This article provides an in-depth look at all aspects of the climate of the Sahel, including the pervasive dust in the Sahelian atmosphere. Emphasis is on two aspects: West African monsoon and the region’s rainfall regime. This includes an overview of the prevailing atmospheric circulation at the surface and aloft and the relationship between this and the rainfall regime. Aspects of the rainfall regime that are considered include its unique characteristics, its changes over time, the storm systems that produce rainfall, and factors governing its variability on interannual and decadal time scales. Variability is examined on three time scales: millennial (as seen is the paleo records of the last 20,000 years), multi-decadal (as seen over the last few centuries as seen from proxy data and, more recently, in observations), and interannual to decadal (quantified by observations from the late 19th century and onward). A unique feature of Sahel climate is that is rainfall regime is perhaps the most sensitive in the world and this sensitivity is apparent on all of these time scales.
Sharon E. Nicholson
Anna Hurlimann and Sarah Bell
Some of the most significant impacts of climate change are likely to be felt in water resources management, but climate change is not the only uncertainty facing water managers and policymakers. The concept of water security has emerged to address social, economic, political, and environmental factors, as well as the physical determinants of water availability. There are significant challenges for communicating about water security under a changing climate. Water security shares many of the characteristics of climate change with regards to communication. It is a complex concept involving interactions between dynamic human and natural systems, requiring public deliberation and engagement to inform political debate and to facilitate behavioral and cultural change. Knowledge and values about water and climate change are communicated through material experiences as well as through language. Communication about water security and climate change takes many forms, which can be characterized as five key modes—policy, communication campaigns, media, cultures, and environments. More effective communication about climate change and water is needed across these different modes to support meaningful participation and deliberation in policy decisions by a wide range of stakeholders. Integrating climate change into communication campaigns about water security provides opportunities to challenge and reframe traditional formulations of the role of water in society and culture and how to manage water in human settlements, the economy, and the environment. The central challenge for communicating the impacts of climate change on water scarcity lies in the complex interactions between society, policy, technology, infrastructure, the economy, and the environment in modern water systems. Different modes of communication are useful to enable public and stakeholder engagement in understanding the issues and making decisions about how to ensure water security in a changing society and environment.
Rainfall over Africa varies across timescales of a few days to several weeks due to several tropical and extratropical modes of variability. Excessive rains or prolonged drought regularly result in natural disasters and have thus a severe impact on the local economy, agriculture, spread of diseases, and entire ecosystems. The dynamical nature of the atmosphere allows the existence of planetary balanced modes, which are called Rossby waves, and smaller-scale unbalanced inertio-gravity (IG) waves. The former, which are more rotational, arise from the horizontal pressure gradient force, while for the latter gravity acts as the restoring force, making their flow pattern more divergent. The main source of variability in the extratropics stems from Rossby waves. At the equator, further types of convectively coupled equatorial waves (CCEWs) exist, namely Kelvin and mixed Rossby-gravity (MRG) waves. As the slowest intraseasonal tropical mode, the Madden–Julian Oscillation (MJO), which is related to Kelvin and Rossby waves, acts on a timescale of 30 to 90 days. Although it is primarily a planetary mode, the MJO has a specific “flavor” over the African continent. On the short intraseasonal timescale of 10 to 25 days, equatorial Rossby (ER) waves and the internal modes of the West African monsoon, the quasi-biweekly zonal dipole (QBZD) and the Sahel mode, modulate rainfall. On the synoptic timescale of a few days to a week, African easterly waves (AEWs) are a dominant mode over West Africa, whereas Kelvin waves predominantly modulate rainfall over equatorial Africa. Extratropical influences on northern and southern Africa manifest themselves in Rossby wave trains, which modulate synoptic to intraseasonal rainfall through tropical rainfall plumes, cold air surges, and upper-tropospheric dry air intrusions. Furthermore, the Saharan heat low (SHL) acts as a link between the northern hemispheric extratropics and tropics. Finally, the Indian monsoon, the Atlantic, Indian, and the Pacific Oceans can remotely affect the intraseasonal variability of African rainfall. Forecasting synoptic to intraseasonal rainfall variability is an integral part of seamless prediction between the weather and climate regimes. In the early 21st century, numerical weather prediction (NWP) systems can forecast larger intraseasonal signals such as the MJO several weeks into the future, but they still struggle to forecast shorter scale features reliably. Besides NWP, statistical models can successfully forecast intraseasonal variability of rainfall. Due to the relevance of synoptic to intraseasonal rainfall variability for African societies, early warning systems (EWSs) have been developed to mitigate impacts.
