During the Southern Oscillation, winds are weaker in the west; thus the wind anomaly, or deviation from an average wind for that time of year is from west to east. This causes a change in the Ekman transport in this region. Instead of flowing away from the equator, it flows toward the equator. To balance this anomalous influx of water, the warm water in the upper layer downwells to the lower layer. However, the warm water is lighter than the cooler water, and is naturally more buoyant. The forcing of the warmer water into the cooler water will cause an oscillation, normally at the steepest density gradient, the thermocline. These oscillations will propagate away from the source of the wind anomaly as very long waves. Along the equator, these are called Kelvin waves and they change the thickness of warm water in both the west and the east, which causes large sea-level changes and leads to El Niņo.
Cross-Section Along Equator, Looking North
Cross-Section Looking West;
Anomalous Winds west to east