Extreme Weather #5 – Trends in "Intensification Rates" of Tropical Cyclones

We’ve been looking at various aspects of Tropical Cyclones (TCs) in the section “Observed Trends” from the latest IPCC report, AR6 (the 6th Assessment Report). Chapter 11 is all about extreme weather.

The report says, p.1585:

..there is evidence that TC intensification rates and the frequency of rapid intensification events have increased within the satellite era.

The satellite era is 1980 to present, so we have about 40 years of global data from satellites.

What is “intensification rate”?

Here is one of the papers cited for this section, Tropical cyclone intensification trends during satellite era (1986–2010), Kishtawal et al 2012:

The trends indicate that the TCs now intensify from 64 kt to 104 kt nearly 9 hours earlier than they did 25 years back. The maximum reduction in intensification time is noticed over the North Atlantic Ocean where the average time needed for TC to intensify from 64 kt to 112 kt has reduced by nearly 20 hours during the past 25-year period.

So basically this is the time for a tropical cyclone to go from “not so bad” to “bad”. And it appears to have reduced. This could make forecasting harder. It could definitely make it more difficult to prepare people living on the coast when a tropical cyclone is heading for them. Overall, this is bad news.

By the way, the units are “kt” which is knots, that strange unit for measurement of speed that nautical people use when the ocean is involved. Who knows why.

The report cites three papers for this phenomenon.

Kishtawal et al 2012 - global over 25 years; they find a significant signal even when removing outliers. They didn’t attribute a cause.

Balaguru et al 2018 - Atlantic over 30 years; they find that the central and eastern tropical Atlantic had an increase, but the western Atlantic did not. They attribute the change to a climate mode known as the Atlantic Multidecadal Oscillation (AMO). They didn’t give a reason for not considering the global data.

Bhatia et al 2019 - Atlantic over 28 years, “We focus primarily on the Atlantic basin because it is the basin with the most consistent high-quality observations”. They attribute the change to global warming, based on results from a climate model - HiFLOR - which is claimed to be the best at simulating TCs.

Bhatia et al 2019 include the note:

On the other hand, both observational datasets have larger uncertainties in global intensity changes. Therefore, any conclusions stemming from the global trend analysis must be treated with caution.

The conclusion of that section of AR6 says:

In summary, there is mounting evidence that a variety of TC characteristics have changed over various time periods. It is likely that.. the frequency of rapid intensification events have increased globally over the past 40 years.

Unfortunately, taken together, the three references cited don’t make that case at all.

The one global paper was over 25 years and didn’t attribute any cause. Another paper was over 30 years for the Atlantic, finding a difference between east and west and attributing change in the east to a natural cause (the AMO), the other paper was over the whole Atlantic over almost 30 years and attributing changes to global warming, while also including the note that global trends in intensity are suspect.

This seems like a sloppy section of the IPCC report. Maybe none of the lead authors were very interested in this idea.

But it does seem that this parameter has changed in some part of the Atlantic.

References

Seneviratne et al, 2021: Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change

Tropical cyclone intensification trends during satellite era (1986–2010), C. M. Kishtawal et al, GRL (2012)

Increasing Magnitude of Hurricane Rapid Intensification in the Central and Eastern Tropical Atlantic, Karthik Balaguru et al, GRL (2018)

Recent increases in tropical cyclone intensification rates, Kieran T. Bhatia et al, Nature (2019)