Definition and predictability of an OLR‐based West African monsoon onset

Abstract

ABSTRACT:The monsoon onset is documented in terms of latitudinal shift of deep convection areas within the ITCZ using an interpolated version of the National Oceanic and Atmospheric Administration (NOAA) Outgoing Longwave Radiation (OLR) at a 5-day time-step over West Africa for the period 1979-2004. Signals in moist convection derived from OLR values lower than 180 W/m2 allow better determination of onset dates (ODs) than the use of other thresholds or of the raw values of OLR. Such ODs are defined without any time filtering or spatial averaging along the meridional plane. They are also significantly correlated with ODs based on other datasets such as the CMAP and Global Precipitation Climatology Project (GPCP) rainfall estimates, and seem more realistic, especially during years when the latitudinal shift is unclear and delayed. However, the respective means [30 June for ODs based on OLR (vs) 25 June for ODs based on CMAP and GPCP] and standard deviations [15.6 (vs) 8.1 days] are slightly different. These differences illustrate the fact that the monsoon onset corresponds to a 10-15-day transition in the West African monsoon seasonal evolution which does not affect, concomitantly, all variables describing the system.The onset is clearly linked to the time evolution of a few key-descriptors of the monsoon system at regional scale such as the installation of northward meridional gradients of moist static energy in low levels and of the monsoon cell over the continent. It is also associated with specific variations of the relative vorticity in low troposphere and of the velocity potential in upper levels over the Sahel region. Moreover, OD time series exhibits good predictability. Multivariate linear regressions have been performed in a leave-one-out cross-validation way using OLR and selected atmospheric predictors in May. The best results explain 60% of the OD variance and are obtained with 2 types of predictors: the OLR gradients between the Gulf of Guinea and the African continent, and the northward migration of the West African Monsoon cell.

Guinean coastal rainfall of the West African Monsoon

“…There is a 10-15 day uncertainty, which is the time necessary for the rainfall environment to shift from the coast to the Sahel (Sultan and Janicot 2003;Fontaine et al, 2008). This 10-15 day period actually coincides with the transitional phase during the first half of July defined by Thorncroft et al (2011).…”

“…However the Sahelian rainy season is established only 15 to 20 days after that, after the transitional phase as described in Thorncroft et al(2011). The transitional phase is also omitted (or included in the Sahelian phase) in Fontaine and Louvet (2006) and Fontaine et al (2008).…”

“…Despite the fact that the rainy season near the coast is the wettest period of the whole West African monsoon (WAM) (Gu and Adler, 2004;Thorncroft et al, 2011), most research effort, especially in recent years, has been devoted to the nature and variability of the Sahelian rainfall (e.g. Hagos and Cook, 2007;Fontaine et al, 2008). There is a continued lack of detailed knowledge and understanding of the nature and variability of the coastal rainy season.…”

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

“…Hence, negative OLR anomalies indicate enhanced moist convection. Several evidences have already shown that OLR is a powerful proxy for the WAM onset (e.g., Fontaine et al, 2008). Finally, we used the Hadley Centre Global Sea Ice and SST (HadISST) dataset version 1.1 (Rayner et al, 2003), which is a monthly dataset of 1 • × 1 • spatial resolution covering the period from 1870 to present.…”

Seasonal forecasts for regional onset of the West African monsoon

“…A single definition is unlikely to fully cover all aspects of monsoon onset which occurs through various stages (Fontaine et al 2008;Gazeaux et al 2011). Basing a forecast on a single indicator could therefore result in an over-confident forecast.…”