Theory and observations of West African monsoon onset

Onset of the West African monsoon in the Sahel (also known as the monsoon jump) is characterized by a shift in the precipitation maximum from the Guinean coast into the continental interior over a few days' time. Analysis of rainfall observations shows that the rainfall maximum lingers along the Guinean coast as the solar forcing proceeds northward in the spring, while Sahel rainfall rates increase steadily and smoothly through the spring. The apparent jump of the rainfall maximum into the Sahel occurs when rainfall is suddenly suppressed over the coast. In the fall, the rainfall maximum retreats southward smoothly.

Circulation features associated with the monsoon onset are examined in higher-resolution reanalysis products, and the presence of inertially unstable flow over the Guinean coast in late spring is established. The analysis shows that the presence of inertially stable flow supports springtime rainfall along the Guinean coast that persists in the spring longer than rainfall at the same latitude over the adjacent Atlantic Ocean. A parameter to monitor for the presence of inertial instability is developed and applied to show that sudden decreases in rainfall on the coast occur when the threshold for inertial instability is crossed on synoptic time scales. Interannual variability in the monsoon onset date, which can differ by a month or more from one year to the next, is associated with the timing of the development of the inertial instability.