Storms and Climate Change

Fig.2: Indications of maximum wind speed and minimum pressure of the most severe tropical storm per season.

The analysis in Fig. 2 indicates a systematic tendency where the strongest winds associated with tropical storms have been in the most recent decades. The graphic also shows a deepening of the minimum pressure over time, i.e. an indication of increased severity – at least if the data represents what actually happened in the past. This trend is consistent with the model results of Knutson & Tuleya (2004) and with a recent statement made by Trenberth, disputed by Chris Landsea. On the one hand, the data presented in Fig. 2 seem to speak for themselves, suggesting there has been a trend in cyclone severity. Fig. 1, on the other hand, suggests there is little indication of a trend. The impression we get from the two figures seem to be contradicting, and we should rightly ask: Are the data consistent, and is their quality reliable? Before 1950, the data is scarce, especially with respect to pressure observations. The wind data is probably less accurate in the early part of the record also. Question is, was there a trend before 1950, or were the minimum pressure values more stable then?

In an independent study, Gettleman et al. (2002) noted a 0.4 degC/decade trend in the dew-point temperature in the 1958-1997 radiosonde observations from the Tropics. They found indications that the Convective Available Potential Energy (CAPE) may have increased. The CAPE and dew-point temperature are independent measurements that can give us clues about cyclone trends, but they are also a potential indicator of climate change.

World-wide trends?

The media attention on tropical cyclones was not been limited to the U.S., south Atlantic or Japan. Australia, has recently been ravished by Tropical Cyclone Ingrid, which was “unusual in that it is the only cyclone in recorded history to impact, as a severe tropical cyclone, on the coastline of three different States or Territories”.

Other storms

The issue of storm trends and global warming has also received attention in Europe. A fairly recent devastating storm over Fance (December 26-27, 1999) made the headlines. Mid-latitude storms, such as those sweeping over western Europe, are distinct to tropical cyclones (more about this later). Some researchers have found that the frequency of mid-latitude storms may have dropped slightly over the European continent, but there have also been indications that the frequency of storms has increased elsewhere (the North Atlantic storm track – Iceland/Norwegian sea). Geng & Sugi (2001) noted an upward trend in the number of North Atlantic storms, albeit with strong decade-to-decade variations. The storm activity around Scandinavia during the most recent winter (2004-2005) was unusually high, and there was a great deal of media attention on the storm trains. These systems were responsible for a mild winter over northern Europe for most of the winter (in fact, March was colder than the other winter months, which is unusual) as well as extensive damage in southern Sweden.

Theoretical considerations

Why do we think that tropical cyclones would change because of a global warming? We have some basic physics-based principles that may possibly provide some insight. The main driving mechanism for all storms is the condensation of water vapour releasing energy/heat. Temperature differences cause instabilities and drive winds, and unstable disturbances grow into powerful storms. Tropical cyclones differ from mid-latitude storms in their primary driving mechanism. So-called ‘baroclinic instabilities’ are more important for the mid-latitude storms, whereas plain convective instabilities are the primary cause for the tropical cyclones – such as an increase in the CAPE.

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