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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Quoting 1319. Grothar:
Good morning Grothar and fellow bloggers. My goodness that blob of mess ( hope its ok Grothar to say blob) by the Bahamas looks darn good. What do you think Gro will become of it if anything?
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1324. vis0

Credit: NOAA / SERVICIO METEOROLÓGICO NACIONAL - MÉXICO
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1323. LargoFl
ONLY ITEM OF NOTE WAS IN THE 00Z CANADIAN AND 00Z NAM. BOTH OF THESE
MODELS SPUN UP A LOW PRESSURE SYSTEM FROM AN AREA OF DISTURBED
WEATHER IN THE BAHAMAS. BOTH MODELS TAKE THIS SYSTEM NORTH OFF THE
FLORIDA ATLANTIC COAST BEFORE THE UPPER LEVEL TROUGH PUSHES IT INTO
THE ATLANTIC OFF THE CAROLINA COAST. THE 00Z GFS DOES FEATURE A LOW
COMING NEAR SOUTH FLORIDA FROM THE BAHAMAS ON FRIDAY...BUT DOES NOT
DEVELOP THE SYSTEM AS WELL AS THE CMC AND NAM DO INTO SATURDAY.
THINKING THE ONLY CURRENT UNCERTAINTY IN THE WEST CENTRAL AND
SOUTHWEST FLORIDA FORECAST WITH THIS SYSTEM IS THAT IT COULD CHANGE
THE CLOUD COVER A BIT AND POSSIBLY AFFECT RAIN CHANCES DEPENDING ON
ITS STRUCTURE AND PATH.

&&
Member Since: August 6, 2011 Posts: 4 Comments: 46605
wow!!
Member Since: March 12, 2013 Posts: 143 Comments: 123731
Woke up to thunder and more rain.
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i see a new tropical wave with a spin here!
Member Since: March 12, 2013 Posts: 143 Comments: 123731
1319. Grothar
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TROPICAL WEATHER OUTLOOK
NWS NATIONAL HURRICANE CENTER MIAMI FL
200 AM EDT THU AUG 1 2013

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE NEXT 48 HOURS.

BEGINNING TODAY WITH THE 8 AM EDT TROPICAL WEATHER OUTLOOK...THE
NATIONAL HURRICANE CENTER WILL BEGIN INCLUDING INFORMATION ABOUT
THE POTENTIAL FOR TROPICAL CYCLONE FORMATION DURING THE FOLLOWING
FIVE DAYS. THIS INFORMATION WILL BE PROVIDED PROBABILISTICALLY IN
10-PERCENT INCREMENTS...AND WILL SUPPLEMENT THE 48-HOUR
PROBABILISTIC FORMATION POTENTIAL ALREADY PROVIDED. THE CURRENT
GRAPHICAL TROPICAL WEATHER OUTLOOK THAT SHOWS THE 48-HOUR GENESIS
POTENTIAL WILL REMAIN UNCHANGED. NHC IS CURRENTLY DEVELOPING A
FIVE-DAY GENESIS POTENTIAL GRAPHIC THAT MAY BECOME AVAILABLE LATER
IN THE SEASON.

are you ready!!
Member Since: March 12, 2013 Posts: 143 Comments: 123731
have noticed its a very wet yr in latin america rainy season is in full swing. its a good time of yr to visit there if yu cant handle the heat
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If you look at the radar images it makes you think that the Bahama blob is a continuation of Dorian...
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Quoting 1301. PensacolaDoug:



I've been called troll a couple of times by Tom Taylor. No worries tho.


We may disagree on just about everything, Doug, but you are certainly no troll. :) I think the "troll" label should be used sparingly around here. Although I have noticed over the years some folks have a very difficult time recognizing the trolls, in that case it is helpful to point them out.
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1314. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 46605
1313. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 46605
Member Since: March 12, 2013 Posts: 143 Comments: 123731
Ha, on the last infrared frame for the Atlantic, it looks like xDorian is pretending to have an eye. Also, looks like the deep convection in about to begin. Shear is low, moisture and SSTs are high.. if it's going to do anything interesting, now is the time to do it.
Member Since: October 24, 2012 Posts: 0 Comments: 214
1310. fmbill
Quoting 1309. GatorWX:
Morning folks. Look who's finally paying us a visit.



