Understanding the double peaked El Niño in coupled GCMs

Abstract

Coupled general circulation models (CGCMs) simulate a diverse range of El Nino–Southern Oscillation behaviors. “Double peaked” El Nino events—where two separate centers of positive sea surface temperature (SST) anomalies evolve concurrently in the eastern and western equatorial Pacific—have been evidenced in Coupled Model Intercomparison Project version 5 CGCMs and are without precedent in observations. The characteristic CGCM double peaked El Nino may be mistaken for a central Pacific warming event in El Nino composites, shifted westwards due to the cold tongue bias. In results from the Australian Community Climate and Earth System Simulator coupled model, we find that the western Pacific warm peak of the double peaked El Nino event emerges due to an excessive westward extension of the climatological cold tongue, displacing the region of strong zonal SST gradients towards the west Pacific. A coincident westward shift in the zonal current anomalies reinforces the western peak in SST anomalies, leading to a zonal separation between the warming effect of zonal advection (in the west Pacific) and that of vertical advection (in the east Pacific). Meridional advection and net surface heat fluxes further drive growth of the western Pacific warm peak. Our results demonstrate that understanding historical CGCM El Nino behaviors is a necessary precursor to interpreting projections of future CGCM El Nino behaviors, such as changes in the frequency of eastern Pacific El Nino events, under global warming scenarios.