U.S. hurricanes and the North Atlantic oscillation

One thing I find surprising about the debate on climate change and hurricanes is the lack of discussion on the North Atlantic oscillation (NAO). Some in politics and insurance suggest greater attention be placed on understanding future hurricane activity as it relates to the United States. We've published 13 scientific papers on this topic since 2001 (here). The research identifies and elucidates the role of the NAO in portending hurricane tracks across the Atlantic Ocean. A weak NAO phase tends to favor tracks that parallel lines of latitude. In contrast a strong NAO phase tends to favor tracks that cross latitudes (hurricanes that, in general, get steered away from the U.S. coast). We speculate the reason for this is related to the position and strength of the subtropical high pressure system.

Interestingly, the NAO was in a positive phase for much of the 1970s and 1980s with historic highs in the early 1990s and speculation about a link to global warming has been made. Osborn et al. (1999) show that the NAO from the 1960s to early 1990s is outside the range of earlier variability in the instrumental record and also outside the range of variability simulated using UK Hadley Centre's numerical model. Thus with greater warmth and perhaps more Atlantic hurricanes it is possible that the threat to the United States as defined by the probability of a strike will remain relatively constant rather than increase.

In fact there is some evidence for this in the historical record of U.S. hurricane counts which show no long term trend but a tendency for a smaller ratio of landfall counts to basin-wide counts. The differential influence of improvements in observing technologies on landfall and total counts tends to confound attempts to understand this tendency as noted in Elsner and Kara (1999). Moreover, conditional on the phase of the NAO, there are statistically significant positive relationships between Atlantic sea-surface temperature (SST) and both U.S. hurricane counts (Elsner and Jagger 2006) and insured losses (Jagger et al. 2007).

Hurricane evidence

The debate on hurricanes and climate change can sometimes devolve into issues of data reliability. Unfortunately some of what is said about these issues is nonsense, or worse, self serving. As one example, during the middle 1990's, the high priest of NOAA's best-track data argued vehemently that the hurricane intensities during the 1950's and '60s were biased upward. I checked with my colleague Noel LaSeur, who flew into these early storms, and he said "If anything, we underestimated the intensity" suggesting a possible downward bias. Noel is correct. With this light, the intensity of the hurricanes of 2004 & 2005 is not that unusual against the backdrop of the formidable mid century hurricanes. Enthusiasts and partisans should not be tinkering with these data. Moreover, while it stands to reason (a priori) that the historical information will be less precise than data collected today with modern technologies, to ignore these earlier records is scientifically indefensible. Inspired by Edward Tufte recommendations for truth-telling in graphical presentations (Visual Explanations, Graphics Press, 1997), I suggest that one way to enforce data standards is to insist that the original, unprocessed data be posted alongside the manipulated data, and that the manipulators and their methods be identified.

Hurricanes and climate change

The World Meteorological Organization has just released their consensus statement on tropical cyclones and climate change which mentions that because of the rapid advances being made in this area findings may be soon superceded by new results. Please consider joining us for the First International Summit on Hurricanes and Climate Change to hear all about the latest discoveries.