I’m looking at some of the details of the Webster, Curry et al 2005 claim that the proportion of intense hurricanes has increased. While I was doing so, I noticed an interesting issue in the Northern Indian Ocean tropical storm counts. Here is an excerpt from Webster et al 2005 Figure showing the count of cyclonic storms per year. The brown curve is the North Indian Ocean.
Webster et al 2005 Figure 3. Number of Cyclonic Storms/Year. Brown is North Indian Ocean.
I’ve collated storm track data and here is what I obtained when I tried to replicate the count for the Indian Ocean. Note that the period covered in the graphs are different. The solid line is the count according to the archive of storm tracks. However in this area, most storms do not have any wind speed estimates. For example, the 1970 Bhola Cyclone killed over 300,000 people in Bangladesh, but has not wind estimates in the archive is not counted as a storm in the Webster et al graph. The red dashed line shows the number of storms with wind speeds exceeding 18 msec-1 (and this is virtually identical to the number of storms with any data). If you look closely, the corresponding counts in Webster et al differ in detail for unknown reasons.
North Indian Ocean – Number of Tropical Storms. Collated from information at http://weather.unisys.com/hurricane/indian_oc/
A couple of questions obviously arise – which may or may not be easy to resolve. There’s a sharp decrease in the number of tropical storms whose storm tracks are archived in 1976 – with seemingly different average levels before and after 1976. Is this due to a measurement artefact or to some climatic change? I have no idea. There is no discussion of the issue by Webster et al. It’s too bad that climate scientists reporting on hurricane counts in Nature and Science are not obligated to provide such accounting details.
Webster et al observed of the Pacific data:
The need for reprocessing the western North Pacific tropical cyclone data set is very clear.
This would also seem to be true of the North Indian Ocean data.
It’s also worth noting that North Indian Ocean SSTs were supposedly rising particularly quickly according to Hansen’s PNAS article, discussed here recently, but the number of tropical storms in the North Indian Ocean has not increased according to any metric.