September 12, 2006

More Support for an AMO/Atlantic Hurricanes Link

Filed under: Hurricanes

Last week, WCR commented on research by Jeffrey Knight and his colleagues, tying natural variations in the Atlantic Multidecadal Oscillation (AMO) to long-term patterns of Atlantic hurricane activity (including the enhanced level of storm activity beginning in 1995). Immediately another paper appeared Geophysical Research Letters, by Rong Zang and Thomas Delworth titled “Impact of Atlantic multidecadal oscillations on India/Sahel rainfall and Atlantic hurricanes.” Zang and Delworth work for the Department of Commerce’s Geophysical Fluid Dynamics Laboratory at Princeton.

Here’s the abstract:

Prominent multidecadal fluctuations of India summer rainfall, Sahel summer rainfall, and Atlantic Hurricane activity have been observed during the 20th century. Understanding their mechanism(s) will have enormous social and economic implications. We first use statistical analyses to show that these climate phenomena are coherently linked. Next, we use the GFDL CM2.1 climate model to show that the multidecadal variability in the Atlantic ocean can cause the observed multidecadal variations of India summer rainfall, Sahel summer rainfall and Atlantic Hurricane activity (as inferred from vertical wind shear changes). These results suggest that to interpret recent climate change we cannot ignore the important role of Atlantic multidecadal variability.

Just like Knight et al., Zhang and Delworth find the AMO to be both a real phenomenon and one that is an influence on regional climate in various locations across the world, including in the tropical Atlantic. These results are unlike the conclusions of an earlier paper by Mike Mann and Kerry Emanuel that suggests that the AMO is not really a physical process, but instead is simply anthropogenic global warming, and that there is no evidence of “any detectable multidecadal cyclicity” in factors which may influence Atlantic hurricane activity.

Mann and Emanuel believe that the upswing in Atlantic hurricane activity during the past several decades is predominately a result of anthropogenic alterations to the earth’s atmospheric composition. This concept, in turn, fuels the repeating breathless message “We are all going to die if we don’t do something right now about global warming.”

Other hurricane researchers counter that there are natural, pseudo-cyclic oscillations in tropical cyclone intensity and frequency that may explain a good deal of the observed variations. Further, the massive demographic changes that have taken place along our Gulf and Atlantic shorelines greatly exceed anything that global warming would ever have to offer in terms of upping the ante for disaster (the alternative repeating message should be, “relax, we are all going to die anyway.”)

The new results published by Zhang and Delworth (and Knight et al.) clearly demonstrate that the debate over the causes of the increase in Atlantic hurricane activity is far from being settled. But, undeterred, climate alarmists continue to push the idea that greenhouse emissions lead to drastically stronger hurricanes with dramatic consequences. Disagree with this notion and you are labeled a contrarian, or worse yet, a “denier.”

What is perhaps even more disturbing is the self-righteousness that has intruded into this debate. Take for instance a recent posting on the blogsite by guest columnist Thomas Crowley. In describing the current debate on hurricane trends and their potential causes, Crowley had this to say about the scientific behavior of the two “camps:”

The average reader of newspaper articles on this [topic] might well have concluded that there are in fact two camps on this subject – one global warming, one natural variability. But I think there is a one-way commingling of the camps. I suspect that most people leaning toward global warming as a contributor to the unusual increase in the magnitude of Atlantic tropical storms and hurricanes would readily concede the possibility that the AMO could, in addition to the El Nino/Southern Oscillation, modulate any greenhouse gas contribution to the enhanced activities of tropical storms.

It is not at all clear to me that the natural variability camp sees any need to commingle with the global warming community.

Like we said, self-righteous. But also wrong.

Here is how the authors of the recent papers described their own work. First Zhang and Delworth, who come from the “camp” that natural cycles are at work:

Our modeling results suggest that if the current warm phase [which began in 1995] of the AMO persists in the coming decade, it will strengthen…the Atlantic hurricane activity. Such a scenario will be complicated by the influence of changing external forcings that may contribute to some of the multidecadal variability as well as the long-term trends of these phenomena. Nevertheless, our results indicate that the impact of AMO is very important for our understanding of the future climate change.

And here is how Knight et al., also natural cycles campers, describe their findings:

The model results also provide physical evidence for the observed link between the AMO and tropospheric vertical shear in the main hurricane development region. Simulated correlations are consistent with those derived from reanalyses, implying the AMO may explain at least part of the observed multidecadal variability in hurricane activity. This does not preclude, however, the possibility of an influence from anthropogenic climate change.

Both sets of authors report their findings that the naturally occurring AMO influences Atlantic hurricane patterns, and that other factors, such as anthropogenic activities, may also play a role.

And here is what Mann and Emanuel—scoutmasters for the global warming campers—conclude in their latest work:

There is a strong historical relationship between tropical Atlantic SST and tropical cyclone activity extending back through the late nineteenth century. There is no apparent role of the AMO. The underlying factors appear to be the influence of (primarily anthropogenic) forced large-scale warming, and an offsetting regional cooling overprint due to late twentieth century anthropogenic tropospheric aerosol forcing.

In other words, the evenhandedness of attribution is much more operative among natural cycle campers than global warmers. Perhaps the gentlemen at RealClimate ought to work for a little more internal consistency on their blog, and a little less self-righteousness.


Knight, J.R., et al., 2006. Climate impacts of the Atlantic Multidecadal Oscillation. Geophysical Research Letters, L17706, doi:10.1029/2006GL026242.

Mann, M.E., and K. A. Emanuel, 2006. Atlantic hurricane trends linked to climate change. Eos: Transactions of the American Geophysical Union, 87, 233-244.

Zhang, R., and T. Delworth, 2006. Impact of Atlantic Multidecadal Oscillation on India/Sahel rainfall and Atlantic hurricanes. Geophysical Research Letters, 33, L17712, doi:10.1029/2006GL026267.

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