Atmospheric tropical modes are important drivers of Sahelian springtime heatwaves

Abstract

Heatwaves pose a serious threat to human health worldwide but remain poorly documented over Africa. This study uses mainly the ERA5 dataset to investigate their large-scale drivers over the Sahel region during boreal spring, with a focus on the role of tropical modes of variability including the Madden–Julian Oscillation (MJO) and the equatorial Rossby and Kelvin waves. Heatwaves were defined from daily minimum and maximum temperatures using a methodology that retains only intraseasonal scale events of large spatial extent. The results show that tropical modes have a large influence on the occurrence of Sahelian heatwaves, and, to a lesser extent, on their intensity. Depending on their convective phase, they can either increase or inhibit heatwave occurrence, with the MJO being the most important of the investigated drivers. A certain sensitivity to the geographic location and the diurnal cycle is observed, with nighttime heatwaves more impacted by the modes over the eastern Sahel and daytime heatwaves more affected over the western Sahel. The examination of the physical mechanisms shows that the modulation is made possible through the perturbation of regional circulation. Tropical modes thus exert a control on moisture and the subsequent longwave radiation, as well as on the advection of hot air. A detailed case study of a major event, which took place in April 2003, further supports these findings. Given the potential predictability offered by tropical modes at the intraseasonal scale, this study has key implications for heatwave risk management in the Sahel.

Prediction skill of Sahelian heatwaves out to subseasonal lead times and importance of atmospheric tropical modes of variability

“…These recommendations are in tune with previous work by Guigma et al (2020b), who identified tropical modes of variability as important large-scale drivers of Sahelian heatwaves. Precisely, the activity of the Madden Julian Oscillation (MJO), the equatorial Rossby (ER) and Kelvin (EK) waves in the Equatorial West Africa sector (0-10°N), where convection peaks in spring, significantly modulates the frequency and spatial distribution of heatwaves in the Sahel.…”

“…Then, each segment undergoes a wavenumber frequency filtering similarly to (Wheeler and Kiladis 1999), to retain the harmonics of the MJO, ER and EK waves. The exact characteristic wavenumbers, periodicities and equivalent depths used to detect each of these three modes of tropical variability are the same as those used in Guigma et al (2020b), and are shown in Table 1. The outcome of the filtering for each mode and for each segment is a 1506-day long timeseries of filtered OLR data at each of the global tropics grid-cell.…”

“…Then, another set of methods is used to assess the activity of tropical modes locally over the Equatorial West Africa sector (this set of methods is not applied to the EM data). The forecast and observed 1506-day long mode-filtered data segments are each averaged at the characteristic 5°E longitude between the equator and 10°N (this band of latitudes corresponds to the region of maximum convection over West Africa in spring; Guigma et al 2020b). For each segment, the resulting unidimensional timeseries and its first order time derivative are standardised (using the standard deviation from ERA5 for both the forecast and observed segments), and, through trigonometric operations, they are combined to identify wave angle and amplitudes for each day.…”

“…SM1 for each mode of variability. The reader is referred to Guigma et al (2020b) for a thorough description of the method. At the end of this process, the days corresponding to padded data (a total of 1460 days for each segment) are removed from the data segments, such that only the mode phases of the effective 46 days of the forecast and the corresponding observation are retained.…”

“…Precisely, the activity of the Madden Julian Oscillation (MJO), the equatorial Rossby (ER) and Kelvin (EK) waves in the Equatorial West Africa sector (0-10°N), where convection peaks in spring, significantly modulates the frequency and spatial distribution of heatwaves in the Sahel. Given the spatio-temporal properties of these modes, Guigma et al (2020b) suggested that they could provide heatwave predictability at subseasonal timescales. Subseasonal predictability has received increasing attention over recent years, given the range of new opportunities for risk management in several sectors (health, disaster preparedness, water management, energy and agriculture) that it brings (White et al 2017).…”