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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3826. TerraformingMaster
1:23 AM GMT on August 03, 2013
Have there ever been cases where a tropical wave or depression or storm has split into two storms? In which one or the other later became a cane? ex-Dorian appears now and has appeared a few times in the last 10 days to have two different COCs. Might explain why the NHC can't pinpoint the COC.
Member Since: July 27, 2013 Posts: 0 Comments: 8
3825. GatorWX
8:44 PM GMT on August 02, 2013
It has that yeah, I'm still here look.

Member Since: January 1, 2008 Posts: 0 Comments: 2684
3824. unknowncomic
7:39 PM GMT on August 02, 2013
We're about to get pounded here in West Palm Beach.
Member Since: August 2, 2011 Posts: 0 Comments: 1773
3823. MahFL
2:21 PM GMT on August 02, 2013
Quoting 3822. DaytonaBill:



Actually you can. Fog is the obvious humidity that is visible, but at lower RH is is visible as haze.


Humidity is the amount of water vapor in the air. Water vapor is the gas phase of water and is invisible.

Fog is a form of precipitation, haze is dust particles and other pollutants in the atmosphere.
Member Since: June 9, 2004 Posts: 0 Comments: 3286
3822. DaytonaBill
1:57 PM GMT on August 02, 2013
Quoting 3749. MahFL:


You can't see humidity, lol.



Actually you can. Fog is the obvious humidity that is visible, but at lower RH is is visible as haze.
Member Since: August 30, 2010 Posts: 0 Comments: 6
3821. SLU
1:46 PM GMT on August 02, 2013
Breaking news:

COLORADO STATE UNIVERSITY FORECAST OF ATLANTIC HURRICANEACTIVITY FROM AUGUST 2 – AUGUST 15, 2013

We expect that the next two weeks will
be characterized by below-average amounts
(<70%) of activity relative to climatology.
(as of 2 August 2013)
Member Since: July 13, 2006 Posts: 12 Comments: 4859
3820. wunderkidcayman
1:46 PM GMT on August 02, 2013
Check out the surface map on this page

Link
Member Since: June 13, 2009 Posts: 2 Comments: 11023
3819. Skyepony (Mod)
1:44 PM GMT on August 02, 2013
Sprinkled in Melbourne from ex-Dorian. Winds are light & from the north here, gusting to 5mph in the last hour.

Member Since: August 10, 2005 Posts: 161 Comments: 37346
3818. WunderAlertBot (Admin)
1:44 PM GMT on August 02, 2013
JeffMasters has created a new entry.
3817. Catherdr
1:43 PM GMT on August 02, 2013
How come the WU graphic of Invest 91 shows "7PM Wed Dec 31 1969" on the time/date?
Member Since: July 9, 2008 Posts: 0 Comments: 53
3816. Stoopid1
1:41 PM GMT on August 02, 2013
There are reports of a brief EF-2 tornado in the Fort Caroline area of Jacksonville from yesterday's storm. Craig also recorded 0.95" of rain in a 17 minute period as well. 2.56" total.
Member Since: August 6, 2007 Posts: 24 Comments: 2688
3815. GetReal
1:43 PM GMT on August 02, 2013

That's All Folks!!! Nothing to come out of this zombie....
Member Since: July 4, 2005 Posts: 204 Comments: 8806
3814. ncstorm
1:42 PM GMT on August 02, 2013
Quoting 3800. wunderkidcayman:

I doubt that almost every storm that was to come off Africa was to recurve according to CFS and they did developed storms like it thought it was CMC
Anyway if CFS had its way we would probably have been up to our 5 or 6 storm already All which would have recurved


If the pattern stays..storms just may..its a possibility you have to take in account..
Member Since: August 19, 2006 Posts: 13 Comments: 14451
3813. MisterPerfect
1:42 PM GMT on August 02, 2013
Member Since: November 1, 2006 Posts: 71 Comments: 20135
3812. ncstorm
1:41 PM GMT on August 02, 2013
Quoting 3810. SLU:


The CFS should never be used to determine storm tracks. It is a climate model and can only be used to guess the climatic trends that may occur.


