During the past 6-8 weeks, the southern Sahara, primarily known as the Sahel or Sub-Saharan Africa, has been… strangely rainy. Locations in the likes of Sudan and Mauritania, which typically see 60-70 mm during that time (equivalent to a typical March in southern England - the Sahel is not part of the Sahara desert), have instead observed close to 200 mm.
Not only that, but at the time of writing (early Sep 2024), forecast models are predicting even more excessive amounts for the next fortnight.
Meanwhile, locations south of Sahel, in the likes of Cameroon and Ghana for example, have seen substantial shortages of rainfall. This is challenging for local agriculture, such as the cocoa plantations of southern Chana and Côte d'Ivoire (Ivory Coast).
This all corresponds to an unusually large northward shift in the inter-tropical convergence zone (ITCZ), within which strong thunderstorm activity tends to be very frequent for long stretches of time.
Back in the 1970s-80s, the Sahel region experienced a drought, after which it took several decades for rainfall amounts to recover to what they were in the 1950s-60s. Scientific research has mainly attributed this recovery to anthropogenic warming and associated temperature increases in regions such as the North Atlantic and Mediterranean.
Is 2024 the hallmark of this trend continuing, to the point that it overshoots, leading to an even wetter Sahel climate? Climate projection models are undecided on this, with scenarios ranging from wetter throughout, to wetter in the central Sahel but drier in the west, to drier throughout.
However, those with a widely wetter Sahel tend to be ones that also predict considerable warming in the North Atlantic and Mediterranean, which is what we’ve seen in recent years. Accordingly, when we look at the Sahel precipitation in the past ten years (see below), there are signs of a northward shift in rainfall distribution.
Note the increased prominence when looking at the latest 5 years. A hint that the trend might be accelerating, though this is too short a time span for us to have confidence in that.
Importantly, we can’t expect this trend to be a steady one. Due to variation in, for example, sea surface temperature (SST) patterns, we should expect the position of the ITCZ to vary around the long-term trend.
Alongside the North Atlantic and Mediterranean (which we’ll get to later), among the strongest influences on ITCZ position comes from the relative warmth of the tropical Atlantic each side of the equator.
North v South Tropical Atlantic
The maps below show the mean precipitation rate anomalies for a cool north tropical Atlantic and warm south tropical Atlantic (left), then the inverse (right).
It’s clear that the first configuration encourages the ITCZ to be well south, the second well north.
This year, the north tropical Atlantic is very warm, while the south tropical Atlantic is near average.
If we were to remove the long-term warming trend from this, we would see a warm north, cool south pattern. Therefore, we can reasonably state that the sea surface temperature pattern in the tropical Atlantic has helped shift the ITCZ exceptionally far north in Jul-Aug 2024.
Of course, when it comes to the Earth’s ocean-atmosphere system, you don’t tend to get nice simple ‘If A then B’ connections. Sure enough, there are other key factors affecting Sahel rainfall.
A research publication released last August picked out the North Atlantic and the Mediterranean as the top two regions where the mean temperatures within have a remote influence on how wet the Sahel is.
Picking out the top 5 for those two respectively produces the composites on the left and right shown below.
You may have spotted a lot of overlap here. Both regions have substantial warming trends since the 1980s, with a dramatic step upward in 2022-2023. In fact, it’s arguable that only those latest two years have much relevance to the climate in the coming decades.
A northward shift of the ITCZ is only hinted at in these composites, and then only for the western Sahel. This may well be because until 2024, we’d never seen a very warm North Atlantic and Mediterranean coincide with a warm north and cool south tropical Atlantic. It’s possible that 2022 or 2023 are the first year(s) to have become sufficiently warm to have a meaningful effect on the ITCZ, only to be countered by the tropical Atlantic SST pattern.
Using ERA5 reanalysis data to identify the wettest 5 historical years in the Sahel since 1979, they are indicated to be 1988, 1994, 1998, 1999, and 2020.
All these years have one other thing in common: A negative Pacific Decadal Oscillation (PDO). This is characterised by a ‘horseshoe’ pattern of SST anomalies in the North Pacific Ocean, where a cooler horseshoe around a warmer middle corresponds to a negative PDO, and vice-versa.
It’s evident in the SST anomaly map for 4th-31st August shown earlier that we have a negative PDO in 2024.
As you can see below on the left, a precipitation rate composite of years with a negative PDO features a north-shifted ITCZ.
Further filtering to only years with a warm north, cool south tropical Atlantic combination (below right), the signal strengthens.
Notice, however, that this is comprised of just two historical years. The combination we have this year has been very rare since at least 1950.
The logical conclusion is that we have seen a rare case of both the PDO and tropical Atlantic favouring a north-shifted ITCZ, with this being overlaid onto a long-term northward trend. A recipe for extreme, potentially record-breaking rainfall in the Sahel.
Attempting to further filter to a warm North Atlantic and Mediterranean, we get no results at all. 2024 is unique! However, there is one match if we drop the Mediterranean requirement: 2012. Lo and behold, that year had a substantially north-shifted ITCZ.
While most climate models predict a generally wetter Sahel, the rarity at which 3 or more of the tropical Atlantic, North Atlantic, Mediterranean, and PDO have been in favour of a north-shifted ITCZ suggests that we probably won’t see a 2024-style outcome feature every year this coming decade or so.
However, there’s no escaping that the North Atlantic and Mediterranean have become so warm and demonstrated so much persistence of anomalous warmth in recent years, that the dice is heavily weighted toward that combination going forward.
What’s more, the PDO tends to favour either a positive or negative state for 5-10 years at a time, and is currently on its 4th year negative, so it could well remain negative for another 1-6 years.
In which case, how the tropical Atlantic behaves could be the most important factor for at least the next few years. This begs the question: Is there any trend there?
Well, since 1950, the rates of warming in the north and south tropical Atlantic have been very similar. This has made for no significant trend in the difference between the two.
On which basis, there’s no obvious reason to expect this year’s relatively warm north, cool south pattern to occur more often in the coming decades… but there’s a nagging possibility that this year’s pattern has some connection to the drop in sulphur dioxide emissions from shipping that began in 2020.
A research publication released May 2024 found that this facilitates widespread warming of the oceans, which is only now approaching peak effect, with the subtropical and tropical North Atlantic seeing among the strongest impacts. The implication being that a warm north, cool south tropical Atlantic pattern may occur more often in the years ahead.
This meteorologist’s overall impression is that 2024-style conditions are likely to occur markedly more often in the years ahead compared to the frequency during 1950-2023, but still with some appreciable breaks where the ITCZ stays further south.
Interestingly, most climate projection models predict little overall change in typical rainfall to the south of the Sahel, even when the Sahel turns wetter. Perhaps this is due to increased available moisture from warming oceans, such that when the ITCZ stays south, the rains are heavier than the historical norm. If so, effective management of water reserves will be essential for handling the associated increase in year-to-year variability.
James Peacock
Head Meteorologist at MetSwift
