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Why does the Equatorial Undercurrent flow eastward below the ocean surface?

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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.

The Equatorial Undercurrent flows eastward below the ocean surface primarily due to a horizontal pressure gradient.1

This gradient is created by the accumulation of water in the western margins of the tropical ocean, which results in a higher sea level in the west compared to the east. Because the Coriolis force is absent or negligible at the geographic Equator, this pressure difference is not balanced by the typical forces that deflect currents at higher latitudes. Consequently, the water is driven from the area of high pressure in the west toward the lower pressure in the east, forming a jetlike, eastward-flowing current beneath the surface layer.1

While the surface layer itself is dominated by the westward-flowing South Equatorial Current driven by trade winds, the Equatorial Undercurrent remains distinct, flowing in the opposite direction at depths of approximately 160 to 500 feet (49 to 152 m).21