Planet Earth is a living planet, so much so that when temperatures rise in one place, they fall in another to maintain global thermal balance. Something similar is going to happen with the Mediterranean and the Sahel: in the last 20 years, the Mediterranean region has been experiencing an increase in temperatures and a decrease in rainfall, rains that seem to have moved to the Sahel, as revealed in a study published in the journal Nature Climate Change, prepared by the Max Planck Institute for Meteorology.
Due to the increase in temperatures in the Mare Nostrum, the humidity that reaches the southern limit of the Sahara at the beginning of the West African monsoon in the month of June is also higher, so the Sahel becomes greener.
The climate of the Sahel is highly variable, dominated by the West African monsoon, which brings rains from June to September. The rest of the year, the drought is very intense. The earth heats up more than the ocean in summer, since the sun is in a higher position and, in addition, the oceans do not absorb heat as quickly as the earth. The air rises from the mainland and, in doing so, produces moisture flows from the sea towards the Sahel.
The intensity of the monsoon has been varying over time. Between the years 1950 and 1960, the Sahel experienced a wet period; In the 1980s, the drought was so intense that more than 100.000 people lost their lives. Since then, the rainfall returned.
The cause according to scientists is Mediterranean warming. To reach that conclusion, different scenarios were studied using various simulations. Thus, they were able to find out that if the temperature in the Mediterranean area remains more or less constant, the precipitation in the Sahel does not increase; on the contrary, if the Mediterranean warms, in the Sahel it rains more.
This is because not only the temperature rises, but also the humidity, which is what "activates" the West African monsoon. In this way, in this part of Africa, they can enjoy more rains at the beginning of the rainy season.
You can read the study here (In English).