Impact of Tropical Modes of Variability on Sahelian Heat Waves: A Case Study in April 2003

Heatwaves are increasingly recognised as one of the most harmful weather hazards to human health but their dynamical drivers are largely unknown in tropical regions. This study investigates the role of tropical synoptic to intraseasonal modes of variability on the occurrence of a large-scale heat wave which took place from 02 to 20 April 2003 over the semi-arid West African Sahel.

The surface energy budget and various dynamical fields are examined to understand the meteorological conditions that shaped the event. To extract the tropical modes a wavenumber-frequency decomposition of OLR anomalies is performed using their respective characteristic equivalent depths, wavenumbers and periods. The modes investigated are the MJO and equatorial waves including Kelvin, Rossby, Mixed Rossby-Gravity and African Easterly waves. The activity of each of them is projected onto the dynamical and physical fields by simple linear regression.

Results show that the event was globally fostered by sensible heat flux and more importantly longwave warming at the surface. The heatwave was initiated during the latest stage of a convectively suppressed phase of the MJO which allowed the advection of warm air from the climatological location of the West African Heat Low to northeastern Sahel, leading to a positive anomaly of sensible heat flux at the surface. Then a convectively enhanced equatorial Rossby wave crossed the region bringing moisture to maintain the heatwave but progressively turned the underlying physical process from sensible flux to longwave radiation. The latter reached its peak and eventually became the sole process with the arrival of the enhanced phase of the MJO, siding with the Rossby wave. Overall the higher frequency modes had a relatively limited impact on the physical and dynamical patterns although they appeared to be important some days. A noticeable aspect of the modulation by tropical modes is that notwithstanding their coupling with convection, their radiative forcing is significantly more efficient through water vapour than clouds.

The present study evidences that tropical modes may underpin the occurrence of heatwaves in the Sahel. This has key implications in terms of heatwave predictability and monitoring at the synoptic to intraseasonal scale over the region although a statistical characterisation is still needed to generalise these findings.