Cyclones atlantiques : un lien avec les sables africains ?

[charles.muller]

Une étude intéressante dans les GRL : en analysant les cyclones 1982-2005 de la région Atlantique et en comparant avec les nuages de poussières émanant des côtes d'Afrique occidentale et du Sahara, tels que les mesurent les satellites depuis 25 ans, des chercheurs ont trouvé une corrélation "robuste" (je n'en sais pas plus, je n'ai pas encore le papier) : plus les nuages de poussières sont faibles, plus la saison cyclonique postérieure est intense. Ils ne se prononcent pas sur le mécanisme physique qui pourrait être à l'oeuvre. Certains suggèrent que ces nuages diminuent la température et l'humidité entre la surface océanique et la basse troposphère, ce qui priverait les cyclones de leur potentiel énergétique, au moment de leur formation ou durant leur parcours. Il semble que les satellites ont permis de réévaluer l'ampleur des nuages, que l'on sous-estimait auparavant : ainsi, les couchers de soleil de Porto Rico sont souvent teintés de ces aérosols venus... d'Afrique !

Cela montre en tout cas la complexité du phénomène cyclonique et la diversité des facteurs à prendre en compte pour évaluer leurs tendances. Je ne sais pas ce qui détermine les flux Ouest (Afrique > Amériques) à l'origine d'une plus ou moins grande quantité de poussières en suspension sur l'Atlantique.

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L19813, doi:10.1029/2006GL026408, 2006

New evidence for a relationship between Atlantic tropical cyclone activity and African dust outbreaks

Amato T. Evan, Jason Dunion, Jonathan A. Foley, Andrew K. Heidinger, Christopher S. Velden

Abstract - It is well known that Atlantic tropical cyclone activity varies strongly over time, and that summertime dust transport over the North Atlantic also varies from year to year, but any connection between tropical cyclone activity and atmospheric dust has been limited to a few case studies. Here we report new results that demonstrate a strong relationship between interannual variations in North Atlantic tropical cyclone activity and atmospheric dust cover as measured by satellite, for the years 1982–2005. While we cannot conclusively demonstrate a direct causal relationship, there appears to be robust link between tropical cyclone activity and dust transport over the Tropical Atlantic.

***

Dust may dampen hurricane fury

MADISON - After more than a dozen hurricanes battered the Atlantic Ocean last year, scientists are wondering what - if anything - might be causing stronger and more frequent storms.

Some have pointed to rising ocean temperatures, brought on by global warming. Others say the upswing is simply part of a natural cycle in which hurricanes get worse for a decade or two before dying down again.

Now, researchers at the University of Wisconsin-Madison have put forward an intriguing theory that introduces a whole new dimension to the debate.

Writing today (Oct. 10, 2006) in the journal Geophysical Research Letters, the scientists discuss a surprising link between hurricane frequency in the Atlantic and thick clouds of dust that periodically rise from the Sahara Desert and blow off Africa's western coast. Lead author Amato Evan, a researcher at UW-Madison's Cooperative Institute for Meteorological Satellite Studies (CIMSS), pored over 25 years of satellite data - dating from 1981 to 2006 - and noticed the correlation. During periods of intense hurricane activity, he found, dust was relatively scarce in the atmosphere. In years when stronger dust storms rose up, on the other hand, fewer hurricanes swept through the Atlantic.

"These findings are important because they show that long-term changes in hurricanes may be related to many different factors," says co-author Jonathan Foley, director of UW-Madison's Center for Sustainability and the Global Environment. "While a great deal of work has focused on the links between [hurricanes] and warming ocean temperatures, this research adds another piece to the puzzle." If scientists conclusively prove that dust storms help to squelch hurricanes, weather forecasters could one day begin to track atmospheric dust, factoring it into their predictions for the first time.

Researchers have increasingly turned their attention to the environmental impact of dust, after it became clear that in some years, many million tons of sand rise up from the Sahara Desert and float right across the Atlantic Ocean, sometimes in as few as five days. "People didn't understand the potential impact of dust until satellites allowed us to see how incredibly expansive these dust storms can be," says Evan. "Sometimes during the summer, sunsets in Puerto Rico are beautiful because of all the dust in the sky. Well, that dust comes all the way from Africa."

