Seasonal evolution of the West African heat low: a climatological perspective

Abstract

The West African heat low (WAHL), a region of high surface temperatures and low surface pressures, is a key element of the West African monsoon system. In this study, we propose a method to detect the WAHL in order to monitor its climatological seasonal displacement over West Africa during the period 1979-2001, using the European Centre for Medium-range Weather Forecast (ECMWF) ERA-40 reanalyses. The low-level atmospheric thickness (LLAT), a variable defined as the difference of geopotential heights at 700 and 925 hPa, is used to detect the dilatation of these levels generated by an increase of the temperature. We define grid points with 10% highest values of the LLAT as the WAHL. We show that our method reliably positions the WAHL over areas of high surface temperatures and low surface pressures, and that it is effective at detecting heat lows. In the course of the year,

Dust impact on the West African heat low in summertime

“…This result shows the consistency between the WAHL location and the 925 hPa wind field. Note that the position of the WAHL in 2006 is very close to its climatological position in summer shown in Lavaysse et al (2009). Between 14 and 20 July 2006, the WAHL is centred on 25 • N, moving from 0 • E on 14 July to 5 • W on 20 July, as typically observed in the summertime.…”

“…The location is calculated using the low-level atmospheric thickness between 925 and 700 hPa taken from the 0600 UTC ECMWF operational analysis . As in Lavaysse et al (2009), the WAHL is defined as the area associated with 10% of the largest thickness over West Africa. Between 1 June and 30 September 2006, the largest occurrence probability is located over northern Mali, just west of the Hoggar Mountains (Figure 1(a)).…”

“…During this period, the WAHL is considered to be a major dynamic element of the West African monsoon (WAM) system, and the main driver of precipitation over the Sahel (see Lavaysse et al, 2009, and references therein). In the low troposphere, the cyclonic circulation generated by the depression tends to organize and reinforce the southwesterly monsoon flow along its eastern flank and the northeasterly harmattan flow along its western flank.…”

3-Step dynamical downscaling with empirical correction of sea-surface conditions: application to a CORDEX Africa simulation

“…On their side, Laprise et al (2013) found that biases present in the CGCMs used for their RCM simulations had detrimental impact on the simulated WAM precipitation in the region of the Guinea Coast (WA-S). As it is the evolution of the interaction between the cold tongue in the region of the Guinea Gulf and the thermal low located over the Sahara (Saharan Heat Low, SHL; Lavaysse et al 2009) that regulates the WAM rainfall migration (Thorncroft et al 2010;Lafore et al 2010), it is evident that having good sea-surface BC and its corresponding atmospheric BC is of paramount importance for the simulation of the WAM precipitation in RCMs.…”

Recent climatological trend of the Saharan heat low and its impact on the West African climate

“…The West African heat low is the thermal response of the lower troposphere over the northern African continent to the seasonal surface warming (Lavaysse et al 2009). In summertime, this low pressure is located over the Sahara, and hence termed Saharan heat low (SHL), and has been identified as a major component of the West African monsoon (WAM) system (Sultan and Janicot 2003;Parker et al 2005;Peyrillé and Lafore 2007;Lavaysse et al 2009) and a driver of precipitation over the Sahel (Lavaysse et al 2010a;Evan et al 2015).…”