Description of the thesis topic

Southern Ocean acts as a central thermostat that sets the Earth's temperature response to greenhouse gas emissions. A delicate balance between its large-scale circulation (referred to as the Antarctic overturning circulation) and its surface temperature, which is shaped by ocean—ice processes, strongly governs the amount of energy accumulation by the planet.
Evidence of ongoing decline of the Antarctic overturning circulation have escalated in recent years, both from observation and numerical modelling. Current understanding is that increased upper ocean stratification, driven by increased high latitude precipitation and icesheet mass loss, reduces the production of very dense waters along the Antarctic continent (Dense Shelf Waters; DSW), which itself reduce the subsequent sinking of Antarctic Bottom Water (AABW) that lies at the bottom of the World's Ocean abysses. This, in turn, could drive substantial warming of the abysses.
These mechanisms remain, however, poorly understood due to the persisting lack of observations in this extreme environment, and challenge to model this part of the world. In particular, since the mid 2010s, rapid, if not abrupt, changes are ongoing in the Southern Ocean—ice system, and the impact of these change on DSW and AABW remain unclear. The overall objective of the PhD project is to understand and quantify how ocean–ice–atmosphere processes affect the Antarctic overturning circulation, and how these are linked to abyssal warming.
The project will be divided in three main parts/questions that will allow to achieve the overall objective:
1 – How does the reduction in the Antarctic sea-ice cover, observed since 2016, affect Dense Shelf Water characteristics? To address this question, the phd student will use historical observations of salinity on the Antarctic continental shelf (from marine mammal, ships, floats), as well as sea-ice cover and thickness observations from satellite to unveil water-mass characteristic variability on the continental shelf associated with sea-ice production variability. Particular attention will be brought, as far as observational approach allows, on link between overall sea-ice extent and production of sea-ice on the continental shelf.
2 – How does different scale of variability in sea-ice production and change in ocean stratification sets the volume of Dense Shelf Water and affect the characteristics of Antarctic Bottom Water ? To address this question, the phd student will use state-of-the-art ocean model developed at LOCEAN to explore the scale of variability of DSW production and link them to the causes at the ocean surface, and consequences in the abyssal ocean. This analysis will extend the observational analysis of step 1.
3 – Can we delineate the causes of the observed abyssal warming between circulation slowdown and adiabatic warming coming from the surface ? To address this question, the phd student will re-assess abyssal warming from historical observation, augmented with the rapidly developing observations from deep Argo float program, and with the mechanistic knowledge brought from the modelling approach developed at step 2. This task will be developed in close collaboration with the Global Energy Budget initiative of the Global Climate Observing System.

Work Context

The future PhD student will work at the LOCEAN laboratory under the joint supervision of Jean-Baptiste Sallée and Casimir de Lavergne. The Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques (LOCEAN) conducts studies on the physical and biogeochemical processes of the ocean and their role in climate in interaction with marine ecosystems. Its teams, widely recognised at international level, work on a wide range of time and space scales to gain a better understanding of ocean dynamics and variations within the climate system, as well as present, past and future trends. They also contribute to the development of analysis, modelling and observation methods, as well as systematic observation of the ocean, both in situ and from space.

Constraints and risks

N/A