On the origin of equatorial Atlantic biases in coupled general circulation models
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Many coupled ocean–atmosphere general circulation models (GCMs) suffer serious biases in the tropical Atlantic including a southward shift of the intertropical convergence zone (ITCZ) in the annual mean, a westerly bias in equatorial surface winds, and a failure to reproduce the eastern equatorial cold tongue in boreal summer. The present study examines an ensemble of coupled GCMs and their uncoupled atmospheric component to identify common sources of error. It is found that the westerly wind bias also exists in the atmospheric GCMs forced with observed sea surface temperature, but only in boreal spring. During this time sea-level pressure is anomalously high (low) in the western (eastern) equatorial Atlantic, which appears to be related to deficient (excessive) precipitation over tropical South America (Africa). In coupled simulations, this westerly bias leads to a deepening of the thermocline in the east, which prevents the equatorial cold tongue from developing in boreal summer. Thus reducing atmospheric model errors during boreal spring may lead to improved coupled simulations of tropical Atlantic climate.
KeywordsCMIP Model Cold Tongue West African Monsoon Atmospheric Model Intercomparison Project Wind Bias
This study was supported by the NOAA CLIVAR Program and the Japan Agency for Marine-Earth Science and Technology through its sponsorship of the International Pacific Research Center. All the model output was downloaded from The IPCC Data Archive at Lawrence Livermore National Laboratory, which is supported by the Office of Science, U.S. Department of Energy. The authors would like to thank Justin Small and two anonymous reviewers for their helpful suggestions. IPRC publication #498.
- Breugem W-P, Hazeleger W, Haarsma RJ (2006) Multimodel study of tropical Atlantic variability and change. Geophys Res Lett 33. doi: 10.1029/2006GL027831
- Chang P, Coauthors (2006) Climate fluctuations of tropical coupled system—the role of ocean dynamics. J Clim 19:5122–5174Google Scholar
- Conkright ME, Locarnini R, Garcia H, O’Brien T, Boyer TP, Stephens C, Antonov J (2002) World Ocean Atlas 2001, objective analyses, data statistics and figures, CD-ROM documentation, National Oceanographic Data Center. Silver Spring, MDGoogle Scholar
- Davey MK, Coauthors (2002) STOIC: a study of coupled model climatology and variability in tropical ocean regions. Clim Dyn 18:403–420Google Scholar
- de Szoeke SP, Xie S-P (2008) The tropical eastern Pacific seasonal cycle: assessment of errors and mechanisms in IPCC AR4 coupled ocean–atmosphere general circulation models. J Clim (in press)Google Scholar
- Kalnay E, Coauthors (1996) The NCEP/NCAR 40-year reanalysis project. Bull Am Meteor Soc 77:437–471Google Scholar
- Mechoso CR, Roberston AW Coauthors (1995) The seasonal cycle over the tropical Pacific in general circulation models. Mon Weather Rev 123:2825–2835Google Scholar
- Richter I, Mechoso CR, Robertson AW (2008) What determines the position and intensity of the South Atlantic anticyclone in austral winter?—an AGCM study. J Clim 21:214–229Google Scholar
- Rouault M, Florenchie P, Fauchereau N, Reason CJC (2003) South East tropical Atlantic warm events and southern African rainfall. Geophys Res Lett 30. doi: 10.1029/2002GL014840
- Xie S-P, Carton JA (2004) Tropical Atlantic variability: patterns, mechanisms, and impacts. In: Earth climate: the ocean–atmosphere interaction. Geophys Monograph, vol 147. AGU, Washington DC, pp 121–142Google Scholar