Intraseasonal variability of the Saharan Heat Low and its link with mid-latitudes

The West African Monsoon (WAM) is, at the first order, driven by the latitudinal thermal gradient between the cold sea surface temperatures in the Guinea Gulf and the high surface temperatures in the Saharan Heat Low (SHL) region. The SHL is a key feature of the WAM, which might play a role in the WAM intraseasonal variability (Sultan et al. 2003). The objective of this study is to characterize the SHL intraseasonal variability during the rainy season, and its relationship with that of the WAM.

In order to assess the SHL intraseasonal variability, real and complex Empirical Orthogonal Function (EOF) analyses are performed on the 850-hPa potential temperature field over Northern Africa and the Mediterranean, using the NCEP/DOE AMIP-II reanalysis. A robust propagative mode is highlighted over North Africa. It consists in two phases. West phases correspond to a maximum temperature anomaly over the Moroccan coast, propagating southwestward, and a minimum temperature anomaly between Libya and Sicily, propagating southeastward, up to the Eastern Sahel. East phases correspond to the opposite temperature structure, with the same propagation as in West phases.

A lag-composite analysis reveals that this SHL intraseasonal mode is preceded by large-scale mid-latitude intraseasonal fluctuations of the atmosphere. The southward penetration of a Rossby wave disturbance over Europe and North Africa generates modulations of the three-dimensional atmospheric structure. The low-level ventilations and the Harmattan circulation are particularly impacted as well as the subtropical westerly and polar jets in the upper troposphere.

West phases are also concomitant with an enhanced convective signal over the Darfur region, which propagates westward, as far as the middle of the Atlantic, at a speed similar to the well-known African Easterly Waves. Four days after the West phase maximum, a convective break follows over the Eastern Sahel and progressively reaches the Central and Western Sahel.

Thus, the SHL appears as a bridge between mid-latitudes and the WAM. Such a result may offer promising sources of predictability over the Sahel at the intraseasonal timescale.