Abyssal circulation in the Somali Basin

The bottom and deep circulation in the Somali Basin are investigated on the basis of hydrographic and direct velocity profiles from three shipboard surveys carried out during the southwest monsoon in 1995 and of velocity time series from the WOCE mooring array ICM7. The inflow of bottom water into the Somali Basin through the Amirante Passage drives a thermohaline circulation, which may be modulated by the monsoon wind forcing. Details of the abyssal circulation have been discussed controversially. Deep velocity records from the mooring array in the northern Somali Basin are dominated by fluctuations with periods of 30–50 days and amplitudes above 5 cm s−1. Despite this strong variability annual record averages indicate the existence of a deep western boundary current (DWBC) below 3000 m at the base of the continental slope south of Socotra Island as part of a cyclonic bottom circulation. The southwestward DWBC transport off Socotra Island is estimated to 2±1.3 Sv. The bottom and deep water exchange between the Somali and Arabian Basin north of 7°N is estimated from two cross-basin geostrophic velocity sections referenced by vertically averaged LADCP currents. For the bottom water, an eastward transport into the Arabian Basin of 1.4±0.5 Sv and 2.1±0.6 Sv was determined in June and August, respectively, while for the deep-water layer above 3500 m eastward transports of 3.6±2.3 Sv in June and 4.0±2.4 Sv in August were obtained.