Ricardo García-Herrera and David Barriopedro
The Mediterranean is a semi-enclosed sea surrounded by Europe to the north, Asia to the east, and Africa to the south. It covers an area of approximately 2.5 million km2, between 30–46 °N latitude and 6 °W and 36 °E longitude. The term Mediterranean climate is applied beyond the Mediterranean region itself and has been used since the early 20th century to classify other regions of the world, such as California or South Africa, usually located in the 30º–40º latitudinal band. The Mediterranean climate can be broadly characterized by warm to hot dry summers and mild wet winters. However, this broad picture hides important variations, which can be explained through the existence of two geographical gradients: North/South, with a warmer and drier south, and West/East, more influenced by Atlantic/Asian circulation. The region is located at a crossroad between the mid-latitudes and the subtropical regimes. Thus, small changes in the Atlantic storm track may lead to dramatic changes in the precipitation of the northwestern area of the basin. The variability of the descending northern branch of the Hadley cell influences the climate of the southern margin, while the eastern border climate is conditioned by the Siberian High in winter and the Indian Summer Monsoon during summer. All these large-scale factors are modulated by the complex orography of the region, the contrasting albedo, and the moisture and heat supplied by the Mediterranean Sea. The interactions occurring among all these factors lead to a complex picture with some relevant phenomena characteristic of the Mediterranean region, such as heatwaves and droughts, Saharan dust intrusions, or specific types of cyclogenesis. Climate model projections generally agree in characterizing the region as a climate change hotspot, considering that it is one of the areas of the globe likely to suffer pronounced climate changes. Anthropogenic influences are not new, since the region is densely populated and is the home of some the oldest civilizations on Earth. This has produced multiple and continuous modifications in the land cover, with measurable impacts on climate that can be traced from the rich available documentary evidence and high-resolution natural proxies.
The Sahel of Africa has been identified as having the strongest land–atmosphere (L/A) interactions on Earth. The Sahelian L/A interaction studies started in the late 1970s. However, due to controversies surrounding the early studies, in which only a single land parameter was considered in L/A interactions, the credibility of land-surface effects on the Sahel’s climate has long been challenged. Using general circulation models and regional climate models coupled with biogeophysical and dynamic vegetation models as well as applying analyses of satellite-derived data, field measurements, and assimilation data, the effects of land-surface processes on West African monsoon variability, which dominates the Sahel climate system at intraseasonal, seasonal, interannual, and decadal scales, as well as mesoscale, have been extensively investigated to realistically explore the Sahel L/A interaction: its effects and the mechanisms involved. The Sahel suffered the longest and most severe drought on the planet in the 20th century. The devastating environmental and socioeconomic consequences resulting from drought-induced famines in the Sahel have provided strong motivation for the scientific community and society to understand the causes of the drought and its impact. It was controversial and under debate whether the drought was a natural process, mainly induced by sea-surface temperature variability, or was affected by anthropogenic activities. Diagnostic and modeling studies of the sea-surface temperature have consistently demonstrated it exerts great influence on the Sahel climate system, but sea-surface temperature is unable to explain the full scope of the Sahel climate variability and the later 20th century’s drought. The effect of land-surface processes, especially land-cover and land-use change, on the drought have also been extensively investigated. The results with more realistic land-surface models suggest land processes are a first-order contributor to the Sahel climate and to its drought during the later 1960s to the 1980s, comparable to sea surface temperature effects. The issues that caused controversies in the early studies have been properly addressed in the studies with state-of-the-art models and available data. The mechanisms through which land processes affect the atmosphere are also elucidated in a number of studies. Land-surface processes not only affect vertical transfer of radiative fluxes and heat fluxes but also affect horizontal advections through their effect on the atmospheric heating rate and moisture flux convergence/divergence as well as horizontal temperature gradients.