Seasonal Characteristics of the Circulation Structure in the Northern Red Sea and Their Relationship with the Thermohaline and Wind Forcing

A combination of glider observations, satellite imagery, atmospheric reanalysis products during winter and summer 2019 are used to identify the seasonal thermohaline and dynamic characteristics of the Northern Red Sea (NRS). In addition, we examine the rapid biogeochemical response to physical processes that occurred in both seasons. After winter convection and mixing, an anticyclonic eddy (AE) was found in response to intensification of the eastward wind. The AE was observed in the area for approximately 15 days. The eddy was characterized by salinity (~40.3) and temperature (<23˚C), which is consistent with the higher concentrations of oxygen and chlorophyll. Remote sensing observations (SST, OC and SSH) indicate that the AC was migrated southward in the basin. The sea level anomaly shows a local sea height elevation consistent with a southward direction of the geostrophic current. We attributed the presence of the AE to change in the wind stress as sea level changes within the time scale from 4-30 days are likely responding to atmospheric forcing. This AE in the north appears, temporarily, to have blocked the northward flow of eastern boundary current as previous studies have indicated in the central Red Sea. After the southward migration of the AE, the quasi-permanent cyclonic eddy (CE) was re-established in the NRS. With this transition, warmer, lower salinity water was once again advected into the NRS. This fresher, warmer water capped the water column and re-established the seasonal thermocline (pycnocline). Associated with these low salinity intrusions, the deep chlorophyll maximum underlay the low salinity surface layer. As Gulf of Aden water arrives, the chlorophyll maximum shifts to lower density isopycnals on the nearshore half of the transect, modifying the plankton distribution, and perhaps advecting a distinct phytoplankton population into the region. These observations clearly demonstrate that despite the thermohaline forcing of the Red Sea, local winds can contribute into changes in the polarity of the eddy field at the synoptic atmospheric forcing time scales.