Detection of Intraseasonal Large-Scale Heat Waves: Characteristics and Historical Trends during the Sahelian Spring

D Bouniol,F Couvreux,F Guichard,J Barbier,R Roehrig

doi:10.1175/jcli-d-17-0244.1

Abstract

Abstract In the Sahel very high temperatures prevail in spring, but little is known about heat waves in this region at that time of year. This study documents Sahelian heat waves with a new methodology that allows selecting heat waves at specific spatiotemporal scales and can be used in other parts of the world. It is applied separately to daily maximum and minimum temperatures, as they lead to the identification of distinct events. Synoptic–intraseasonal Sahelian heat waves are characterized from March to July over the period 1950–2012 with the Berkeley Earth Surface Temperature (BEST) gridded dataset. Morphological and temperature-related characteristics of the selected heat waves are presented. From March to July, the further into the season, the shorter and the less frequent the heat waves become. From 1950 to 2012, these synoptic–intraseasonal heat waves do not tend to be more frequent; however, they become warmer, and this trend follows the Sahelian climatic trend. Compared to other commonly used indices, the present index tends to select heat waves with more uniform intensities. This comparison of indices also underlined the importance of the heat index definition on the estimated climatic heat wave trends in a changing climate. Finally, heat waves were identified with data from three meteorological reanalyses: ERA-Interim, MERRA, and NCEP-2. The spreads in temperature variabilities, seasonal cycles, and trends among reanalyses lead to differences in the characteristics, interannual variability, and climatic trends of heat waves, with fewer departures from BEST for ERA-Interim.

Highlights

From April to June, the subtropical Sahelian region experiences very high temperature during both nighttime and daytime, when monthly mean temperatures can range up to 308 and 408C respectively
The heat wave detection is based either on Tmax or Tmin because processes controlling daytime and nighttime temperatures in the Sahel are likely to be distinct: for instance, maximum temperature is often influenced by the incoming surface shortwave radiation whereas the minimum temperature is very sensitive to the amount of water vapor during this time of the year (Guichard et al 2009; Slingo et al 2009; Gounou et al 2012; Largeron et al 2017, manuscript submitted to Climate Dyn.)
The selection of heat waves over the Sahelian domain leads to retaining fewer heat waves at the edge of the Sahelian domain and the sample is substantially reduced. To avoid these edge effects, all heat waves detected over the whole African domain are accounted for, those overlapping over 20% of the Sahel domain

Summary

Introduction

From April to June, the subtropical Sahelian region experiences very high temperature during both nighttime and daytime, when monthly mean temperatures can range up to 308 and 408C respectively. The heat wave detection is based either on Tmax or Tmin because processes controlling daytime and nighttime temperatures in the Sahel are likely to be distinct: for instance, maximum temperature is often influenced by the incoming surface shortwave radiation whereas the minimum temperature is very sensitive to the amount of water vapor during this time of the year (Guichard et al 2009; Slingo et al 2009; Gounou et al 2012; Largeron et al 2017, manuscript submitted to Climate Dyn.) Another noteworthy difference for this heat index is that the percentile is fixed over the whole March–July period instead of over a moving window: it enables a seasonal analysis of the heat wave occurrence. For comparison with changes occurring at larger scales, climatic trends of Tmin and Tmax are computed over the Sahelian domain

Morphology and intensity of Sahelian heat waves

Long-term evolution of heat waves since 1950

Findings

Conclusions