Massive African Dust Storm Cooling Atlantic Hurricane Odds for Early August

By: Dr. Jeff Masters , 3:10 PM GMT on July 31, 2013

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A massive dust storm that formed over the Sahara Desert early this week has now pushed out over the tropical Atlantic, and will sharply reduce the odds of tropical storm formation during the first week of August. The dust is accompanied by a large amount of dry air, which is making the Saharan Air Layer (SAL) much drier than usual this week. June and July are the peak months for dust storms in the Southwest Sahara, and this week's dust storm is a typical one for this time of year. Due in large part to all the dry and dusty air predicted to dominate the tropical Atlantic over the next seven days, none of the reliable computer models is predicting Atlantic tropical cyclone formation during the first week of August.


Figure 1. A massive dust storm moves off the coast of Africa in this MODIS image taken at 1:40 UTC July 30, 2013. Image credit: NASA.


Video 1. The predicted movement through August 3 of this week's Africam dust storm, using the NOAA NGAC aerosol model. Image credit: NOAA Visualization Laboratory.

How dust affects hurricanes
Saharan dust can affect hurricane activity in several ways:

1) 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) from the coast of Africa to the Caribbean, providing hurricanes with less energy to form and grow. Ocean temperatures in the MDR are currently 0.7°F above average, and this anomaly should cool this week as the dust blocks sunlight.

2) The Saharan Air Layer (SAL) is a 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.

3) Dust may also act to produce more clouds, but this effect needs much more study. If the dust particles are of the right size to serve as "condensation nuclei"--centers around which raindrops can form and grow--the dust can act to make more clouds. Thus, dust could potentially aid in the formation and intensification of hurricanes. However, if the dust acts to make more low-level clouds over the tropical Atlantic, this will reduce the amount of sunlight reaching the ocean, cooling the sea surface temperatures and discouraging hurricane formation (Kaufman et al., 2005.)


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.

Dust in Africa's Sahel region and Atlantic hurricane activity
The summertime dust that affects Atlantic tropical storms originates over the southwestern Sahara (18° - 22° N) and the northwestern Sahel (15° - 18° N) (Figure 3.) The dust from the Southwest Sahara stays relatively constant from year to year, but the dust from the Northwest Sahel varies significantly, so 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 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. In 2012 (Figure 3), precipitation across the northwestern Sahel was much above average, which should result in less dust than usual over the Atlantic this fall, increasing the odds of a busy 2013 hurricane season.


Figure 3. Rainfall over the Northwest Sahel region of Africa was about 200% of average during the 2012 rainy season. The heavy rains promoted vigorous vegetation growth in 2013, resulting in less bare ground capable of generating dust. Image credit: NOAA/Climate Prediction Center.

The future of African dust: highly uncertain
A September 2013 paper in the Bulletin of the American Meteorological Society by Joseph Prospero and Olga Mayol-Bracero, "Understanding the Transport and Impact of African Dust on the Caribbean Basin," discusses the large uncertainties on how African dust may change due to climate change. Over the past decade, there has been no clear relationship between African dust and climate indices such as rainfall in the Sahel or the El Niño/La Niña cycle, which "makes it difficult to predict how dust emissions and transport might change over the coming decades as climate changes. The problem is exacerbated by the inability of models (IPCC 2007) to agree on future rainfall trends over large areas of North Africa (including the Sahel) that are known to be major dust sources today and in the recent past."

Links
Saharan Air Layer Analysis from the University of Wisconsin

Atlantic dust forecast from the Tel-Aviv University Weather Research Center

The Saharan Air Layer (SAL) was first described in 1972, in this classic paper: Carlson, T. N., and J. M. Prospero (1972), The Large-Scale Movement of Saharan Air Outbreaks over the Northern Equatorial Atlantic, Journal of Applied Meteorology, 11(2), 283-297

Dr. Amato Evan published a study in Science magazine March 2009 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.

