Circulation and Water Mass Formation in the Northern Red Sea Response to Wind and Thermohaline Forcing

Abstract

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.