6 Conclusions and Perspectives
While long involved in documenting the reasons for the great Sahelian drought that struck the region at the end of the 1960s for almost 30 years, climate scientists are now challenged with the understanding of the new turn taken by rainfall since the end of the 1990s: a return to wetter conditions, though still dryer than the two decades preceding the drought; and a strong interannual variability of annual totals. The last 25 years have experienced both low agricultural yields and an increasing number of damaging floods, raising the question of a more extreme climate with more dry spells and more precipitations extremes.
Tarhule (2005) was the first to signal the growing societal importance of floods in the Sahel. Intuitively, he suggested that “[i]t is reasonable to expect […] that more frequent high-intensity rainfall would characterize a return to wetter conditions, with a concomitant increase in flood risk.” Eleven years after, are there robust scientific arguments to confirm this intuition? More generally, what do we know about rainfall extremes in the Sahel? This chapter summarizes the state of knowledge in that regard.
We show that some advances have been made in characterizing the space–time structure of extreme Sahelian storms. In particular, scale invariance properties of extreme rainfall have been demonstrated in the framework of the fractal theory. This helped in characterizing the dependence of extreme rainfall distribution on time duration (between 1 and 24 hours) and spatial area (from point to 10,000 km2). The rainfall scaling properties allowed for the generation of IDAF curves for the surroundings of Niamey (Niger), which are very useful tools for hydrological engineering and for assessing the severity of rainfall events in observations or atmospheric model simulations.
Although the recent evolution of the annual rainfall regime has been well documented in the Sahel, there are only a few papers dealing with the recent changes in Sahelian MSC properties, and even fewer that consider the most intense of them. MCSs are, however, key elements for understanding the hydroclimatic variability in the Sahel as they produce the great majority of seasonal rainfall. As shown in this chapter, their occurrence and intensity directly influence the hydrological response of catchments. Studies dealing with trends in extreme rainfall in the Sahel often suffer from the difficulty of (1) obtaining time series with long enough periods of records and/or (2) identifying robust methodologies to detect significant trends from notoriously noisy time series of extremes. Dealing with the detection issues, Panthou et al. (2012, 2013) proposed a robust regional statistical methodology based on the EVT. This allowed Panthou et al. (2014a) to assess the recent trends in extreme Sahelian rainfall. They showed that the recent return to higher annual rainfall in the Central Sahel has happened in a context of a persisting deficit of the number of rain events, compensated by a larger share of strong rainfall events. This new rainfall regime is typical of a more extreme climate characterized by harsher dry spells during the rainy season and more precipitation extremes. This trend seems to be accompanied by an increasing mean size of MCSs. This is coherent with the fact that bigger MCSs are more likely to produce higher rainfall amounts (e.g., Roca et al., 2014).
This chapter provided an original analysis of the atmospheric processes responsible for the 20 most extreme events that have happened in Ouagadougou since 1997. A composite analysis of these 20 events revealed the presence of consistent atmospheric features. First, favorable conditions for the development and maintenance of convection are provided by a large-scale moist anomaly, already visible in the eastern Sahel 3 days before the event and arriving at Ouagadougou in phase with a marked AEW. The concurrence of synoptic (AEW) and large-scale (moist anomaly) atmospheric drivers is a key feature of the occurrence of extreme rainfall events in Ouagadougou. It allows for the moistening associated with the well-developed vortex to be found in the composite with a northerly transport of moist air to the west of the MCS and advection by the monsoon flow on its eastern flank.
According to the recent evolution of the rainfall regime shown in this chapter, we recommend using with caution the term “recovery,” often used to qualify the rainfall of the last two decades. This word only reflects the evolution of mean annual rainfall levels, and it masks a strong interannual variability and a significant intensification of the rainfall regime at the daily scale. When used without caution it can also inadvertently downplay the societal impacts of climate extremes that remain in force in the Sahel. Dry years have continued to affect crop yields and water resources, and heavy rainfall has more than ever destroyed crops and increased the risk of flooding. By combining an increase in extreme precipitation and in dry spells, the Sahelian rainfall regime actually exhibits all the signs and symptoms of a “higher hydroclimatic intensity” as defined by Giorgi et al. (2011). This term or “period of rainfall intensification” is preferred to “recovery” when describing the Sahelian climate of the last two decades.
Despite great advances, several questions related to extreme rainfall are still open and deserve to be clarified through further research.
