Extreme Air-sea interactions at latitudes

Position: 26-057 Extreme Air-sea interactions at high latitudes
## 26-057 Extreme Air-sea interactions at high latitudes
* Doctorat, 36 mois
* Temps plein
* Expérience : pas de préférence
* Maitrise, IEP, IUP, Bac+4
* Oceanography Polar lows (PL) are short-lived mesoscale cyclones that develop over high-latitude oceans, associated with strong winds, heavy precipitation, and large waves. They have a significant impact on coastal and maritime human activities, as well as on ocean circulation, sea ice, and the polar climate. Several studies (e.g., Bresson et al., 2022) have shown that the number of PLs and their intensity are projected to substantially decrease (by over 60%) by the end of the 21st century due to the faster air temperature increase relative to water temperature and therefore higher stability of the atmospheric boundary layer.

Meanwhile, the rapid evolution of the Arctic Sea ice extent further opens new regions for the PL formation (Smirnova et al., 2015).
By cooling the water of the ocean surface in polar regions, the whole forcing exerted by PLs that develop every year can have an important impact on the thermohaline circulation by modifying the formation rate of the ocean deep water and can cause major events of deep convection (Garcia-Quintana et al., 2019).  Taking into account polar lows is extremely important when modelling large scale oceanic circulation and composing short-term weather forecasts (Condron et al.,

2013): the predicted decrease in number of polar cyclones could lead to a weakening of deep-water mixing and large-scale oceanic circulation.

The impact of polar lows on the ocean circulation at high latitudes, as well as the role of ocean feedbacks on the evolution of these atmospheric systems themselves, is however still under debate. In particular, the relevant role of surface sensible heat flux and latent heat release in PL development has often been highlighted. The diabatic fluxes from the oceanic surface and the vertical mixing associated with PLs can in fact cause significant decrease in the Sea Surface temperature (SST, Føre & Nordeng, 2012).

In contrast, a warming ocean response has also been reported in regions with an ocean temperature inversion (near the sea ice edge). These differences in SST responses contribute to significant differences in surface thermal heat fluxes from the ocean to the PL (Wu, 2021).Most of the previous studies used limited in situ observation data, and a comprehensive understanding is insufficient.

Alternatively, satellite-based analysis products have just been recently used to characterize the SST responses for an ensemble of PLs in the Nordic Seas, highlighting the bi-modal distributions of SST response to PLs (Tomita and Tanaka, 2024).  As of today, the statistical SST responses in other key oceanic PL’s development zones worldwide (Labrador Sea, Gulf of Alaska, Bering and Chukchi Seas, Southern Ocean.)

remain unknow. In addition, the Arctic and Antarctic Oceans are strongly stratified by salinity in the uppermost layers with major changes expected in that stratification under global warming (Hordoir et al., 2022). Given the potential presence of strong haloclines in PL development regions, one expects significant variability in the Sea Surface Salinity (SSS) responses to their passage: these have never been characterized in the past.

A first objective will therefore be to investigate the statistical characteristics of the ocean response to the passage of PLs at global scale based on observational (satellite/in situ) and model re-analyses data. Following a storm-centric approach, statistical composites will help analyse the spatio-temporal distributions of SSS, SST and SSH responses (including the recent SWOT high-resolution sea-level observations) to the passage of Polar Lows in all their regions of development (Arctic and Antarctic) using the most recent PL tracking databases (Stoll, 2022).

The aim will then be to better identify the influence of PLs on surface heat flux and its relationship with the vertical structure of the high-latitude ocean, to help answer its climatic evolution.  For the first time, the spatio-temporal distributions of ocean responses (SSS, SST, SSH) to these extreme events will be studied in relation to regional preconditioning (vertical ocean/atmosphere structures, fronts), as well as to storm characteristics (size, speed, intensity, regions).

Compared with those better known following tropical cyclone passages, physically- or data-driven relationships leading to scaling laws shall then be derived, to better identify key processes leading to better explain PL life-cycles under various conditions. The third objective is to establish the potential of future space observation in monitoring Polar Lows and their interactions with the ocean.  In particular, one will consider the potential of the future high priority mission Copernicus CIMR which will provide for the first time simultaneous SSS, SST, wind speed vector, water vapor, as well an ensemble of sea ice characteristics…