Adam Lea wrote:


"Graham P Davis" wrote in message
...
Adam Lea wrote:


"Yokel" wrote in message
...
Another major reason for the low hurricane activity was higher wind
shear
this season, which disrupted many of the storms before they had a
chance to
become full-blown hurricanes.

This isn't true. I have looked at the average August-September 200-850mb
wind shear over the Atlantic main development region (10-20N, 20-60W)
and it is actually below average this year. If you look at the tropical
cyclone formation probability throughout this summer:

http://rammb.cira.colostate.edu/proj...ent/tatfpr.gif

you will notice that it has been well above climatology virtually all
summer and is similar to 2005 and 2004. August-September Atlantic sea
surface temperatures have been warmer than average and trade winds have
been weaker than normal and together have been consistent with seasons
with twice as much activity as this year. I have also looked at mid
level relative humidity and that has been slightly unfavourable this
summer but was similar to 2005 and 2004. Overall it seems the reason why
the activity
this year has been supressed is a mystery.


That's odd. Hurricane activity this year was expected to be reduced by El
Nino. Part of the reasoning behind this was an associated increase in
shear. However, another El Nino effect, the eastward displacement of
hurricane tracks, has occurred.


Yep - it's odd. In other El Nino years the low level trade winds are
stronger than normal which is correlated with reduced cyclonic vorticity
over the tropical atlantic and thus reduced hurricane activity. That is
not the case this year.

I've been having another think about this. What I've seen this season with
the development - or non-development - of tropical storms in the Atlantic
and East Pacific seems typical of an El Nino year. East Pacific storms have
been moving eastwards towards the USA and Atlantic storms have been
diverted away from the Caribbean and East Coast of the US. Any Atlantic
tropical depression, if it hasn't been pushed N then NE, has been destroyed
by SW shear.

A difference I've seen between this year and last is that last year the
development of storms and hurricanes occurred in the west of the area
you've looked at for shear and some formed in the Caribbean, which is not
in your area. This year, most if not all development has been in the east
of the area. Although shear in your area may be lower overall than usual,
in the important part of that area, and to the west of it, where sea
temperatures are highest, I suspect that shear has been higher than usual.

One small depression just east of the Caribbean at the moment is moving
north and being demolished by strong SW upper winds. Actually, if
90L.Invest is included, it makes two being destroyed, as the one I was
watching is to the SE of 90L.Invest.

--
Graham Davis
Bracknell