Hurricane Heat

There is observational evidence for an increase of intense tropical cyclone activity in the North Atlantic since about 1970, correlated with increases of tropical sea surface temperatures. There are also suggestions of increased intense tropical cyclone activity in some other regions where concerns over data quality are greater. Multi-decadal variability and the quality of the tropical cyclone records prior to routine satellite observations in about 1970 complicate the detection of long-term trends in tropical cyclone activity. There is no clear trend in the annual numbers of tropical cyclones.

From the results presented in Kossin et al. the “suggestions” of increases in intense tropical cyclone activity in regions other than the Atlantic basin are not really so well supported, at least for the last 23 years.

We’ve tried to make this point a bit more clearly here, despite the implications of the headline of Kossin’s press release.


(disclosure: I have, to some degree, been funded by the fossil fuel industry since 1992)

2. Dr. Kevin Trenberth:

1) The methodology is trained on the Atlantic. It has no parameters to allow for different structures or size of storms, and there is no good reason why it should work well on storms in other basins. Given the different land-sea configurations and the different role of ENSO in the different basins, and the fact that disturbances in other basins do not form from easterly waves from off of Africa, there is every reason to expect that storms in other basins have different characteristics. For instance, there is greater activity in the Pacific Northwest, and the tropopause is higher in the western Pacific, and this affects brightness temperatures at tops of clouds. If the size of storms differs then the fixed form of EOFs will not be able to capture that form. The analysis must be able to account for differences among basins in order to have confidence in variability or trends. It would be easy enough to test whether the storms in other basins had different characteristics by also performing an EOF analysis for each region. This basic test was not done. It should be.

2) The results are suggestive of these problems. In the SIO where the method gives 0, 1 or 2 storms vs up to 6 in the best track data, there is a serious bias. Similar large biases exist in the SPAC (up to 2 vs 5 in best track). Obviously the threshold is effectively different and it is a comparison of apples and oranges.

3) In addition, this version of the paper deals with PDI. The earlier version of the paper dealt with intensity of storms and that was abandoned because the results were not very good. In particular, the presumption is that the older results were the problem because operational methods have improved. But the Kossin et al results showed bigger and greatest discrepancies with those from best track in recent years: there is no convergence over time. This is harder to see with PDI, because the biggest storms are emphasized, but the question of why is there not good agreement in recent years is not answered.

3. Dr. Judith Curry:

The most vexing thing about the tropical cyclone data sets is the uncertainty that analyst subjectivity contributes to this. The Dvorak scheme for determining tropical cyclone intensity is notoriously subjective, see the recent BAMS article on this. The importance of what Jim Kossin has done is to take this subjectivity out of the analysis.

Kossin’s method matches well the historical data in the North Atlantic (NATL) and East Pacific (EPAC). The method was trained using North Atlantic data, and the East Pacific regime in terms of dynamical and thermodynamical conditions is very close to the North Atlantic conditions. However, Kossin’s method diverges substantially from the established data sets in the Western Pacific, South Pacific, and Indian Oceans. Does this mean that the established data sets or in error, or that Kossin’s method (trained in the Atlantic) does not translate well to the other ocean regions?

Owing to problems with dealing with historical satellite data, Kossin’s study was extended only back to 1983 (the period for which the satellite data are well calibrated), and it is almost certain that this data set cannot be extended back prior to 1977. By itself, this data set is too short to say anything about a trend in intensity. But it can in principle be used to assess uncertainties in the established data sets.

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