ENSO Warming: Ekman Transport in Eastern Pacific

The El Niño Southern Oscillation (ENSO) phenomenon is characterized by significant changes in sea surface temperatures across the equatorial Pacific Ocean.

During the El Niño phase of ENSO, the Eastern Equatorial Pacific experiences anomalous warming.

Understanding Ekman Transport

Ocean Ekman transport is the net movement of surface water caused by wind. In the tropical Pacific, prevailing winds typically drive surface water away from the equator (poleward Ekman transport).

This poleward movement facilitates the upwelling of cold, deep ocean water along the equator. This upwelling process normally keeps the Eastern Equatorial Pacific relatively cool.

El Niño Mechanism

• Weakened Trade Winds: During El Niño events, the trade winds across the Pacific weaken significantly.
• Reduced Wind Stress: The weakening of these winds reduces the east-to-west wind stress on the ocean surface.
• Decreased Ekman Transport: Consequently, the poleward Ekman transport of surface water from the equator diminishes.
• Less Upwelling: With reduced Ekman transport, the upwelling of cold deep water lessens.
• Surface Warming: The reduced upwelling allows the normally warm surface waters to accumulate in the Eastern Equatorial Pacific, leading to anomalous warming.

Therefore, the warming of the Eastern Equatorial Pacific during El Niño is attributed to the reduction in poleward ocean Ekman transport.