Tropical cyclones are becoming more powerful with the most dramatic increase occurring over the North Atlantic. The increase is correlated with an increase in ocean temperature. A debate concerns the nature of this increase with some researchers attributing it to natural climate fluctuations while other researchers attributing it to anthropogenic increases in forcing from greenhouse gases. A Summit on Hurricanes and Climate Change held May 27-30, 2007 at the Aldemar Knossos Royal Village in Hersonissos, Crete brought together leading academics and researchers to discuss the issues and to address what research is needed to advance the science of hurricane climate.
The Summit was hosted by Aegean Conferences and supported by the Bermuda Institute for Ocean Sciences (BIOS) Risk Prediction Initiative and by the U.S. National Science Foundation. It was organized to provide a venue for encouraging a lively, spirited exchange of ideas. In this spirit, it was appropriate to convene at the birthplace of the Socratic method. This volume is a collection of research papers from participants of the Summit.
Tropical cyclones are typically analyzed as a passive response to climate forcing: the hurricane as a product of its environment. A warm ocean provides sustenance, a calm atmosphere nurturing, and a subtropical high-pressure cell forward direction. An increase in oceanic heat will raise a hurricane's potential intensity, yet an increase in shearing winds could counter by dispersing the heat in a fledgling storm. This perspective is useful for identifying the mechanisms responsible for making some seasons active while others inactive. A point of emphasis at the Summit was that statistical modeling is superior to data analysis (trend lines, etc) as it avoids cherry-picking the evidence and provides a framework for making use of older, less reliable data.
For example, a Poisson distribution is useful for modeling tropical storm counts over time. The benefit of a statistical approach is that it provides a context that is consistent with the nature of the underlying physical process, analogous to the way the laws of physics provide a context for studying meteorology. It was shown at the Summit that smoothing (filtering) the hurricane count data introduces low frequency patterns that may not be significant and that a statistical model of Atlantic hurricanes indicates a recent upswing in the number of strongest hurricanes with little or no multidecadal variation.
Although the question of whether we can ascribe a change in tropical cyclone intensity to anthropogenic climate change (attribution) is still open, it was argued based on statistical models for extreme winds that the difference in hurricane intensity for storms near the U.S. coast between globally warm and cool years is consistent in sign and magnitude with theory and simulations. In this regard it was noted that the discrepancy between numerical model results and observations is likely due to a reliance on data analysis rather than statistical models.
The collective role that hurricanes play in changing the climate was another point of emphasis at the Summit. Over the Atlantic Ocean, heat and moisture transport out of the tropics by an ensemble of hurricanes moving poleward in a given season was shown to have a detectable influence on the baroclinic activity at high latitudes the following winter, which in turn influences the preferred hurricane track type (recurving or straight-moving) during the subsequent hurricane season. Thus a communication between the tropics and the middle latitudes on the biennial time scale is accomplished through tropical cyclone track changes and middle latitude baroclinicity. This finding has important implications for financial markets because it provides a way to hedge risk through diversification.
Also, the relationship between global warming and ENSO was explained in terms of warming rather than warmth. A warming planet is associated with more El Nino events, which on the biennial time scale leads to cooling. These are intriguing hypotheses about climate change and tropical cyclones that merit further investigation. It was also shown that super typhoons in the western North Pacific need a deep ocean mixed layer for rapid intensification only in regions where the sub surface water temperatures are marginally supportive of tropical cyclone intensification. It was demonstrated that high aerosol concentrations lead to an invigoration of the convection in tropical cyclones through enhancement of the ice/water microphysi-cal processes inside the clouds.
Another important theme of the Summit was paleotempestology—the study of prehistoric storms from geological and biological evidence. For instance, coastal wetlands and lakes are subject to overwash processes during hurricane strikes when barrier sand dunes are overtopped by storm surge. The assumption is that during landfall the waves and wind-driven storm surge reach high enough over the barrier to deposit sand in the lake. In a sediment core taken from the lake bottom, a sand layer will appear distinct from the fine organic mud that accumulates slowly under normal conditions. Sediment cores taken from the northeastern Caribbean show more sand layers during the second half of the Little Ice Age when sea temperatures near Puerto Rico were a few degrees C cooler than today providing some evidence that today's warmth is not needed for increased storminess. Not surprisingly intervals of more hurricanes correspond with periods of fewer El Nino events. It was shown that sedimentary ridges in Australia left behind by ancient tropical cyclones indicate activity from the last century under represents the continent's stormy past. It was argued that proxy techniques based on oxygen isotopes from tree rings and cave deposits show promise for studying prehistoric tropical cyclone events because of the signature left in the annual layers by the isotopically lighter tropical cyclone rainwater.
It was mentioned that a spatially limited set of proxies or historical records are not able to resolve changes in overall activity from changes in local activity due to shifts in tracks. While the northeastern Caribbean region is in the direct path of today's hurricanes, was it always? Network analysis of hurricane activity might be able to shed light on this question. The answer is important as more hurricanes locally could mean changes in steering rather than changes in abundance. Proxy data from the U.S. Gulf coast show a pattern of frequent hurricanes between 3800 and 1000 years ago followed by relatively few hurricanes during the most recent millennium which is explained in terms of the position of the subtropical North Atlantic High. Moreover it was shown that recent increases in typhoon intensities affecting Korea can be explained by an eastward shift in the subtropical North Pacific High allowing the storms to recurve over the warmer waters of the Kuroshio Current rather than over the colder subsurface waters of the Yellow Sea. In order to understand how climate influences local changes in tropical cyclone activity, it was remarked that more research is needed to identify factors influencing tropical cyclone tracks.
Results from high-resolution numerical models, including a 20 km-mesh model, were consistent in showing stronger tropical cyclones in a warmer future. Most models indicate an overall decrease in the number of storms, attributable in one study to greater atmospheric stability and a decrease in the vertical mass flux. Not all models agree on the change in individual basin numbers with some models showing an increase in the Atlantic and others a decrease. It was shown that models without tropical cyclones remove the oceanic heat in the tropics through stronger trade winds. It was noted that models may be better at identifying changes to the large-scale genesis fields and that models still do not have the resolution to be useful to society. Climate model projections can be downscaled to construct tropical cyclone climatologies using a method that combines rejection sampling by numerical models to determine genesis points with simple physical models for storm motion and winds. A few participants focused on the perception and politics of tropical cyclone risk in a changing climate.
This volume provides a cross-section of the topics that were covered during the Summit. It is broadly organized around study type with empirical analyses first followed by statistical models, then by numerical simulations.
We would like to extend our gratitude to the following individuals for helping with the review process: George Kallos. Kevin Hodges, Constantin Adronache. Anastasios Tsonis. Kyle Swanson. Fabrice Chauvin, Auguste Boissonnade, Bob Rohli, Claudia Mora, William Read, Kevin Walsh, Kerry Emanuel, Byron Daynes, and Greg Holland. Special thanks goes to Robert Hodges for his help with copy editing.
James B. Elsner Thomas H. Jagger
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