Capturing a Mode of Intermediate Water Formation in the Red Sea

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.

Extreme heat loss in the Northern Red Sea and associated atmospheric forcing

“…The model outputs of the last 24 hours are combined every day to generate the long-term high-resolution reanalysis for the AP during the period 1980-2020. The details of the AP-RA are well documented by several previous studies (e.g., Viswanadhapalli et al, 2017;Ellis et al, 2019;Dasari et al, 2019;Dwivedi et al, 2020;Langodan et al, 2020;Asfahani et al, 2020;Hoteit et al, 2020;Viswanadhapalli et al, 2020;Dasari et al, 2021;). The Rapid Radiative Transfer Model for Global circulation model was used for both shortwave and longwave radiation (Iacono et al, 2008).…”

“…In the northern part of the open Red Sea and in its two northward extensions, the gulfs of Aqaba and Suez, heat loss via the sea surface during winter cools the already hypersaline water and forces it to sink. This dense water according to its density can reach intermediate or deep layers and fuel the two loops of overturning circulation in the basin (Cember, 1988;Woelk and Quadfasel, 1996;Sofianos and Johns, 2003;Manasrah et al, 2004;Biton and Gildor, 2011;Papadopoulos et al, 2013;Abualnaja et al, 2015;Yao and Hoteit, 2018;Asfahani et al, 2020). Extreme surface cooling, which leads to dense water formation events, is directly related to the energy transferred from the sea to the atmosphere.…”

“…Therefore, the characteristics of the heat exchange at the air-sea interface and its link to distinct atmospheric circulation patterns are of great interest. In particular, attention needs to be paid to the characterization of extreme heat loss, which is a decisive driving factor in the formation of dense water in the NRS (Sofianos and Johns, 2003;Yao et al, 2014a;Yao et al, 2014b;Papadopoulos et al, 2015;Yao and Hoteit, 2018;Asfahani et al, 2020).…”

Phytoplankton Biomass and the Hydrodynamic Regime in NEOM, Red Sea

“…The permanent northwestern surface circulation in the open sea, which intensifies during winter, drives northward currents that flow near the coast and have been described previously as the Eastern Boundary Current [79,82]. This current affects all coastal regions (excluding lagoons) and thus they generally show common characteristics in terms of their physical properties, nutrient concentrations and seasonal variability.…”

“…For example, the seasonality of Chl-a in the Port of Duba is very similar to the open sea. The boundary current [62,79,82] also creates several small eddies and bifurcating currents along natural barriers in the shallow regions near the coasts. Among them, a cyclonic eddy persists during all seasons to the south of Sindala (Figure 5) and could be a source of colder waters and nutrients for the region via upwelling.…”