Buoyancy-driven circulation in the Red Sea

Abstract

This thesis explores the buoyancy-driven circulation in the Red Sea, using a combination of observations, as well as numerical modeling and analytical method.The first part of the thesis investigates the formation mechanism and spreading of Red Sea Overflow Water (RSOW) in the Red Sea. The preconditions required for open-ocean convection, which is suggested to be the formation mechanism of RSOW, are examined. The RSOW is identified and tracked as a layer with minimum potential vorticity and maximum chlorofluorocarbon-12. The pathway of the RSOW is also explored using numerical simulation. If diffusivity is not considered, the production rate of the RSOW is estimated to be 0.63 Sv using Walin's method. By comparing this 0.63 Sv to the actual RSOW transport at the Strait of Bab el Mandeb, it is implied that the vertical diffusivity is about The second part of the thesis studies buoyancy-forced circulation in an idealized Red Sea. Buoyancy-loss driven circulation in marginal seas is usually dominated by cyclonic boundary currents on f-plane, as suggested by previous observations and numerical modeling. This thesis suggests that by including β-effect and buoyancy loss that increases linearly with latitude, the resultant mean Red Sea circulation consists of an anticyclonic gyre in the south and a cyclonic gyre in the north. In mid-basin, the northward surface flow crosses from the western boundary to the eastern boundary. The observational support is also reviewed. The mechanism that controls the crossover of boundary currents is further explored using an ad hoc analytical model based on PV dynamics. This ad hoc analytical model successfully predicts the crossover latitude of boundary currents. It suggests that the competition between advection of planetary vorticity and buoyancy-loss related term determines the crossover latitude.The third part of the thesis investigates three mechanisms that might account for eddy generation in the Red Sea, by conducting a series of numerical experiments. The three mechanisms are: i) baroclinic instability; ii) meridional structure of surface buoyancy losses; iii) cross-basin wind fields.

The eddy kinetic energy budget in the Red Sea

“…Based on a high-resolution MITgcm, we investigated the budget of eddy energy sources, redistribution and sink in the Red Sea during different seasons. In the Red Sea, overturning circulation has been reported to Journal of Geophysical Research: Oceans 10.1002/2015JC011589 exhibit a distinct seasonally reversing pattern [Yao et al, 2014a[Yao et al, , 2014bSofianos and Johns, 2003;Zhai, 2014], consequently, eddies are calculated with regards to a climatologically monthly mean of flow to eliminate the seasonally varying signals. Our results agree with Zhan et al [2014] who found that eddy activity has a significant seasonality that peaks in winter and troughs in summer.…”

“…The tilted isopycnal in the central and northern basins generates available potential energy, which, when accompanied with a relatively smaller Richardson number in winter, is likely to be released by triggering baroclinic instability that subsequently form eddies [ Zhan et al ., ]. Eddies generated by such baroclinic instability can balance surface buoyancy loss in a marginal sea [ Visbeck et al ., ; Marshall and Schott , ], and, therefore, have a considerable effect on the buoyancy‐driven circulation in the Red Sea [ Zhai , ]. In contrast, in summer, the entire basin is subjected to a positive net heat flux.…”

“…The eddy‐associated transports are reported to be several times greater than the mean overturning transport [ Bower and Farrar , ], and the intensity of eddies may affect deep water formation and overturning circulation in the basin [ Papadopoulos et al ., ]. In addition, eddies in the Red Sea also have a strong impact on the marine ecosystem: the eddies' relatively well‐maintained integrities and energetic flow allow them to transport nutrients, heat, carbon, and other biogeochemical particles from one coast to the other, processes sometimes enhanced by a series of flanking eddies, and vertically eddy‐induced pumping brings nutrient‐enriched subsurface water into the euphotic zone to help sustain primary production [ Zhai , ; Xiu and Chai , ; Stramma et al ., ].…”

“…Zhai and Bower [] identified the dipole eddy system near 18°N–19°N as an ocean response to the Tokar wind jet using observations and an idealized numerical model of the Red Sea. And Zhai [] discussed other possible mechanisms as the source of eddy generation in the Red Sea including baroclinic instability, bottom topography, cross‐basin wind, and meridional varying buoyancy forcing.…”

Identifying Algal Bloom ‘Hotspots’ in Marginal Productive Seas: A Review and Geospatial Analysis

“…NEM wind causes counterclockwise circulations and cools the surface waters, causing deep convective mixing, whereas SWM wind causes clockwise circulations and upwelling in the Arabian Sea. At the same time, waters are driven into and out of the Sea of Oman, the Arabian Gulf, and the Red Sea steered by boundaries, with gyres being found in the eastern part of the Sea of Oman, the central southern and northern Arabian Gulf, and in the southern and northern Red Sea [23], as shown in Figure 2. As for the biogeochemical properties, there are two distinct periods of high biological activity that emerge as a result of the semiannual wind reversals associated with the monsoon phenomenon.…”