Hurricane season 2005--why so active?
This will be my last blog entry until August 12; I'm vacationing far from the tropics (Yellowstone!) to appreciate some mountain weather.
Today's monthly summary of hurricane activity for July issued by the National Hurricane Center (NHC) states:
"The month of July saw unprecedented tropical cyclone activity in the Atlantic Basin...with the development of five named storms...Tropical Storm Cindy...Hurricane Dennis...Hurricane Emily... Tropical Storm Franklin...and Tropical Storm Gert. The previous record for named storms that formed in July was four. The two major hurricanes that developed during the month tied a record set in 1916. The July activity follows an unusually active month of June ...And the seven named storms that have formed thus far in 2005 represent a record level of activity for the first two months of the season."
Why has this hurricane season been so active? Part of the reason lies in a decades-long natural cycle in hurricane activity that in 1995 switched to a high-hurricane activity mode. Hurricane activity has been above normal since 1995, and will likely continue to be for the rest of this decade and the next.
Additionally, there are six key ingredients are necessary for tropical cyclone formation (you can read about these in full detail in the Tropical Cyclone FAQ. We'll focus on three of them in particular that have been highly conducive to tropical cyclone formation during this remarkable hurricane season of 2005.
Vertical Wind Shear
Hurricanes need low values of vertical wind shear between the surface and the upper atmosphere (the jet stream level, typically 35,000 - 40,000 feet high in the tropics). Vertical wind shear is the magnitude of wind change with height. High vertical wind shear can disrupt a tropical cyclone trying to form by literally tearing it apart. High wind shear also can weaken or destroy a healthy tropical cyclone by interfering with the organization of deep convection around the cyclone center. Typically, 20 knots (23 mph or 10 m/s) or less difference in wind speed between the surface and upper atmosphere is considered favorable for hurricanes. In June and July of 2005, wind shear values were 20 - 40% below normal for the Gulf of Mexico and Caribbean Sea, the primary genesis locations for the seven tropical cyclones that formed. Wind shear values this low are highly favorable for tropical cyclone formation (see plots below).
Figure 1. Average amount of vertical wind shear (in black) and observed wind shear (in blue) for 2005 for the western Caribbean. Credit: Colorado State University (NOAA/CIRA)
Figure 2. Average amount of vertical wind shear (in black) and observed wind shear (in blue) for 2005 for the eastern Caribbean. Credit: Colorado State University (NOAA/CIRA)
Sea Surface Temperatures
Hurricanes need ocean waters of at least 26.5C (80 F) through a depth of about 50 meters to form or maintain their strength. The warmer the water, the better, since a hurricane is a huge heat engine. Sea surface temperatures (SSTs) are at the highest levels ever observed in the Atlantic, for the 50 years we have records. As of July 31, typical tropical Atlantic SSTs were about 2F (1.1C) above normal.
Figure 3. The Sea Surface Temperature departure from normal (in degrees C) for July 31, 2005. A large area of above normal SSTs (yellows and light greens) covers virtually the entire North Atlantic Ocean. The cold wake of Hurricane Emily is still apparent between the Yucatan Peninsula and southern Texas. Credit: U.S. Navy.
Moist Air
Hurricanes need moist air in the mid-troposphere (5 km or 3 mi altitude). Dry air interferes with the development of the large thunderstorm complexes needed to get a tropical storm going. Until the last week of July, the air over the tropical Atlantic and Caribbean Sea has been very moist. Since then, several large dust storms have moved off of the coast of Africa, accompanied by copious amounts of dry air that has interfered with tropical storm formation. TOMS aerosol data shows a large area of dust covering the entire tropical eastern Atlantic today.
Is Global Warming to Blame?
How much, if any, of this year's activity is due to global warming? That's a difficult question to answer. The research published so far shows that global warming cannot be linked to an increase in the number of hurricanes. So, this season's exceptional number of storms is probably unrelated to global warming. However, there is considerable debate whether or not sea surface temperatures and hurricane intensity have been affected by global warming. It is possible that the remarkable intensity of the hurricanes seen so far this season can be partially blamed on global warming. However, much more research needs to be done on this subject before we can link global warming with hurricane intensity. I plan to write a detailed article on the subject later this season, after I've had time to read the new research linking hurricane intensity to global warming, due to be published in Nature magazine on Sunday, August 7.
Dr. Jeff Masters
Reader Comments
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A POSSIBLE MIDDLE TO UPPER LEVEL CYCLONIC
CIRCULATION CENTER MAY FORM NEAR 32N89W IN SOUTH CENTRAL
MISSISSIPPI DURING THE NEXT 12 TO 24 HOURS AS SEEN ON WATER
VAPOR IMAGERY.
By the way Raysfan is that - TB Devil Rays fan. I'm a bostonian displaced in Orlando. I go to every Sox vs. Rays games in tampa. I have become a huge fan of TB.
Posted By: 147257 at 12:44 PM GMT op 05 Augustus, 2005.
site gives this warning
Currently, a weak upper-level low is situated over southern Mississippi-southern Alabama. Every once in a while, underneath such systems, a low-level circulation can form. It is an area to watch over the next few days.
look here en click Trop FCst on and watch it now it still taking the path they predicted
Alec is it still raining by you?
I'm not the best forecaster in the world.....but I don't post irresponsible statements because it's what I think.
Anyway, I'm done discussing STORMTOP.....I'm sticking to my new 'skip-over' rule.
evolution dont you feel like a dam fool posting this...you are a real critic..you should look at yourself in the mirror..you know absolutely nada about forecasting hurricanes..stick to knitting..lol
still too soon to write td9 off yet...climatology speaking this season hasn't followed the rules.
LOL, have we forgotten about emily, gert, the depression that never formed over puerto rice and the explosive growth of nothing over the bay of campeche!!!!!!!
Stormtop, you really need to quit blasting people on the blog who do not agree with you. GROW UP.
i can see that another wave is getting ready to come off Africa. Conditions are more condusive over there since the african dust plume has let up.
wrong again alec i gave you the article and i cant believe you said the dust is letting up ....the dust will be there another 2 weeks alec and anything that comes off the coast will have a very hard time developing.
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