Abstract
Abstract. By analysing ARGO float data over the last four years, a few aspects of the mesoscale variability of water masses in the Arabian Sea are described. The Red Sea Outflow Water (RSOW) is concentrated in the Southwestern Gulf of Aden, in particular when a cyclonic gyre predominates in this region. Salinities of 36.5 and temperatures of 16 °C are found in this area at depths between 600 and 1000 m. RSOW is more dilute in the eastern part of the Gulf, where intense and relatively barotropic gyres mix it with Indian ocean Central Water. RSOW is also detected along the northeastern coast of Socotra, and fragments of RSOW are found between one and three degrees of latitude north of this island. In the whole Gulf of Aden, the correlation between the deep motions of the floats and the sea-level anomaly measured by altimetry is strong, at regional scale. The finer scale details of the float trajectories are not sampled by altimetry and are often related to the anomalous water masses that the floats encounter. The Persian Gulf Water (PGW) is found in the float profiles near Ras ash Sharbatat (near 57° E, 18° N), again with 36.5 in salinity and about 18–19 °C in temperature. These observations were achieved in winter when the southwestward monsoon currents can advect PGW along the South Arabian coast. Fragments of PGW were also observed in the Arabian Sea between 18 and 20° N and 63 and 65° E in summer, showing that this water mass can escape the Gulf of Oman southeastward, during that season. Kinetic energy distributions of floats with respect to distance or angle share common features between the two regions (Gulf of Aden and Arabian Sea), in particular peaks at 30, 50 and 150 km scales and along the axis of monsoon currents. Hydrological measurements by floats are also influenced by the seasonal variations of PGW and RSOW in these regions.
Highlights
The Northwestern Indian Ocean is composed of the 3000– 5000 m deep Arabian Basin bounded east by the shallower Chagos-Laccadive ridge and the Indian continental shelf and west by the shallower Gulf of Aden
Southwest of the Arabian Sea, the Somali Basin has the largest depths. This oceanic region is connected to two evaporation basins, the Persian Gulf, and the Red Sea
The circulation and water masses in the Arabian Sea are strongly influenced by the monsoon winds and by the intense air-sea heat fluxes (Johns et al, 2000)
Summary
The Northwestern Indian Ocean is composed of the 3000– 5000 m deep Arabian Basin bounded east by the shallower Chagos-Laccadive ridge and the Indian continental shelf and west by the shallower Gulf of Aden. The Red Sea Water enters the Gulf of Aden with a temperature close to 22 ◦C and a salinity close to 40 These values decrease to 16 ◦C and 36 once this water has mixed with surrounding waters (Indian ocean Central Water, ICW) and the RSOW settles at depths of 600 to 900 m in the Arabian Sea; from there, it follows the East African coast southward. Strong mixing with Indian Ocean Surface Water (IOSW) decrease the thermohaline characteristics of the outflowing water to about 20 ◦C and to 37.5 in the Gulf of Oman where the equilibrium depth of PGW is about 200–250 m; the PGW characteristics decrease to 17 ◦C and 36.2 in the northern Arabian Sea where its equilibrium depth is about 300 m (Prasad et al, 2001). Deep mesoscale displacements are studied in relation with surface motion provided by altimetric maps
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