Two organisations that issue extended range tropical storm forecasts for the Atlantic basin predict a return to above normal activity in the 2007 hurricane season.
Philip Klotzbach, 26, and William Gray, 76, of Colorado State University estimate there will be 14 names storms, seven hurricanes and three intense hurricanes of Category 3 or greater in their report released last month.
‘We expect Atlantic basin Net Tropical Cyclone activity in 2007 to be about 140 per cent of the long term average,’ the report states.
Klotzbach and Gray base their estimates on 52 years of past data of certain interacting atmospheric and oceanic conditions recorded through November 2006. The scientists also compare the conditions with previous years since 1949 that have had similar conditions.
‘Our research team has shown that a sizable portion of the year-to-year variability of Atlantic tropical cyclone activity can be hindcast with skill exceeding climatology,’ the report states.
‘It is only through hindcast skill that one can demonstrate that seasonal forecast skill is possible. This is a valid methodology provided that the atmosphere continues to behave in the future as it has in the past.’
The two Colorado State University scientists noted atmospheric conditions that suggest a weakening of the El Niño, a warming of the water surface temperatures in the Pacific Ocean. The El Niño, which formed unexpectedly in the late summer of 2006, is thought to have curtailed tropical cyclone activity in the Atlantic basin during the peak of last year’s hurricane season.
Klotzbach and Gray also note atmospheric conditions that usually precede the formation of a La Niña – or cooler than normal ocean surface temperatures – in the Pacific, which historically has enhanced tropical cyclone activity in Atlantic.
Searching for past years that showed similar atmospheric or sea surface temperatures in the Pacific and Atlantic, the scientists found that the years 1952, 1958, 1966 and 2003 compared the best. Those years averaged 11 named storms, 6.8 hurricanes and 3.3 intense hurricanes.
In the opinion of Klotzbach and Gray, the increased Atlantic basin cyclone activity in not a result of global warming.
Instead it is associated with a multi-decadal cycle that is expected to last another decade or two, after which the Atlantic basin would return to an era of less activity, as was the case from 1901-1925 and 1970-1994.
Also predicting high activity for the Atlantic hurricane season were Mark Saunders and Adam Lea of Tropical Storm Risk and the Benfield UCL Hazard Research Centre of the University College London.
Based on the current and projected climate signals, the two scientists estimate the 2007 hurricane season will experience activity 60 per cent above normal. They also believe that 2007 has about an 80 per cent likelihood to be in the top one-third of years historically with regard to tropical cyclone activity.
Looking at the forecast trade wind speed over the Caribbean and tropical North Atlantic, and the forecast sea surface temperature in the tropical North Atlantic, Saunders and Lea forecast there will be 15.7 (plus or minus 4.6) tropical storms; 8.8 (plus or minus 2.9) hurricanes; and 3.9 (plus or minus 1.8) intense hurricanes of Category 3 or above.
Both of the Klotzbach/Gray and the Tropical Storm Risk predictions last year of high activity where considerably over estimated, as 2006 ended up being below normal in terms of tropical cyclone activity.
Saunders gave some possible reasons why last year’s predictions were so far off in an article titled Winds of Change that he wrote for Post Magazine Risk Report in November.
Besides the onset of the unexpected El Niña, Saunders said another significant factor in curtailing last year’s hurricane activity was the presence of considerable African dry air and Saharan dust over the main area of hurricane development in the Atlantic during the peak season months of August and September. Those two conditions inhibited thunderstorm activity and thus tropical storm development.
Still, Saunders sees 2006 as an anomaly and said that it should not badly reflect on the general capability of forecasting hurricane activity.