Is all of that finally vertically stacked? It sure has taken long enough.
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1309. GatorWX
Morning folks. Look who's finally paying us a visit.

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1308. VR46L
Gil looks to be very strong



Member Since: March 1, 2012 Posts: 0 Comments: 7077
000
ABNT20 KNHC 010543
TWOAT

TROPICAL WEATHER OUTLOOK
NWS NATIONAL HURRICANE CENTER MIAMI FL
200 AM EDT THU AUG 1 2013

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE NEXT 48 HOURS.

BEGINNING TODAY WITH THE 8 AM EDT TROPICAL WEATHER OUTLOOK...THE
NATIONAL HURRICANE CENTER WILL BEGIN INCLUDING INFORMATION ABOUT
THE POTENTIAL FOR TROPICAL CYCLONE FORMATION DURING THE FOLLOWING
FIVE DAYS. THIS INFORMATION WILL BE PROVIDED PROBABILISTICALLY IN
10-PERCENT INCREMENTS...AND WILL SUPPLEMENT THE 48-HOUR
PROBABILISTIC FORMATION POTENTIAL ALREADY PROVIDED. THE CURRENT
GRAPHICAL TROPICAL WEATHER OUTLOOK THAT SHOWS THE 48-HOUR GENESIS
POTENTIAL WILL REMAIN UNCHANGED. NHC IS CURRENTLY DEVELOPING A
FIVE-DAY GENESIS POTENTIAL GRAPHIC THAT MAY BECOME AVAILABLE LATER
IN THE SEASON.

here we go.. 8am coming soon
Member Since: March 12, 2013 Posts: 143 Comments: 123731
vis. very impressive sal moving across the atlantic
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1304. Gearsts
The trolls are out early today.
We really need something to track soon or they will take over the blog lol
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Condions are extremely favorable for intensifcation in the bahamas into a hurricane
Member Since: July 2, 2013 Posts: 0 Comments: 1525
1302. VR46L
Quoting 1301. PensacolaDoug:



I've been called troll a couple of times by Tom Taylor. No worries tho.


Yeah .. Wasn't him that called me one but it was people I disagreed with and I was debating fair and all of a sudden your a troll was thrown up ... Just cause I disagreed with them ...
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Quoting 1296. VR46L:


Agreed ... I loathe the he is a troll statements ! Its bores me ... When I first started here I got called a troll ..by two of the "respected " climate bloggers..But I don't think I am considered one by the weather bloggers .

I find it a bullying tactic as I said earlier this morning . If they are doing wrong I am guessing the Tase gun and Banhammer the moderators have will get put into action ...



I've been called troll a couple of times by Tom Taylor. No worries tho.
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1300. VR46L
Look forward to seeing this guy on visual later on

Member Since: March 1, 2012 Posts: 0 Comments: 7077
xDorian is still stacking better and better; maybe, just maybe the model about a depression or weak TS forming out of it will come to fruition. Needs better convection than what it has right now though, but being at DMAX I think that should be no issue. Should be fun to monitor.
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1298. barbamz

Aaah, this loop really shows very nicely the direction of the jet stream over Europe, sending hot air from Africa to us. --- Bye for now, have to work unfortunately.
Member Since: October 25, 2008 Posts: 68 Comments: 7630
Quoting 1295. ecflweatherfan:


Well, that's great... We're glad to have you on here :-). Many of us are on here for that same purpose... but we have a bunch of trolls that muck it up for the majority who do want to learn a thing or two about weather. One posts a few maps, interprets them a little with his/her slant in what they are seeing... others then take that, blow it WAAAAY out of proportion like we got some Cat 5 in the making by tomorrow that is not even an Invest today. And make the same ridiculous comments over and over again to get attention. It gets annoying... I'm sure you can agree!

you don't know what you are babbling about this will form today!
Member Since: July 2, 2013 Posts: 0 Comments: 1525
1296. VR46L
Quoting 1284. islander101010:
hear to learn about weather not read about characters acting like teenagers discussing how many ignores they got


Agreed ... I loathe the he is a troll statements ! Its bores me ... When I first started here I got called a troll ..by two of the "respected " climate bloggers..But I don't think I am considered one by the weather bloggers .

