Abstract
Using satellite-derived surface current products, we examined annual current variations in the equatorial Pacific Ocean and associated zonal displacement of the Pacific warm pool. Annual current variations, generated mainly by annual equatorial Rossby waves, displaced the warm pool eastward (westward) during boreal spring-summer (winter), with accompanying increase (decrease) in the zonal heat advection at a rate of 0.2-0.4 °C mon-1 in the warm pool region in normal years when neither El Niño nor La Niña event take place; similar seasonal increases (decreases) have also been observed in El Niño years. Annual zonal currents variations attributable to oceanic Rossby waves provide more (less) favorable background sea surface conditions for the onset of El Niño during boreal summer (winter) by weakening (strengthening) the zonal sea surface temperature gradient through eastward (westward) heat transport near the equator in the warm pool region. Annual zonal currents variations might play a key role in seasonally locking the onset of El Niño events in the warm pool region.
