Notice the counter
currents along the equator, in orange, which flow in the opposite
direction of each gyre.
Think of it this way.....the northern
and southern equatorial currents, pushed westward by the trade
winds, pile up water in the western portion of each ocean basin.
For example, the sea surface is about 0.5 meter (20 inches) higher
in the western Pacific near Indonesia than it is in the eastern
Pacific near South America. At some point, gravity will cause
this pile of warm water to flow back across the ocean basin where
the winds are weak, which is along the equator. This current
is especially strong during years of the El Nino climatic event.