Abstract
Formation of intermediate waters in the northern Red Sea was captured by Seagliders during the winter of 2016. The analysis is based on measurements collected by consequent deployments of Seaglider autonomous underwater vehicles along a 70 km transect, which was run repeatedly from the coast to midbasin during the period from October 2015 to May 2016. The observations revealed typical open ocean convection processes including a preconditioning phase followed by deep convection and spreading of the newly formed waters through lateral exchange. Winter conditions were characterized by surface cooling and enhanced evaporation, which were observed from late October until late February. During this period, the gradual surface cooling and salinity increase resulted in the weakening of the surface stratification. Recurrent cyclonic gyres forced by strong atmospheric events led to episodic convection and the formation of intermediate seawater typical of the upper overturning circulation cell of the Red Sea. Following deep mixing, a remarkable bloom of chlorophyll was detected at the top layer of the water column in response to the nutrients delivery from the deeper layers. The end of winter was marked by the intrusion of lower salinity water presumably advected from the south and the reestablishment of the general cyclonic circulation, typical for the northernmost part of the Red Sea. To the best of our knowledge, this is the first observation study of such convection events in the northern Red Sea.
Plain Language Summary Water formation in the northern Red Sea during winter plays an important role in the basin's overturning circulation. Dense hypersaline water formed in the northern parts of the basin travels at intermediate depths towards the south and finally exits into the Indian Ocean. We describe for the first time a mechanism for the intermediate water formation, which was deduced from Seaglider records captured during the winter of 2016. We present the response of the upper seawater column to episodic atmospheric forcing events and the crucial role of cyclonic gyres in the convection process. During the period of observations, several events with simultaneous activation of the cyclonic gyre favored the winter surface cooling and led to open ocean convection. Vertical mixing in the water column supplied the near surface layer with nutrients, which caused a remarkable chlorophyll bloom. The deep mixing events occurred over very short periods, typically less than a week, and were followed by rapid restratification of the surface layer.