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A model for super El Ninos

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Two aspects of ENSO that have attracted widespread attention in recent times are the diversity in the spatial structure of ENSO events and the existence of super El Nino events. We have revisited the problem of how the tropical atmosphere responds to deep convective heating patterns of relevance to ENSO , using a dry atmospheric model that incorporates a realistic background state. We show that atmospheric state changes brought about by El Nino constrain thermally forced solutions in such a way that SST variability is suppressed (enhanced) over the eastern (central) Pacific. This implies that coupled dynamics inherent to the Pacific cannot account for the so-called canonical ENSO pattern that is characterized by strongest SST anomalies in the eastern Pacific. Next, we consider the importance of the downstream tropical circulation forced by Indian Ocean Dipole (IOD) events on surface winds over the Pacific. We show that this downstream effect is strong enough to have a notable impact on SST variations over the Pacific, and consequently in perturbing the evolution of El Nino. Further, the impact of IOD events are amplified (diminished) when it co-occurs with an El Nino (La Nina) event. Our results suggest that a synchrony of IOD and ENSO dynamics is necessary for the so-called super El Nino events to develop. Finally, we argue that a large part of ENSO diversity, often presented as the existence of two kind of El Nino events, is an artefact, arising from the lumping of the rare but extreme super El Nino events together with more moderate events

This talk is part of the DAMTP Atmosphere-Ocean Dynamics series.

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