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Why do currents in the northern Indian Ocean reverse with the monsoons?

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World map showing major ocean current systems, with arrows indicating the direction and speed of warm and cold currents in the Atlantic, Pacific, Indian, Southern, and Arctic Oceans. Warm currents are marked in red and cold currents in blue, with currents labeled by name and symbols denoting those that reverse direction seasonally. A legend explains current speed categories and color codes. — ocean current Major ocean current systems of the world.

Ocean surface circulation is primarily wind-driven, and the northern Indian Ocean is unique because its surface currents reverse direction every half year in response to the seasonal monsoon winds.12

During the northeast monsoon (November–March), the prevailing winds drive the North Equatorial Current westward across the northern Indian Ocean. However, when the southwest monsoon sets in from April to October, the wind direction shifts. These winds, combined with southeast trade winds that cross the Equator, push the surface waters eastward, creating the Monsoon Current.12

This seasonal reversal also impacts major regional currents, such as the Somali Current. During the summer, the southwest monsoon drives the Somali Current northeastward along the coast of East Africa; as the monsoon weakens and reverses in the winter, the current also reverses to a slow southwestward flow. This biannual shift creates two opposing gyres that are separated by the Indian subcontinent, distinguishing the Indian Ocean's circulation from the more stable patterns found in the Atlantic and Pacific oceans.3412