November 16, 2011

Atlantic Hurricanes: Fewer, Worse…Less Menacing?

Filed under: Climate Extremes, Hurricanes

Judging by the number of posts to date in each of our “Categories” (listed in the right-hand sidebar), it seems that, as far as individual categories go, we’ve treated you to more articles on “Hurricanes” than on anything else besides “Climate Politics”—and that’s saying a lot! And while we’ve spent a lot of time talking about the observational record of hurricanes and what it reveals (namely how weak the relationship is between global warming and hurricane characteristics), as well as projections as to what the future may hold in store for hurricane frequency (declines) and intensity (slight increases), we’ve haven’t really talked much about potential changes to the preferred hurricane tracks that may evolve under “global warming.” So, here, we’ll set out to change that.

To do so, we’ll highlight a couple of new articles by scientists at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (a less-visible part of the National Hurricane Center where an awful lot of hurricane research takes place) that suggests that global warming may alter the environment in the Atlantic Ocean basin in such a way as to steer hurricanes away from the U.S. coastline. So while they may get stronger, they would become less of a threat to our coastal communities.

A lot of recent research has focused on the influence of the size of the Atlantic Warm Pool (AWP)—an area of warm water spanning parts of the Gulf of Mexico, Caribbean, and western North Atlantic Ocean—on the strength and frequency, and preferred tracks of Atlantic Basin tropical cyclones. In particular, work by Wang et al. (2008) established that large AWPs tended to produce both more and stronger hurricanes—and that the AWP may be the primary mechanism through which multi-decadal variations in the patterns of Atlantic sea surface temperatures (known as the Atlantic Multidecal Oscillation, or AMO) modulate hurricane activity (see this WCR for more on the AMO-hurricane link). A large AWP alters the environment in the main hurricane development region (MDR) of the Atlantic Ocean including weakening the vertical wind shear and increasing the atmospheric instability—both conditions which favor tropical cyclone genesis and growth.

Wang and colleagues (2008) developed a timeseries of the size of the AWP dating back to the mid-19th century (Figure 1) and also showed that there were several distinct patterns which formed the AWP history.


Figure 1. Area anomaly indices (%) if the Atlantic Warm Pool (AWP) during the Atlantic hurricane season of June–November (JJASON). The area index is calculated as the anomalies of the area of sea surface temperature higher than 28.5°C divided by the climatological JJASON AWP area (figure adapted from Wang et al., 2008).

Figure 2 shows the evidence of signals in the AWP index from an overall global warming (top panel), El Niño-La Niña cycles (middle panel), and the AMO (lower panel).


Figure 2. Projections of the total AWP area index of Figure 1 onto (a) global warming mode, (b) ENSO-like variation, and (c) the AMO (source: Wang et al., 2008).

Based upon the positive relationship between the size of the AWP and the global temperature anomaly, it would be reasonable to infer continued expansion of the AWP with rising temperatures. On the surface of things, this relationship would seem to indicate that continued global warming would lead to more Atlantic hurricanes, stronger Atlantic hurricanes, and consequently a greater threat to the U.S. mainland. However, the surface of things is not always the best judge of what really is to be expected.

More insight, in fact, is gained through the further studies of Wang and colleagues, the results of some of which were just published a few weeks ago in the journal Geophysical Research Letters. In this work, the authors examined the “Impact of the Atlantic warm pool on United States landfalling hurricanes” (as the title suggests). After all, if hurricanes don’t make landfall, the damage that they may cause is greatly limited.

For this investigation, Wang and colleagues divided the observed history of Atlantic tropical cyclones (since 1970) into years with large AWPs and those with small AWPs and then tallied up within each division storm characteristics such as number and tracking tendency. What they found was that large AWPs were associated with more storms, but—and this is important—storms which preferentially stayed off-shore and did not make landfall along the U.S. coast. In other words, the hurricanes which formed became less menacing.

The reason for these observed changes was that as the AWP increased in size it made conditions more favorable for tropical cyclone formation and growth further out in the Atlantic Ocean. And at the same time, it altered the atmospheric circulation patterns over the central Atlantic Ocean such that the storms which did form were steered more northward into the open Atlantic and away from the U.S. mainland. The Figure 3 shows this typical pattern. A hypothetical storm which formed in the Atlantic’s primary hurricane genesis region (indicated by the black box in Figure 3) has a tendency to recurve northwards in years with a large AWP (top panel) while during years with a small AWP, the same storm would have the tendency to be steered towards the southeastern U.S Atlantic coast and Gulf of Mexico (bottom panel). Clearly, coastal interests in the U.S. would prefer the former situation rather than the latter.


Figure 3. Tropical cyclones which form in the Atlantic Ocean’s main development region (MDR) (black box) during years when the Atlantic Warm Pool (AWP) is large in spatial extent have a tendency to track into the central Atlantic and away from the U.S. coastline (top panel), while tropical cyclones which form in the MDR during years with a relatively small AWP have a tendency to track towards the southeastern Atlantic Coast and Gulf Coastal regions (bottom panel) (image source: Wang et al., 2011).

So, in combination, these two papers by the researchers at the AOML, indicate that while global warming may lead to changes in the AWP that have a tendency to favor more and stronger tropical cyclones, that the same changes will also tend to direct the storms into the open Atlantic Ocean and away from the U.S. coast. Consequently, the idea that global warming will “make hurricanes worse” is not as worrisome as it is often pitched.

As with everything climatological, the actual situation as it develops is filled with complexities each of which acts to complicate the final outcome not only for each individual storm system, but each hurricane season as well. As we noted up front, the level of natural variability manifest through the interactions of many different processes ultimately overwhelms the pure signal from anthropogenic “global warming” and so detecting an influence like that identified by Wang and colleagues, much less assigning global warming as the cause (even partially) becomes virtually impossible. But at least this work helps dispel notions that landfalling hurricanes in the U.S. are “consistent” with expectations from “global warming.”

References:

Wang, C.L., S-K. Lee, and D.B. Enfield, 2008. Atlantic Warm Pool acting as a link between Atlantic Multidecadal Oscillation and Atlantic tropical cyclone activity. Geochemistry, Geophysics, Geosystems, 9, Q05V03, doi:10.1029/2007GC001809.

Wang, C., L. Hailong, S-K. Lee, and R. Atlas, 2011. Impact of the Atlantic warm pool on United States landfalling hurricanes. Geophysical Research Letters, 38, L19702, doi:10.1029/2011GL049265.




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