Phenomenology and Thermodynamical Characteristics of West African Heat Waves

Abstract:

Most of the studies on heat waves focus on their occurrences in the USA, Europe, China and Australia. Only a few articles deal with heat waves over the Sahel, even though monthly maximum temperatures in this region can rise to up to 40°C before the arrival of the summer monsoon. The focus here is on identifying and comprehending the physical mechanisms involved in the occurrence and phenomenology of springtime heat waves over the Sahel.

In order to analyse those physical mechanisms, a heat index based on synoptic and intra-seasonal changes was defined. This new detection method was applied separately to daily maximum and minimum temperatures, leading to heat indexes respectively called HWmax and HWmin. This separation matters because physical processes linked to these two temperatures are expected to differ: maximum temperature fluctuations may be connected to cloud-aerosol-induced changes in surface shortwave radiation, whereas the influence of atmospheric humidity may be predominant for the minimum temperature, via its impact on surface longwave fluxes during nightime.

Using the ERA-Interim reanalysis over the period 1979-2014, a hundred heat waves per heat index have been detected, that is roughly three heat waves per year, of mean length 5 to 6 days. HWmax perceived heat waves from March to July whereas HWmin recorded heat waves from March to May only. The temperature and other dynamic and thermodynamic variables, as well as severity, were further analysed in order to provide the major heat wave properties, and to assess whether archetypes can be identified.

Finally, the detected heat waves have been compared to those picked up by classical heat indexes like the NOAA index and quantile-based indexes. The differences between the heat waves identified with the ERA-Interim, ERA-40, MERRA and NCEP2 reanalyses, the observationally-based BEST data and those obtained with local data will also be discussed, in particular their fluctuations over the past decades.