SST fronts, as in Western Boundary Currents, substantially affect the troposphere. In particular, large SST gradients are responsible for a spatial localization of the storm-track. Numerical simulations that take into account the high spatial resolution in the ocean or the high frequency associated with the diurnal cycle of the oceanic mixed layer demonstrate the importance of the resolution in the ocean for air-sea interactions. On the other hand, very high-resolution simulations of the ocean have shown that the meso (200km) and submesoscales (10km) are strongly energetic and are associated with enhanced SST gradients. We propose in this project to combine these bodies of recent work by using state-of-the-art "very high resolution" simulations of the atmosphere and oceans in idealized or more realistic settings. We want to address the problem of ocean-atmosphere interactions in this new setting and study their effect on atmospheric variability (in the tropics, in the midlatitudes and the planetary teleconnections) taking into account these new processes. It seems to us that one needs a better understanding on how ocean and atmosphere interact in presence of strong SST gradients, due to oceanic meso- and submesoscales. Another related question is the sensitivity of coupled ocean-atmosphere processes to the representation of the oceanic mixed layer and its sub-daily variability. The key questions we have identified are: how SST gradients and oceanic eddies affect the different atmospheric entities that form the so-called storm tracks? Does water vapor play a catalytic role in the ocean-atmosphere coupling? How strong SST gradients variability impact the atmospheric low-frequency variability? What is the impact of the representation of tropical ocean small scale and rapid fluctuations on the teleconnections between tropics and extratropics? This project will aim in a better understanding on the mechanisms of air-sea interactions. It should help to answer the still open questions of the role of air-sea interactions in seasonal forecasting (associated with tropics-extratropics teleconnections). Key processes of air sea coupling that are lacking in current GCMs will be identified and the impact of the resolution will be assessed. To this end, we will make use of two types of models, from regional models to global ones. Both forced and coupled configurations will be examined. The effect of oceanic variability on the atmospheric (synoptic and low-frequency) variability will be examined and the atmospheric feedback on the ocean as well using these simulations. We will also investigate using the new datasets of SST of higher resolution if such relationship between the atmosphere and the ocean exists or not.