Active phases and pauses during the installation of the West African monsoon through 5-day CMAP rainfall data (1979-2001)

Objective diagnostic analyses based on PCA decomposition, Varimax rotation and low-pass filters (greater than or equal to1 month) are performed on the 5-day CMAP rainfall data over West Africa. The northward excursion of the rainbelt can be divided into 4 basic subperiods each composed of an equilibrium period (pause) followed by a rapid increase (active phase) of the system. We suggest easy criteria to automatically detect these events and propose a calendar for the 23 years. The pauses are centered around 27-28 March (pause #1), between 29 April and 2 May (pause #2), 11 June (pause #3) and 24 July (pause #4) with standard deviations of 7-11 days. The phase of any pause impacts on the phase of other pauses. Pause #4, starting earlier, is preceded by earlier pauses #2 and #3, then is followed by an earlier annual rainfall maximum and more abundant June-August amounts.