Climate and Weather Distillery for Africa and the Africa Diaspora: Cabo (Cape) Verde Season poised to go

Many of the tropical storms or hurricanes that form over the Atlantic develop directly from African Easterly Waves (AEWs). Normally, the pass across West Africa every 3-5 days and are most pronounced at 10,000 feet or 3000 meters. These waves provide important rains for West African farmers, however, this year the waves have not been so regular. It has been felt to most in the Western Sahel.. (Senegal); for example, Saturday night was only the second rain with more than 10 mm of rain.

Caption: Rainfall recorded in Dakar Senegal from May 17 through August 17

It is not just Dakar, that has been dry but much of Senegal during this period.

Caption: May-August rainfall anomalies (brown means dry)

The dryness in Senegal can be attributed to several items which I explain

1.) El Nino year. During most of the summer, the Pacific was warmer than normal (El Nino conditions) and this leads to more westerly shear, which does not allow thunderstorms to persist for very long. The El Nino has now collapsed.

2.) Saharan Air Layer Intrusion. There were many episodes of the Saharan air moving across the Atlantic during June and July. With each intrusion, there is very dry air, which is stable and contains dust particles. The dry air is normally between 3000 and 15000 feet and easy to see when you fly above it on a commercial airline.

Caption: Flying about Saharan Air Layer on June 16 early afternoon between Dakar and Cabo Verde(left) and early evening (right)
between Praia, Cabo Verde and Sal Cabo Verde

3. Warming in the Indian Ocean. We now know that the more than 2 decades of drying over much of West Africa was linked to warming in the Indian Ocean which is related to anthropogenic warming from greenhouse gases. When the Indian Ocean is warm, it tends to promote more sinking of dry air over Africa, thereby reducing rain.

Caption: Warmer waters are present over much of the Indian Ocean and extending toward Africa

A shift towards stronger waves and wetter conditions??

Yes. Over the last week, we have seen signs that AEWs will begin to look more like the normal conditions. A stream of fast winds called the African Easterly Jet (AEJ) has shifted towards northern Senegal and should provide fuel for waves to grow. The Saharan Air layer in response also seems to be moving further north giving the waves a chance to grow.

What else might happen and is it related to climate change?

Studies undertaken by Dr. Bamba Sylla (a good colleague that I helped to mentor) suggest that June and July will be dryer over the Sahel region, but during August and September there will be intense and possibly extreme rainfall when it occurs as the climate continues to warm throughout the 21st century. This pattern of extreme rainfall which poses a serious threat to agriculture and food security is lessened if we act to reduce greenhouse gas emissions.

So will there be more tropical storms, hurricanes in the next 2-3 months?

I think that the likelihood of tropical systems developing from AEWs over the next 10 days will significantly increase. Ocean temperatures across most of the Atlantic and Gulf of Mexico are above normal but conditions still need to be suitable for tropical cyclones to form (limited dry air, low shear).

Take Away: Be prepared, especially if you live on an island or within 10 miles of a coastline!! Remember it is not just the wind or the storm surge it is the rain, overflowing streams, and rivers.