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Why do equatorial currents generally flow westward?

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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.

Equatorial currents generally flow westward primarily because they are driven by the trade winds. These persistent east-to-west winds exert a shearing stress on the ocean surface, which pushes the surface waters in a westward direction.123

Specifically, the North Equatorial Current is propelled by the Northeast Trade Winds, while the South Equatorial Current is driven by the Southeast Trade Winds. While the Coriolis effect influences the movement of ocean currents globally by deflecting them, its horizontal component is negligible at the equator, allowing the wind-driven force to dominate the direction of these currents.124

In contrast, the eastward-flowing Equatorial Counter Current exists near the thermal equator, where the wind field is offset, and the Equatorial Undercurrent flows eastward beneath the surface due to a pressure gradient caused by higher sea levels in the western margins of the tropical oceans.12