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6.3 Interpretations by Emanuel (2005)

Emanuel’s interpretations of the effects of increasing sea surface temperatures were

expressed in terms of a ‘power dissipation index’ (PDI) defined as follows.

τ

PDI = ∫ V max 3 dt

0

where V max is the maximum sustained wind speed at 10 metres height, and τ is the

lifetime of a storm.

On the assumption that the economic loss in windstorms varies as the cube of the

wind speed, the PDI is assumed to represent the ‘destructiveness’ of a tropical

cyclone.

Emanuel summed the PDIs over all storms in each calendar year for the North

Atlantic and North-west Pacific Basins, and found significant increases from about

1990 and 1980 respectively. Significant apparent correlations with changing sea

surface temperatures were found. However, thermodynamic considerations would

indicate only a 6-9% increase in PDI for the 0.5 o C observed increase in sea surface

temperature, whereas the observed changes in PDI, with about a 50% increase, greatly

exceeded this range in both basins. Emanuel therefore concluded that only part of the

observed apparent increase in PDI can be attributed to increased sea surface

temperatures.

6.4 Interpretations by Klotzbach (2006)

Klotzbach (2006) extended the analysis to all basins with tropical cyclone activity,

and excluded data before 1986 on the basis that, before the mid 1980s, only visible

satellite information was available and hence nighttime observations were excluded;

also the quality and resolution of satellite imagery had improved greatly by the later

period.

Klotzbach used an ‘Accumulated Cyclone Energy Index’ (ACE) as an indicator of

trends. The ACE is similar to the PDI but incorporates the square instead of the cube

of the maximum surface wind speeds.

Only in the North Atlantic Basin was a significant increasing trend in ACE found. In

fact, decreasing trends were found in the Northeast, Northwest and Southwest Pacific

Basins. Klotzbach’s analysis, using the more recent (and more reliable) data, found

only a small increase in Category 4-5 hurricanes in the North Atlantic and Northwest

Pacific during the 20-year study period. Klotzbach’s findings were stated to be

‘..contradictory to those of Emanuel (2005) and Webster et al (2005)’.

6.5 Interpretations by Kossin et al (2007)

To eliminate the variability in global hurricane intensity records due to improvements

in satellite technology, Kossin et al (2007) constructed a more homogeneous data by

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