From: stvjas@meteor.wisc.edu (Stephen Jascourt)
Newsgroups: sci.geo.meteorology
Subject: Re: Relationship of violent weather to greenhouse effect
Date: 30 Sep 92 23:39:32 GMT
Organization: University of Wisconsin, Meteorology and Space Science

In article <1992Sep30.192607.28327@vexcel.com> dean@vexcel.com (Dean Alaska) writes:
>Most of this discussion seems to be about whether there already are more
>or stronger hurricanes because of global warming.  Since the long-term
>threat of global warming is not directly tied to a possible current
>warming, maybe we could talk about hypothetical warming in the future.
>
>Can generalizations be made about the frequency and or strength of
>hurricanes if the IPCC projection for warming happens (1.5 to 4.5 C)?
>
>		dingo in boulder	(dean@vexcel.com)

The maximum strength a hurricane can achieve depends on how low it can reduce
the pressure in the eye, which depends on how much it can warm the air, which
in turn depends on the water content it has available, which is primarily
determined by the sea temperature. Therefore, assuming the mid and upper
troposphere warm by smaller amount, increased subtropical ocean temperatures
will increase the maximum strength POSSIBLE for a hurricane to achieve. 

Next, we have to ask what fraction of hurricanes will come close to achieving
that strength. That depends on the flow conditions around the incipient
disturbance. To the extent that global climate change causes different flow
regimes (or different frequency of particular flow regimes) in the mid and
upper troposphere over the subtropical oceans, this presents an important
factor. Since we don't know how this will be yet, this important factor is
an unknown. ENSO events tend to produce lower-latitude strong westerly
jets, which shear apart developing hurricanes, reducing the number of
Atlantic hurricanes; but in the Pacific, the warmer waters allow hurricanes
to exist where they normally wouldn't (such as Iniki hitting Hawaii).
But GCM's indicate global warming will primarily increase high latitude
temperatures, not affect low-latitude temperatures as much, and this would
decrease the mid-latitude temperature gradients, hence weaker jets across
the Atlantic, more favorable conditions for hurricane development. Until
we know these kinds of characteristics of what the climates of the future
will be like, we cannot properly address this issue.

Additionally, there is the question of how many incipient disturbances there
will be in the first place (or how many strong enough disturbances). In the
Atlantic, this depends largely on how wet the Sahel is. With global climate
change altering the predominant flow patterns somewhat, this may change or
some other feature may play a more important role in the strength of the
easterly waves coming off of Africa, and their latitude may change somewhat
as well-- these are all unknowns of great importance in answering your
hurricane question.

In summary, increased sea temperatures alone would increase the frequency and
strength of hurricanes, but other changes may act to offset (or perhaps
enhance) that effect, and these other changes are big ??? now. 

Stephen Jascourt   stvjas@meteor.wisc.edu
Newsgroups: sci.geo.meteorology