A unique rainfall event is about to unfold across the Sahara desert, otherwise known as the driest place on Earth. The amount of rainfall might not seem large by normal standards, but a large part of the Sahara will get well over 500% of normal monthly rainfall in August and September.It's not very often that the Sahara desert experiences these rainfall events. They are very rare, less than once per decade on average, but they are usually a sign that something is perhaps changing in the Earth's weather system, indicating an unusual state of the Atmosphere.
The driest place on EarthSahara is called the driest place on Earth because it receives little to no yearly precipitation. It is the world's largest and hottest desert, extending from the Atlantic Ocean in the west to the Red Sea in the east. It covers over 9.2 million square kilometers (3.6 million square miles).
This region is hot and dry because it is under the subtropical ridge, a permanent high-pressure system. This ridge causes the air to descend, making the atmosphere dry and stable and preventing cloud formation and precipitation.
But the Sahara was not always a hot wasteland. Just a
few thousand years ago (6000-11000), it was a green region with lakes, rivers, and lush vegetation. But changes in the Earth's orbit caused shifts in the world's weather patterns. That abruptly transformed the green region into some of the driest land on Earth.
Below is an image that compares what scientists believe northern Africa looked like a few thousand years ago. It was a green area filled with life, but weather changes have transformed it into the dry, hot region that we know today.
Weather and precipitationThe image below shows the average monthly rainfall for August in the past 53 years. You can see that the Saharan region is mostly void of precipitation, with only some low amounts in the more southern regions. But notice the very strong belt of precipitation across the Equator.
That tropical belt of rainfall is called the Inter-Tropical Convergence Zone (ITCZ). It's basically a band of clouds, showers, storms, and rainfall that is spread around the whole Equator. This feature exists because the trade winds from both hemispheres meet, creating rising motion, clouds, and rain.
You can see an example of the ITCZ in the image below. Tropical winds from both hemispheres meet near the Equator, creating rising motion in the atmosphere, producing clouds, storms, and precipitation.
The ITCZ is very important because it affects the weather and climate across tropical countries. It also plays a role in the global weather system and can also impact tropical storm formation. The rainfall events in the Sahara also depend on the changes and shifts of the ITCZ.
In the image below, you can see the latest ITCZ analysis by NOAA. The black line shows the normal (expected) position of the ITCZ for this time of year. But the red line shows the actual position of this anomaly, and you can see it has shifted much to the north.
The graph data shows an even better picture of the ITCZ position over Africa. We can see that since June, the ITCZ has been shifting much more to the north than usual. This can drastically change the weather patterns over the region and also affect the Atlantic Hurricane Season.
Because the whole ITCZ has shifted to the north, strong thunderstorm systems move into the Atlantic Ocean at a higher latitude over colder waters. That means we can get fewer tropical systems powering up as they move out of Africa at a bad location to organize and intensify.
This is just one of the indirect impacts that the ITCZ position in Africa can have on the weather in North America, especially during the Hurricane Season in the United States.
But the more immediate affect of this northerly shift is that it brings rare rainfall events deep into the Sahara desert.
Rain in the desert?Over half of the Sahara desert receives less than 25mm (1 inch) of rainfall per year. Occasionally, more rainfall is found along the southern portions as the ITCZ shifts up and down. But a full on rainfall event across most of the desert is not something we see every year, or perhaps even every decade.
Below is the latest GFS model forecast for total rainfall in the next 16 days. What you see is rainfall covering a large part of the Saharan desert. The amounts might not look high, but if we consider the total yearly amount, many regions have a few years worth of rain in a few days. Image by
weathermodels.com using a commercial license.
Looking at anomalies also gives us a very clear picture. An anomaly is a deviation from the normal. The ensemble rainfall anomaly forecast below shows that a large part of the Saharan desert will experience a widespread rainfall anomaly in the next two weeks and likely beyond.
Looking at the ECMWF ensemble total precipitation, we can see that actual total amounts are perhaps not super high. Again, it shows us that many regions are forecast to get a few years worth of rain in a span of two weeks.
The ECMWF extended precipitation anomaly for early September also shows a large part of the Sahara desert under significant rainfall anomalies. A large part of the desert is forecast to experience rainfall even going into next month.
Looking even deeper into September, we have an image below that shows the forecast for % of normal precipitation. as you can see, the Saharan desert is maxed out at 500% around the edges. This indicates that central regions will get over 1000% of the normal rainfall in the coming days and weeks ahead.
But how rare are these events really?
Rare weather eventWe decided to produce a graph for precipitation over the Sahara, covering the month of August. You see the rainfall anomalies for each August in the past 53 years, with only 4 years showing a strong anomaly and more rainfall. With four events in the past five decades, it gives as less than one such event per decade.
Because the precipitation forecast also shows strong anomalies in September, we decided to look at September data for the same period and area. As you can see, only one strong event occurred in 1994. We don't have the 2024 data yet to understand the full context, but based on the forecast, it will be at least the second wettest year in the past decades.
Looking closer at the 1994 precipitation anomaly, you can see that the overall anomalies were not as strongly widespread as we can see in some current forecasts for September. This could indicate that the current event could be placed very high in the history books, if not among the wettest years.
We have not yet completed our research on potential indications for Autumn or Winter across the United States or Europe. But one thing is certain: Such strong and rare weather anomalies can indicate a big instability of the global weather system.
We will keep you updated on the developing weather trends in the coming seasons, so make sure to bookmark our page. Also, if you have seen this article in the Google App (Discover) feed, click the like button (♥) there to see more of our forecasts and our latest articles on weather and nature in general.
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