I find it a bullying tactic as I said earlier this morning . If they are doing wrong I am guessing the Tase gun and Banhammer the moderators have will get put into action ...
Member Since: March 1, 2012 Posts: 0 Comments: 7077
Quoting 1284. islander101010:
hear to learn about weather not read about characters acting like teenagers discussing how many ignores they got


Well, that's great... We're glad to have you on here :-). Many of us are on here for that same purpose... but we have a bunch of trolls that muck it up for the majority who do want to learn a thing or two about weather. One posts a few maps, interprets them a little with his/her slant in what they are seeing... others then take that, blow it WAAAAY out of proportion like we got some Cat 5 in the making by tomorrow that is not even an Invest today. And make the same ridiculous comments over and over again to get attention. It gets annoying... I'm sure you can agree!
Member Since: Posts: Comments:
1294. barbamz
China media: Heat crisis
BBC, August 1, 2013

Media say a spate of violent incidents and the government's slow response to a heatwave are putting the public on edge.

"In some local places, public servants have reduced their working hours because of high temperatures, which has stirred criticism. A subway station in Hangzhou is said to have turned off its air conditioning to prevent people from going there to enjoy the cool. Such reports have aroused public anger," notes the Global Times.

"If the government acts slowly, problems generated by high temperature may turn to political matters," it warns.

"Yes, this summer has indeed put citizens on edge. The reason for this is not only sustained high temperatures, but also a spate of vicious incidents," the Southern Metropolis Daily adds.


Member Since: October 25, 2008 Posts: 68 Comments: 7630
Good morning to all and welcome to August.

July is gone and the meat of the season begins today. Let's see how things evolve down the road but be prepared for anything that mother nature brings as it only takes one strike to do all the harm to an area.
Member Since: April 29, 2009 Posts: 75 Comments: 15489
1291. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 46605
Quoting 1289. slugworthspin:
will that be posted by the nhc? and where is it going ?

bermuda!!
Member Since: July 2, 2013 Posts: 0 Comments: 1525
1288. LargoFl
hey NWS..looks good...............
Member Since: August 6, 2011 Posts: 4 Comments: 46605
1286. LargoFl
by the way..nws might be testing its 5-day forecast starting at the 8am discussion...
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1285. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 46605
hear to learn about weather not read about characters acting like teenagers discussing how many ignores they got
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1283. LargoFl
might be some bad storms in the middle of the country today
Member Since: August 6, 2011 Posts: 4 Comments: 46605
1281. barbamz
Short hello: Good morning with "some" (cough!) rain from southeastern China:



Published July 31, 2013
After red alerts issued for heatwave in southeast China, a torrential rain fall stroke Ningbo, a southeast coastal city in Zhejiang province on July 30. Heavy rain dropped down with flashing Lightning and rumbling thunder.
Member Since: October 25, 2008 Posts: 68 Comments: 7630
1280. LargoFl
the Blob comes close to east coast of florida then gets swept up the east coast and gets absorbed..nothing for florida but some added moisture friday..nothing so far to worry about
Member Since: August 6, 2011 Posts: 4 Comments: 46605
1279. beell
If I had taken an extended break from weather watching over the past week or two and popped open a WV loop this morning I might come to the conclusion that this entire area appeared "hostile" to meaningful development.

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1278. LargoFl
GFS has nothing thru Aug 9th....................
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Quoting 1274. tj175:


Camille is a troll and has been put on my ignore list


Yep. On mine too. I'm up to like 15 of em I think.
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Quoting 1260. Gearsts:
Bahamas blob just a TUTT

No it is not
Member Since: July 2, 2013 Posts: 0 Comments: 1525

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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.

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