The easterly Equatorial Counter-Current between about 4° and 10° N may be explained as follows: The overall effect of the Trade Winds is to drive water towards the west, but the flow is blocked by the land masses along the western boundaries. As a result, in equatorial regions the sea-surface slopes up towards the west, causing an eastward horizontal pressure gradient force. Because winds are light in the Doldrums, water is able to flow down the horizontal pressure gradient in a current that is contrary (i.e. 'counter') to the prevailing wind direction. The zone of calm is the necessary and dominant factor here, though other factors play their role as well.