Does Global Warming Change Hurricane Intensification Rates?

On Sept. 13, as the surprise Hurricane Humberto careened towards the Louisiana-Texas border region, National Hurricane Center forecaster James Franklin wrote this disarmingly frank commentary on its progress:

BASED ON OPERATIONAL ESTIMATES ... HUMBERTO STRENGTHENED FROM A 30 KT DEPRESSION AT 15Z YESTERDAY TO A 75 KT HURRICANE AT 09Z THIS MORNING ... AN INCREASE OF 45 KT IN 18 HOURS. TO PUT THIS DEVELOPMENT IN PERSPECTIVE ... NO TROPICAL CYCLONE IN THE HISTORICAL RECORD HAS EVER REACHED THIS INTENSITY AT A FASTER RATE NEAR LANDFALL. IT WOULD BE NICE TO KNOW ... SOMEDAY ... WHY THIS HAPPENED.

Humberto was a true shocker -- and Franklin's frustrated words underscore just how little we understand about how the shock came about. So it's hardly surprising that Humberto has promoted serious chatter (and worry).

In particular, in the wakes of Humberto and the similarly rapidly-intensifying Category 5 Hurricane Felix, the question has been quite legitimately raised: How might global warming affect hurricane intensification rates?

Hurricanes and typhoons tend to rapidly intensify when they run over stretches of very deep warm water in an atmospheric environment conducive to strengthening (e.g., one characterized by weak vertical wind shear). Rapid intensification can be particularly dangerous when it occurs just before landfall, as in the case of Humberto. In these situations, a population led to expect a relatively weak storm can suddenly find itself staring down a killer with no time left to evacuate.

For this reason, if we want to better defend lives and property, hurricane rapid intensification is a very important subject of inquiry. But it's also a subject that, at the current juncture, remains very little understood.

Still, there's no doubt we've recently seen some scary storm behavior in the Atlantic region lately. As previously mentioned, Humberto came literally out of nowhere and strengthened from a disorganized group of clouds into a Category 1 hurricane in just 18 hours (officially). In other words, 18 hours before Humberto was a Category 1 hurricane slamming the coast, the National Hurricane Center was not even issuing advisories about it. That makes the storm "unique in the Atlantic hurricane record," explains hurricane guru Jeff Masters.

Meanwhile, we saw a different but also worrisome intensification rate record from Hurricane Felix last month. Post 1970 in the Atlantic region, Felix had the most rapid known intensification from the first advisory issued to Category 5 strength -- 54 hours. (Before 1970 there's a rival, 1960's Hurricane Ethel, but measurement techniques were very different then. For further discussion see here). Finally, two years ago Hurricane Wilma set yet another Atlantic hurricane intensification rate: The storm's minimum pressure dropped 97 millibars in just 24 hours.

These are the facts when it comes to rapid intensification in the Atlantic -- but can we generalize further about them? Jeff Masters, pretty much the chief expert among hurricane bloggers, puts it like this:

No scientist has published a paper linking rapid hurricane intensification rates with global warming. While the cases of Humberto and Felix are certainly unique, the year 1969 also had two storms that were very similar in their intensification rates. A quick look I did at historical intensification rates doesn't show any noticeable trends, and I think that the rapid intensification rates of Felix, Humberto, and Wilma the past three years are not far enough outside the statistical norms that we need to invoke climate change as an explanation. Still, it does leave one wondering, and climate change could be affecting hurricane intensification rates.

But how might it be affecting them? What would the mechanism of action be? Masters doesn't say, but I'm going to assume that he means more heat stored in the ocean creates an environment more conducive to rapid strengthening. But that raises some obvious questions: Is there an upper limit imposed by physics on how fast strengthening can occur? And do our data totally limit us in saying anything more about trends in intensification rates at the current juncture?

To answer these questions, the "Storm Pundit" will be canvassing hurricane experts for more illumination about how global warming may affect hurricane rapid intensification rates -- a subject that may well inspire much more scientific research in the near future. Stay tuned.

Typhoon Wipha

How fitting that as I write this post, Typhoon Wipha is rapidly intensifying in the Northwest Pacific region. Wipha could soon become our thirteenth Category 4 or higher hurricane of the year, and the second Category 4 or higher typhoon in this region in a week (the former being the recently dissipated Typhoon Nari).

Moreover, while it's still early, Wipha could be en route to Shanghai. The storm would presumably have weakened from peak intensity by the time it gets there, but nevertheless, it would be quite a month indeed if both Tokyo and Shanghai experienced serious typhoons. Here's Wipha's latest forecast track, courtesy of the Joint Typhoon Warning Center: