Coastal rainfall onset in the West African monsoon

The West African coastal rainfall onset is determined by analysis of rainfall in the 10oW-10oE zonal band. The coastal onset is defined when the southern boundary of the rainband reaches the equator. Using (1979-2001) 23-year GPCP pentad datasets, the mean coastal onset occurs on 12 May with a standard deviation of 2.7 pentads. This result is robust and is independent of the data used. It is shown that the coastal onset is primarily driven by the Atlantic cold tongue (ACT) development; especially there is a 301 K threshold below which the ACT develops rapidly and leads to the coastal onset. The large interannual variability of the coastal onset is also shown to be closely linked to the large variability of the ACT formation. On the other hand, in terms of intensity and length the coastal rainfall is related to the SSTs between the coast and the equator and to a lesser extent to the Saharan heat low (SHL). The SST cooling in this area is assumed to be driven by equatorial upwelling via the ACT and coastal upwelling from the east. It is further shown that the coastal regime is related to the Sahel onset, especially one condition for the Sahel onset is the decline of the coastal regime.