Decadal prediction of the dominant West African monsoon rainfall modes

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Abstract

[1] The present study offers for the first time the validation of decadal prediction systems upon the West African monsoon (WAM) variability. The ENSEMBLES multimodel and perturbed parameter decadal reforecasts are used to assess multiyear prediction skill for the dominant WAM precipitation regimes. Thus, the focus of the assessment is on time scales longer than seasonal to interannual. To retain lower-frequency predictability (interannual to decadal), a 4 year average is applied, which indeed has been shown to remove most of the interannual variability that is unpredictable beyond 1 year in dynamical forecasting (e.g., El Niño–Southern Oscillation). First, the decadal hindcasts are analyzed to assess forecast skill of Guinean and Sahelian area-averaged rainfall indices. Findings suggest that there is no significant skill in predicting these rainfall indices, probably due to the distinctive representations of deep tropical convection in each forecast system. This is further addressed by computing and comparing the leading modes of WAM variability in the ENSEMBLES decadal reforecasts against observations. Results show that while in the observations, global warming has an important role, in the forecast systems, the Atlantic Ocean is the main player. The Atlantic Niño represents the leading forcing for the simulated Guinean precipitation. Sea surface temperature (SST) anomalies associated with the simulated Sahelian precipitation project onto the Atlantic multidecadal variability (AMV), in which the subtropical branch shows consistency across the forecast systems. No significant skill has been found, however, to predict these WAM precipitation modes, although the Sahelian pattern presents systematic positive correlation scores and lower root mean square errors along the whole forecast range. This is reflected in a tendency for reproducing the Sahel dry period around the 1980s. Likewise, the good performance across the models in simulating the relationship between the leading rainfall modes and the surrounding SST forcings points out encouraging prospects for decadal forecasting. Previous studies show multiyear prediction skill of the AMV in the ENSEMBLES decadal reforecasts. Here the skill of the Atlantic-3 SST index is discussed.

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