Why does the surface circulation of the northern Indian Ocean reverse direction twice a year?

The surface circulation of the northern Indian Ocean reverses direction twice a year primarily because it is wind-driven and dictated by the seasonal monsoon system.1

Unlike the Atlantic and Pacific Oceans, where currents remain relatively stable, the northern Indian Ocean is subject to the shifting winds of the monsoon. These winds reverse their direction approximately every six months due to the differential heating of land and sea:2

• Northeast Monsoon (November–March): During this period, the winds blow from the northeast, driving the North Equatorial Current (or Northeast Monsoon Drift) in a southwest and westerly direction.21
• Southwest Monsoon (April–October): As the winds shift to blow from the southwest, they reverse the flow of the surface currents, pushing the Monsoon Current eastward and sending branches into the Arabian Sea and Bay of Bengal.21

This seasonal reversal creates opposing gyres that are separated by the Indian subcontinent, fundamentally altering the ocean's hydrology twice annually.1