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Impacts of the West African monsoon

on food security Lecturers: Doug Parker (The West African Monsoon)

Andy Challinor (Climate change impacts on food security)

Acronym list

AEJ African Easterly Jet ITZC Inter-Tropical Convergence Zone AEW African Easterly Waves SAL Saharan Air Layer AMMA African Monsoon Multidisciplinary Analyses SST Sea Surface Temperature CAPE Convective Available Potential Energy WAM West African Monsoon

The monsoon weather system

In West Africa, three main monsoon mesoscale systems determine weather and climate variability, which are the African Easterly Jet (AEJ), the African Easterly Waves (AEW) and the organized convective storms (Leroux, 2001). The AEJ results from the temperature gradient between the Guinea coast and the Sahara desert, where the wet and dry adiabats from each region meet and result in a easterly jet at roughly 650 hPa (Parker et al., 2005). The Saharan Air Layer (SAL), known as the Harmattan Layer, is the north-easterly dry and warm component of the monsoon system and is shown in pink in Figure 1 together with the ITCZ (named ITD in the figure), the AEJ , the tropical easterly jet and the Harmattan winds represented as northerly dry air (Ramel et al., 2006). The AEJ directs thunderstorms from east to west leading to the formation of African Easterly Waves. The AEW are the principal synoptic-scale weather system in the West African Monsoon (WAM) and modulate the distribution of precipitation (Cook, 1999; Janicot et al., 2008).

The meridional migration of the Inter-Tropical Convergence Zone (ITCZ) determines the amount of rainfall over West Africa (Thorncroft et al., 2011). The offset of the WAM coincides with a northward shift of the ITCZ during the spring, reaching 10°N during the summer and altering significantly the wind and precipitation patterns over the west of Africa related to the reversal of the winds in the lower

Figure 1 | Schematic of the West African Monsoon System. FIT stands for ITD (InterTropical Discontinuity), “Air chaud Saharien” stands for ‘Warm Saharian Air”, JEA stands for AEJ (African Easterly Jet), JET stands for TEJ (Tropical Easterly Jet), “Air sec” stands for “Dry Air”. (Lafore, 2007).

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troposphere relative to wintertime (Le Barbé et al., 2002). During the WAM, the AEJ transports moisture over the land during the night and vertically during the day, leading to the formation of convective systems (Parker et al., 2005). The Sahara desert plays a key role here as it contributes to increase the surface pressure gradient by reinforcing the ITZC depression (AMMA, 2012).

It must be highlighted the importance of understanding the coupled atmosphere-land-ocean processes that drive the WAM as they interact on different spatial scales. At global scale, the WAM is influenced by the El Niño Southern oscillation at seasonal to decadal timescales; interactions between the land, the tropical Atlantic Ocean and the atmosphere as well as the monsoon processes occur at regional scales (AMMA, 2012). Mesoscale meteorology explores the insides of the WAM, its interactions with the AEJ, AEW and its associated convective systems. Figure 2 below show the different spatial and temporal scales of the WAM and the implications in the principal components of the ecosystems. The present work will be focused on the local or sub-mesoscale impacts of the WAM, encompassing the impacts on food security through changes in the hydrology and agriculture of the region.

The dry nature of the west African region means that the natural irrigation and agricultural production of this area depend strongly on the WAM, and hence any slight change on its onset date or strength has a significant impact on the agricultural, economic and social sectors of this part of the African continent (AMMA, 2012). The Sahel encompasses some of the poorest countries in the world (Argelia, Niger, Mali, Chad, Sudan and Eritrea).

One of most important challenges in terms of adaptation is the ability to predict the onset date of the monsoon, which roughly coincides with the sowing date and is crucial to guarantee continuous water availability for the crops (Ati et al., 2002; Vellinga et al., 2013). The typical onset date averaged between 1979 and 2009 is the 29 of June (with a standard deviation of 8 days) (Xie et al., 2003).

Although many studies agree on the link between the northwards migration of the WAM and the pressure difference between the Sahara and the Gulf of Guinea, there are different theories to explain the cause of the sudden onset of the monsoon (Vellinga et al., 2013). From a change in the position of the Saharan heat low (Ramel et al., 2006) to the influence of topographic features that lead to dry subsidence (Sultan et al., 2005) and the independent rainfall patterns in the coastal (driven by tropical changes in sea surface temperature (SST)) and Sahelian areas (driven by changes in the AEJ and AEW) (Gu and Adler, 2004). Thorncroft et al., (2001) associated the onset date with a previous reduction in rainfall led by a reduction in SST in the Gulf of Guinea together with the advent of a dry westerly flow.

Considering all the different theories, the onset of the WAM can be identified by a sudden decrease in SST in the Gulf of Guinea together with a decrease in precipitation along the coast, the shift of the maximum solar heating over the Sahara, a strengthening of the EAJ and a weakening of the Harmattan winds from the northeast (Vellinga et al., 2013). The uncertainties regarding the onset

Figure 2 | Schematic of the processes in the West African Monsoon and their different time and spatial scales (AMMA, 2012).

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day of the monsoon and the projections of precipitation increase the vulnerability of the population of West Africa to climate variability and represent an added difficulty in terms of agricultural planning.

Also, while long-term climate projections show changes in the hydrological cycle and an increase in moisture in the atmosphere, whether the dynamics of the WAM will see variations on a future climate remains uncertain (Brown et al., 2009). Climate models still have difficulties to simulate the complex dynamics of the weather systems in the West of Africa.

