Intraseasonal variability of the West African monsoon

Abstract

The intraseasonal time scale is critical in West Africa where resources are highly rainfall dependent. Three main modes of variability have been identified, two with a mean periodicity of 15 days and one with a mean periodicity around 40 days. These modes have a regional scale and can strongly influence precipitation and convective activity. They are mainly controlled by atmospheric dynamics and land-surface interactions. They can also modulate the very specific phase of the African summer monsoon onset. A better knowledge of the mechanisms controlling this scale is necessary to improve its predictability.

On the spatial coherence of rainfall over the Saloum delta (Senegal) from seasonal to decadal time scales

“…At seasonal time scales, statistically significant regressions are found over the whole western half of sahelian Africa, including Niger. This finding is similar to the results of Janicot et al (2011) computed from a Sahel index.…”

“…At seasonal time scales (Figure 5), regressions are positive, greater than 0.5 mm/d and statistically significant up to Niger and Chad. This homogeneous regression over the Sahel region is similar to the results of Janicot et al (2011, their Figure 5 at lag 0) computed from a Sahel index. Note however that this latter study uses daily data and thus includes shorter time scales than the one displayed here.…”

“…The sub-seasonal, or intra-seasonal, rainfall variability during the summer monsoon season results from complex interactions between processes with various time and space scales (e.g., Diedhiou et al, 1999;Janicot et al, 2011). Intermittent regional modes have been identified, which represent an envelope modulating the occurrences of mesoscale convective systems Fritsch, 1993, 1997;Hodges and Thorncroft, 1997;Mathon and Laurent, 2001).…”

Simulation of the West African monsoon onset using the HadGEM3-RA regional climate model

“…Rainfall maxima occur during the peak of the West African monsoon (WAM) in June-July-August (JJA) and are associated with the meridional displacement of the intertropical convergence zone (ITCZ). The migration of the ITCZ is not smooth but characterized by successive phases of intense rainfall and pauses (Janicot et al 2011;Sultan and Janicot 2003). Firstly, the ITCZ holds a quasi-stationary position around the Guinea Coast near 5°N between April and June.…”

New perspectives on land–atmosphere feedbacks from the African Monsoon Multidisciplinary Analysis

“…A major topic of interest across AMMA was the intraseasonal variability in rainfall (Janicot et al, 2011). Rainfall fluctuations on periods ∼15 days in the Sahel induce a coherent, large-scale response of sensible heat flux from the land due to soil moisture (Taylor, 2008) and insolation (Lavender et al, 2010).…”