Quiet Atlantic; more on Supertyphoon Saomai

High wind shear continues to dominate the tropical Atlantic, and there's little to be concerned with today. The remains of the tropical wave that the Hurricane Hunters investigated earlier this week as it moved through the Lesser Antilles Islands are just south of Haiti. A hint of a circulation at mid levels of the atmosphere developed this afternoon, but the associated heavy thunderstorm activity is limited. The wave is under about 10-20 knots of wind shear, and any development of this system will be slow.

Supertyphoon Saomai:
In China, the death toll has risen to over 100 in the wake of Supertyphoon Saomai, which slammed into the coast south of Shanghai Thursday as a Category 4 storm with 135 mph winds. The death toll will no doubt rise higher today as the remains of Saomai spread heavy rains through the same region of China hit by Tropical Storm Bilis, which killed more than 600 people last month.


Figure 1. Supertyphoon Saomai as it passed north of Taiwan, August 10, 2006 at 1:22 GMT. At maximum strength, Saomai was a Category 5 storm with 160 mph winds. The image was taken by the Department of Defense F-15 satellite. Image credit: NOAA Environmental Visualization Project.

The media is calling Saomai the worst typhoon to hit China in 50 years, but there is some dispute about just how strong the storm was at landfall. Here is comparison of intensities from three different agencies at Saomai's landfall at 12 GMT August 10:

U.S. Joint Typhoon Warning Center: 1-min sustained winds of 135 mph, Cat 4.
Japan Meteorological Agency: 1-min sustained winds of 100 mph, Cat 2.
Hong Kong Observatory: 1-min sustained winds of 115 mph, Cat 3.

So, these three agencies all using the same satellite data couldn't agree on the strength of this typhoon within two Saffir-Simpson categories! This underscores the difficulty of trying to determine if global warming is causing an increase in Category 4 and 5 hurricanes--even today with much better tools and training, experts still can't agree on storm intensities with the accuracy needed for such a study.

This was discussed in more detail in a paper published this year by Kamahori, Yamazaki, Mannoji, and Takahashi of the the Japan Meteorological Agency (JMA) in the on-line journal Scientific Online Letters on the Atmosphere - a new journal produced by the Meteorological Society of Japan. The study compares typhoon intensities in the Northwest Pacific since 1977 as compiled by the Joint Typhoon Warning Center (JTWC) and the JMA. The JTWC data was used in the famous Webster et. al study from 2005 that found a worldwide 80% increase in Category 4 and 5 tropical cyclones since 1970. A key element of their conclusions was the data from the Northwest Pacific, which make up about 50% of global Category 4 and 5 storms. The JMA group found that using JTWC's dataset, the number of days when a Category 4 or 5 typhoon was present increased from about 10 per year in 1977-90, to 17 per year during 1991-2004--a 70% increase. However, the JMA data for the same time period showed a 40% decrease in Category 4 and 5 typhoon days. The authors concluded, "We do not have sufficient evidence to judge which dataset is reasonable." I would have to agree--until we get a coordinated major re-analysis effort of all the tropical cyclone data for the globe, it is dangerous to make conclusions about whether global warming is causing an increase in tropical cyclone intensities. I think it is likely there has been some increase, but it is nowhere nearly as large as the 80% increase reported by Webster et. al.

Jeff Masters