Deep currents in the Arabian Sea in 1987

Abstract

New features of the deep circulation in the Arabian Sea were revealed by four sections of deep CTD-O 2 stations occupied by the R.R.S. Charles Darwin late in 1986 and early in 1987, during the northeast monsoon, and repeated during the succeeding southwest monsoon. The most prominent elements in the bottom-water layer were (1) a northward flowing, western-boundary current of volume transport of about 4×10 6 m 3 s −1 in the southern Somali Basin, which appeared to turn eastward at the equator and supply water to the northern Somali Basin from along the equator, and (2) a southeastward flowing boundary current along the northeastern flank of the Carlsberg Ridge in the Arabian Basin, which demonstrated that the bottom water of the Arabian Basin enters mainly from the Somali Basin through the Owen Fracture Zone, rather than from the Central Indian Basin. The strength of the boundary current in the Somali Basin implies an unusually large upward velocity at the top of the bottom water, about 12×10 −5 cm s −1. Mapping of water properties in the deep-water layer above the bottom water during successive monsoons was undertaken because three earlier surveys had shown differing horizontal distributions there, and had thus raised the possibility of a monsoonal reversal of the circulation, paralleling that in the near-surface water. However, the patterns of variation in the salinity and oxygen fields were essentially the same on the two Darwin cruises, suggesting a broad southwestward flow in the deep-water layer of the Somali Basin during both monsoons. We propose that this pattern was not forced by the monsoons but reflected mainly the mean circulation of the deep water as driven by the upwelling of the bottom water from below, that that pattern might be enhanced during the northeast monsoon if the wind-forcing penetrates into the deep water, and that a moderately strong southwest monsoon (in contrast to the extraordinarily weak monsoon of 1987) would be required to disrupt this distribution of water properties.