On 11/09/11 22:11, Graham P Davis wrote:
On Sun, 11 Sep 2011 20:30:47 +0100, Adam Lea wrote:

After the very destructive 2004& 2005 hurricane seasons there was some
suggestion that a warming climate was increasing the intensity of
tropical cyclones. The general consensus now is that in a warmer
climate, tropical cyclone numbers globally will decrease

The "suggestion" that a warming climate would increase the number and
intensity of cyclones predates 2004 by a few decades. With increasing sea
temperatures I'd expect this to happen and for them to spread their area
of influence.

Is the counter-argument something to do with the upper troposphere being
forecast to warm more than lower down and so reducing instability?
Against that, the stratosphere is cooling as expected and so I'd expect
the depth of the troposphere to be increasing and hence, depth of
instability.


One counter-argument is that it is not just the surface that would warm,
but the whole depth of the troposphere thus the vertical instability
would remain unchanged and the sea surface temperature threshold for
tropical storm formation would increase.

Another counter-argument is a (fairly) recent paper suggesting (via
climate model studies) that vertical wind shear will increase over the
Atlantic in a warming climate (something to do with changes in the
Walker circulation, I believe).

http://www.eurekalert.org/pub_releas...-aci041607.php

The last few years, hurricanes seem to have been hampered by shear more
than I've seen before. Has that got anything to do with the forecast
reductions?


Well last year was one of the most active on record, it was just no
hurricanes hit the U.S. 2009 was suppressed by a combination of Saharan
air outbreaks and an El Nino. 2008 was active and destructive. 2007 was
an oddball, much less active than it should have been given La Nina
conditions (I think that was due to wind shear). This year, it seems to
be reduced vertical instability that is hampering developing storms.
There are plenty of storms forming, just they are struggling to
intensify. This was similar to the case in early-mid August 2010 where
there was a break in TC activity until the lid came off in the third
week (that was due to reduced vertical instability as well).

We have just gone back into a La Nina so it may be we get some enhanced
late season activity this year (and a possibility of going into the
Greek alphabet again).

I have to say this years hurricane season is one of the most ridiculous
I have seen - 14 named storms but only two hurricanes so far. It also
looks like, since Irene, the U.S. hurricane deflector shield is back in
place with Atlantic storms recurving and TC Nate being driven into Mexico.