No Se Nada

In my spare time I'm a hurricanes expert. NOT!! (I'll let others keeping pretending they are, though.) But my phud is in phys. oceanography and climatology, so I keep my eye on all this hurricane busyness.

A new paper is out in Geophysical Research Letters by Virmani and Weisberg (Univ. of South Florida) titled The 2005 hurricane season: An echo of the past or a harbinger of the future? Unfortunately you need to be an AGU member or at a university with full access to get the paper. (But what do you really need to read the whole paper for when you have bloggers to tell you about it?)

[Apparently the Webster group, home to my favorite cat fighter Judy Curry (note: read the byline on that interview to understand an upcoming reference...), has another hurricanes paper coming out Friday in Science Express. It's probably a follow-up to Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment. More on that when it comes out.]

The Virmani/Weisberg GRL paper (let's just call it VW06 for the rest of this post) doesn't actually answer the question they pose in their title ("echo of the past or a harbinger of the future?"), but they do make some interesting deconstructions of various influences on the 2005 hurricane season. This is the take-home message as I read it:

What controlled the anomalous 2004 and 2005 seasons was a slackening in the easterly winds in the tropical Atlantic (for you non- atmospheric nerds, the "ly" at the end of a direction means "coming from" that direction). The winds usually cool the upper few dozen meters of the ocean surface, so when the winds aren't there the ocean gets anomalously warm, which leads to stronger hurricanes.

When this slackening of the easterlies occurs concurrently with a couple of other climatological factors that favor hurricane development, you get a big hurricane season. This has happened a few times in the past, with no apparent connection to any known climatological cycle or to anthropogenic global warming (AGW):

These years of anomalously high SST and related variables have been classified as having active hurricane seasons, with 10 or more tropical storms, of which at least seven (1958), twelve (1969), nine (1980), eleven (1995) and ten (1998) were hurricanes [Staff, Weather Bureau Office, 1958; Simpson et al., 1970; Lawrence and Pelissier, 1981; Lawrence et al., 1998; Pasch et al., 2001]. The average long-term mean is 6 hurricanes per year [Pasch et al., 2001].

and, from the Conclusions:

While anomalous, these conditions in the Atlantic and Caribbean during 2004 and 2005 were not unprecedented and were equally favorable during the active hurricane seasons of 1958, 1969, 1980, 1995 and 1998.

However, the Atlantic Multidecadal Oscillation (AMO) is also a controlling factor on hurricanes. VM06 show the AMO as having both its well-known multidecadal signal, as well as a positive linear trend of about 0.5°C/120 years. If you've been following the hurricane science wars over the past few months, you know that the AMO is the crux of the hurricanes/global warming fight. The positions have roughly been staked out as:

• NOAA/hurricane forecasters: trends in hurricanes is all due to the natural cycle (AMO)
• Webster's group and Kerry Emanuel: trends in the frequency of the strongest hurricanes can be linked to AGW

But without saying anything about the overall trend of all hurricanes, VW06 note that the anomalously big hurricane years over the past five decades happen independently of the AMO or its long-term upward trend:

Hurricane frequency is generally greater (lower) when the AMO is in its positive (negative) phase [Goldenberg et al., 2001]. Since the mid-1990s the AMO has been in the positive phase and hurricane activity has gradually increased. However we note that the anomalous years referred to in section 3 discriminate neither on the AMO nor the long term trend.

So what does this say about hurricanes and global warming? It says that the most active years seem to happen at random and cannot be linked to a secular trend in rising sea-surface temperatures due to AGW. And that the reason the SSTs get high in these big hurricane years is in large part due to slackening easterly winds, which happens because of natural variability, not AGW.

But VW06 also says that there is a long-term trend to the AMO. Since the AMO plays a role in hurricane development, this long-term trend probably has an influence on hurricanes (that's my take). Is this long-term trend anthropogenic global warming? VW06 say:

Without detrending, a gradual increase in temperature occurs over this time period in addition to the AMO. Whether this trend is part of a longer natural cycle or due to anthropogenic effects remains to be determined.

For my money, the long-term trend in the AMO is AGW, but it doesn't kill their point.

In the end, though:

While multidecadal oscillations in SST and SLP have been established in the Atlantic [Deser and Blackmon, 1993; Halliwell, 1997; Enfield and Mestaz-Nunez, 1999] and connected to the frequency of hurricanes [Landsea et al., 1999; Goldenberg et al., 2001] there is not a clear link between the AMO or the long term trend and individual active hurricane years, confirming the importance of other factors in hurricane formation.

So VW06 seems to give fodder to both AGW-hurricane is!!/is not!! positions. The biggest years are due to multiple factors, mostly driven by slackening easterly winds, and to my reading seem to happen at random (in time). But there is an upward trend in the AMO, which is an important controlling factor on hurricanes. The answer hinted at in VW06 is that there is no AGW signal on the biggest hurricane years, but there might be an AGW signal in the overall, long-term trend of hurricanes. So in the end, there's something in VW06 for everybody. It's natural and there's a trend.

Now let's make this relevant. Before each side gets all fired up for another media-page battle where Paul Thacker can get somebody on "one side" to say mean things about somebody on "the other side," let's first ask this question on behalf of the junior Pielke:

Why do you care about hurricanes? Is it because they are storms like your average tornado or stationary cold front, and you're just a weather geek? Or is it because they come on land and wreck stuff? Do you really care about the hurricanes that never make landfall and just spin around in the Sargasso Sea? Or do you only care when a hurricane does earthquake-like damage?

I assume that the honest answer for the vast majority of you is the latter. You don't track hurricanes out to sea and so the only reason you might care about hurricanes in a global warming world is because they might do more damage upon landfall than hurricanes in a non-global warming world.

And they might, but if you're truly worried about damage, it is by now very obvious (and follow up paper is forthcoming, I gather) that coastal building and demographics patterns are much more important to hurricane damage trends than AGW likely ever will be.

So do you want to keep believing Ross Gelbspan's BS and keep bemoaning that W ain't doing nuttin' about climate change and he's really to blame for larger future hurricane damages? Or do you want to blame coastal zoning and population management? For your taxpaying money, I think the latter is a lot easier and cheaper to tackle than the former, and will be about 50 times more effective to boot.