The IPCC report on extreme climate and weather events

Whether the characteristics of tropical cyclones have changed or will change in a warming climate — and if so, how — has been the subject of considerable investigation, often with conflicting results. Large amplitude fluctuations in the frequency and intensity of tropical cyclones greatly complicate both the detection of long-term trends and their attribution to rising levels of atmospheric greenhouse gases. Trend detection is further impeded by substantial limitations in the availability and quality of global historical records of tropical cyclones. Therefore, it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes. However, future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%. Balanced against this, higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre. For all cyclone parameters, projected changes for individual basins show large variations between different modelling studies.

In GCMs, these phenomena appear as vortex-like features in the discrete representation of the flow represented by neighboring grid boxes. It’s quite remarkable that these phenomena are present at all in these models (sometimes they are not, though), even though they may have too weak or exaggerated features. In the real world, the definition of a tropical storm is a synoptic scale low-pressure system with maximum sustained surface wind speed greater than 17 m/s, and in hurricanes greater than 33 m/s.

If we look at wind speed measurements at a given location, we see that there are relatively few days with zero wind, more often there are moderate wind speeds, and it is typically rare when the wind speed exceed the threshold defining a tropical storm or a hurricane. In statistical terms, the wind speed may be described by a distribution function – e.g. a Weibull distribution (e.g. here and here). The situation is illustrated below showing wind speed statistics, where the curve is the probability distribution function (pdf) for the wind speed and where the x-axis represents the wind speed and the y-axis the likelihood (frequency) of occurrence. The threshold marking tropical storms is shown as the first vertical line (the others mark typical TC categories), and the area under the curve to the left of this treshold (denoted “a” in the diagram) is proportional to number of observations (e.g. days) with no tropical storms. The area above (“A”) is proportional to the frequency of tropical cyclone occurrence.

Fig. 2. Wind speed statistics and tropical cyclones.

Let’s consider the implication of fewer tropical cyclones but an increase in their intensity. In terms of wind speed statistics, this suggests a shift in the pdf (grey gurve in Fig. 3), with an increase in the area under the curve with wind speeds lower than 17 m/s (“a”). This also implies a decrease in the area under the curve for which wind speeds exceed 17 m/s (“A”), as the area under the total curve of a pdf must be constant (unity by definition). But if the tropical cyclones are getting more intense (increased mean TC maximum wind speed), there must be a second threshold, e.g. 33 m/s for which the area under the curve for the new pdf is greater than for the old curve.

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