Interaction of the South Asian and West African monsoons on intraseasonal to multidecadal timescales

Recent studies have identified intraseasonal variability in the African monsoon system (Sultan and Janicot, 2003; and Mathews, 2004), and a relationship between African variability, the large-scale system and the South Asian monsoon (Hoyos and Webster, 2004; and Collins and Webster, 2005). We explore possible physical mechanisms of the link between African and Asian intraseasonal and interannual variability using the operational ECMWF 1˚x1˚ reanalysis atmospheric circulation data and Reynolds sea surface temperature data. Timescales from intraseasonal to multidecadal climate change are separated using statistical tools, such as multivariate singular spectral analysis and wavelet analysis. Cross-wavelet analysis and wavelet coherence are used to examine the coherency of regional monsoon signals and there relationship to large-scale variability and SST on timescales from intraseasonal to interannual. Signals at different timescales are extracted from large-scale variables using composites relative to monsoon or SST variability. To determine the forcing of the monsoon system by anthropogenic climate change in the 20th century, we make use of a set of coupled global GCM runs made under the auspices of the IPCC. Large-scale atmospheric circulation forced by atmospheric composition in these model simulations, allows us to assess the response of monsoon systems to multidecadal climate change. As a first step, the ability of these GCMs to simulate the large-scale circulation associated with African and Asian monsoons is evaluated.