Mechanisms of the Overturning Circulation in the Northern Red Sea, more than Convective Mixing

Abstract. The northern Red Sea (NRS) is where Red Sea Outflow Water and, occasionally, Red Sea Deep Water, are formed. Glider observations are used to describe the formation mechanisms and pathways of the intermediate waters in the NRS in late winter from 31 January to 18 April 2019. Utilizing glider observations, atmospheric reanalysis products, and satellite datasets, we evaluated the mesoscale activity and the atmospheric conditions that contribute to outflow water formation. The cyclonic circulation in the region surfaces dense water, which exposes it to the atmosphere, ventilating the water column and contributing to phytoplankton growth (enhancement of chlorophyll concentration) due to the nutrients upwelled into the euphotic zone. Subduction of this water in the 3-dimensional cyclonic circulation transported oxygenated, elevated chlorophyll water to depths between 150 m and 250 m along the 28.2 kg/m3 isopycnal. Unlike previous observations, in late February, a strong anticyclonic circulation blocked the inflow of warmer, fresher water into the region. It was accompanied by a negative heat flux and an uplifting of dense water to the surface. Net cooling through mid-March cooled the incoming surface waters from the south. At the end of the observational period, the intrusion of warmer, fresher waters from the south coincided with the re-establishment of cyclonic circulation and capped the dense surface water that had formed during March. These observations demonstrate that multiple processes contribute to Red Sea Outflow Water formation - convective mixing, cyclonic uplifting of dense water, subduction, and meso(submeso-)scale processes.