The Sahara sand rises when hot desert air collides with the cooler, dryer air of the Sahel region-just south of the Sahara-and forms wind. As particles swirl upwards, strong trade winds begin to blow them west into the northern Atlantic. Dust storms form primarily during summer and winter months, but in some years - for reasons that aren't understood - they barely form at all.

Evan decided to explore the correlations between dust and hurricane activity after CIMSS research scientist Christopher Velden and others suggested that dust storms moving over the tropical North Atlantic might be able to suppress the development of hurricanes.

The UW-Madison researchers say that makes sense because dry, dust-ridden layers of air probably helps to "dampen" brewing hurricanes, which need heat and moisture to fuel them. That effect, Velden adds, could also mean that dust storms have the potential to shift a hurricane's direction further to the west, which unfortunately means it would have a higher chance of hitting U.S. land.

While the UW-Madison work doesn't confirm that dust storms directly influence hurricanes, it does provide compelling evidence that the two phenomena are linked in some way. "What we don't know is whether the dust affects the hurricanes directly, or whether both [dust and hurricanes] are responding to the same large scale atmospheric changes around the tropical Atlantic," says Foley. "That's what future research needs to find out."

###

- Paroma Basu, (608) 262-9772, basu1@wisc.edu

EMBARGOED FOR RELEASE OCT. 10, 2006 AT 12:01 A.M. EST

PHOTO EDITORS: High-resolution images are available for downloading at: http://www.news.wisc.edu/newsphotos/dustCloud.html

CONTACT: Amato Evan, (608) 263-3951, amatoe@ssec.wisc.edu; Christopher Velden, (608) 262-9168, chrisv@ssec.wisc.edu; Jonathan Foley, (608) 265-9119, jfoley@wisc.edu

[charles.muller]

Ils m'ont déjà envoyé le papier. Voici les courbes de tendance (sur le second graphique, ils ont retiré les années à fort ou faible ENSO). La corrélation est de 0,58 (sur la période hors 1994, année de mesures incomplètes en septembre) ou de 0,71 (hors signal ENSO).

[lc30]

c'est curieux mais c'est la formulation de Gray et Landsea, sur l'influence des pluies sahariennes sur la fréquence des cyclones, cela date bien ( Landsea avait un site de belle pédagogie dans un temps il existe peut-être encore : chercher Landsea avec google).

--

lc30

[geoman]

Une étude intéressante dans les GRL : en analysant les cyclones 1982-2005 de la région Atlantique et en comparant avec les nuages de poussières émanant des côtes d'Afrique occidentale et du Sahara, tels que les mesurent les satellites depuis 25 ans, des chercheurs ont trouvé une corrélation "robuste" (je n'en sais pas plus, je n'ai pas encore le papier) : plus les nuages de poussières sont faibles, plus la saison cyclonique postérieure est intense. Ils ne se prononcent pas sur le mécanisme physique qui pourrait être à l'oeuvre. Certains suggèrent que ces nuages diminuent la température et l'humidité entre la surface océanique et la basse troposphère, ce qui priverait les cyclones de leur potentiel énergétique, au moment de leur formation ou durant leur parcours. Il semble que les satellites ont permis de réévaluer l'ampleur des nuages, que l'on sous-estimait auparavant : ainsi, les couchers de soleil de Porto Rico sont souvent teintés de ces aérosols venus... d'Afrique !

Cela montre en tout cas la complexité du phénomène cyclonique et la diversité des facteurs à prendre en compte pour évaluer leurs tendances. Je ne sais pas ce qui détermine les flux Ouest (Afrique > Amériques) à l'origine d'une plus ou moins grande quantité de poussières en suspension sur l'Atlantique.

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L19813, doi:10.1029/2006GL026408, 2006

New evidence for a relationship between Atlantic tropical cyclone activity and African dust outbreaks

Amato T. Evan, Jason Dunion, Jonathan A. Foley, Andrew K. Heidinger, Christopher S. Velden

Abstract - It is well known that Atlantic tropical cyclone activity varies strongly over time, and that summertime dust transport over the North Atlantic also varies from year to year, but any connection between tropical cyclone activity and atmospheric dust has been limited to a few case studies. Here we report new results that demonstrate a strong relationship between interannual variations in North Atlantic tropical cyclone activity and atmospheric dust cover as measured by satellite, for the years 1982–2005. While we cannot conclusively demonstrate a direct causal relationship, there appears to be robust link between tropical cyclone activity and dust transport over the Tropical Atlantic.

