Storms and Climate Change

The Atlantic hurricane season will soon be upon us again , and no doubt many people will recall last year’s devastating Hurricanes that swept across Florida. There was a great deal of press about these storms, as 3 major hurricanes and 5 tropical storms made landfall in the US. According to HurricaneProtection.com, the last time eight different tropical cyclones impacted the United States coastline in a single season was 1916. There were a total of 15 tropical storms and hurricanes, which means that the total number of storms that year was higher than 95% of the previous years of hurricane observations. There was also a record number of Typhoons over Japan in 2004 (10! The previous record was 6 from 1996) . Typhoons are the same as Hurricanes, but have a different name over the Indian ocean and the western Pacific. They are also known as ‘tropical cyclones’. Furthermore, it was the first time that a tropical cyclone had been observed in the south Atlantic (see WMO Climate News, Jan 2005, p. 12)! So, what’s going on?

The topic regarding tropical storms have also been a contriversial topic in the scientific community. Has there really been a systematic long-term trend in the storm statistics? Chan and Liu (2004) found no systematic change in typhoons that could be related to changes in the sea surface temperature. Because trends in sea surface temperature (SST) have been relative weak over the past 50 years, Knutson & Tuleya (2004) have argued that CO2-induced tropical cyclone intensity changes are unlikely to be detectable in historical observations and will probably not be detectable for decades to come. Although there is no clear linear trend in the Atlantic hurricane number (see Fig. 1), there may nevertheless be other indications which may suggest that the tropical cyclone statistics are changing.

Fig.1: Number of storms every 5-year interval since 1850 divided in 6 different categories of severities, with ‘Tropical Storm’ as the least and ‘Category 5′ as the most powerful cyclones. The whole column indicates the total number of tropical cyclones over the given 5-year interval.

We know for sure that the number of Atlantic cyclones is affected by the presence El Ninos (the frequency is almost halved) and La Ninas (many hurricanes). Hence, we know that there must at least be something that influences the hurricane statistics. We know that one necessary condition for Atlantic cyclones to be spawned is that the sea surface in parts of the tropical Atlantic must be higher than a threshold value (~27deg C). The state of the atmosphere must also be favourable, that is, the winds should not change too much with height, as a wind shear might ‘tear’ the growing structures.

GCMs tend to be too coarse to resolve cyclones, but high-resolution regional models for storm studies exist. Knutson & Tuleya (2004) have studied how the hurricane activity may respond to increased CO2 levels, given known physical laws, and found a deepening of the central pressure and more intensive rainfall. If we choose to look at other statistics from the same data as in Fig. 1 – say the maximum wind speeds or minimum pressure associated with the Atlantic hurricanes – there appears to have been a trend after all (Fig. 2).

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