Morocco after heavy rainfall, Image source: CCTV News
Expert consultant: Wang Chenghai, professor of College of Atmospheric Sciences, Lanzhou University
Rare rainfall in parts of the Sahara desert has led to spectacular scenes of water emerging from sand dunes and even heavy rains in some arid areas, triggering flooding.
As the world's largest desert, the Sahara spans over a dozen countries in North Africa, Central Africa, and West Africa. It has a tropical desert climate, making it one of the hottest and driest regions on Earth, with annual precipitation less than 50 millimeters, and some areas experiencing no rain at all throughout the year. Why is there an unusual flood here now? What are its impacts? Is it due to global warming? Will such extreme weather events become more frequent in the future?
The "push" behind the desert floods
The Sahara Desert is located in the subsurface area of the subtropical high pressure system, with precipitation mainly concentrated from July to September. Generally, the western part receives relatively more annual rainfall, while the eastern part receives relatively less. The region that experienced significant rainfall this time is Morocco, situated at the northwest corner of North Africa, adjacent to the Atlantic Ocean. This area typically has an annual average precipitation of less than 250 millimeters, but at the end of September this year, in the village of Taghounit, about 450 kilometers south of Rabat, the capital of Morocco, the 24-hour rainfall exceeded 100 millimeters, and the total rainfall over two consecutive days surpassed the region's annual average precipitation for the previous years. Such a large amount of rainfall in such a short period has not been seen in the past three or four decades.
The southern edge of the Sahara Desert has also experienced unusual precipitation this year. Satellite images show that recent storms have broken out in the long-dry regions of the southern Sahara, bringing green plants to what was once barren desert. Data from the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center indicates that parts of Niger, Chad, and Sudan have received more than four times their normal rainfall since mid-July.
Why does heavy precipitation occur in the desert?
The main weather systems affecting the Sahara Desert region include the West African monsoon, the tropical convergence zone, and temperate cyclones. The West African monsoon is the primary system influencing precipitation in western North Africa, particularly in the Sahel region south of the Sahara. Every summer (June to September), warm and moist air from the South Hemisphere moves northward from the Atlantic Ocean, bringing monsoon rains.
Experts analyze that the heavy rain in Morocco this time is related to the temperate cyclone over the eastern Atlantic, specifically Hurricane "Kirk," which has brought moisture from the eastern Atlantic into the arid region. The temperate cyclone is a significant weather system affecting the coastal areas of North Africa, especially during autumn and winter. When low-pressure systems over the central Mediterranean combine with moist air, they can bring precipitation to Morocco, Algeria, Tunisia, and other Mediterranean coastal regions of North Africa. From September 7 to 8, similarly influenced by the temperate cyclone, parts of Morocco, Algeria, Tunisia, and Libya located in the Sahara Desert recorded precipitation ranging from 10 millimeters to over 200 millimeters. In September 2023, Hurricane "Daniel" made landfall along the coast of Libya, bringing heavy rainfall equivalent to 2 to 10 times the annual precipitation in the area, with some regions experiencing over 10 times the amount.
Floods in the desert are a double-edged sword
Since September this year, heavy rains and floods have hit several areas in the southeastern provinces of Tatta and Rashidiyah. Heavy rainfall has disrupted roads and destroyed infrastructure in many places, seriously affecting people's lives.
After the rain, water gushed out of the sand in parts of the desert and through dunes and desert plant communities to fill Lake Iraki, which had been dry for 50 years. In Algeria, the rare sight of the Mirahe salt marsh being filled with rainwater was also seen.
After the rain, why does water seep out of the dunes? This is mainly due to the permeability of the sand and the terrain. Since sand has high permeability, rainwater quickly seeps into the groundwater layer. When a large amount of rainwater infiltrates underground, it significantly raises the groundwater level and pressure. If the water pressure in the groundwater exceeds the capacity of the surface layer, water will seep through cracks or pores in the sand dunes or on the surface to the ground. Additionally, in desert landscapes, dunes often encircle low-lying areas. Extreme rainfall can cause water to accumulate in these low-lying areas, increasing local groundwater pressure, which leads to water seeping through or emerging from the sand dunes.
Desert arid zone ecosystem is fragile and lacks the natural regulation capacity to deal with precipitation. Therefore, once there is heavy precipitation in the desert, it will often bring damage to facilities and human settlements with weak protection measures, and its harm is even greater than that in inland areas.
Most of Morocco has been experiencing drought for six consecutive years, forcing farmers to fallow their land and cities and villages to implement water rationing. However, extreme rainfall can also trigger floods, destroying farmland, irrigation systems, and agricultural infrastructure. Due to the lack of effective flood control systems in desert areas, the destructive power of floods is often severe, leading to reduced crop yields or even total crop failure. Therefore, the development of desert agriculture largely depends on the moderation and stability of precipitation.
It is worth noting that while precipitation benefits plant growth, animals and plants in the desert have long adapted to arid conditions. Sudden heavy rainfall can lead to an excess of water resources, posing a threat to some animals and plants. Moreover, desert areas are typically dry, with pathogens having weaker reproductive capabilities. However, after heavy rainfall, standing water may breed mosquitoes, spreading diseases such as malaria and dengue fever.
Global warming may exacerbate extreme weather and climate events in deserts
As global warming intensifies, the hydrological cycle will accelerate on one hand, and become more unstable and unpredictable on the other. At the same time, due to the localized nature of extreme weather and climate events, regions around the world are facing increasingly severe floods or droughts, as well as disaster challenges brought about by sudden shifts between dry and wet conditions. Studies have shown that under the backdrop of global warming, the frequency of heavy rainfall in arid areas is also increasing. In recent years, heavy rain events have been frequent in the northwest arid region of China, and there have also been floods in the Taklamakan Desert.
Extreme weather and climate events, while turning some long-dry desert regions green, disrupt the rhythm of Atlantic hurricanes, causing drought in areas that should receive rain, and flooding in regions that have been long dry. A study published in Nature this June shows that with the increase in global atmospheric carbon dioxide levels and global warming, the tropical convergence zone may move further north over the next few decades, meaning that precipitation in parts of the Sahara could increase.
Research shows that over the past nearly 40 years, precipitation in the Sahara Desert has shown an increasing trend, with an average increase of 15.2% per decade. The Sahara Desert's precipitation has also experienced significant inter-decadal variations. From the early 1970s to the early 1990s, there were notable drought events during the summer months, which may be related to global precipitation decreases and droughts in the Sahel region from the 1950s to the late 1980s. From the early 1990s to 2010, there was a clear recovery trend in drought conditions, but a sudden change occurred in 1992, followed by an 18% increase in precipitation per decade. The Intergovernmental Panel on Climate Change (IPCC) released its Sixth Assessment Report, indicating that since the 1960s, the number of days with precipitation exceeding 10 millimeters in eastern North Africa has decreased, while the number of consecutive dry days has increased. In contrast, heavy rainfall and flooding events have increased in western North Africa.
Will extreme precipitation events such as "a year or even a decade of rain in one day" in the Sahara become more frequent in the future?
The simulation predictions of future precipitation in the Saharan Desert region under climate models show that when global warming reaches 2℃ or higher, extreme precipitation intensity in the northeastern part of the Sahara will weaken, while it will increase in the northwestern and southern regions. Extreme precipitation events such as "a year's or even ten years' worth of rain in one day" may become more frequent. This is related to factors such as rising sea temperatures and the northward shift of the tropical convergence zone under global warming, which lead to increased intensity, frequency, and northward movement of tropical cyclones.