***

Dust may dampen hurricane fury

MADISON - After more than a dozen hurricanes battered the Atlantic Ocean last year, scientists are wondering what - if anything - might be causing stronger and more frequent storms.

Some have pointed to rising ocean temperatures, brought on by global warming. Others say the upswing is simply part of a natural cycle in which hurricanes get worse for a decade or two before dying down again.

Now, researchers at the University of Wisconsin-Madison have put forward an intriguing theory that introduces a whole new dimension to the debate.

Writing today (Oct. 10, 2006) in the journal Geophysical Research Letters, the scientists discuss a surprising link between hurricane frequency in the Atlantic and thick clouds of dust that periodically rise from the Sahara Desert and blow off Africa's western coast. Lead author Amato Evan, a researcher at UW-Madison's Cooperative Institute for Meteorological Satellite Studies (CIMSS), pored over 25 years of satellite data - dating from 1981 to 2006 - and noticed the correlation. During periods of intense hurricane activity, he found, dust was relatively scarce in the atmosphere. In years when stronger dust storms rose up, on the other hand, fewer hurricanes swept through the Atlantic.

"These findings are important because they show that long-term changes in hurricanes may be related to many different factors," says co-author Jonathan Foley, director of UW-Madison's Center for Sustainability and the Global Environment. "While a great deal of work has focused on the links between [hurricanes] and warming ocean temperatures, this research adds another piece to the puzzle." If scientists conclusively prove that dust storms help to squelch hurricanes, weather forecasters could one day begin to track atmospheric dust, factoring it into their predictions for the first time.

Researchers have increasingly turned their attention to the environmental impact of dust, after it became clear that in some years, many million tons of sand rise up from the Sahara Desert and float right across the Atlantic Ocean, sometimes in as few as five days. "People didn't understand the potential impact of dust until satellites allowed us to see how incredibly expansive these dust storms can be," says Evan. "Sometimes during the summer, sunsets in Puerto Rico are beautiful because of all the dust in the sky. Well, that dust comes all the way from Africa."

The Sahara sand rises when hot desert air collides with the cooler, dryer air of the Sahel region-just south of the Sahara-and forms wind. As particles swirl upwards, strong trade winds begin to blow them west into the northern Atlantic. Dust storms form primarily during summer and winter months, but in some years - for reasons that aren't understood - they barely form at all.

Evan decided to explore the correlations between dust and hurricane activity after CIMSS research scientist Christopher Velden and others suggested that dust storms moving over the tropical North Atlantic might be able to suppress the development of hurricanes.

The UW-Madison researchers say that makes sense because dry, dust-ridden layers of air probably helps to "dampen" brewing hurricanes, which need heat and moisture to fuel them. That effect, Velden adds, could also mean that dust storms have the potential to shift a hurricane's direction further to the west, which unfortunately means it would have a higher chance of hitting U.S. land.

While the UW-Madison work doesn't confirm that dust storms directly influence hurricanes, it does provide compelling evidence that the two phenomena are linked in some way. "What we don't know is whether the dust affects the hurricanes directly, or whether both [dust and hurricanes] are responding to the same large scale atmospheric changes around the tropical Atlantic," says Foley. "That's what future research needs to find out."

###

- Paroma Basu, (608) 262-9772, basu1@wisc.edu

EMBARGOED FOR RELEASE OCT. 10, 2006 AT 12:01 A.M. EST

PHOTO EDITORS: High-resolution images are available for downloading at: http://www.news.wisc.edu/newsphotos/dustCloud.html

CONTACT: Amato Evan, (608) 263-3951, amatoe@ssec.wisc.edu; Christopher Velden, (608) 262-9168, chrisv@ssec.wisc.edu; Jonathan Foley, (608) 265-9119, jfoley@wisc.edu

Tres interressant, je me posais justement la question dans la partie question reponse sur la meteo, si les sables sahariens avaient une influence sur l'ampleur des precipitations.

[mottoth]

Un peu hors-sujet (mais pas tant que ça), j'ai vu une emission cet été qui faisait état d'études montrant une correlation entre l'augmentation de la fréquence des nuages de sable Sahariens atteignant Trinidad et Tobago et l'apparitions de maladies auparavant rares dans l'archipel, comme l'asthme chez l'homme ou certaines maladies affectant le corail. Il a été montré que les germes responsables étaient portés par ces nuages de sable.