Guinean coastal rainfall of the West African Monsoon

The nature and variability of the springtime rainfall onset at the Guinean coast is explored. The coastal onset is defined as the time when the oceanic extent of the rain band crosses the Equator. The oceanic extent is defined as the location where the rain rate falls below 2 mm day(-1). The mean coastal onset averaged over 1979-2009 from Global Precipitation Climatology Project (GPCP) rainfall data is on 11 May with a standard deviation of 14.5 days. This result is robust and seen in different rainfall products. The coastal rainfall demise is determined as the time when simultaneously the peak rainfall at the coast weakens and the rainfall over the Sahel intensifies. The mean coastal demise is 26 June with a standard deviation of 9.5 days. This implies that the mean length of the coastal phase is 47 days with a standard deviation of 13 days. The coastal onset and its demise are primarily driven by changes in sea-surface temperature (SST) between the Guinean coast and the Equator. There exists a 301 K threshold below which the equatorial cold tongue develops rapidly and leads to the coastal onset 10 days later. The same threshold applies to SSTs near the coast, where the water cooling precedes the end of the coastal rain phase by 16 days. Sea-surface temperature anomalies in the southeast Atlantic in winter are potentially a predictor for the coastal rainfall variability in spring. Copyright (C) 2011 Royal Meteorological Society