Archive of Spotlight Feature Articles

Global Warming and Hurricanes

Geophysical Fluid Dynamics Laboratory

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 in the warmer climate due to the higher sea surface temperatures.

According to a new 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°C (Figure 1). Recent preliminary findings indicate that these results may apply to the other tropical cyclone basins as well. Such an increase in the upper-limit intensity of hurricanes with global warming was suggested on theoretical grounds a decade ago, but the NOAA investigation is the first to examine the question using a hurricane prediction model that can simulate realistic hurricane structures.

GFDL modeling results... indicate that both sea surface temperatures and the atmospheric temperature profile in the environment around the storm are important factors in determining how strong a tropical cyclone can become.

Fig 1: Maximum surface windspeeds for the hurricanes simulated for control (thin line) and high CO₂ (thick line) conditions.

In the present day climate, the maximum observed intensities of tropical cyclones appear to be correlated with the underlying sea surface temperatures, with the strongest storms observed over relatively warm ocean waters. However, the GFDL modeling results, as well as theories of maximum tropical cyclone intensities, indicate that both sea surface temperatures and the atmospheric temperature profile in the environment around the storm are among the important factors in determining how strong a tropical cyclone can become. Therefore, assessing the possible influence of global warming on tropical cyclone intensities involves accounting for the influence of both warmer ocean waters and of changes in the atmospheric temperature profile.

Fig 2 Top: a tropical storm as simulated in the global climate model. Shown are surface temperature (shading), winds and sea level pressure. 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.)

To address this problem, the NOAA group used the high-resolution GFDL hurricane prediction model to "telescope in" on a selected sample of storm cases from long (120 year) simulations of the GFDL global climate model (Figure 2). The coarsely resolved storms in the global model were replaced by more realistic storm initial conditions in the hurricane model using a procedure analogous to that employed for operational hurricane predictions at NOAA's National Centers for Environmental Prediction. Importantly, the large-scale environments from the global climate model were retained and allowed to influence the development of the storms in the regional model.

A large sample (n=51) of individual storm cases were then simulated for present day climate conditions and compared with a second sample of 51 cases under greenhouse gas-warmed climate conditions. The comparison showed a 5-12% increase in surface wind speeds for the the high CO₂ storms (Figure 1). The simulations also showed a substantial increase (28%) in near-storm rainfall in the high CO₂ cases (Figure 3). Such changes in wind intensity and precipitation, if they occurred, could have important societal consequences.

The approach of linking the large scale environments from the global climate model together with the high resolution hurricane model was necessary because the global climate models currently used for greenhouse warming studies have a spatial resolution that is too coarse (with model gridpoints spaced typically 250 km apart) to simulate the most intense hurricanes or certain features of hurricanes such as the eye. Future efforts will involve incorporating other possibly important effects in the simulations, such as allowing the storm in the hurricane model to influence the underlying sea surface temperature through mixing of cooler ocean waters up to the surface. The present study also does not address the issue of possible future changes in the frequency of occurrence of tropical cyclones.

Fig 3: Six-hour accumulated rainfall in the vicinity of the storm center (within about 180 km) for hurricanes simulated under control (thin line) and high CO₂ (thick line) conditions.

The GFDL hurricane prediction model used for the study is currently the operational hurricane prediction model at NOAA's National Centers for Environmental Prediction and has been used successfully 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.

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 results were published by T. Knutson, R. Tuleya, and Y. Kurihara in the Feb 13, 1998 issue of Science and in an upcoming issue of Climate Dynamics.

(Image of Hurricane Mitch courtesy of NOAA/NCDC.)

model image of Hurricane Mitch

View an mpeg animation
of a GFDL hurricane model supertyphoon (1.96mb)

The Geophysical Fluid Dynamics Laboratory (GFDL), located in Princeton, New Jersey, is engaged in comprehensive mathematical modeling research in the primary areas of the National Oceanic and Atmospheric Administration (NOAA) mission, such as weather forecasting, climate prediction, and ocean services. Its fundamental goal is to expand the scientific understanding of those physical processes that govern the behavior of the atmosphere and the oceans as complex fluid systems. Its practical goal is to improve predictive skill for weather and climate in a wide variety of key NOAA emphasis areas. For example, modeling research at GFDL is focussed on the following: new approaches for forecasting hurricanes and mid-latitude severe weather; short-term forecasts of the coastal ocean environment; prediction of El Niño events and their weather implications a year in advance; understanding and prediction of stratospheric ozone depletion; and improved prediction of global and regional climate changes associated with human activities such as fossil fuel burning, chemical releases, and land use practices.

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