HOUSTON – Today’s I’m ‘dusting’ off a blog I wrote last year regarding the origins of our Saharan dust which I find fascinating. Here are some of the salient points:
Most of the dust comes from ONE spot in Africa. We’ve known about the dust since the 1960s. The dust is iron and phosphorous rich which greatly helps marine and plant life, especially the Amazon Forest in South America. Here’s more:
Believe it or not, half of Saharan dust that comes our way begins in one place: the Bodélé Depression (pronounced Bah-del). Just what and where is this dust source in the Sahara?
First the what. Over thousands of years, the once huge Lake Chad (about the size of Lake Erie and now only 5% of that) dried up leaving silt and sediment in the hot desert sun which baked into a fine dust. This bowl, or depression, of dust is northeast of the current Lake Chad and is 310 miles long, 93 miles wide and about 500 feet deep. In fact, it only takes up .2% of the whole Sahara Desert and yet is THE primary source of all this dust--hundreds of thousands of tons of it every year! You can see on the map below the exact location of the Bodélé Depression. Other countries to the west--Algeria, Mali, Mauritania and Algiers supply airborne dust due to the very hot 100+ temperatures causing rising air which takes the dust upward where winds transport it westward.
Imagine, this relatively small geographical spot produces tons of dust that travels all the way across the Atlantic to Texas!
Of course, the dust affects much of the rest of the southeast United States, Caribbean, South America, and even Europe. To be sure, the dust is full of iron rich minerals and phosphorus and without this transport of rich nutrients to the Amazon forests in South American, those forests would likely die. And the dust inhibits hurricanes by producing a dry atmosphere and downward motion of air. We’ve seen that this year. On the downside, the dust causes serious nasal congestion and even deadly breathing problems for some.
So how does such a small space produce so much dust? To the north of the Bodélé Depression are two mountain ranges, the Tibesti and the Ennedi, and between the two a wind tunnel is created. Those strong winds lift the dust into the atmosphere and then upper level easterly trade winds move it across Africa and the Atlantic. Here’s a map illustrating the geography and I’ve drawn in the wind tunnel arrows:
In addition, storms to the south of the Sahara Desert in the Sahel region, move across the continent creating winds that also carry dust. You can see the forecast there for more storms this week and they are pretty constant this time of year:
Why has there been so much dust this year? I’m not honestly sure we can know. An active upper level wind pattern, more storms crossing the continent, perhaps a layer of finer silt and sediment in the Bodélé Depression. Certainly a warmer atmosphere creates stronger winds (a matter of steeper pressure gradients which is a more meteorological answer), and a warmer atmosphere may create more rising dusty air from the desert. This one small area of the Sahara is not easy to study, but it IS being studied. As for how long these dusty days last, they usually wane by mid-August.
The big upside is that these dusty, dry conditions inhibit tropical formation--be glad of that. Just like this time last year, there is a tropical system in the Atlantic with a 40% chance to develop!
Have a great weekend--it’s going to get very hot and humid again by the end of the weekend and into next week!
Frank
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