In the last decade, a continuous endeavor has been devoted to documenting and understanding the WAM synoptic-to-intraseasonal variability. Three main and well-separated timescales have been emphasized. Each of them is associated with one or several modes of variability, i.e. large and coherent fluctuations in rainfall and wind fields. The 25-90-day main mode is likely related to the Madden-Julian oscillation. In the 10-25-day band, two main modes have been described, the so-called quasi-biweekly zonal dipole and Sahelian mode. Recently, the last one has been shown to be related sometimes to equatorial Rossby waves, sometimes to the Saharan heat low variability and the associated tropical-extratropical interactions. Finally, the synoptic timescale (< 10 days) is mainly modulated by the well-known African easterly waves, and by Kelvin waves, but also associated with fluctuations in the precipitable water field, which can be tracked over the whole Sahel band and during more than one week.
A new stage has thus been reached now where it can be asked how relevant this knowledge about the WAM synoptic-to-intraseasonal variability is from a more operational point of view. Indeed, the various modes are often detected using specific filtering and statistical methods, for which the utilization in real time is not always straightforward. Their documentation mainly remains on composite or regression maps, so that one specific event might deeply differ from the mean one.
We therefore developed several real-time and simple diagnostics to monitor and forecast the WAM synoptic-to-intraseasonal variability and some of the associated modes, and tested them during the 2011 WAM season. In the present study, we present and evaluate some of them, through a synthesis of the WAM intraseasonal variability during the summer 2011. Further key improvements in terms of both diagnostics and physical understanding are also identified.
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