Thursday, June 07, 2007

Summary: Summit on Hurricanes and Climate Change

An International Summit on Hurricanes and Climate Change was held May 27-30, 2007 at the Aldemar Knossos Royal Village in Hersonissos, Crete. It 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 by myself and Richard Murnane (BIOS). There were 77 participants from at least 18 countries with a mix of academics and stakeholders (insurance and risk modelers). The purpose was to gather leading researchers in the field of tropical cyclone climate for sustained discussions on the state of the science and to elevate the discourse above the fray. In this spirit, it was appropriate to convene at the birth place of the Socratic method.

Traditionally tropical cyclones are 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. In this regard it was argued that data 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 this approach is that it provides a context that is consistent with the nature of underlying physical processes, analogous to the way the laws of physics provide a context for studying meteorology. It was shown that smoothing (filtering) the count data introduces low frequency patterns that may not be significant and that a data model of Atlantic hurricanes indicates a recent upswing in the number of strongest hurricanes with little multidecadal variation. The figure above shows annual hurricane rates using a data model of basin-wide Atlantic hurricane counts. Results are shown for different categories of hurricane intensity. Note that for each year the model provides a distribution on the estimated annual rate as indicated by a box plot. The model also provides a distribution for the number of rate changes over the 63-year period (1943-2005). Note the absence of a multidecadal pattern.

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 data models for extreme winds that the difference in U.S. hurricane intensity 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 data models.

The collective role that hurricanes play in changing the climate was a point of emphasis at the Summit. In the Atlantic, 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. 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 enhancement of the ice/water cloud microphysics leading to the invigoration of the convection inside a tropical cyclone.

Another important theme of the Summit was paleotempestology. Paleotempestology is 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 provides 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. Sedimentary ridges in Australia left behind by ancient tropical cyclones indicate that activity from the last century under represents the continent's stormy past. Proxy techniques based on oxygen isotopes from tree rings and cave deposits also show promise for studying prehistoric tropical cyclone events.

It was mentioned that a spatially limited set of proxies or historical records is 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? 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 the colder subsurface waters of the Yellow Sea. In order to understand how climate influences local changes in tropical cyclone activity, more research is needed to identify factors influencing tropical cyclone tracks.

Results from various 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 need to 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 of genesis points with simple physical models for storm motion and winds. A few presentations focused on the perception and politics of tropical cyclone risk in a changing climate.

The summit was a success. All sessions were plenary and attendance averaged greater than 90%. There were 4 days of talks with each day broken into an early and late morning session of 4 to 5 speakers each. Coffee, tea, soft drinks, and snacks were served during the poster session between the oral sessions. The posters were available for viewing during the entire summit. Lunch was served on the beach after the late morning session followed by free time. In the evening participants and their company were treated to a full course meal at a local restaurant. Wednesday afternoon featured a tour of the Minoan Palace of Knossos near the city of Heraklion. The Summit concluded with a gala dinner featuring traditional Greek and Cretan music and dancing. Invited speakers were provided full accommodations. Travel awards were given to six students and two additional participants. The conference proceedings will be published by Springer as an edited volume in 2008.