Soil-atmosphere interactions

There is a bidirectional relationship between the WAM and the agriculture in the Sahel region. The monsoon is the main source of rainfall and determines the agricultural cycles, and in turn the vegetation alter the moisture and energy flows and therefore the convective available potential energy (CAPE) (Garcia‐Carreras et al., 2010).

During the last decades, shrub and forestland have been converted into cropland in West Africa and it can be noticed the transition to semiarid land cover from north to south that lead to strongly marked vegetation and hence moisture gradients (Garcia‐Carreras et al., 2010). Figure 3 shows the atmospheric circulation that results from differences in vegetation types that are key to understand the temperature and moisture fluxes related with the WAM.

Agricultural impacts

During the last decades, the trend towards drier weather conditions in the west African region have had deleterious agricultural and socioeconomic impacts such as the increase in the food prices due to a decrease in agricultural production (Redelsperger et al., 2006).

Agriculture is a strongly weather-dependent human activity (Hansen, 2002). Higher than usual mean temperatures result on a shorter crop growth period and lower yields. Also, more precipitation extremes increase the variability in crop growth and yield, meaning that while one crop’s yield increases, the other crop’s one decreases (e.g. El Niño effects on soybean and maize (Iizumi et al., 2014).

The agriculture in the Sahel region is based on small-scale systems especially vulnerable to climate variability since the populations living in that area rely strongly on agricultural productivity. In agreement with Sultan et al (2005), the Sahel is the only region in the world where the food production per capita is decreasing instead of increasing due to the recent long drought periods. Ickowicz et al. (2012) estimated the interannual variability of precipitation in the Sahel region as

Figure 3 | Schematic of the atmospheric circulation resulting from vegetation heterogeneities. The temperatures over the forest cover are cooler in comparison to the adjacent land, leading to a moisture flow towards the cropland. In presence of northerly winds, updrafts are enhanced, leading to cumulus clouds in the southern side of the warm anomalies (Garcia‐Carreras et al., 2010).

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around 30%, and together with an also high spatial variability of the rainfall associated with the WAM, demand strongly adapted agricultural practices. On the following years, water availability will be reduced in West Africa due to the economic development and a rapid population growth, situation that will be exacerbate by climate change and increased climate variability (Cooper, 2004).

Both the price of the food and the climatology in West Africa follow a seasonal cycle and are subject to interannual variation as shown in Figure 4 (Brown et al., 2009). The limited food availability during the summer, especially on very dry years increases the price of the food and makes it more difficult for the local farmers to sell the food and increase their income. In turn, due to the food insecurity the farmers cannot afford to buy more technologically advanced agricultural machinery that would allow to increase their production and crop yields (Brown et al., 2009).

Food insecurity means in this case that because of the uncertainties about the net agricultural productivity, farmers will secure their production to cover their own demand, and it is not enough to produce at subsistence levels if the year has been especially dry or negative in terms of agricultural production.

In regards to climate projections, some authors have associated the slight increase in average annual rainfall of the last decades in the Sahel with a “Sahel regreening”, but these projections still remain unclear with the added complexity of the monsoon systems (Ickowicz et al., 2012). At local scale, the impacts of extreme events as a result of interannual variability can mask the regional trend (Thornton et al., 2009). To adapt to droughts and climate variability, farmers and policymakers are designing agricultural strategies to increase the crop productivity and adaptation to climate changes to guarantee the agricultural production in a region that irrigation methods and fertilisers are not broadly used (Sultan et al., 2005).

The African Monsoon Multidisciplinary Analyses project (AMMA)

It must be clarified the difference on associated risks of climate change and the WAM. Although the adaptation mechanisms could be similar in both cases, techniques of agricultural planning linked to the monsoon season have been developed during the last decades by local farmers (AMMA, 2012).

Figure 4 | Millet prices in Bamako, Mali (blue), Niamey, Niger (green), and Ouagadougou, Burkina Faso (purple) plotted with world corn prices (yellow) (Brown et al., 2009).

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The grand challenge is to adapt crops to new climate risks associated to climate change such as the decrease of water availability that causes devastation in the populations of the region. Local farmers, together with local and international institutions are working together to evaluate their ability to response to old and new risks associated to climate change in order to reduce impacts on food security.

The best example of international cooperation and study of the physical mechanisms of the WAM, its interannual variability and effects on the agriculture, economy and society of West Africa is the African Monsoon Multidisciplinary Analyses (AMMA) project. One of the key outputs of AMMA in regards to food security is the better understanding of the WAM that will allow to predict the onset date of the monsoon and estimate the accumulated rainfall in the wet season. With this information, farmers can adapt to weather conditions by changing the sowing and harvesting dates, selecting specific crop species that are less demanding or can adapt better to variable weather conditions and widening the range of economic (Thornton et al., 2009; Ickowicz et al., 2012; Dietz et al., 2006).

Projects like the AMMA are gaining importance as the impacts of the WAM and climate variability in West Africa mean a reduction in the quality of life of the populations from an economic, environmental and social point of view. Understanding the dynamics of the WAM will integrate the research carried out by AMMA by implementing monitoring and prediction strategies to reduce the vulnerability of the populations in West Africa to the WAM.

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REFERENCES

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