Please see my second post to the CFS..
Member Since: August 19, 2006 Posts: 13 Comments: 14451
3811. 2manytimes
1:40 PM GMT on August 02, 2013

Quoting 3790. CaribBoy:


Yes I saw nothing on the GFS... first half of august could be boring/ a BUST!


Good morning!
But that's a good thing. Isn't it?
Member Since: August 4, 2009 Posts: 0 Comments: 8
3810. SLU
1:40 PM GMT on August 02, 2013
Quoting 3791. ncstorm:
Yesterday's CFS long range run showed nothing but recurvatures..might be another Bermuda year..


The CFS should never be used to determine storm tracks. It is a climate model and can only be used to guess the climatic trends that may occur.
Member Since: July 13, 2006 Posts: 12 Comments: 4859
3808. ncstorm
1:38 PM GMT on August 02, 2013
Quoting 3797. VR46L:


NC , Now now ... you know the CFS is to be viewed like a fantasy film .... at the moment alot of te shorter term models need to be veiwed the same way..


not so much for storm development but what pattern its showing..August is already predicting to have some strong troughs come down as far as the midatlantic states..I could see what its saying..If anything should happen for the US Conus, it will be the GOM and home grown mischief..

Member Since: August 19, 2006 Posts: 13 Comments: 14451
3807. RitaEvac
1:38 PM GMT on August 02, 2013
Ladies and Gentlemen of TX....The Dog days of summer....are fully entrenched for ya, good luck, and God speed.
Member Since: July 14, 2008 Posts: 1 Comments: 9628
3806. SFLWeatherman
1:37 PM GMT on August 02, 2013
For me
Wind Speed NW at 5 mph
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3802. RitaEvac
1:34 PM GMT on August 02, 2013
Quoting 3794. SouthernIllinois:
I'm a happy girl.

The accumulated precip out to 84 hours for the 18Z, 00Z & 06Z NAM are all looking spectacular for Southern Illinois. Yes...what you are seeing is the NAM is pretty confident we will get something out of this next system. Yay!!!


Another 0.35"?
Member Since: July 14, 2008 Posts: 1 Comments: 9628
3800. wunderkidcayman
1:33 PM GMT on August 02, 2013
Quoting 3791. ncstorm:
Yesterday's CFS long range run showed nothing but recurvatures..might be another Bermuda year..

I doubt that almost every storm that was to come off Africa was to recurve according to CFS and they did developed storms like it thought it was CMC
Anyway if CFS had its way we would probably have been up to our 5 or 6 storm already All which would have recurved
Member Since: June 13, 2009 Posts: 2 Comments: 11023
3799. StormPro
1:32 PM GMT on August 02, 2013
Quoting 3775. GetReal:




It would seem that the center of attention has shifted significantly NNW.

t easy to see it on the radar, even for a dummy like me!
Member Since: August 4, 2010 Posts: 0 Comments: 606
3798. CaribBoy
1:32 PM GMT on August 02, 2013
Quoting 3791. ncstorm:
Yesterday's CFS long range run showed nothing but recurvatures..might be another Bermuda year..


If Bermuda wants the SAL we would be delighted to send it north for them too XD
Member Since: October 6, 2007 Posts: 0 Comments: 5930
3797. VR46L
1:32 PM GMT on August 02, 2013
Quoting 3791. ncstorm:
Yesterday's CFS long range run showed nothing but recurvatures..might be another Bermuda year..


NC , Now now ... you know the CFS is to be viewed like a fantasy film .... at the moment alot of te shorter term models need to be veiwed the same way..
Member Since: March 1, 2012 Posts: 0 Comments: 6832
3796. GetReal
1:31 PM GMT on August 02, 2013
Weather stations along central Florida Atl coast are reporting light W and NW winds, but high barometer readings 1014mb and above.
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3795. Stoopid1
1:30 PM GMT on August 02, 2013
91L should move north until it's parallel to Cape Canaveral, then we should see the eastern element to the track occur from frontal interaction.
Member Since: August 6, 2007 Posts: 24 Comments: 2688
3793. VR46L
1:30 PM GMT on August 02, 2013
Quoting 3790. CaribBoy:


Yes I saw nothing on the GFS... first half of august could be boring/ a BUST!


