Contributions of physical processes to the recent variability (1980-2015) of nutrient concentrations in the North Atlantic Subpolar Gyre

• Main Author: Dale, Alizée
• Other Authors: Laboratoire des Sciences du Climat et de l'Environnement Gif-sur-Yvette (LSCE), Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris-Saclay-Centre National de la Recherche Scientifique (CNRS)-Direction de Recherche Fondamentale (CEA) (DRF (CEA)), Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA), Université Paris-Saclay, Marion Gehlen, Douglas Wallace
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2023
• Subjects: Subpolar Gyre; Nemo-Pisces; Nutrients; Transport; Entrainment; Nao; Gyre Subpolaire; Nutriments; Entrainement; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; Labrador Sea; north atlantic current; North Atlantic; North Atlantic oscillation
• Online Access: https://theses.hal.science/tel-04460570; https://theses.hal.science/tel-04460570/document; https://theses.hal.science/tel-04460570/file/116241_DALE_2023_archivage.pdf

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. Macro-nutrients (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 the case from the mid-1990s to the end of the 2000s, physical conditions are similar to those projected under climate change (i.e. , warming and freshening, weakening of deep convection, slowing down of the Atlantic Meridional Overturning Circulation, increasing straatification. During the same period, a decline in macro-nutrient 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 macro-nutrient mixed layer concentrations over the SPG between 1980 and 2015. The analysis used 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 macro-nutrient concentrations and the main physical processes, such as deep convection in the Labrador Sea, and the 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 ...