Numerical simulation and remote sensing have indicated that the northern half of the Red Sea has a significant role in the thermohaline circulation within the basin. However, very few studies with in situ observation have been performed in a region where the formation of Red Sea Outflow Water (RSOW) and occasionally of Red Sea Deep Water (RSDW) take place during the winter in the northern Red Sea (NRS). This study provides new insights into the seasonal variability and the mechanisms that drive the thermohaline circulation of the north half Red Sea using high-resolution glider observations combined with reanalysis and satellite datasets. The study describes the water masses characteristics, the mesoscale activity, and the forcing mechanisms. In addition, we examine the biogeochemical responses to the physical drivers in the northern half of the Red Sea and how these processes alter the marine ecosystem. During winter, the mesoscale eddy activity and heat fluxes create the necessary conditions for the formation of RSOW in the NRS. The cyclonic circulation elevates relatively denser water in the surface, which is exposed to the atmosphere exchange. Thus, it leads to subduction of the surface layer forming of RSOW. The subducted water has been characterized by high oxygen as it has recently been ventilated. In addition, chlorophyll fluorescence has subducted along the isopycnals, contributing to exporting material below the sunlit layer. After the formation of RSOW, a period of strong anticyclonic circulation was observed In late February, which stirred and mixed the advected waters from the south in the northern region. It is accompanied by heat flux transition, and at the periphery of the observed Anticyclonic Eddy, an uplifting of the densest water to the surface occurred. The presence of the anticyclonic circulation enables the water advection from the south and extends the time of the surface water for atmospheric exposure. In April, the warmer intrusion of fresher waters from the south dominated the eastern part of the NRS, reestablishing the cyclonic circulation. To the best of our knowledge, this is the first in situ observation in the NRS that captured the seasonal progression of the transition of heat flux in wintertime and water advection that terminates the formation of RSOW. A continuous supply of northward warmer, lower salinity near the coast from the south is observed throughout the summertime period. Strong stratification with surface mixed layers no deeper than 25-30 meters due to the advection of lower salinity surface water and local heating. Another change that occurred during the summer period is that the source of low salinity inflow into the region transitioned from Gulf of Aden Surface Water (GASW) to Gulf of Aden Intermediate Water (GAIW)—assuming that the inflow of GAIW began with the onset of the Southwest Monsoonal winds in the south. The summertime heating and along basin evaporation set up the system for the wintertime cooling and additional evaporation that contributes to the formation of RSOW and RSDW. The mixed layer Price-Weller-Pinkel (PWP) model (Price et al., 1986) is implemented to quantify the influence of local heat fluxes compared with horizontal advection of the Gulf of Aden Water on the upper layer. Simulation of the mixed layer showed that advection was the major contributor to the seasonally integrated heat content and mixed layer simulation in summer. In contrast to winter, the timing of the mesoscale eddy activity, significant cooling, and advection add complexity to the region. The difference in the heat content was significant, and the PWP predicted an increasing mixed layer depth, while the observed mixed layer depth remained relatively constant. The differences between the calculated and simulated heat content were minimum during the absence of the mesoscale eddy and advection from the south. Overall, the quantification suggests a complex relationship between atmospheric forcing and advection on the heat content and the mixed layer depth.

La température de surface de l'océan (SST) est depuis l'avènement des satellites, l'une des variables océaniques la mieux observée. Les modèles réalistes de circulation générale océanique ne la prennent pourtant pas en compte explicitement dans leur fonction de forçage. Dans cette dernière, seules interviennent les variables atmosphériques à proximité de la surface (température, humidité, vitesse du vent, radiations descendantes et précipitations) connues pour être entachées d'incertitudes importantes dès lors qu'on considère l'objectif d'étudier la variabilité à long terme de l'océan et son rôle climatique. La SST est alors classiquement utilisée en assimilation de données pour contraindre l'état du modèle vers une solution en accord avec les observations mais sans corriger la fonction de forçage. Cette approche présente cependant les inconvénients de l'incohérence existant potentiellement entre la solution « forcée » et « assimilée ». On se propose dans cette thèse de développer dans un contexte réaliste une méthode d'assimilation de données de SST observée pour corriger les paramètres de forçage atmosphérique sans correction de l'état océanique. Le jeu de forçage faisant l'objet de ces corrections est composé des variables atmosphériques issues de la réanalyse ERAinterim entre 1989 et 2007. On utilise pour l'estimation de paramètres une méthode séquentielle basée sur le filtre de Kalman, où le vecteur d'état est augmenté des variables de forçage dont la distribution de probabilité a priori est évaluée via des expériences d'ensemble. On évalue ainsi des corrections de forçage mensuelles applicables dans un modèle libre pour la période 1989-2007 en assimilant la SST issue de la base de données de Hurrel (Hurrel, 2008), ainsi qu'une climatologie de salinité de surface (Levitus, 1994). Cette étude démontre la faisabilité d'une telle démarche dans un contexte réaliste, ainsi que l'amélioration de la représentation des flux océan-atmosphère par l'exploitation d'observations de la surface de l'océan. / Sea surface temperature (SST) is more accurately observed from space than near-surface atmospheric variables and air-sea fluxes. But ocean general circulation models for operational forecasting or simulations of the recent ocean variability use, as surface boundary conditions, bulk formulae which do not directly involve the observed SST. In brief, models do not use explicitly in their forcing one of the best observed ocean surface variable, except when assimilated to correct the model state. This classical approach presents however some inconsistency between the “assimilated” solution of the model and the “forced” one. The objective of this research is to develop in a realistic context a new assimilation scheme based on statistical methods that will use SST satellite observations to constrain (within observation-based air-sea flux uncertainties) the surface forcing function (surface atmospheric input variables) of ocean circulation simulations. The idea is to estimate a set of corrections for the atmospheric input data from ERAinterim reanalysis that cover the period from 1989 to 2007. We use a sequential method based on the SEEK filter, with an ensemble experiment to evaluate parameters uncertainties. The control vector is extended to correct forcing parameters (air temperature, air humidity, downward longwave and shortwave radiations, precipitation, wind velocity). Over experiments of one month duration, we assimilate observed monthly SST products (Hurrel, 2008) and SSS seasonal climatology (Levitus, 1994) data, to obtain monthly parameters corrections that we can use in a free run model This study shows that we can thus produce in a realistic case, on a global scale, and over a large time period, an optimal flux correction set that improves the forcing function of an ocean model using sea surface observations.