High-resolution simulation of monsoon variability of the Indian Ocean currents

A numerical model of the Indian Ocean circulation is developed at the Institute of Numerical Mathematics, Russian Academy of Sciences, and the results of simulation of monsoon current variability are presented. The model resolution is 1/8° in longitude, 1/12° in latitude with 21 unevenly vertical levels. The model was run for 15 years using monthly mean atmospheric data from NCEP reanalysis. The annual cycle of surface and subsurface currents, temperature and salinity fields are analysed for the final 15th year. The model reproduces the Summer Monsoon and Winter Monsoon Currents, their annual cycles, space structures and magnitudes. The modelled Somali Current has a good resemblance to observations with model velocities exceeding 2 ms⁻¹ and the transport about 70 Sv during the Summer Monsoon. The model results show that a turn of the Somali Current from the summer to winter direction is accompanied by the generation of anticyclonic eddies that drift westward by the β -effect and dissipate in the Somalia shore and the Gulf of Aden. The monsoon variability of equatorial surface currents and equatorial undercurrents is analysed. It is shown that these currents are mainly generated by zonal wind stress, which is dominated at the equator by a semi-annual harmonic. Therefore, the equatorial surface currents and the compensating equatorial subsurface undercurrents also change directions with a semi-annual periodicity. It is found that gradient currents caused by river run-off make an important contribution to the circulation in the Bay of Bengal. The main features of quasistationary vigorous eddies like Great Whirl, Socotra High, Lakshadweep High, and Lakshadweep Low are well reproduced.