Kaufman, Y. J., I. Koren, L. A. Remer, D. Rosenfeld, and Y. Rudich, 2005a: The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean. Proc. Natl. Acad. Sci. USA, 102, 11 207–11 212.

Wang, Chunzai, Shenfu Dong, Amato T. Evan, Gregory R. Foltz, Sang-Ki Lee, 2012, Multidecadal Covariability of North Atlantic Sea Surface Temperature, African Dust, Sahel Rainfall, and Atlantic Hurricanes, J. Climate, 25, 5404–5415.
doi: http://dx.doi.org/10.1175/JCLI-D-11-00413.1

Jeff Masters

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526. auburn (Mod)
But I might add this.. Please do not engage in personal attacks or bickering.
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525. FOREX
Quoting 517. wayne0224:
Ardvarks I cant say but we do have Armadillos.


Can't a moderator remove Buzzard so he cannot post anymore?
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Ok.......The children have spoken............BYE.Thank you VR..
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It's a fish!
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522. auburn (Mod)
I personally dont have a problem with the word crap..if admin does will they please advise?
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img src=""> Ts Gil!!
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Quoting 500. Tazmanian:



yet the mods take care of it plzs your not a mod pat
You have got to be joking Taz..
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518. VR46L
Quoting 508. PalmBeachWeather:
I said knock off the crap and I got chastised... For real??


I didn't know that was a bad language... but then again to me saying Poof is foul and nasty
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Quoting 501. buzzardswrath:
who was it saying theres ardvarks in south florida tearing up everyones lawns? never heard of that in florida before..... up here we got racoon , possum, buzzard , deer. lizards but no ardvarks... now thats foolish
Ardvarks I cant say but we do have Armadillos.
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Time for me to take a break from the blog i see that is that weard time of the day and is hard to watch. LOL
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Member Since: July 3, 2005 Posts: 426 Comments: 128609
Quoting 498. Gearsts:
Ignore the trolls please everyone.


And also don't quote them.
Member Since: April 29, 2009 Posts: 75 Comments: 14301
Quoting 505. Hurricane1956:
Very well said!!!,from somebody who should be respected on this blog.
I said knock off the crap and I got chastised... For real??
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Quoting 496. Patrap:
PalmBeachWeather, if your going to chastise anyone here, I'd advise you to quit using the "C" word as that's a explicit violation of the "Rules of the road" here.

#5 Foul or obscene language or content of any kind is not allowed.

Very well said!!!,from somebody who should be respected on this blog.
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Quoting 496. Patrap:
PalmBeachWeather, if your going to chastise anyone here, I'd advise you to quit using the "C" word as that's a explicit violation of the "Rules of the road".

#5 Foul or obscene language or content of any kind is not allowed.

Crap???
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Hurricane Gil Rainbow Loop



Member Since: July 3, 2005 Posts: 426 Comments: 128609
Quoting 498. Gearsts:
Ignore the trolls please everyone.
Gearsts....I tried too freekin' many.....
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Quoting 496. Patrap:
PalmBeachWeather, if your going to chastise anyone here, I'd advise you to quit using the "C" word as that's a explicit violation of the "Rules of the road" here.

#5 Foul or obscene language or content of any kind is not allowed.

Quoting 496. Patrap:
PalmBeachWeather, if your going to chastise anyone here, I'd advise you to quit using the "C" word as that's a explicit violation of the "Rules of the road" here.

#5 Foul or obscene language or content of any kind is not allowed.




yet the mods take care of it plzs your not a mod pat
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So many immature people here now.Sure isn't what it used to be.......I may have to leave for a while and hope things will get better here...It' embarrassing listening to such BS....BYE until things get better.
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Ignore the trolls please everyone.
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Quoting 388. 1900hurricane:
000
NOUS64 KFWD 310408
FTMGRK

MESSAGE DATE: JUL 31 2013 04:08:50

THE KGRK WSR-88D WILL BE DOWN UNTIL FURTHER NOTICE. THEY ARE STILL
WAITING FOR A PART TO ARRIVE SO THAT THEY CAN FIX THE PEDESTAL
FAILURE.

