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 Subject: G3) What may happen with tropical cyclone activity due to global warming? The United Nation's Intergovernmental Panel on Climate Change (IPCC) has speculated that climate change due to increasing amounts of anthropogenic "greenhouse" gases may result in increased tropical sea surface temperatures (SSTs) and increased tropical rainfall associated with a slightly stronger intertropical convergence zone (ITCZ) (Houghton et al., 1990, 1992, 1996). Because tropical cyclones extract latent and sensible heat from the warm tropical oceans and release the heat in its upper tropospheric outflow to fuel the storm's spin up, early work of the IPCC expressed concern that warmer SSTs will lead to more frequent and intense hurricanes, typhoons and severe tropical cyclones. These concerns prompted the IPCC (Houghton et al. 1990) to suggest in 1990 that:
"There is some evidence from model simulations
and empirical considerations that the frequency
per year, intensity and area of occurrence of
tropical disturbances may increase [in a
doubled carbon dioxide world], though it is
not yet compelling."
However, any changes in tropical cyclone activity are intrinsically
also tied to large-scale changes in the tropical atmosphere. As
a result, SSTs by themselves cannot be considered without corresponding
information regarding the moisture and stability in the tropical
troposphere. What has been identified in the current climate as
being necessary for genesis and maintenance for tropical cyclones
(e.g. SSTs of at least 26.5°C [80°F] - Gray
1968) would change in an enhanced doubled CO2 world because
of possible changes in the moisture or stability. It is quite reasonable
that an increase in tropical and subtropical SSTs would be also
accompanied by an increase in the SST threshold value needed for
cyclogenesis because of compensating changes in the tropospheric
moist static stability (Emanuel 1995).
In addition to the thermodynamic variables, changes in the tropical
dynamics also play a large role in determining changes in tropical
cyclone activity. For example, if the vertical wind shear over the
tropical North Atlantic moderately increased during the hurricane
season in an increased CO2 world - as what is typically seen during
El Nino-Southern Oscillation warm phases (El Nino events), then
we would most likely see a significant decrease in tropical cyclone
activity. This is due to the Atlantic basin having a marginal climatology
for tropical cyclone activity because of its sensitivity to changes
in vertical wind shear and lack of an oceanic monsoon trough (Gray
et al. 1993). In other less marginal tropical cyclone basins, changes
in the vertical shear profile typically result in alterations in
the preferred location of development (e.g.
Nicholls 1979, Chan 1985,
Revell and Goulter 1986, and
Lander 1994). These complications
along with conflicting global circulation modeling (GCM) runs compelled
the 1995 IPCC (Houghton et al. 1996)
to express greater uncertainty about the nature of tropical cyclones
in an enhanced CO2 environment:
"The formation of tropical cyclones depends not
only on sea surface temperature (SST), but also
on a number of atmospheric factors. Although
some models now represent tropical storms with
some realism for present day climate, the state
of the science does not allow assessment of
future changes."
Most recently, Henderson-Sellers et al. (1998)
addressed a few of the tropical cyclone-greenhouse warming problems.
The first is that "there is
no evidence to suggest any major changes in the area or global location of
tropical cyclone genesis in greenhouse conditions." This conclusion is
based upon Holland's (1997) thermodynamic tropical cyclone model which does
show that in a greenhouse-warmed climate there is an upward alteration
in the minimum SST from 26.5° to 28°C (80° to 83°F) needed for tropical
cyclogenesis. The additional conclusion from Henderson-Sellers et al.
(1998) suggests "an increase in [maximum potential intensity] MPI of
10%-20% [in central pressure or 5%-10% in maximum
sustained winds] for a doubled CO2 climate but the known omissions
(ocean spray, momentum restriction, and possibly also surface to 300
hPa lapse rate changes) all act to reduce these increases." This
second finding is also based upon the thermodynamic models of
Emanuel (1986) and
Holland (1997), which also appear
to corroborate similar findings for Northwest Pacific typhoons from a
"downscaled" GCM to mesoscale model approach by Knutson et al. (1998).
Henderson-Sellers et al. (1998) does not provide guidance for possible
changes in tropical cyclone frequency, mean intensity, or area of occurrence.
The most helpful paper that may predict changes in hurricane
and typhoon frequency with some realism is the recent work by
Royer et al. (1998). Based upon
alterations to the large scale atmospheric and oceanic conditions
(vertical shear, vorticity and thermodynamic stability),
they suggest that only small changes to the tropical cyclone frequencies
may result: up to 10% increase in numbers in the Northern Hemisphere
(primarily in the Northwest Pacific) and up to a 5% decrease in numbers
in the Southern Hemisphere. These values should be considered very
preliminary.
To summarize, our current assessment of how global warming may alter
hurricanes, typhoons and tropical cyclones is as follows (from
Henderson-Sellers et al. 1998,
Knutson et al. 1998, and
Royer et al. 1998):
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