Link: FORECAST OF ATLANTIC HURRICANE ACTIVITY FOR SEPTEMBER AND OCTOBER 2005.
Following a record amount of June-July tropical cyclone activity and an active August in which Hurricane Katrina caused the greatest economic loss ever inflicted by a hurricane on the United States, we are continuing the bad news by predicting above-average activity for September and October. This year should be one of the most active and is already the most destructive hurricane season on record.
This cannot be good news for the hurricane-ravaged gulf coast. CSU was also right on the money for their August forecast:
Eric Blake spent from 1998-2001 as a graduate student at Colorado State University. His research efforts went into the development of an Atlantic basin August-only hurricane forecast scheme which was used for our 5 August 2005 forecast. See Blake (2002) or Blake and Gray (2004) for background information. Our August 2005 forecast called for well above-average activity, and this forecast verified remarkably well.
CSU forecast and verification of August-only hurricane activity made in early August
Tropical Cyclone Parameters and August 2005 Adjusted August August 2005 1950-2000 August Average (in parentheses) Statistical Forecast 2005 Forecast Verification Named Storms (NS) (2.8) 3.2 5 5 Named Storm Days (NSD) (11.8) 12.1 20 21 Hurricanes (H) (1.6) 1.3 3 2 Hurricane Days (HD) (5.7) 6.7 10 7 Intense Hurricanes (IH) (0.6) 0.9 1 1 Intense Hurricane Days (IHD) (1.2) 2.8 3 2.5 Net Tropical Cyclone Activity (NTC) (26.4) 33.7 50 42
The CSU hurricane experts do not believe that global warming has anything to do with additional hurricanes.
Many individuals have queried whether the unprecedented landfall of four destructive hurricanes in a seven-week period during August-September 2004 and the landfall of two more major hurricanes in the early part of the 2005 season is related in any way to human-induced climate changes. There is no evidence that this is the case. If global warming were the cause of the increase in United States hurricane landfalls in 2004 and 2005 and the overall increase in Atlantic basin major hurricane activity of the past eleven years (1995-2005), one would expect to see an increase in tropical cyclone activity in the other storm basins as well (ie., West Pacific, East Pacific, Indian Ocean, etc.). This has not occurred. When tropical cyclones worldwide are summed, there has actually been a slight decrease since 1995. In addition, it has been well-documented that the measured global warming during the 25-year period of 1970-1994 was accompanied by a downturn in Atlantic basin major hurricane activity over what was experienced during the 1930s through the 1960s.
We attribute the heightened Atlantic major hurricane activity between 1995-2005 to be a consequence of the multidecadal fluctuations in the Atlantic Ocean thermohaline circulation (THC) as we have been discussing in our Atlantic basin seasonal hurricane forecasts for several years. Major hurricane activity in the Atlantic has been shown to undergo marked multidecadal fluctuations that are directly related to North Atlantic sea surface temperature anomalies. When the Atlantic Ocean thermohaline circulation is running strong, the central Atlantic equatorial trough (ITCZ) becomes stronger. The stronger the Atlantic equatorial trough becomes, the more favorable are conditions for the development of major hurricanes in the central Atlantic. Since 1995, the THC has been flowing more strongly, and there has been a concomitant increase in major hurricanes in the tropical Atlantic.
Dr. Gray gave an explanation in English :-) for National Geographic. Note thermohaline circulation above is a more technical description of why salt content affects sea surface temperature which is a main driver for hurricane intensity. One should note that global warming would melt the polar ice caps, decreasing the salinity, reducing sea surface temperature, and thus potentially decreasing the intensity of hurricanes.
CSU's Gray and Klotzbach say the stormier summers are part of a well-established cycle of fluctuating hurricane activity. The cycles are caused by ocean currents that alter the salt content of the water, Klotzbach said. When the salt content is higher in the Atlantic—as it is now—the water is warmer, and that causes more tropical storms to form.
When the salt content is lower, the water is cooler and fewer hurricanes form, Klotzbach said.
The cycles take 25 to 40 years to run their courses. The present cycle of increased hurricanes started in 1995 and is expected to continue for at least another decade, perhaps longer.
Klotzbach said that if global warming were affecting hurricane formation, more storms would be forming around the world instead of only in the Atlantic Basin, which includes the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico.
"I really can't pick out a trend of increased hurricanes around the world," Klotzbach said. "If you look at the eastern Pacific Ocean, hurricane activity there has gone way down since 1995. There's no trend toward more storms there. It's a tricky problem."
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