Spatio‐temporal characteristics of the recent rainfall recovery in West Africa

Abstract

Using daily (monthly) rainfall data from 167 (254) stations across West Africa with at least 80% data availability for the 31-year period 1980-2010 and the gridded African Rainfall Climatology Version 2 (ARC2) for the period 1983-2010, linear trends in yearly and monthly rainfall totals were investigated. Measures of the Expert Team on Climate Change Detection and Indices (ETCCDI) and two rainy season onset and retreat definitions were employed to assess the corresponding trends in frequency and intensity of daily rainfall and changes to monsoon season length. A rotated Empirical Orthogonal Function analysis yielded two homogeneous rainfall regions, the Sahel and Guinea Coast, in terms of interannual to decadal rainfall variability, and this led to analysis of station data and Standardised Precipitation Index for the two regions. Results show that the majority of stations in the Sahel between the West Coast and 15 ∘ E shows a statistically significant positive rainfall trend for annual totals. The August-October period exhibits the largest rainfall recovery in the Sahel and the date of the retreat of the rainy season significantly moved later into the year by 2 days decade −1 . The recovery is reflected both in more rainy days associated with longer wet spell duration and more extreme rainfall events. Trends along the Guinea Coast are weak and non-significant except for extreme rainfall related indices. This missing significance is partly related to the hiatus in rainfall increase in the 1990s, but also to the larger interannual rainfall variability. However, the tendency towards a more intense second rainy season suggests a later withdrawal of rains from the West African subcontinent. ARC2 trends are broadly consistent where ground calibration was undertaken, but are dubious for Nigeria and Ghana, and especially for the Guinea, Jos and Cameroon Line highlands due to missing gauge data.

Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review

“…Taylor et al [5] identified, using satellite infrared temperature (IRT) data, a robust and strong positive trend in the frequency of intense Sahelian mesoscale convective systems (MCS) since 1982 (in which MCS-mean IRT was used as a measure of intensity). Similar conclusions emerge from Sanogo et al [106] and Zhang et al [107].…”

Impacts of the seasonal distribution of rainfall on vegetation productivity across the Sahel

“…Moreover, an increase in heavy-rainfall events and prolonged dry spells have been observed and are projected for the future (Sanogo et al, 2015;Taylor et al, 2017;Zhang et al, 2017). Our results showed that the semi-arid zone will be most prone to these projected changes, and an increase in heavy-rainfall events, a delay in the onset of the wet season and dry spells exceeding 14 days will cause a significant reduction in vegetation productivity, although the annual rainfall amount may be constant or even increasing.…”

“…Several studies have reported a tendency towards an earlier start of the wet season (Sanogo et al, 2015;Zhang et al, 2017), but projections predict a delay of the wet season in the later 21th century (Biasutti and Sobel, 2009;Guan et al, 2014). Moreover, an increase in heavy-rainfall events and prolonged dry spells have been observed and are projected for the future (Sanogo et al, 2015;Taylor et al, 2017;Zhang et al, 2017).…”

“…A few studies show that there is a strong dependency of Sahelian vegetation growth on the timing of the wet season (Diouf et al, 2016) and the frequency and distribution of dry spells (Proud and Rasmussen, 2011), but currently no regional-scale study have systematically analysed the importance of a variety of rainfall metrics for vegetation growth as a function of mean annual rainfall. Further, there is evidence showing that not only a shift in the timing of the wet season but also increasing extreme events occur in the Sahel (Panthou et al, 2014;Sanogo et al, 2015;Taylor et al, 2017;Zhang et al, 2017), suggesting the need for a comprehensive understanding of how rainfall seasonality impacts vegetation production.…”

Desertification, resilience, and re-greening in the African Sahel – a matter of the observation period?

“…The contrast between a dryer western Sahel and a wetter eastern Sahel is becoming more significant (Lebel and Ali, 2009;Nicholson, 2013). Sanogo et al (2015) found a statistically significant positive rainfall trend between 1980 and 2010; however, it did not reach the degree of wetness of the 1950s.…”