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| 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 |
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| Global warming causes heavier
rainfall |
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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. |
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| Carbon dioxide (CO2) is one of
the major factors influencing the number of tropical cyclones |
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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.
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- Simulated number of tropical cyclones when separately varying
the sea surface temperature and CO2 concentrations (Annual global
average)
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| Toward the next IPCC report |
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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 |
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