Dust forecast for the 2009 Atlantic hurricane season
There will be less African dust than usual over the tropical Atlantic during this year's hurricane season, according to a new experimental dust forecast issued by Dr. Amato Evan of the University of Wisconsin. Dr. Evan used a statistical model that correlated levels of dust activity in past years with rainfall over the Sahel region of Africa and a natural regional wind pattern known as the North Atlantic Oscillation (NAO). He forecasts that dust levels over the Main Development Region (MDR, 8 - 20°N & 15 - 65°W) for Atlantic hurricanes during this year's hurricane season will be similar to last year's below-average levels, thanks in large part to plentiful rains over the Sahel region of Africa during the 2008 rainy season (Figure 1). However, the dust levels expected this year do not approach the record lows seen in 1994 and 2005. Dust forecasts made in May or June are skillful going out five months, with a skill 11 - 16% better than a "no-skill" forecast using climatology.
Figure 1. Rainfall over the Sahel region of Africa was generally 50 - 100 mm (2 - 4 inches) above average during the 2008 rainy season (about 20 - 80% above average). The heavy rains promoted vigorous vegetation growth in 2009, resulting in less bare ground capable of generating dust. Image credit: NOAA/Climate Prediction Center.
The Sahara and the Sahel: significant sources of dust
The summertime dust that affects Atlantic tropical storms originates over the southwestern Sahara (18° - 22° N) and the northwestern Sahel (15° - 18° N) (Figure 2). The dust that originates in the Southwest Sahara stays relatively constant from year to year. However, the dust from the northwestern Sahel varies significantly from year to year, and understanding this variation may be a key factor in improving our forecasts of seasonal hurricane activity in the Atlantic. The amount of dust that gets transported over the Atlantic depends on a mix of three main factors: the large scale and local scale weather patterns (windy weather transports more dust), how wet the current rainy season is (wet weather will wash out dust before it gets transported over the Atlantic), and how dry and drought-damaged the soil is. The level of drought experienced in the northwestern Sahel during the previous year's rainy season (June - October) is the key factor of the three in determining how much dust gets transported over the Atlantic during hurricane season, according to a January 2004 study published in Geophysical Research Letters published by C. Moulin and I. Chiapello. A dry rainy season the previous year will make an expanded area of loose soil which can create dust. It is also possible that the corresponding changes in vegetation can alter the regional weather patterns, causing more dust production.
Figure 2. Map of the mean summer dust optical thickness derived from satellite measurements between 1979 and 2000. Maximum dust amounts originate in the northern Sahel (15° to 18° N) and the Sahara (18° to 22° N). The Bodele depression in Chad is also an active dust source. Image credit: Evidence of the control of summer atmospheric transport of African dust over the Atlantic by Sahel sources from TOMS satellites (1979-2000) by C. Moulin and I. Chiapello, published in January 2004 in Geophysical Research Letters.
How dust suppresses hurricanes
Dust acts as a shield which keeps sunlight from reaching the surface. Thus, large amounts of dust can keep the sea surface temperatures up to 1°C cooler than average in the hurricane Main Development Region (MDR) off the coast of Africa, providing hurricanes with less energy to form and grow. Dust also affects the Saharan Air Layer (SAL), an layer of dry, dusty Saharan air that rides up over the low-level moist air over the tropical Atlantic. At the boundary between the SAL and low-level moist air where the trade winds blow is the trade wind inversion--a region of the atmosphere where the temperature increases with height. Since atmospheric temperature normally decreases with height, this "inversion" acts to but the brakes on any thunderstorms that try to punch through it. This happens because the air in a thunderstorm's updraft suddenly encounters a region where the updraft air is cooler and less buoyant than the surrounding air, and thus will not be able to keep moving upward. The dust in the SAL absorbs solar radiation, which heats the air in the trade wind inversion. This makes the inversion stronger, which inhibits the thunderstorms that power a hurricane. The dust may also act to interfere with the formation of cloud drops and rain drops that these thunderstorms need to grow, but little is known about such effects. It is possible that dust may act to help hurricanes by serving as "condensation nuclei"--centers around which raindrops can form and grow.
For additional reading
Dr. Evan published a study in Science magazine this March showing that 69% of the increase in Atlantic sea surface temperatures over the past 26 years could be attributed to decreases in the amount of dust in the atmosphere.
Jeff Masters
Reader Comments
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Should we get some Trough syrup? :)
What is your take Drak,the heat is pumping up and something has to give ?
The Earth needs to vent, right?
A little dry on the jokes... lol!
Looks like we will get somewhat of a cool down as the trough sets up of the east coast.
