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Climate Dynamics

, Volume 42, Issue 1–2, pp 171–188 | Cite as

Equatorial Atlantic variability and its relation to mean state biases in CMIP5

  • Ingo Richter
  • Shang-Ping Xie
  • Swadhin K. Behera
  • Takeshi Doi
  • Yukio Masumoto
Article

Abstract

Coupled general circulation model (GCM) simulations participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are analyzed with respect to their performance in the equatorial Atlantic. In terms of the mean state, 29 out of 33 models examined continue to suffer from serious biases including an annual mean zonal equatorial SST gradient whose sign is opposite to observations. Westerly surface wind biases in boreal spring play an important role in the reversed SST gradient by deepening the thermocline in the eastern equatorial Atlantic and thus reducing upwelling efficiency and SST cooling in the following months. Both magnitude and seasonal evolution of the biases are very similar to what was found previously for CMIP3 models, indicating that improvements have only been modest. The weaker than observed equatorial easterlies are also simulated by atmospheric GCMs forced with observed SST. They are related to both continental convection and the latitudinal position of the intertropical convergence zone (ITCZ). Particularly the latter has a strong influence on equatorial zonal winds in both the seasonal cycle and interannual variability. The dependence of equatorial easterlies on ITCZ latitude shows a marked asymmetry. From the equator to 15°N, the equatorial easterlies intensify approximately linearly with ITCZ latitude. When the ITCZ is south of the equator, on the other hand, the equatorial easterlies are uniformly weak. Despite serious mean state biases, several models are able to capture some aspects of the equatorial mode of interannual SST variability, including amplitude, pattern, phase locking to boreal summer, and duration of events. The latitudinal position of the boreal spring ITCZ, through its influence on equatorial surface winds, appears to play an important role in initiating warm events.

Keywords

Equatorial Atlantic GCM biases Interannual variability Double ITCZ CMIP5 Bjerknes feedback Surface winds Equatorial Kelvin waves 

Notes

Acknowledgments

We thank the anonymous reviewers for their helpful comments. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, the U.S. Department of Energy’s Program for Climate Model Diagnosis and Intercomparison which provides coordinating support and led development of software infrastructure for CMIP, and the climate modeling groups for making available their model output. SPX is supported by NOAA.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2012

Authors and Affiliations

  • Ingo Richter
    • 1
    • 2
  • Shang-Ping Xie
    • 3
    • 4
    • 5
  • Swadhin K. Behera
    • 1
    • 2
  • Takeshi Doi
    • 1
    • 2
  • Yukio Masumoto
    • 1
  1. 1.Research Institute for Global ChangeJAMSTECYokohamaJapan
  2. 2.Application LaboratoryJAMSTECYokohamaJapan
  3. 3.International Pacific Research CenterUniversity of Hawaii at ManoaHonoluluUSA
  4. 4.Department of MeteorologyUniversity of Hawaii at ManoaHonoluluUSA
  5. 5.Scripps Institution of OceanographyUniversity of California at San DiegoLa JollaUSA

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