The Subpolar Gyre (SPG) of the North Atlantic plays a key role in the carbon cycle and climate variability. It is the site of a vigorous spring phytoplankton bloom, maintained by the seasonal transport of nutrients in association with light. Macronutrients (NO₃, PO₄, DSi) are supplied predominantly to the mixed layer by lateral transport from high latitudes (mainly through the Davis and Hudson Straits), from lower latitudes by the North Atlantic Current, or by vertical transport from below the mixed layer where higher concentrations are present. These supply processes have been suggested to vary in response to the North Atlantic Oscillation, a major mode of natural climate variability.

When the NAO index is negative, as it was from the mid-1990s to the end of the 2000s, physical conditions are similar to those projected forclimate change (i.e. , warming and freshening, weakening of deep convection, slowing down of the Atlantic Meridional Overturning Circulation, increasing stratification. During the same period, a decline in macronutrient concentrations was observed throughout the region leading to the hypothesis that underlying processes could be similar to those acting under global warming to reduce mixed layer nutrient levels.

The main objective of this thesis was to analyze and quantify the contributions of dynamic processes (i.e., lateral and vertical transport) to the observed variability in macronutrient mixed layer concentrations within the SPG between 1980 and 2015. The analysis used output from a coupled physical-biogeochemical model (NEMO-PISCES) discretized on a quarter-degree grid.

An assessment of the model’s representation of the spatial and temporal variability of macronutrient concentrations and the main physical processes, such as deep convection in the Labrador Sea, and lateral transport of water and nutrients, was carried out in comparison with data from observations. An initial focus was on the Labrador Sea, which is a region characterized by deep, intense, and variable winter convection, making it an ideal laboratory for distinguishing the role of variability in deep winter convection from that of the Subpolar Gyre circulation and inputs from the Arctic Ocean. Despite some weakening, the contribution of changes of Arctic DSi transport through the Davis and Hudson Straits to observed nutrient variability was shown to be negligible (= 3$\%$). Deep convection was identified as the main driver of the decline in pre-bloom DSi concentrations in the Labrador Sea. The study was extended next to the broader SPG, with assessment of the recent variability of macronutrient concentrations in the mixed layer through analysis of the variability of lateral and vertical transports between a period of strong positive NAO, a period of negative NAO, and the period following.

In addition to the temporal variability of concentrations in response to atmospheric forcing, regional differences emerged, with a contribution dominated by vertical transport in the Labrador and Irminger Seas, led by variations in the depth of the mixed layer. Zonally-integrated nutrient transport east and north of the SPG are coherent with the variability of nutrients within the SPG but decoupled from nutrients transported by the Gulf stream via the North Atlantic Current west of 38°N. In this thesis, I highlighted the predominance of vertical mixing in the contemporary variability of nutrient concentrations, compared with lateral inputs.

I show that a slowing of the general circulation associated with stratification of the water column leading to the weakening of vertical nutrient fluxes, as is the case projected for climate change conditions, would lead to a reduction in macronutrient concentrations and potential future primary production in the SPG.

La soutenance se tiendra le jeudi 26 octobre à 14:00, en amphi Bloch, bâtiment 772, CEA, Orme des Merisiers. Elle sera aussi accessible en visioconférence via le lien suivant : https://uvsq-fr.zoom.us/j/97972979237?pwd=NFVKY0NUSmxKbDV3eHNzbUIvanRBUT09