The Sahara is considered one of the driest places in the world, with many regions receiving little or no rain for months or even years. In the coming days and weeks, however, some places are likely to experience an unusual rainfall event.
Rain in the Sahara
The Sahara is considered one of the driest places in the world; in many regions there is little or no rain for months or even years. As the following illustration shows, the rainfall peak at this time of year lies on an axis around 10° north of the equator; the Sahara (area shown in black) is literally as dry as dust, with a few exceptions.
Fig. 1: Average precipitation distribution in the month of August over the years 1970-2023; Source: Andrej Fils
The reason for this precipitation pattern can be found in the so-called Intertropical Convergence Zone (ITCZ). An area around the equator where the trade winds from the northern hemisphere and the southern hemisphere meet. The winds are weak here, but the humidity is very high. This zone is known for its heavy rainfall and thunderstorms, which occur when the warm, moist air rises, cools and then falls back to earth as rain (see also the blog on the monsoon). The ITCZ is a kind of low-pressure trough that stretches around the globe and follows the zenith of the sun with a delay of around 3 to 4 weeks. Depending on the ratio of land and water masses, the ITCZ is deflected regionally more to the north or south. As land masses have a significantly lower heat storage capacity compared to water masses, the land warms up faster than the sea and the ITCZ meanders more strongly here.
For some time now, however, the ITCZ has been much further north than normal. This northward shift has become even more pronounced over the past ten days. Depending on the region, the offset is between 1 and 3 degrees, i.e. around 100 to 300 kilometers.
Fig. 2: Seasonal course of the ITCZ; Source: Climate Prediction Center
Fig. 3: Current and usual position of the ITCZ; Source: Climate Prediction Center
Precipitation shift to the north
This shift in the ITCZ has also resulted in a direct shift in precipitation further north. According to the GFS model, rainfall is forecast for the next two weeks, particularly in parts of Mauritania, Mali, Algeria and Niger, but also in Chad, Libya and Sudan. The totals of 5 to 50 mm may not sound like much by Central European standards, but for this region of the world this is record-breakingly high!
Fig. 4: Precipitation totals until mid-September according to GFS; Source: Tropical Tidbits
According to the current model status, the unusual weather pattern is likely to continue for some time into September. By the end of the month, precipitation deviations of more than 500% are likely in many areas.
Fig. 5: Precipitation deviation (%) from the norm in the next 30 days; Source: Weathermodels
An analysis of the weather data since 1970 shows that such expected precipitation deviations are extremely unusual, especially for the month of September. The last time there were somewhat larger deviations was in 1994, although at that time practically only regions in Algeria and Mali were affected – It is therefore quite possible that this year could be even more extreme.
Fig. 6: Precipitation deviations in September 1994 compared to the current norm; Source: Severe-Weather
Influence on the Atlantic hurricane season
Normally, wave-like weather systems (so-called African Easterly Waves, AEW) develop at this time of year, which move from east to west and typically move from the Guinea Highlands across the equatorial Atlantic and act as a kind of "seedling" of later hurricanes. This process is currently somewhat suppressed, which is currently reducing tropical activity in the Atlantic. At the beginning of the hurricane season, a much more active season was forecast for this year due to the high water temperatures and the transition from El Niño to La Niña.
— Ben Noll (@BenNollWeather) August 29, 2024The weather over Africa is very weird right now and it's affecting hurricane season.
Typically, African Easterly Waves, or hurricane seedlings, swirl from the Guinea Highlands in West Africa into the tropical Atlantic where they strengthen during late August and September.
