Contrasting the termination of moderate and extreme El Nio events in coupled general circulation models

As in the observed record, the termination of El Nio in the coupled IPCC-AR4 climate models involves meridional processes tied to the seasonal cycle. These meridional processes both precondition the termination of El Nio events in general and lead to a peculiar termination of extreme El Nio events (such as those of 1982-83 and 1997-98), in which the eastern equatorial Pacific warm sea surface temperature anomalies (SSTA) persist well into boreal spring/early-summer. The mechanisms controlling the peculiar termination of extreme El Nio events, which involves to the development of an equatorially centred intertropical convergence zone, are consistent across the four models that exhibit extreme El Nios and observational record, suggesting that this peculiar termination represents a general feature of extreme El Nios. Further, due to their unusual termination, extreme El Nios exhibit an apparent eastward propagation of their SSTA, which can strongly influence estimates of the apparent propagation of ENSO over multi-decadal periods. Interpreting these propagation changes as evidence of changes in the underlying dynamical feedbacks behind El Nio could therefore be misleading, given the strong influence of a single extreme event.