Understanding the double peaked El Niņo in coupled GCMs
Graham, FS and Wittenberg, AT and Brown, JN and Marsland, SJ and Holbrook, NJ, Understanding the double peaked El Nino in coupled GCMs, Climate Dynamics, 48, (5) pp. 2045-2063. ISSN 0930-7575 (2017) [Refereed Article]
Coupled general circulation models (CGCMs) simulate a diverse range of El Niņo-Southern Oscillation behaviors. "Double peaked" El Niņo 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 Niņo may be mistaken for a central Pacific warming event in El Niņo 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 Niņo 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 Niņo behaviors is a necessary precursor to interpreting projections of future CGCM El Niņo behaviors, such as changes in the frequency of eastern Pacific El Niņo events, under global warming scenarios.
ENSO, El Niņo evolution, coupled general circulation model, CMIP5, cold tongue bias, climate change