A model of the seasonal circulation in the Arabian Sea forced by observed winds

Abstract

Results of a numerical model of the wind driven seasonal circulation in the Arabian Sea are presented, with particular emphasis on the ocean's response to the monsoon winds. The model equations are the fully nonlinear reduced gravity transport equations in spherical coordinates. The model resolution is 1/8° in the east-west direction and 1/4° in the north-south direction. The model basin geometry corresponds as closely as possible to that of the north-west Indian Ocean from 40°E to 73°E and from 10°S to 25°N, and includes the gulfs of Aden and Oman, and the islands of Socotra and the Seychelles. The southern boundary and a portion of the eastern boundary, from the equator to 6°S, representing the opening between the Maldives and the Chagos Archipelago, are open boundaries. At other boundaries, the no-slip condition is applied. The wind stress data used to force the model comes from the NOAA National Climate Center's TD-9757 Global Marine Sums data, which consists of monthly mean winds compiled on 1° squares from over 60 years of ship observations. These data are interpolated in time using the mean and first five Fourier harmonics at each point, and then interpolated linearly to the model grid. The model equations are integrated in time using centered finite differences in time and space (a leap-frog scheme), with lateral friction treated by a Dufort-Frankel scheme.

After a one year spin up, the model settles into a regular periodic seasonal cycle, even though the solution to the model equations is locally highly nonlinear, with large nonlinear eddies developing in the same location at the same time of year from one year to the next. The development of the model Somali Current system with the onset of the (northern hemisphere) summer monsoon is consistent with the available observations in the region. The model reproduces many of the observed features in this region, such as the two-gyre circulation pattern, and the timing and movement of these features corresponds well with their real world counterparts. The model also shows an eastward jet forming in late June to early July at 13°N, just to the east of Socotra. This jet is fed by flow coming out of the great whirl. The break down of the two-gyre pattern occurs in mid to late August, when the southern gyre breaks up into several smaller eddies, the northern-most of which coalesces with the great whirl. Numerous small cyclonic eddies develop along the Arabian coast, from the Gulf of Oman into the Gulf of Aden, in early to mid August, and persist well into the winter monsoon. The model shows that it is possible to simulate very complicated flows, if one has sufficient wind data, using fairly simple models with a realistic basin geometry.