High-resolution glider and remote sensing observations reveal the annual physical and biogeochemical variability of the central Red Sea

The Red Sea’s significant latitudinal range from 12˚N to 30˚N creates a thermohaline circulation that drives a northward-flowing eastern boundary current (EBC). Both numerical simulations and observations demonstrate that the ubiquitous mesoscale and submesoscale features within the basin affect the northward trajectory of the EBC. A sustained glider study was initiated in the central Red Sea (CRS) to evaluate the physical and biogeochemical variability of this complex regime. Glider observations, coupled with remote sensing, reanalysis products, and a 1-D mixing model, have been used to identify and characterize these mesoscale processes over an annual cycle. Comparison of the glider data with the 1-D mixing model results shows that the magnitude of restratification of the mixed layer (ML) is not fully explained by heat, freshwater, and momentum fluxes. This study has aimed 1) to investigate the seasonal physical and biogeochemical variability, and 2) to assess where and when lateral advection influences mixed layer evolution. The relationship between horizontal and vertical distributions of temperature and salinity suggests that EBC contributes to slumping of the horizontal density gradient, thus increasing stratification. The EBC was clearly present in early spring as evidenced by the transport of warm, low-salinity water into the CRS. This low salinity, warmer water enhances stratification, sharpens horizontal gradients, and may cap mesoscale eddy activity. The influx of Gulf of Aden water varies with season. Glider observations reveal the seasonal contrasts of the water mass characteristics and their transition from summer stratification into winter mixing. Changes in the water mass significantly affect the distribution and dispersal of biogeochemical properties, with the deep chlorophyll maximum and phytoplankton population present during summer and early fall when subsurface Gulf of Aden Intermediate Water is present.