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Influence des tourbillons océaniques sur le rail des dépressions atmosphériques

Abstract : This thesis aims to understand how sea surface temperature (SST) anomalies associated with mesoscale oceanic eddies may contribute to mid-latitude storm tracks variability. Based on idealized high resolution atmospheric simulations, local and large scale remote influences of the eddies onto the boundary layer and the free troposphere are investigated. Basic configuration is a generic baroclinic channel above an eddying SST front. The first part of the manuscript deals with the response of the atmosphere in the boundary layer, based on the well-known mechanisms of pressure adjustment and vertical mixing of momentum. As expected from theoretical scaling arguments, the former is important for weak wind conditions. Due to the large scale SST front, direction of strong background wind also controls stability : a transition, from a forcing by mixing of momentum to a forcing by SST-driven pressure forces, occurs for equatorward winds as cold air is advected above warmer SST. It shows how sub-weekly atmospheric variability modulates the low level response. Wind stress divergence matches the linear relationship observed with downwind SST gradient. In specific conditions, horizontal divergence is however proportional to the laplacian of boundary layer temperature, within a large range of horizontal scales (500-100~km). In a second part, the experiment with oceanic eddies is compared to its zonally symmetric counterpart. It reveals weak but robust modifications of atmospheric large-scale circulation and synoptic variability : the Eulerian storm track and the jet stream are shifted poleward. Surface forcing is associated with a local response in the mid-troposphere, which is intermittent and dominated by the day-to-day variability. Because of an asymmetry in the response to cold and warm eddies, the oceanic eddies are also responsible for a net increase of both latent and sensible surface heat fluxes. It accounts for a large part of the differences obtained between eddying and zonally symmetric configurations. Based on heat and mechanical energy budgets, an interpretation of the poleward displacements of the storm track and the jet is given. The contribution of latent heat release to both budgets, occurring mainly within synoptic storms, plays a major role in the atmospheric response.
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Submitted on : Wednesday, May 20, 2020 - 1:13:13 PM
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Alexis Foussard. Influence des tourbillons océaniques sur le rail des dépressions atmosphériques. Océan, Atmosphère. Sorbonne Université, 2018. Français. ⟨NNT : 2018SORUS492⟩. ⟨tel-02613784⟩



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