MOORE/WFO FWD


GRK has to be in the running for the site with the most downtime. I like the upgrades to the electronics they performed last year, but get the feeling the mechanical pieces cause the most trouble.

If the NWS would just integrate the ADRAD that sits on top of your building into the network...


Not that it looks like we will need it anytime soon.
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PalmBeachWeather, if your going to chastise anyone here, I'd advise you to quit using the "C" word as that's a explicit violation of the "Rules of the road".

#5 Foul or obscene language or content of any kind is not allowed.

Member Since: July 3, 2005 Posts: 426 Comments: 128609
Quoting 483. Gearsts:
Just and upper low there.
This is from one of the serious moderators on this blog.
"Quoting 433. Skyepony:
GEOS-5 has the blob as a weak depression on SE FL sat night sun morning, that moves up the coast til about the Cape then turns near 90� east & gets smaller. I was surprised. It's usually more conservative.
Quoting 483. Gearsts:
Just and upper low there.
Well,this is from one of the serious moderators on this blog.
"Quoting 433. Skyepony:
GEOS-5 has the blob as a weak depression on SE FL sat night sun morning, that moves up the coast til about the Cape then turns near 90� east & gets smaller. I was surprised. It's usually more conservative.
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Quoting 479. Bluestorm5:
Some of you aren't doing us any favor by quoting buzzardswrath instead of ignoring him, as many of us got him on ignored list. Uninformed are smart enough to know he's a troll.




I think I told you guys the quote and the ignored do not go well


I wish the admins would re move the quote from this blog
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buzzardswrath @

Lol.I'm talking about local weather.
Member Since: August 14, 2010 Posts: 10 Comments: 17063
2013 East Pacific Hurricane Season is now 7-5-0 entering the peak year. Very impressive.
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Gonna be a stormy day here tomorrow...
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Quoting 477. buzzardswrath:
not scaring people . just need to be prepared this dorian could whip up quick,,, its cane season and anything goes
I will say this one more time buzzard......You have "NO" idea what you are talking about.I have been here for many years.You may be my first ignore person....Knock off the crap because you have no idea what the hell you are talkin' bout...GOT IT???
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Hurricane Gil Viz Loop

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Quoting 478. Hurricane1956:
That thing is looking very interesting!! from the Tropical point of view IMO.
It's not worth an invest... -__-
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Quoting 475. Civicane49:
BULLETIN
HURRICANE GIL ADVISORY NUMBER 6
NWS NATIONAL HURRICANE CENTER MIAMI FL EP072013
200 PM PDT WED JUL 31 2013

...GIL NOW A HURRICANE...


SUMMARY OF 200 PM PDT...2100 UTC...INFORMATION
----------------------------------------------
LOCATION...13.9N 120.6W
ABOUT 935 MI...1505 KM SW OF THE SOUTHERN TIP OF BAJA CALIFORNIA
MAXIMUM SUSTAINED WINDS...75 MPH...120 KM/H
PRESENT MOVEMENT...WNW OR 285 DEGREES AT 12 MPH...19 KM/H
MINIMUM CENTRAL PRESSURE...992 MB...29.29 INCHES

I beat you :) But don't worry, i'll leave the rest of the updates to you :p I am just bored so I was waiting for the update
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Quoting 467. Hurricane1956:
I believe the NHC have to move the blob by the Bahamas to Invest Status as soon as possible!!!.
Just and upper low there.
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Quoting 427. Grothar:


Trust me. Perfect. Especially your use of idioms. And you and I know, the most difficult part.