Root question is, at what point does the total energy absorbed, in a given area, require exhausting by virtue of weather? Sooner or later we get atmospheric TNT /
see every one in the morning
iam out
It just looks like the most interesting thing I've seen in the Atlantic in weeks lol
You could use the build of of SSTs as an analogy. SSTs support stronger storms that consume the energy built up in the ocean once the heat potential is transferred into kinetic energy the storm can use there is a depletion in the potential energy until it can be regained again.
And for all the people whose heartfelt response gave peace and comfort.
They who sailed the sea in ships, trading on the deep waters, these saw the works of the Lord and His wonders in the abyss.
His command raised up a storm wind which tossed its waves on high. They mounted up to Heaven; they sank to the depths; their hearts melted away in their plight.
They cried to the Lord in their distress: from their straights He rescued them, He hushed the storm to a gentle breeze, and the billows of the sea were stilled.
They rejoiced that they were calmed, and He brought them to their desired haven. Let them give thanks to the Lord for his kindness and His wonderous deeds to the children of men.
Psalm 107
Well said. Obviously there is much more at play, but where is the quantifiable threshold? We still don't know until we see the Sat/Quicksat. That's not good enough. We need to make it better! and can. Night all.----
Why rule out a Cape Verde season? SAL should be low, and CV's don't get crankin until late August anyway.
I don't like to have to keep mentioning 2004 but the first storm didn't form until unusually late.
how many of those cat 4's hit land?
all four of them effected land at that intensity.
2007 was the only season that featured 2 Category 5's making landfall at Category 5 status.
2005 you know was insane with the Category 4 storms.
2004 Charley, Frances, Ivan all effected land at Category 4 intensity.
2003 Isabel was the only Atlantic Category 5 so far this decade. The rest were all GOMEX and Caribbean Canes. Fabian also made landfall at Bermuda at Category 4 intensity.
2002 featured Hurricane Lili as a Category 4.
2001 featured Iris and Michelle.
2000 Featured Keith and Issac. Issac never effected land.
they one hurricane was a there where 3 cat 2 hurricanes and olny one of them wish was the G storm that had 150mph winds that made land fall some where but i dont re call any cat 4s makeing land fall any where other then that G storm
Ike did on an Island (Forgot which)
Omar was upgraded post season to Category 4 status.
That's absolutely insane. It is no wonder Wilma was able to explosively deepen to become the strongest Atlantic hurricane on record.
I made a post about 7 hours ago asking the same question and no one seamed intrested. what are you thoughts
When do you think we will get classification?
the tropical atlantic is the same refrain. Troughiness and high vertical wind shear. although the wind shear tendency is for a drop in the shear in the gomex and carib, there are no threat ares to talk about this morning. The models are not hinting of tropical development for the next seven days. it is beginning to appear that we may not get a named storm in the atlantic this month
We should start seeing interesting WX features developing this upcoming week all the way from the W Carib to the Mid Atl. NOGAPS has been rather persistant at developing a strong disturb region in the W Carib in response to the upward MJO this upcoming week but no model support yet from the others. So we'll have to see.
I see what u mean.....
Summer Is Here!; Tropical Update; Other Stuff
It's only a disturbance, there's no isobars clusters around that.
I had just worked a Spay/Neuter clinic there that spring. It's a beautiful island with the nicest people I have met from any of the Bahamian islands! I was sick watching Ike make a bee line for them and then trying to find out how they faired. They did better than expected, no human deaths and not many animal deaths either.
Will you be there in your hot dress?
I've heard it said that tropical cyclones transport a lot of heat poleward. Don't they just put it right back into the ocean (friction, waves, mixing)? Except for the poleward outflow, and what warm air/vapor is dragged along in the storm, how much heat is actually transported? It seems the ocean currents, and straight line winds do most of the transporting, and can relieve that built up TCHP without all of the drama. Anyone got a link to a page/document with concise #s on ocean vs atmospheric heat transport?
Ike check it out!
Looks like an artic blast went through Kissimmee, FL as the low was 33 degrees in the State Extremes in Florida. Kissimmee you mind sharing some of that with the rest of the state, lol.
Tropical cyclone help to balance the Earth's heat budget by enhancing the Hadley Circulation. It is not the tropical cyclone itself but the role it plays in the Hadley circulation, where heat store in the tropics, eventually rises and is expelled poleward. Recurving TCs that leave the Hadley circulation do this by a different but less effective process, ie, baroclinic.
The majority of the heat is transported by ocean currents while contribution by tropical cyclones are unquantified.
After the passing of Hurricane Floyd, I found a coconut on the beach of the Jersey Shore!
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