Mechanism for Asymmetry in Zonal Phase Propagation of ENSO Sea Surface Temperature Anomalies

One manifestation of ENSO non-linear behaviour is an asymmetry in which SST anomalies propagate eastward during strong El Niño events and westward during La Niña events. The cause of this asymmetry has not been previously examined. In this study, heat budget analysis conducted using a reanalysis product which spans the 1959-2006 period reveals a mechanism that can give rise to this asymmetry. The asymmetry, which is more apparent in the post-1976 period, is shown to arise from the fact that the westward flowing equatorial Pacific background current is enhanced during La Niña and weakened during El Niño events. This favours more prominent westward advection of anomalous sea surface temperature during La Niña than during El Niño. During the extreme 1982 and 1997 El Niño events the zonal current anomalies are sufficiently strong to reverse the westward background current. Thus, both ENSO amplitude and the strength of the background currents can play a role in setting the degree of the asymmetry. This finding has implications for understanding ENSO dynamics under global warming and differing ENSO behaviour across climate models.