Equatorial Undercurrent

The Equatorial Undercurrent is a subsurface eastward jet centered on the equator within the thermocline, present in the Pacific (the Cromwell Current), Atlantic, and Indian Oceans. It flows against the westward surface current, driven by the eastward pressure gradient that the trade winds build up by piling warm water in the west. Core speeds reach about 1 to 1.5 meters per second near 100 to 200 meters depth, with transport near 30 to 40 Sverdrups in the Pacific. The Coriolis force vanishing at the equator allows the jet to balance the pressure gradient and stay equatorially trapped.