Structure and variability of the subtropical gyre

• Main Author: Maze, Guillaume
• Other Authors: Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Université de Bretagne Occidentale, Brest, Guillaume Roullet
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2020
• Subjects: Mid-latitude ocean dynamic; water mass life cycle; oceanic processes and air-sea interactions from meso to large scale; seasonal to interannual variability; Dynamique océanique aux moyennes latitudes; cycle de vie des masses d’eau; processus océaniques et d’interactions océan-atmosphère de méso à grande échelle; variabilité saisonnière à interannuelle; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; Pacific; North Atlantic
• Online Access: https://hal.archives-ouvertes.fr/tel-03344568; https://hal.archives-ouvertes.fr/tel-03344568/document; https://hal.archives-ouvertes.fr/tel-03344568/file/gmaze_hdr_jury_20200312_hal.pdf

The subtropical gyre is a large system of ocean currents and water masses stretching across each of the oceans at mid-latitudes. Primarily driven by winds, the upper ocean circulation tends to accumulate water masses in the center of the gyre. This accumulation drives: a specific stratification, a large scale equatorward circulation in the upper ocean and a compensating intense and narrow current flowing poleward along the western boundary of the ocean (eg: the Gulf Stream in the North Atlantic or the Kuroshio in the North Pacific). The gyre circulation is responsible for redistributing heat taken up by the ocean at low-latitudes to the higher latitudes and to the atmosphere, as well as for storing heat and anthropogenic carbon in its water masses at mid-latitudes. The gyre dynamic furthermore leads to a shallow (1000m) ventilation of the ocean on time scale ranging from 1 to 20 years that plays a role in climate by moderating ocean-atmosphere fluxes on these intermediate timescales.Over 14 years, my work has been centered on subtropical gyres with a focus on the North Atlantic gyre and its water masses. I provide a description of my research contributions to key physical oceanographic questions: what is, and what controls, the ocean stratification structure and variability on seasonal to decadal time-scales ? I answer these questions applied to the most important subtropical gyre water mass of the North Atlantic,the "Eighteen Degree water". I show how I used geophysical fluid dynamic principles of the ocean ventilation. A clear emphasis is made on the water-mass transformation process. I will furthermore present new analysis and diagnostic techniques I developed to objectively study the ocean stratification from ocean circulation numerical simulations, ocean state estimates based on data assimilation and direct ocean measurements. Last, I conclude with the main research axes I propose to investigate in the upcoming years: the past and future variability of western boundary currents. Le gyre subtropical est un ...