Today rain is falling in the Sahara desert, one of the driest places on earth. Moisture from the African monsoon has moved far north of its normal tropical extent into the center of the Sahara desert. Rain is falling in places it hasn’t fallen in years. The African monsoon moved north of its normal region in August bringing some locations in north Africa many times their normal rain.
North African rainfall 6August - 5 September 2024 shown as percent of normal rain. Extraordinary rain fell in the Arabian peninsula and the Sahara desert over the 30 day period.
American and European weather and climate models forecast that the monsoon rains will continue for the next two weeks or more as the African monsoon will continue to be displaced far north of it’s normal tropical position.
The African monsoon will be displaced northwards of normal in the second week of September bringing rain to the Sahara desert. Forecast by the European ensemble EPS model.
At Severe Weather Europe Andrej Flis explained how unusual this Saharan rain is.
A unique rainfall event is currently unfolding across the Sahara desert, one of the driest places 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 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 changing in the Earth’s weather system, indicating an unusual state of the Atmosphere as we head into Autumn and Winter.
In normal years the subtropical high pressure area, a heat dome, sits relentlessly over the Sahara. The sands are so reflective, the air so dry and the ground is so hot that heat is actually lost to space over the Sahara under normal conditions. That leads to hot days and cool nights with huge temperature swings especially near the equinoxes. This September’s equinox period will be very abnormal with cooler days, warmer nights and intrusions of tropical air from the African monsoon.
The Sahara wasn’t always a desert. From the beginning of the interglacial period up to 6000 years ago, there were lakes in the Sahara. Megalake Chad and the other Saharan lakes were so large that they kept the climate moist even after the orbital parameters that brought maximum insolation to the Arctic in late spring and early summer ending the last glacial period became less favorable. However, 6000 years ago the solar heating patterns caused by earth’s orbital variations brought more ocean warming to the southern hemisphere and less to the northern hemisphere and the north Atlantic cooled. The cooling of the north Atlantic waters brought on the drying of the Sahara and the desert conditions that have predominated the region for the past 5000 years.
But this summer something is different. The waters of the north Atlantic are relatively hot while the waters of the south Atlantic are relatively cool. And the waters of the Indian ocean on the east coast of Africa are very warm. And the Mediterranean sea is hot.
The waters of the north Atlantic are relatively hot and the south Atlantic relatively cool in early September 2024. The Mediterranean sea is hot.
The high heat content of the north Atlantic and the Mediterranean sea has affected the atmospheric circulation patterns bringing exceptional heat to far northern Europe and allowing the heat dome over western north Africa to break down, bringing monsoon moisture north into the Sahara desert.
The 500mb map for Europe and Africa on 6Sept 2024 shows a blocking high - a heat dome - over Scandinavia and low pressure over the western Sahara.
The heating of the north Atlantic caused by human activities, in particular, greenhouse gas (ghg) emissions, may be creating conditions similar to those that brought the African monsoon into the Sahara 6000 years ago. Climate models have predicted that the Sahara could become wetter with increasing ghg emissions and that appears to be beginning this year.
There is good news in this. The atmospheric waves that move into the north Atlantic that develop into tropical storms have been displaced far north of their normal track this August and early September. They have moved into cooler waters. The waters of the main development region for Atlantic hurricanes is very hot, but without the strong tropical waves over the warm waters the hurricane season has gone quiet.
The bad news is the humidification of the Sahara leads to increased global uptake of solar heat because water vapor is a strong greenhouse gas.
