Importance of oceanic resolution and mean state on the extra-tropical response to El Niño in a matrix of coupled models
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The extra-tropical response to El Niño in configurations of a coupled model with increased horizontal resolution in the oceanic component is shown to be more realistic than in configurations with a low resolution oceanic component. This general conclusion is independent of the atmospheric resolution. Resolving small-scale processes in the ocean produces a more realistic oceanic mean state, with a reduced cold tongue bias, which in turn allows the atmospheric model component to be forced more realistically. A realistic atmospheric basic state is critical in order to represent Rossby wave propagation in response to El Niño, and hence the extra-tropical response to El Niño. Through the use of high and low resolution configurations of the forced atmospheric-only model component we show that, in isolation, atmospheric resolution does not significantly affect the simulation of the extra-tropical response to El Niño. It is demonstrated, through perturbations to the SST forcing of the atmospheric model component, that biases in the climatological SST field typical of coupled model configurations with low oceanic resolution can account for the erroneous atmospheric basic state seen in these coupled model configurations. These results highlight the importance of resolving small-scale oceanic processes in producing a realistic large-scale mean climate in coupled models, and suggest that it might may be possible to “squeeze out” valuable extra performance from coupled models through increases to oceanic resolution alone.
KeywordsNorth Pacific Extra-tropical SST ENSO GCM Basic state
The models described were developed from the Met Office Hadley Centre Model by the UK High-Resolution Modelling (HiGEM) Project and the UK Japan Climate Collaboration (UJCC). HiGEM is supported by a NERC High Resolution Climate Modelling Grant (R8/H12/123). UJCC was supported by the Foreign and Commonwealth Office Global Opportunities Fund, and jointly funded by NERC and the DECC/Defra Met Office Hadley Centre Climate Programme (GA01101). Some model integrations were performed using the Japanese Earth Simulator supercomputer, supported by JAMSTEC. NCEP/NCAR reanalysis data were provided by the NOAA/OAR/ERSL PSD, Boulder Colorado, USA, from their web site at http://www.cdc.noaa.gov/. The UKMO HadISST data were provided by the British Atmospheric Data Centre, from their website at http://badc.nerc.ac.uk/data/hadisst/. AD was supported by a NERC PhD studentship. We thank two anonymous reviewers whose comments helped to improve the manuscript.
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