Did dust kill the 2006 cane season?

[J Kinkaid]

After 2005’s record hurricane season, Caribbean, Gulf Coast, and East Coast residents braced for more destruction in 2006, but the devastating storms never came. Although 2005 had witnessed 12 tropical storms and 15 hurricanes—five of which made landfall—2006 saw only four tropical storms and five hurricanes. No hurricanes made landfall in the United States in 2006.

Warm sea surface temperatures help fuel hurricanes, and in 2006, those temperatures were cooler than expected. These images show average sea surface temperatures in 2006 (top) and 2005 (bottom), as measured by the Advanced Microwave Scanning Radiometer for EOS (AMSR-E), aboard NASA’s Aqua satellite. Cool temperatures appear in blue, and warm temperatures appear in pinkish yellow. The tracks of tropical storms (gray) and hurricanes (black) are overlaid on the maps.

The temperature “threshold†needed for hurricanes to form is about 28 degrees Celsius (about 82 degrees Fahrenheit). In 2005, a much larger area of the Atlantic Ocean and Caribbean Sea was at or near that threshold. The difference between the two seasons is particularly noticeable in the waters east of Florida and north of South America, which have much more yellow in 2005 than 2006.

Why the difference between 2005 and 2006? William Lau of NASA Goddard Space Flight Center, and Kyu-Myong Kim of University of Maryland-Baltimore County, think the answer comes from the Sahara, namely dust. Using dust observations collected by the Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite, they found that the Sahara sent an unusually large amount of dust over the Atlantic during the 2006 hurricane season. By blocking incoming sunlight, the researchers say, the dust could have caused the widespread cooling of the subtropical North Atlantic Ocean, subduing hurricanes. The researchers don’t yet know how great a part dust played in derailing hurricane formation in 2006, but they hope their work will fuel more studies.

http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17617

 

[Bob Schafer]

Of course, the 'canes that DID occur tracked differenly in 2006 than in 2005 (generally speaking), too. IIRC, the 'canes that turned right and up into the N Atlantic depicted in the graphic occurred in August and September. Had they made landfall along USA shores, the potential still existed for them to be major events for the USA. Credit upper wind patterns for that, not dust.

I also thought the dust was more of a factor in inhibiting tropical development off the African coast than a Gulf/Carribean SST issue. Wasn't El Nino credited with keeping Gulf/Carribean SST's down in some way?

One last thing: I could have sworn that one CAT1 hurricane struck extreme S FL early in the season. Not true? Or was it later downgraded to TS?

 

[Jesse V. Bass III]

"One last thing: I could have sworn that one CAT1 hurricane struck extreme S FL early in the season. Not true? Or was it later downgraded to TS?"

no hurricane hit the u.s. last year. alberto came into the big bend area on june 13th with winds near 50mph... and, ernesto was a hurricane for a brief time near haiti but was downgraded... it hit the keys with 40mph winds... then tracked up the atlantic just off the coast coming ashore near wilmington n.c. with winds near 70mph.. i was there for that landfall and it was very close to becoming a hurricane but did not make it before landfall..

the sal was very strong last season off africa and the trades seemed to carry alot of that dust westward.. several times systems had the chance to develop but were hindered by the dust, or dry air... but i dont think that played a role in the gulf... el nino and shear may have had the bigger part..

 

[Jim Leonard]

Its not so much the dust inhibits tropical storm formation it is the strongly stable airmass which is associated with the dust. During later June until early August under the intense heat of the Sahara Desert heat lows develop and are pushed westward off the African coast. A more convective monsoon type systems develop during mid-August through September. These heat lows are filled with Sahara sand and dust and a very warm mid-level temperatures. The sst's ( seasurface temperatures) are too cool in the eastern atlantic to break this horendous cap. This one big reason why there is normally very few tropical cyclones in the deep tropics in July. The problem with 2006 was also aggravated by the developing EL-Nino induced wind shear and drier than average west African continent.

 

[cdcollura]

Jim Leonard is right.

The dust has nothing to do with the air it is suspended in.

Dry air created subsidence, plus a stronger trade-wind inversion (fancy tropical name for "cap"), and thunderstorms could not get going and therefore hurricane formation was quelled.

Also, wind shear, which is great for supercells, is detrimental for developing tropical systems. The El Nino of 2006 enhanced westerly flow above 18,000 feet (500 MB) over the Atlantic, but may have increased activity in the E Pacific a bit.

In rare cases, contrary to this, a dust layer in the mid-levels may absorb sunlight and warm up, increasing mid-level temps a bit, and strengthening any inversion (warmer layer aloft) there too.