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Water depth anomaly in the Gulf of Aden

The water depth anomaly in the Gulf of Aden is shallower than it should be because of eddies, outflows and evaporation due to high and recent rise in sea surface temperatures (SST) during summers seasons. According to Bower et al (2005) the Saline, dense Red Sea Water (RSW) originates in the northern Red Sea because of an excess of evaporation over precipitation. It enters the Gulf of Aden (GOA) in the northwestern Indian Ocean as a dense overflow through the shallow Bab el Mandeb.

The Red Sea Outflow Water (RSOW) is the entrained mixed product water that descends from the 150 m deep Hanish Sill in the northern BAM Strait, less dense fresher water. However, during winter, the Red Sea outflow transport is typically about two times the annual mean because of monsoon winds and seasonal fluctuations in buoyancy forcing. Outflow transport reaches a maximum ( 0. 6 Sv) in winter (October–May), when prevailing monsoon winds over the region are from the south-southeast (Bower et al 2002).

There have been recently numerous and new oceanographic observations in the Gulf of Aden along the northwestern Indian Ocean. Some studies have shown large, energetic, deep-reaching mesoscale eddies (Bower et al 2002) that contribute and influence the spreading rates and pathways of intermediate-depth Red Sea Water (RSW). The other important pieces of the thermal puzzle are the present-day topography, gravity, seismic velocities and all the geological aspects contribute to the shallowness.

Comparison of salinity and direct velocity measurements (Bower et al 2002) indicates that the eddies advect and stir RSW through the Gulf of Aden and the anomalous water properties in the center of the anticyclonic eddy point to a possible formation site in the Somali Current System. The exchange flow has a two-layer structure, with dense, saline RSW flowing out at the bottom, and less dense GOA surface water flowing toward the Red Sea in the upper layer.

During summer (June–September), prevailing winds from the north-northwest drive a surface flow out of the Red Sea, and GOA water flows in via an intermediate layer sandwiched between the surface layer and a thin layer of outflowing dense RSW, producing a three-layer exchange flow. The overflow results from an excess of evaporation over precipitation in the entire Red Sea is estimated to be about 2 m yr_1. The existence of mesoscale eddies and currents contribute to the water depth.

Low oxygen water perists throughout a wide depth range in the intermediate waters of the Arabian Sea. The extensive exchange of water between the Red Sea, the Gulf of Aden and the Arabian Sea, the strong evaporation and the monsoonal winds that blow over the region, all assist in the formation of complex vertical structures in the water column of the Gulf of Aden (Al Saafani 2008). As argued by Yamanaka et al (2008), the resultant outflow from the Red Sea through the Gulf of Aden is considered to play a strong role in determining the properties of these intermediate waters

Several studies and research have been done to ascertain the reasons for shallowness in relation to water depth anomalies in this Gulf. Bower et al (2005) conducted a study dubbed the Red Sea Outflow Experiment (REDSOX), which was the first comprehensive field study of the hydrography and circulation of both the descending Red Sea outflow plume, and the equilibrated RSOW in the GOA. The study aimed at achieving the following: 1) describing the pathways and downstream evolution of the descending outflow plumes in the western Gulf of Aden,

2) quantifying the processes that control the final depth of the equilibrated RSOW, and 3) Identifying the transport processes and mechanisms that advect RSOW and its properties through the GOA and into the Indian Ocean. This study have revealed new large, energetic, deep-reaching eddies in the Gulf of Aden that fundamentally influence the spreading of RSW. With the use of Shipboat Acoustic Doppler Current Profiler (SADCP), measurements at 100 and 300 m reveal that currents in the Gulf of Aden were strong and organized into coherent eddy structures in February 2001 (Bower et al 2002).

The most conspicuous feature is an energetic cyclonic eddy adjacent to the Somali coast in the southwestern part of the gulf. From these studies we can deduce that the outflow waters reached neutral buoyancy where the bathymetric channels empty into the deep Tadjura Rift during winters and the northern channel was the source for the most saline, deepest and densest salinity maximum. Outflow currents are lower during summer due to evaporation.

The outflow water reached neutral buoyancy somewhat upstream of the channel exits, with lower salinity, temperature, and potential density in comparison with winter (Bower et al 2002). It is also evident that the two deeper salinity maxima at nominal depths of 600 and 800 m, are initially more confined by the walls of the Tadjura Rift. Waters associated with both maxima are generally swept southward out of the rift through gaps in the southern rift wall and also along the continental slope. , Other theories suggest that magma accumulation, heat and changing atmospheric pressure.

Lucazeau et al (2008) alleges that accumulation of magma below the OCT crust could produce the expected thermal and density anomaly by advection of hotter and lighter material and release of latent heat. He further explains that another alternative is serpentinization of the continental mantle, which might be exposed and fractured in the OCT and then percolated by water flows, as observed at several locations on mid-ocean ridges. Al Saafani (2008) noted that the atmospheric pressure at mean sea-level is highest during January and lowest during July at Aden with a range of 10 m bar.

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