September 7, 2006

A Knight’s Tale

Filed under: Climate Extremes, Hurricanes

A new paper by Jeff Knight and colleagues finds further evidence to support their findings, reported last year, that the Atlantic Multidecadal Oscillation (AMO) is a real, physical phenomenon (as opposed to a statistical artifact) involving multidecadal variations in surface temperature primarily in the North Atlantic region and that its oscillations have consequences on regional climate, including North Atlantic tropical cyclones.

A large number of scientists will be hardly surprised by this. But, as in most active research areas, there’s considerable controversy. Another group of scientists do not believe that the AMO is a real physical phenomenon that affects North Atlantic Hurricanes. The leader of this latter group is Penn State’s Dr. Michael Mann. In collaboration with M.I.T.’s Dr. Kerry Emanuel, Mann published an article in the June 13th, 2006 issue of Eos: Transactions of the American Geophysical Union (you can find the article here.). A short summary of the article was presented on the blog (to which Mann is a major contributor) by the University of Chicago’s Dr. Ray Pierrehumbert:

If I may summarize and oversimplify some very deep conclusions, it would be as this: A lot of what is being called the AMO is really just global warming by another name. –raypierre

As to how the AMO pertains to Atlantic hurricanes, this is what Mann and Emanuel have to say in their Eos article:

It might be argued that other factors [besides rising sea surface temperatures, SSTs] potentially associated with the AMO (e.g., changes in vertical wind shear in the tropical North Atlantic) could be responsible for the observed tropical cyclone changes [e.g., Goldenberg et al., 2001]. This possibility was rejected after examining the residual time series that results from removing the statistical fit of the bivariate model for T(t) from the annual tropical cyclone series. The residual shows no evidence of a multidecadal spectral peak (see additional material). Thus, it can be inferred that any factors unrelated to SST that might influence tropical cyclone activity also do not exhibit any detectable multidecadal cyclicity.

Implications for Future Changes: 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. These findings have implications for potential impacts of various alternative possible future anthropogenic forcing scenarios on Atlantic tropical cyclone trends.

Acknowledgments We thank G. Schmidt, S. Rahmstorf, C. Folland, J. Knight, B. Santer, and K. Trenberth for comments on an early version of this manuscript, and we thank an anonymous reviewer for helpful comments on the manuscript.

It is interesting to see J. Knight thanked in the acknowledgements (as well as C. Folland) for these researchers (along with Adam Scaife) are the ones who have just published a paper in Geophysical Research Letters that concludes:

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 [Emanuel, 2005]. The model results also show an association of hurricane region shear with multidecadal variations in Pacific SSTs.

Since this finding is in direct opposition to the writing of Mann and Emanuel, we are left to wonder whether Knight’s and Folland’s “comments on an early version of this [Mann and Emanuel’s Eos] manuscript” were largely ignored.

That wouldn’t surprise us. Knight et al.’s most recent paper expands upon the results that Knight and colleagues published in 2005—results that established the AMO as a physical phenomenon that is manifested in direct observations, climate proxies, and general circulation models. Interesting, none other than Mike Mann was a co-author of the 2005 Knight paper, probably contributing significantly to the part about the existence of an AMO signal in climate proxies.

However, since the publication of that paper, Dr. Mann has repeatedly tried to distance himself from the results (for a partial example of this comments made at—“partial” that is, because, after repeated attempts to try to engage Dr. Mann in a discussion about the implications of Knight et al. (2005), ceased allowing our comments on this topic) culminating in his Eos article. Basically, he is refuting the claim that anything other than anthropogenic influences on Atlantic SSTs are responsible for the observed increase in Atlantic tropical cyclone activity during recent decades.

Despite Mann’s contrarian opinion, Knight, Folland, and Scaife conclude that the AMO is indeed an important driver of high and low periods of Atlantic hurricane activity as well as playing a role in other regional climate phenomena such as rainfall in Brazil and the African Sahel and seasonal storminess in North America and Western Europe.

Again, the fact that factors other than SST changes influence Atlantic hurricane patterns shouldn’t come as any surprise. A paper by Hoyos et al. earlier this year found that in addition to a significant decrease in vertical wind shear, the Atlantic basin has also experienced a decline in vertical stability—another factor favorable for tropical cyclone development. So along with rising SSTs, this combination of things has proven quite adept at spinning up strong hurricanes (see here for more details).

The new paper by Knight et al. provides additional support to the notion that the enhanced level of Atlantic hurricanes activity since 1995 is likely, to some considerable degree, to be attributable to natural variations in the North Atlantic SST patterns. Some other atmospheric changes promoting hurricanes that accompanied the AMO, such as the decrease in stability, are opposite to what one would expect from global warming. SSTs have also likely warmed a bit from anthropogenic enhancements to the earth’s greenhouse effect. And these elevated SSTs may be having an impact on Atlantic hurricanes (see for example our recent paper published in Geophysical Research Letters (Michaels et al., 2006) which is available here). But, the anthropogenic impact on hurricane intensity is likely small in comparison to natural variations and thus remains largely undetectable.


Hoyos, C.D., et al., 2006. Deconvolution of the factors contributing to the increase in global hurricane intensity. Science, 312, 94-97.

Knight, J.R., et al., 2005. A signal of natural thermohaline cycles in observed climate. Geophysical Research Letters, L20708, doi:10.1029/2005GL24233.

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

Michaels, P.J. et a.l, 2006. Sea-surface temperatures and tropical cyclones in the Atlantic basin. Geophysical Research Letters, 33, doi:10.1029/2006GL025757.

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