Physical and biological characteristics of the winter‐summer transition in the Central Red Sea

Abstract

The Central Red Sea (CRS) lies between two distinct hydrographic and atmospheric regimes. In the southern Red Sea, seasonal monsoon reversal regulates the exchange of water between the Red Sea and the Indian Ocean. In the northern Red Sea, intermediate and occasionally deep water are formed during winter to sustain the basin's overturning circulation. Highly variable mesoscale eddies and the northward flowing eastern boundary current (EBC) determine the physical and biogeochemical characteristics of the CRS. Ship‐based and glider observations in the CRS between March and June 2013 capture key features of the transition from winter to summer and depict the impact of the eddy activity on the EBC flow. Less saline and relatively warmer water of Indian Ocean origin reaches the CRS via the EBC. Initially, an anticyclonic eddy with diameter of 140 km penetrating to 150m depth with maximum velocities up to 30–35 cm s−1 prevails in the CRS. This anticyclonic eddy appears to block or at least redirect the northward flow of the EBC. Dissipation of the eddy permits the near‐coastal, northward flow of the EBC and gives place to a smaller cyclonic eddy with a diameter of about 50 km penetrating to 200 m depth. By the end of May, as the northerly winds become stronger and persistent throughout the basin, characteristic of the summer southwest monsoon wind regime, the EBC, and its associated lower salinity water became less evident, replaced by the saltier surface water that characterizes the onset of the summer stratification in the CRS.

Physical Mechanisms Routing Nutrients in the Central Red Sea

“…Given the interaction of the two eddies, entrainment of cooler, saltier offshore water by the onshore flow between the two eddies may mix with the coastal Gulf of Aden water to increase its salinity (∼39.4) and reduce its temperature (∼26°C). Zarokanellos et al () pointed out that eddies may block or redirect the flow of the EBC. In this example, the EBC appears to be entrained by the southern anticyclonic eddy, and is constrained to the coast and mixed with offshore water by the interaction of the eddy pair.…”

“…It has been suggested that eddy‐induced upwelling events, with variable position and lifespan, interact with the vertical and horizontal thermohaline circulation by injecting deep‐water nutrients into the surface layers (Gaube et al, ; Kürten et al, ; Martin & Richards, ). Elsewhere, it is shown that the interaction of the EBC with mesoscale eddies and may function as a barrier to this northbound thermohaline flow of the EBC, by deflecting the EBC toward the west and offshore (Figure , Zarokanellos et al, ). Models, remote‐sensing of ocean color and altimetry indicate that the CRS is dominated by recurrent and occasionally dipole eddies (Chen et al, ; Raitsos et al, ; Zhan et al, ).…”

Evaporative Implications of Dry‐Air Outbreaks Over the Northern Red Sea

“…Model simulations (e.g., Eshel & Naik, 1997;Sofianos & Johns, 2003;Yao, Hoteit, Pratt, Bower, Zhai, et al, 2014) and some observations (Bower & Farrar, 2015;Zarokanellos, Papadopoulos, et al, 2017) indicate that the surface EBC carries warm and relatively low-salinity surface waters, becoming cooler, more saline, and denser as it progresses northward due to intense air-sea fluxes. The composites suggest that the high evaporation associated with the large-scale westward winds may induce surface water mass transformation in the surface EBC.…”

“…Despite these differences, they have a very similar pattern. During winter, this region is dominated by the northward-flowing surface Eastern Boundary Current (EBC) that is believed to be an essential piece of the Red Sea overturning circulation (Bower & Farrar, 2015;Sofianos & Johns, 2003;Zarokanellos, Papadopoulos, et al, 2017). The WHOI/KAUST mooring is located on the southern flank of this high evaporation pool (Figure 12).…”

Three‐Dimensional Signature of the Red Sea Eddies and Eddy‐Induced Transport

“…These have been captured in various independent observations, including remotely sensed sea surface height (SSH) data (Bower & Farrar, 2015;Raitsos et al, 2017;Zhan et al, 2014), sea surface temperature (SST) data (Papadopoulos et al, 2015), chlorophyll-a data (Raitsos et al, 2013), synthetic aperture radar imagery (Karimova & Gade, 2014), and in situ observations (Sofianos & Johns, 2007;Zarokanellos et al, 2017;Zhai & Bower, 2013). These have been captured in various independent observations, including remotely sensed sea surface height (SSH) data (Bower & Farrar, 2015;Raitsos et al, 2017;Zhan et al, 2014), sea surface temperature (SST) data (Papadopoulos et al, 2015), chlorophyll-a data (Raitsos et al, 2013), synthetic aperture radar imagery (Karimova & Gade, 2014), and in situ observations (Sofianos & Johns, 2007;Zarokanellos et al, 2017;Zhai & Bower, 2013).…”