Masha (mashats) wrote in climatepapers,

Hurricanes and global warming

I'm not a tropical meteorologist, but I've observed a lot of the "heat" around the debate on hurricanes and global warming at conferences. I came across this recent GRL paper and I thought it was really interesting. The authors explain how "both sides of the isle are right and wrong at the same time".

Latif, M., N. Keenlyside, and J. Bader (2007), Tropical sea surface temperature, vertical wind shear, and hurricane development, Geophys. Res. Lett., 34, L01710, doi:10.1029/2006GL027969.

The authors start off with an explanation of ACE [Accumulated Cyclone Energy] index. It is calculated by summing the squares of the estimated maximum sustained velocity of every active tropical storm (wind speed 35 knots or higher), at six-hour intervals. The ACE Index is available on-line at

After that the authors show that the SSTs (sea surface temperatures) of the Atlantic have gone up over the past several decades, whereas the ACE seem to be experiencing some decadal variability.* But unlike other papers, the authors don't stop there, they also look at the trend s in Indo-Pacific SSTs over the several decades. They use the ECHAM5 model to simulate wind shear caused by SST anomalies in three tropical ocean and conclude that the combined index (the tropical Indo-Pacific/tropical North Atlantic SST difference) is the best predictor for the wind shear. Wind shear, on the other hand is negatively correlated with the ACE Index.

This is the final graph of the paper, showing all three variables the authors discuss:

Figure 4: ACE Index (black), inverted simulated vertical wind shear (blue), and tropical North Atlantic/Indo-Pacific SST difference (red). Results are shown from 1940 onwards, since observations are most reliable for this period. The data were normalized with respect to their individual long-term standard deviations to ease comparison. The thin lines are the raw JJASON values. The thick lines denote the low-pass filtered (applying an 11-year running mean) values.

The figure is rather convincing in showing how an increase in the Atlantic SSTs can cause increased hurricane activity, but at the same time the increase in the Indo-Pacific SSTs (think of El Nino years vs. La Nina years ;-) can offset this influence.
The authors didn't address whether and how humans might be responsible for the future evolution of the curve.

*This inconsistency between the Atlantic SSTs and ACE index trends is basically the crucial point of the disagreement between the two sides of the debate. One side ("global warming supporters") says that there's a link between Atlantic SSTs and Atlantic hurricanes, the other side (more skeptical) says that hurricanes follow some natural cycle of variability, yet unknown.
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I suggest taking a look at this paper
P. J. Webster, G. J. Holland, J. A. Curry, H.-R. Chang 2005 Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment. Science Vol. 309. no. 5742, pp. 1844 - 1846

These guys look at all the basins, and look at frequency and intensity of the hurricanes in the last several decades, and show the upward trend in hurricane intensity (not frequency) pretty well (increased amount of cat 4 and 5)
yup, I've seen it/

It all comes to the point of how you measure intensity. For example the ACE index, that is quoted above doesn't take into account the areal extent of a system, which some people believe is a really important factor.

There's a new fight in the most recent GRL. I haven't read it, but people say it's amusing ;-)
I'll have to take a look, not a journal that I read habitually :)