El Niño and La Niña sea surface temperature anomalies: Asymmetry characteristics associated with their wind stress anomalies

The asymmetric nature of El Niño and La Niña sea surface temperature (SST) anomalies is investigated by the use of National Centers for Environmental Prediction reanalysis data and various ocean and atmosphere models. It is demonstrated that the relatively weak SST anomalies during La Niña compared with those of El Niño are related to the westward shift of wind stress anomalies by 10°–15°. The asymmetric characteristics of atmospheric responses are confirmed by the general circulation model experiments with the two different SST anomalies, which have equal amplitude but are of opposite sign from each other. The experiments with an intermediate ocean model and a hybrid coupled model clearly show that the SST anomalies over the equatorial Pacific become weaker as the zonal wind stress shifts to the west. Not only the amplitude but also the oscillation timescale of the SST anomaly is shown to be sensitive to the location of wind stress anomalies. The duration of La Niña, which is rather shorter than that of El Niño, is also related to the longitudinal displacement of the wind stress anomaly.