Tropical SSTs: Natural variations or Global warming?

by Michael Mann and Gavin Schmidt

Roughly a year ago, we summarized the state of play in the ongoing scientific debate over the role of anthropogenic climate change in the observed trends in hurricane activity. This debate (as carefully outlined by Curry et al recently) revolves around a number of elements – whether the hurricane (or tropical cyclone) data show any significant variations, what those variations are linked to, and whether our understanding of the physics of tropical storms is sufficient to explain those links.

Several recent studies such as Emanuel (2005 — previously discussed here) and Hoyos et al (2006 — previously discussed here) have emphasized the role of increasing tropical sea surface temperatures (SSTs) on recent increases in hurricane intensities, both globally and for the Atlantic. The publication this week of a comprehensive paper by Santer et al provides an opportunity to assess the key middle question – to what can we attribute the relevant changes in tropical SSTs? And in particular, what can we say about Atlantic SSTs where we have the best data?

This role of SST remains pivotal in understanding long-term trends in hurricane activity, regardless of whether the SST increases are natural or anthropogenic in origin. Mann and Emanuel (2006) noted that once SST is accounted for as a factor, there is no apparent multidecadal signal in long-term hurricane numbers, regardless of what that signal might be (i.e., SST, wind sheer, or any other factors that potentially influence activity). Other studies (e.g. Bell and Chelliah, 2006) generally agree that the ‘memory’ of any multi-decadal osillation (whether forced or natural) lies in the ocean, and atmospheric simulations using observed SST data in the Atlantic reproduce many of the observed correlations (Zhang and Delworth, 2006). So understanding the origin of the warming SSTs is central to understanding changes in hurricane behavior.

The essential question, then, is what has caused the long-term SST changes? This question is sometimes reduced to the ‘straw man’ of whether tropical SSTs trends (which are very clear in the data) are internal (i.e., due to a natural oscillation of the climate system) or anthropogenic (i.e. forced by some combination of human-related causes)? Of course, this is a over-simplification since there always at least some role for internal variability. So a more useful question is to what extent SST changes can be attributed to various possible causes.

In the Atlantic, the proposition advanced by some is that a natural oscillation known as the Atlantic Multidecadal Oscillation (AMO) is responsible for the observed trends in SST (and by extension, North Atlantic hurricane activity). To examine that question requires a clear analysis of the data, and examination of what models suggest regarding the likely amplitude, spatial extent and physical mechanisms underlying this oscillation.

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