Asymmetry in the Duration of El Nino and La Nina

El Nino and La Nina are not a simple mirror image, but exhibit significant differences in their spatial structure and seasonal evolution. In particular, sea surface temperature (SST) anomalies over the equatorial Pacific cold tongue are larger in magnitude during El Nino compared to La Nina, resulting in positive skewness of interannual SST variations. The associated atmospheric deep convection anomalies are displaced eastward during El Nino compared to La Nina because of the nonlinear atmospheric response to SST. In addition to these well-known features, an analysis of observational data for the past century shows that there is a robust asymmetry in the duration of El Nino and La Nina. Most El Ninos and La Ninas develop in late boreal spring/summer, when the climatological cold tongue is intensifying, and they peak near the end of the calendar year. After the mature phase, El Ninos tend to decay rapidly by next summer, but many La Ninas persist through the following year and often reintensify in the subsequent winter. Throughout the analysis period, this asymmetric feature is evident for strong events in which Nino-3.4 SST anomalies exceed one standard deviation in December. Seasonally stratified composite analysis suggests that the eastward displacement of atmospheric deep convection anomalies during El Nino enables surface winds in the western equatorial Pacific to be more affected by remote forcing from the Indian Ocean, which acts to terminate the Pacific events.