The global tropical cyclone season of 2008: below average

It was a below average year for global tropical cyclone activity, and the destructive power of these storms was close to the lowest levels observed since since reliable records began in the early 1980s. However, the the total number of global deaths from tropical cyclones was the highest since 1991, thanks to the estimated 140,000 people killed in Myanmar from Tropical Cyclone Nargis. The total number of storms world-wide was 90, slightly lower that the average from the past 25 years of 92 (Figure 1). The global number of hurricanes, intense hurricanes (Category 3 and higher), and Category 4 and stronger storms were all below average. Only one Category 5 storm was recorded in 2008--Super Typhoon Jangmi, which attained winds of 165 mph at 06 GMT on September 27, as it approached the north coast of Taiwan. The last time so few Category 5 storms were recorded globally was in 1974, when there were none. The 2008 hurricane season was much above average in the Atlantic, but the Atlantic only accounts for about 13% of all global tropical cyclone activity.


Figure 1. Statistics for the global tropical cyclone season of 2008. The three numbers in each box represent the actual number observed in 2008, followed by the averages from the period 1983-2007 (in parentheses), followed by the record (in red). Averages and records were computed using the December 23, 2008 release of NOAA's new International Best Track Archive for Climate Stewardship.

A notable feature of the 2008 tropical cyclone season was the low Accumulated Cyclone Energy (ACE) for the season. ACE is a measure of the total destructive power of a hurricane season, based on the number of days strong winds are observed. ACE for an individual storm is computed by squaring the maximum sustained winds of the storm at each 6-hourly advisory, and summing up over the entire lifetime of the storm. The ACE value for 2008's storms was close to the low values on record seen in 2000-2001 and the early 1980s (Figure 2). Part of the reason for the low ACE values in 2008 (and in 2007) was due to the presence of a La Niña event in the Eastern Pacific. During such events, the formation region for Western Pacific typhoons moves northwestward, closer to China. Thus, storms that form in the Western Pacific spend less time over water before they encounter land, resulting in less time to intensify. They also accumulate a lower ACE due to their shorter duration. Since the Western Pacific is responsible for 35% of the world's major tropical cyclones, the global ACE value is strongly tied to year-to-year variations in the El Niño/La Niña cycle. The last major La Niña event (2000-2001) is clearly evident in Figure 2 as a minimum in global and Northern Hemisphere ACE.


Figure 2. Global (green) and Northern Hemisphere (blue) Accumulated Cyclone Energy (ACE) 24 month running sum through December 31, 2008. Note that the year indicated represents the value of ACE through the previous 24-months. Image credit: Ryan Maue, Florida State University.

Climate change and the 2008 global tropical cyclone season
The 2008 global tropical cyclone season shows that these storms are subject to large natural variations. Given this high natural variability and the short record of good data we have (just 25 years or so), it will be very difficult at present to prove that climate change is affecting global tropical cyclone activity. The situation is different in the Atlantic, where we have a longer reliable data record, and the storms seem to be more sensitive to changes in Sea Surface Temperature. I'll be putting together a full review of the scientific progress on understanding the link between climate change and Atlantic and global hurricane activity over the coming few months.


Figure 3. Satellite image of 2008's strongest tropical cyclone at maximum intensity: Super Typhoon Jangmi. Jangmi was rated a Category 5 storm with 165 mph winds at 06 GMT Sep 27, 2008, making it the only Category 5 storm of 2008. The storm eventually weakened to a low-end Category Four before striking Taiwan. It left two people dead and caused at least $800 million (2008 USD) in damages. Image credit: NOAA/SSD.

Jeff Masters