Modulation du courant circumpolaire antarctique par les interactions air-mer à méso-échelle

Date de début: 01 Octobre 2024

Date limite de candidature: La date limite de candidature à l’école doctorale est le 30 mai 2024 à minuit, mais les candidats sont encouragés à contacter le directeur de thèse dès que possible.

Directeur de thèse: Lionel Renault

Financement: Concours pour un contrat doctoral

Résumé

This thesis aims to assess the extent to which fine-scale Ocean-Atmosphere interactions (thermal and mechanical) can modulate the dynamics of the Antarctic Circumpolar Current (ACC) with a particular focus on cascades of energy, eddy compensation, eddy saturation, as well as feedback from the Ocean on the atmospheric storm track. To this end, based on previous work by Foussard et al.(2019b,a), we will use an ensemble of idealized coupled atmospheric simulations representing storms and oceanic simulations representing an idealized ACC. The simulations will differ in the interactions they consider, which will allow us to disentangle the different effects of thermal and mechanical interactions
The Antarctic Circumpolar Current (ACC) is an ocean current that flows from west to east around the Antarctic. The ACC is the most intense ocean current in the world carrying around 170 million m^3 of water per second (Sverdrups, Sv) (Donohue et al., 2016). It plays a central role in shaping our climate, as it act as a barrier isolating the huge Antarctic ice sheet from subtropical waters (Naveira Garabato et al., 2011), and is associated with a vertical circulation acting as an important sink for anthropogenic carbon and heat(Frölicher et al., 2015). As highlighted in the latest IPCC report, the processes controlling the ACC strength and variability and the associated vertical circulation remain poorly understood, particularly at fine-scale (mesoscale) (Fox-Kemper, 2021).
The ACC has the particularity to be overlaid by persistent eastward wind and by extra-tropical powerful cyclones characterized by surface wind speeds that can exceed 20 m/s, and up to 35 m/s in extreme storms (Young et al., 2020). Those surface winds drive the ACC and are primarily balanced by the topographic form stress (Rintoul et al., 2001). However, in this region, mesoscale eddies (scalesO(100km)) have been shown to have a tremendous importance in modulating the ACC dynamics by inducing two distinct phenomena:the eddy saturation and the eddy compensation (e.g., Morrison et al. (2013)). Eddy saturation consists in pumping the momentum flux down the bottom ocean, reducing its impact on the mean flow. It causes a flattening of the isopycnal, and, thus, a slow-down of the mean current, causing a weak (though non-zero) ACC transport sensitivity to increased wind stress (Meredith and Hogg, 2006). Eddy compensation acts on the associated Meridional Overturning Circulation (MOC), which is, as the ACC, mainly driven by the wind and its associated Ekman transport. The eddy-induced transport can balance the wind-driven current, causing an insensitivity of the MOC to an accelerating wind forcing (Marshall and Radko, 2003; Morrison et al., 2013).
A number of studies have assessed the response of an idealized ACC to a constant zonal stress, assuming that surface winds exert only a zonal mean force and neglecting the fine-scale oceanic feedback to the atmosphere (e.g., Abernathey et al. (2011); Thompson and Sallée (2012); Jouanno et al. (2016)). On the one hand, the presence of storm causes a heterogeneity in the atmospheric forcing,which by non-linear process may have a strong impact on the ACC dynamics and in particular on mesoscale eddies. On the other hand,in the last decades, two main air-sea fine-scale interactions have been shown to strongly modulate the ocean dynamics: the thermal and the mechanical feedbacks (e.g., (Renault et al., 2019)). The mesoscale Thermal FeedBack (TFB) can weaken the generation of eddies by causing turbulent heat fluxes that reduce the Mesoscale Available Potential Energy in the Ocean (Bishop et al., 2020) and,thus, the baroclinic conversion of energy Ma et al. (2016). The mechanical feedback represents the influence of the surface current ont he overlying atmosphere (Current FeedBack, CFB). CFB has two main impacts on the ocean: a large-scale impact, which consists of a slow-down of the mean circulation; and a mesoscale impact, the so-called “eddy-killing”, i.e., a damping of the mesoscale activity (e.g.,Renault et al. (2016, 2020).