Hurricane Spin

Michael Mann and Gavin Schmidt

A recent paper by Vecchi and Soden (preprint) published in the journal Geophysical Research Letters has been widely touted in the news (and some egregiously bad editorials), and the blogosphere as suggesting that increased vertical wind shear associated with tropical circulation changes may offset any tendencies for increased hurricane activity in the tropical Atlantic due to warming oceans. Some have even gone so far as to state that this study proves that recent trends in hurricane activity are part of a natural cycle. Most of this is just ‘spin’ (pun intended), but as usual, the real story is a little more nuanced.

We have commented on the connections between hurricanes and climate change frequently in the past (see e.g. here, here, here, and here). The bottom line conclusion has consistently remained that, while our knowledge of likely future changes in hurricanes or tropical cyclones (TCs) remains an uncertain area of science, the observed relationship between increased intensity of TCs and rising ocean temperatures appears to be robust (Figure 1). There is nothing in this latest article that changes that.

Figure 1. Measure of total power dissipated annually by tropical cyclones in the North Atlantic (the power dissipation index “PDI”) compared to Aug-Oct tropical North Atlantic SST (from Emanuel, 2005; data)

The Vecchi and Soden (V+S) study suggests that increased ‘vertical wind shear’ in the tropical Atlantic might overcome this effect. Wind shear is related to the rate at which different layers in the atmosphere move – zero shear means that the layers all move together, large shear means that the upper layers are moving very differently to those below – and is inimical to hurricane formation and intensification. The well-known impact of El NiƱo on reducing Atlantic hurricane activity is in fact due to increased shear from the associated atmospheric circulation changes. The V+S results come from analysing the results of 18 different model simulations that were done for the IPCC AR4 and which now provide a superlative database for assessing what models do and do not project. It’s important to be clear that these models do not resolve hurricane processes and that the analysis is related to the large scale ‘background’ environment in which hurricanes form. Nonetheless the idea of looking at these simulations to see what happens to that large scale environment, as V+S have done, is certainly interesting and worthwhile.

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