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In particular, some studies recently showed that during the last two decades, the mean annual rainfall levels remained much longer in deficit over the western Sahel than in the eastern and central Sahel (Lebel and Ali, 2009; Mahé and Paturel, 2009). This east–west dipole is seen in the recent CMIP simulations and seems to be a dominating pattern for the decades to come (Monerie et al., 2012). An important issue is thus the way this regional contrast is reflected in the evolution of the mean and extreme rainfall regimes at mesoscale. More generally, the question of regional contrasts must also be examined in the southern West Africa (Sudanian and Guinean regions) to highlight the impact of the regional north–south gradient and the potential differences between inland and coastal precipitation.
Further research is also needed to identify the mechanisms involved in the evolution of extreme events in the context of climate change. The role played by the atmospheric features shown to be responsible for extreme events in Ouagadougou has to be confirmed at the regional scale and possibly on longer timescales. Then it will be possible to analyze how the occurrence of these features has changed in the recent past and how it will change in the future. Climate models will be helpful in that respect. Although they still struggle in representing convection processes, they most often provide a good representation of synoptic patterns driving rainfall over West Africa.
Another task is to more accurately characterize the typology of MCSs that locally produce extreme rainfall amounts. The increasing mean size of MCSs presented in this chapter suggests that bigger systems might be more frequent over the last 10 years. This preliminary result must be confirmed by deeper analyses of the interaction between local intense rainfall and associated MCSs. In the short term, it is planned to realize a combined analysis of rain gauge–based measurements and infrared data from geostationary satellites available since the beginning of the 1980s. This should allow for assessing changes in size and duration of MCSs and detecting the life cycle stages at which they are the most prone to triggering extremes.
A related issue is improving documentation on the internal structure of extreme rainy systems. Although most of the studies dealing with extreme rainfall evolution are based on daily values, it is also very important to document how rainfall intensities have evolved at subdaily scales. Fig. 4.12 shows 5-min time step records of the two biggest events available in the AMMA-CATCH Niger database over the period 1990–2016 (Lebel et al., 2009). Both events occurred in the morning and accumulated more than 200 mm of rainfall in less than 6 hours (return period estimated around 3000 years). Interestingly, these two extreme events display very distinctive temporal structures characterized for first event (event of Koure 2003) by two main convective cells with maximum intensities around 10 mm/5 min, and for the other (event of Dantiandou 2014) by one single very intense convective cell with a maximum intensity peaking at 14.5 mm/5 min. These time structures probably come from a different degree of convection organization within the MCSs that produced these events. Moreover, it is likely that these two types of hyetographs, if occurring over a similar catchment, would have a very different hydrological impact. Runoff production being very sensitive to short timescale rainfall intensities, especially in the Sahel where soils have very low infiltration capacities, it is likely that an event such as that of Dantiandou 2014 would produce more runoff than the event of Koure 2003.
Figure 4.12. Five-minute time step records of the two biggest rainfall events that occurred over the AMMA-CATCH Niger network since it has operated in 1990.
This illustration raises the question of how the intensification demonstrated at a daily basis has been translated into subdaily rainfall intensities. Are convective cells in extreme events more intense or more numerous, or both? How has the temporal structure of extreme events evolved over time? Investigations based on the high-resolution rainfall data set provided by the Observation Service AMMA-CATCH are ongoing on this subject, mainly with aim of assessing recent trends in maximal short time rainfall intensities. This will contribute to the development of nonstationary IDAF curves that are of high interest for infrastructure design in a changing climate (e.g., Cheng and AghaKouchak, 2014). This will also be of great value for better evaluating the relative contribution of rainfall and land-use changes in the recent increase in floods in the Sahel (Descroix et al., 2012; Aich et al., 2015; Casse et al., 2016).
Finally, it is worth noting that the most significant advances presented in this chapter are the result of combined statistical and physical analyses of both ground-based and satellite observations. They have been fostered by international scientific programs such as the AMMA (Redelsperger et al., 2006) project and its current successor AMMA-2050 (www.amma2050.org), as well as by projects dedicated to climate observations such as the AMMA-CATCH Observing System (Lebel et al., 2009, www.amma-catch.org) and the satellite mission Megha-Tropiques (Roca et al., 2015, meghatropiques.ipsl.polytechnique.fr). All these initiatives help to reinforce two essential pillars required if one wants to respond to the challenges posed by the ongoing global changes: (1) continuing long-term climate observations; and (2) favoring the exchange of scientific expertise beyond disciplinary barriers.