Effects of global warming on typhoon climatology

Tropical cyclones, including typhoons and hurricanes, are generated over tropical oceans and produce strong winds and rain as they develop. Tropical cyclones are one of the important areas addressed by the Global Warming Research Group in the Global Warming Research Program. Mr. Yoshimura explains the effects of global warming on the frequency of tropical cyclones and precipitation patterns.

Mr. Jun Yoshimura
Researcher, Global Warming Research Program

Global warming causes heavier rainfall

 
Our research group applies a global (atmospheric) climate model in which the Earth's surface is divided into a grid of approximately 100 km squares to study the various effects on tropical cyclones caused by global warming. We have conducted a number of numerical experiments, which include one that uses two types of cumulus convection schemes, and another that applies multiple patterns of different sea surface temperature distributions. In each experiment, we obtained results indicating that global warming causes an approximate 20% reduction in the number of tropical cyclones.

We also estimated the change in precipitation near the core of the tropical cyclone due to global warming. As atmospheric stability*1 is increased by global warming, a tropical cyclone cannot obtain a sufficient updraft in its core unless more latent heat*2 is released. In 1999, we presented results indicating that tropical cyclones with the same intensity (wind speed) would have 10-30% more precipitation than present-day tropical cyclones. This result might explain one of the reasons for the decrease in the number of tropical cyclones: since greater thermal energy is required for cumulus convection activities to form them, there are fewer instances in which atmospheric conditions are favorable for the formation of tropical cyclones.

 
Carbon dioxide (CO2) is one of the major factors influencing the number of tropical cyclones

 
In 2001, we presented our experimental results on the effect of separately varied sea surface temperatures and CO2 concentrations (see graph). When the sea surface temperature rose, there was no major difference in the number of tropical cyclones. On the other hand, when the atmospheric CO2 concentration increased without changing the sea surface temperature, the number of tropical cyclones dropped significantly. This result was unexpected, since we believe that the sea surface temperature is the most important factor affecting the climate over tropical oceans. To clarify the reason, we studied the average precipitation at all times across the entire tropical region. We found that the higher the CO2 concentration, at magnitudes of two- or four-fold increases, the greater the decrease in precipitation. This indicates reduction in the total energy of latent heat release that is available for tropical cyclone development, and it is a possible reason for the decrease in the number of tropical cyclones.

  • Simulated number of tropical cyclones when separately varying the sea surface temperature and CO2 concentrations (Annual global average)
 
Toward the next IPCC report

 
It is important to thoroughly understand changes in tropical cyclones in order to recognize the increased risk potential from natural disasters and to take effective countermeasures. The IPCC Third Assessment Report published in 2001 stated that it is 'likely' that tropical cyclone peak wind and precipitation will intensify over some areas. On the other hand, for the number of tropical cyclones, the IPCC summary reads only that "changes in tropical cyclone location and frequency are uncertain," and no conclusion is given, although the results of our study are cited in the main text of the report. We will use more high-resolution models in our experiments and devise analytic methods so that our research results will be reflected in the conclusions of the IPCC Fourth Assessment Report in 2007.

*1 The atmosphere becomes unstable when there is warm air (light air) near the Earth's surface. On the other hand, when the upper air is warm, the atmosphere becomes stable. We obtained experiment results indicating that global warming causes an increase in air temperature near 500hPa that is 1ºC greater than the temperature increase near the Earth's surface.
*2 Thermal energy that is released when water vapor condenses into cloud droplets

Frontier Newsletter/No.23
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