It sure is looking like being Quiet at the moment...
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3792. SFLWeatherman
1:29 PM GMT on August 02, 2013
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3791. ncstorm
1:28 PM GMT on August 02, 2013
Yesterday's CFS long range run showed nothing but recurvatures..might be another Bermuda year..
Member Since: August 19, 2006 Posts: 13 Comments: 14451
3790. CaribBoy
1:27 PM GMT on August 02, 2013
Quoting 3789. VR46L:


It might be a while to the next one....


Yes I saw nothing on the GFS... first half of august could be boring/ a BUST!
Member Since: October 6, 2007 Posts: 0 Comments: 5930
3789. VR46L
1:25 PM GMT on August 02, 2013
Quoting 3784. CaribBoy:


DORIAN need to be forgotten now. Next!


It might be a while to the next one....
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3788. SFLWeatherman
1:24 PM GMT on August 02, 2013
For WPB from Steve Weagle
The remnants of Dorian are right off the coast and moving inland. Potential for heavy rain beginning this afternoon. Several inches possible.
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3787. moonlightcowboy
1:24 PM GMT on August 02, 2013
Good morning, weathergeeks! ;)


xDorian about to be absorbed by the frontal trough? Disappear finally?




This has been some system to follow. Such is the tropics! ;) TGIF! Have a great day, all.
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3786. GetReal
1:21 PM GMT on August 02, 2013
Member Since: July 4, 2005 Posts: 204 Comments: 8806
3785. Stoopid1
1:24 PM GMT on August 02, 2013
Quoting 3778. GetReal:
It would seem that the center of attention is now just off the coast of Port St. Lucie.


The radar RitaEvac posted shows it pretty well.
Member Since: August 6, 2007 Posts: 24 Comments: 2688
3784. CaribBoy
1:23 PM GMT on August 02, 2013
Quoting 3782. SFLWeatherman:
Hey it up now lol


DORIAN needs to be forgotten now. Next!
Member Since: October 6, 2007 Posts: 0 Comments: 5930
3783. ncstorm
1:22 PM GMT on August 02, 2013
Good Morning Folks..another cloudy day here in eastern NC..but Im not complaining..just great to have woken and able to see family and friends once again..

Member Since: August 19, 2006 Posts: 13 Comments: 14451
3782. SFLWeatherman
1:22 PM GMT on August 02, 2013
Hey it up now lol
Member Since: May 23, 2012 Posts: 0 Comments: 4392
3781. CaribBoy
1:21 PM GMT on August 02, 2013
WHEN THEY FORECAST THE SAL 5 DAYS IN ADVANCE THAT's NEVER A BUST FORECAST!
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3780. leftlink
1:21 PM GMT on August 02, 2013
Just in the last 5 minutes the infrared image from the Colorado site relocated Dorian from off the keys to northeast of Fort Lauderdale.

Guess they think it is going to feed moisture from the band to the south and also from the end of the trough which extends to north Florida.

But no surface winds picking up I guess...

Member Since: December 28, 2010 Posts: 0 Comments: 134
3779. RitaEvac
1:20 PM GMT on August 02, 2013
Landfall

Member Since: July 14, 2008 Posts: 1 Comments: 9628
3778. GetReal
1:19 PM GMT on August 02, 2013
It would seem that the center of attention is now just off the coast of Port St. Lucie.
Member Since: July 4, 2005 Posts: 204 Comments: 8806
3777. Stoopid1
1:17 PM GMT on August 02, 2013
Quoting 3751. Newswatcher:



Look at the Bahamas. It is 1008mb. Isn't that TS strength?


Depends on surrounding pressures, but if it were a system that would typically be a tropical depression like pressure.
Member Since: August 6, 2007 Posts: 24 Comments: 2688
3776. MisterPerfect
1:17 PM GMT on August 02, 2013
Member Since: November 1, 2006 Posts: 71 Comments: 20135

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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.