You guys on the blog help me a lot, lol.
(One thing I've learned: I don't have to add: gals. In Germany this would be a capital sin, though. We are very political correct.)
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The long range models show at least the fist half of August being below normal.
Member Since: August 14, 2010 Posts: 10 Comments: 17063
Some of you aren't doing us any favor by quoting buzzardswrath instead of ignoring him, as many of us got him on ignored list. Uninformed are smart enough to know he's a troll.
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Quoting 467. Hurricane1956:
I believe the NHC have to move the blob by the Bahamas to Invest Status as soon as possible!!!.
Quoting 467. Hurricane1956:
I believe the NHC have to move the blob by the Bahamas to Invest Status as soon as possible!!!.
That thing is looking very interesting!! from the Tropical point of view IMO.
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HURRICANE GIL DISCUSSION NUMBER 6
NWS NATIONAL HURRICANE CENTER MIAMI FL EP072013
200 PM PDT WED JUL 31 2013

THE CLOUD PATTERN HAS BECOME BETTER ORGANIZED DURING THE PAST
SEVERAL HOURS AND VISIBLE SATELLITE IMAGES REVEAL A RAGGED EYE
EMBEDDED WITHIN A CENTRAL DENSE OVERCAST...CDO. IN ADDITION...THERE
ARE SEVERAL CONVECTIVE BANDING FEATURES WRAPPING AROUND THE CDO. ON
THIS BASIS...GIL HAS BEEN UPGRADED TO HURRICANE STATUS IN THIS
ADVISORY. GIVEN THAT THE SHEAR IS FORECAST TO REMAIN LIGHT FOR THE
NEXT FEW DAYS AND THE OCEAN IS WARM...THE OFFICIAL FORECAST CALLS
FOR GIL TO INTENSIFY A LITTLE MORE DURING THE NEXT DAY OR SO. THE
GLOBAL MODELS...PRIMARILY THE GFS...FORECAST AN INCREASE IN THE
UPPER-LEVEL WESTERLIES OVER OR NEAR THE CYCLONE DURING THE LATTER
PART OF THE FORECAST PERIOD. THIS PATTERN...IN COMBINATION WITH
COOLER WATERS...SHOULD RESULT IN GRADUAL WEAKENING BEYOND 36 OR 48
HOURS.

GIL HAS SLOWED DOWN A LITTLE AND IS NOW MOVING TOWARD THE
WEST-NORTHWEST OR 285 DEGREES AT 10 KNOTS. GIL CONTINUES TO BE
STEERED BY THE FLOW SOUTH OF A SUBTROPICAL RIDGE WHICH IS FORECAST
TO PERSIST. THIS PATTERN WILL KEEP GIL ON A GENERAL WEST-NORTHWEST
TRACK FOR THE NEXT 2 OR 3 DAYS. AFTER THAT TIME...BOTH THE GFS AND
THE ECMWF MODELS INDICATE THAT GIL WILL BECOME VERTICALLY DECOUPLED
AND A WEAKENED LOWER-LEVEL CYCLONE WILL CONTINUE WESTWARD...STEERED
BY THE LOW-LEVEL FLOW. THE OFFICIAL FORECAST FOLLOWS THE GLOBAL
MODEL SOLUTION AND THE MULTI-MODEL CONSENSUS...TURNING GIL MORE TO
THE WEST BEYOND 3 DAYS.

A RECENT ASCAT-B PASS WAS USED TO ADJUST THE INITIAL WIND RADII
ASSOCIATED WITH GIL WHICH IN FACT IS A SMALL HURRICANE.


FORECAST POSITIONS AND MAX WINDS

INIT 31/2100Z 13.9N 120.6W 65 KT 75 MPH
12H 01/0600Z 14.3N 122.3W 70 KT 80 MPH
24H 01/1800Z 14.7N 124.3W 70 KT 80 MPH
36H 02/0600Z 15.0N 126.2W 70 KT 80 MPH
48H 02/1800Z 15.3N 128.0W 65 KT 75 MPH
72H 03/1800Z 15.5N 131.5W 60 KT 70 MPH
96H 04/1800Z 15.5N 135.0W 45 KT 50 MPH
120H 05/1800Z 15.5N 138.0W 30 KT 35 MPH

$$
FORECASTER AVILA
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.