I've just brought up-to-date my spreadsheet of North Atlantic/European
sea-level pressure, and I noted this morning (I've been waiting for this to
happen for over a week) that now - very marginally - both the westerly
and southerly indices for the last 30 days over the British Isles have
flipped
negative. In other words, the mean sea-level pressure pattern shows, just,
a flow over the BI from a northeasterly quarter.

This is the first instance of a net 30-day flow from the easterly half of
the
compass since mid-April to mid-May last year, and the first from the
northeasterly quarter since April 2000.

Remembering many such months, especially in Spring, in the dim and
distant past, I guessed that this 6-year interval must be unusually long,
perhaps representing a significant change in the climate of NW Europe.
So I checked monthly data since 1950. Calendar months when both the
westerly and southerly indices were negative included:

Jan 1963 1985
Feb 1955 1969 1983
Mar 1955 1962
Apr 1978 1981 1983 1986 1989 1998 2000
May 1951 1968 1975 1977 1978 1980 1984 1985
Jun 1997
Jul 1968
Aug 1976
Sep 1976
Oct 1993
Nov 1965
Dec none

Decadal totals we
1951-60 - 3
1961-70 - 6
1971-80 - 7
1981-90 - 8
1991-00 - 4
2000- - 0

We know that post-1988 there was an increase in winter westerliness
(actually Jan, Feb, Mar, but not Dec). There are signs that this has
come to end, though of course it's impossible to say whether this is
a short-term hiccup or not. But this increased mobility was not
really apparent in other months, which makes the recent absence
of NE-ly months all the odder.

The biggest gap was between Mar 1955 and Mar 1962, but
there is no guarantee yet that this calendar month will end up with
a NE-ly gradient, so the present gap may continue.

Philip Eden

Philip Eden wrote:
I've just brought up-to-date my spreadsheet of North Atlantic/European
sea-level pressure, and I noted this morning (I've been waiting for this to
happen for over a week) that now - very marginally - both the westerly
and southerly indices for the last 30 days over the British Isles have
flipped negative.

In other words, the mean sea-level pressure pattern shows, just,
a flow over the BI from a northeasterly quarter.

This is the first instance of a net 30-day flow from the easterly half of
the compass since mid-April to mid-May last year, and the first from
the northeasterly quarter since April 2000.

I'm not certain what you are on about but the time frame was simiar for
me and the description of polarity was exact. I have noticed this
before; sometime just before I got a computer. That would have been in
the eighties. I can't be less vague than that.

"Richard Dixon" wrote :
"Philip Eden" philipATweatherHYPHENukDOTcom wrote :

We know that post-1988 there was an increase in winter westerliness
(actually Jan, Feb, Mar, but not Dec). There are signs that this has
come to end, though of course it's impossible to say whether this is
a short-term hiccup or not. But this increased mobility was not
really apparent in other months, which makes the recent absence
of NE-ly months all the odder.

Earlier this winter I looked at a box around the Azores and a box in
Scandinavian and compared relative average pressures since 1987. The
Azores
high was dominant during the 1990s but it was clear how it was less since
2000 or so. Jan 1987, predictably, was the most Scandinavian-dominated
month. I'll update this and post to the newsgroup (when I eventually get a
spare hour or two).

Yes, I agree that there has been a weakening of winter mobility, especially
noticeable in February and March which were the two months to reach
historic high levels of westerliness during the 1990s over the NE Atlantic
and NW Europe. Since then we have still had exceptionally westerly
months (eg Feb 2002, Jan 2005) but they have occurred in relative
isolation.

What I find particularly interesting is that, although we are often told
that
increased mobility (i.e. a +ve NAOi) is probably a consequence of climate
change, the British Isles westerliness index has during the last 15 years
reached or approached historic *low* levels in several months ... including,
most oddly, December.

Philip

"Philip Eden" philipATweatherHYPHENukDOTcom wrote in message
...

What I find particularly interesting is that, although we are often told
that
increased mobility (i.e. a +ve NAOi) is probably a consequence of climate
change, the British Isles westerliness index has during the last 15 years
reached or approached historic *low* levels in several months ...
including,
most oddly, December.

Philip

In Cornwall Winters this decade have been generally very quiet compared with
the 1990s. However, (this Winter excepted) the marked decrease in mobility
has not resulted in a fall of temperatures in winter, as might be expected,
but the reverse:-

Penzance
5 Winters 1994/95 to 1998/99
Av No. of Gales 11.2 Av Temp deviation from 1971/2000 LTA +0.4

6 Winters 1999/2000 to 2004/2005
Av. No. of Gales 5.3 (historically exceptionally low) Av. Temp.
deviation from 1971/2000 LTA +0.7

Graham