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Global Warming and Hurricanes
The strongest hurricanes in the present climate may be upstaged by even more intense hurricanes over the next century as the earth's climate is warmed by increasing levels of greenhouse gases in the atmosphere. Most hurricanes do not reach their maximum potential intensity before weakening over land or cooler ocean regions. However, those storms that do approach their upper-limit intensity are expected to be slightly stronger -- and have more rainfall -- in the warmer climate due to the higher sea surface temperatures. And more recent work with more comprehensive models incorporating hurricane-generated "cool SST wakes" continue to support these conclusions.
Figure 1
According to a simulation study by a group of scientists at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL), a 5-12% increase in wind speeds for the strongest hurricanes (typhoons) in the northwest tropical Pacific is projected if tropical sea surfaces warm by a little over 2 degrees C (Figure 1). Although such an increase in the upper-limit intensity of hurricanes with global warming was suggested on theoretical grounds a decade ago, this investigation is the first to examine the question using a hurricane prediction model that is being used operationally to simulate realistic hurricane structures.
Fig. 1. Maximum surface windspeeds for the hurricanes simulated for control (thin line) and high CO2 (thick line) conditions.
Figure 2
The NOAA group simulated samples of hurricanes from both the present-day climate and from a greenhouse-gas warmed climate by linking information from GFDL's global climate model into the high-resolution GFDL hurricane prediction model (Figure 2). This hurricane prediction model is currently the operational hurricane prediction model that has been used successfully by NOAA's National Centers for Environmental Prediction to predict tropical storm tracks for the last several hurricane seasons. The GFDL climate model is one of the leading models used by climate researchers to project possible effects of greenhouse gases on future climate.
Fig. 2. Top: a tropical storm as simulated in the global climate
model. Shown are surface temperature (shading), pressure and winds. Bottom:
the same storm case, but as simulated with the hurricane prediction model.
Shown are surface winds and precipitation on the inner grid of the hurricane
model. The vector spacing illustrates the resolution of the two models
(250 km for the global model vs. 18 km for the hurricane model.)
This research provides an example of the use of high performance computing to provide important new information regarding the potential impact of global climate change upon future weather systems. The research was published by T. Knutson, R. Tuleya and Y. Kurihara in the Feb 13, 1998 issue of Science, with a detailed follow-up paper in Climate Dynamics (1999, vol. 15).
Figure 3
In more recent work, scheduled to appear soon in the Journal of Climate, NOAA scientists Knutson and Tuleya teamed up with Isaac Ginis and Weixing Shen of the University of Rhode Island to explore the climate warming/ hurricane intensity issue using a more advanced hurricane modeling framework. The new model incorporates a full ocean model beneath the hurricane to simulate the "cool SST wake" generated by the moving storm (Figure 3). The model with this ocean coupling effect included simulates a similar percentage increase of hurricane intensity under warm climate conditions as the original model without ocean coupling.
Fig. 3. Sea surface temperatures (SSTs, light contours and color shading, in degrees Celsius) and sea level pressure (dark contours, in millibars) from an idealized coupled hurricane model/ocean model experiment. The "cool wake" in SSTs produced by the hurricane is indicated by the lower SSTs to the east-southeast of the storm. The storm motion is toward the west-northwest.