Is the number of tropical storms increasing?

During the hurricane season of 2004 an unusual number of strong tropical storms hit the Caribbean and the continental United States accounting for large damages and, even worse, a substantial death toll. In this context, the question is often raised, whether the observed storm frequency and intensity does already reflect the anticipated climate change due to increasing greenhouse gases.

Tropical cyclones form over oceans with surface temperatures above 26┬░C as strong low pressure systems that can reach diameters of more than 500 km. Evaporation of immense amounts of water vapour under these high temperature conditions and the related condensation aloft provide ample energy for the cyclone dynamics, leading to wind speeds of up to 300 km/h and extreme precipitation. The core of the strongest storms remains however virtually free of wind and clouds due to sinking air there. Tropical cyclones with maximum wind speeds of more than 33m/s (corresponding to about 120 km/h) are called Hurricanes over the Atlantic and Typhoons over the Pacific. Tropical cyclones are usually found during the late summer and early fall season. They are less abundant over the southern oceans than in the northern hemisphere.

The number and intensity of these cyclones differ from year to year but did not exhibit significant systematic trends during the last hundred years. An existing observation time series of land fall cyclone cases over the continental US indicates a reduction of cases after a maximum around the mid of the 20th century. Concerning global estimates, satellite information is required over most ocean and many land areas due to often very limited conventional observation capacity. But as satellites have only been available for recent decades, the derivation of climatological tends is difficult, especially with respect to very strong and thus rare events.

Therefore, the assessment of the potential influence of anthropogenic caused climate change on tropical cyclones can only be based on climate model calculations. The third IPCC assessment report on climate change from 2001 (see: http://www.ipcc.ch) states in this respect, that most of the model calculations do not exhibit any significant trend in the frequency of intense tropical storms. This is most probably due to the fact that in the models not only the ocean surface but also the upper tropical tropopause become warmer with time, resulting in a rather stabilising influence on tropical cyclone numbers. But the models seem to indicate an increase in intensity of the individual storms due to the generally expected increase of the water cycle intensity in a warmer climate.