Climate Dynamics

, Volume 29, Issue 1, pp 1–20 | Cite as

African monsoon teleconnections with tropical SSTs: validation and evolution in a set of IPCC4 simulations

  • Mathieu Joly
  • Aurore Voldoire
  • Hervé Douville
  • Pascal Terray
  • Jean-François Royer
Article

Abstract

A set of 12 state-of-the-art coupled ocean-atmosphere general circulation models (OAGCMs) is explored to assess their ability to simulate the main teleconnections between the West African monsoon (WAM) and the tropical sea surface temperatures (SSTs) at the interannual to multi-decadal time scales. Such teleconnections are indeed responsible for the main modes of precipitation variability observed over West Africa and represent an interesting benchmark for the models that have contributed to the fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC4). The evaluation is based on a maximum covariance analysis (MCA) applied on tropical SSTs and WAM rainfall. To distinguish between interannual and multi-decadal variability, all datasets are partitioned into low-frequency (LF) and high-frequency (HF) components prior to analysis. First applied to HF observations, the MCA reveals two major teleconnections. The first mode highlights the strong influence of the El Niño Southern Oscillation (ENSO). The second mode reveals a relationship between the SST in the Gulf of Guinea and the northward migration of the monsoon rainbelt over the West African continent. When applied to HF outputs of the twentieth century IPCC4 simulations, the MCA provides heterogeneous results. Most simulations show a single dominant Pacific teleconnection, which is, however, of the wrong sign for half of the models. Only one model shows a significant second mode, emphasizing the OAGCMs’ difficulty in simulating the response of the African rainbelt to Atlantic SST anomalies that are not synchronous with Pacific anomalies. The LF modulation of these HF teleconnections is then explored through running correlations between expansion coefficients (ECs) for SSTs and precipitation. The observed time series indicate that both Pacific and Atlantic teleconnections get stronger during the twentieth century. The IPCC4 simulations of the twentieth and twenty-first centuries do not show any significant change in the pattern of the teleconnections, but the dominant ENSO teleconnection also exhibits a significant strengthening, thereby suggesting that the observed trend could be partly a response to the anthropogenic forcing. Finally, the MCA is also applied to the LF data. The first observed mode reveals a well-known inter-hemispheric SST pattern that is strongly related to the multi-decadal variability of the WAM rainfall dominated by the severe drying trend from the 1950s to the 1980s. Whereas recent studies suggest that this drying could be partly caused by anthropogenic forcings, only 5 among the 12 IPCC4 models capture some features of this LF coupled mode. This result suggests the need for a more detailed validation of the WAM variability, including a dynamical interpretation of the SST–rainfall relationships.

Notes

Acknowledgments

This work was supported by the AMMA program (African Monsoon Multidisciplinary Analysis), and by the ENSEMBLES European project (contract GOCE-CT-2003-505539). The authors are grateful to all IPCC4 participants and to the PCMDI for the build up of the IPCC4 database. The figures have been prepared using GrADS software, and the MTM tool was found on the Web site http://www.atmos.ucla.edu/tcd/ssa/. Thanks are also due to Fabrice Chauvin for his comments, and Serge Janicot for improving the original manuscript. Finally, we acknowledge the two anonymous reviewers for their constructive criticisms and suggestions.

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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  • Mathieu Joly
    • 1
  • Aurore Voldoire
    • 1
  • Hervé Douville
    • 1
  • Pascal Terray
    • 2
  • Jean-François Royer
    • 1
  1. 1.Météo-France, CNRM/GMGEC/UDCToulouse Cedex 1France
  2. 2.IPSLParisFrance

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