Keith (Southend) wrote:

With all this fuss about the impending 'cold' winter I wonder if we
could lay all the cards on the table as to the rational behind this
forcast?

Apart from the predicted -ve NAO due to a cold SST to the south of
Newfoundland and an active hurricane season, one correlation that
doesn't stack up to me, I just don't get it and I thought after a number
of years frequenting this newsgroup I'd picked up a few ideas. Even
Will's becoming a 'tease' :-)

I've wintnessed a raft of Autumn scenarios all that supposedly pointed
to a cold winter, then come the middle of December zonality takes over.
I just don't see it myself.

So what's it all about? (Alfy)


My memories of the use of SST anomalies go back almost 40 years so some of
the explanations may be a bit flaky. This was all before NAO became
fashionable, although measures of zonality were used - which amounts to
much the same thing.

The reason for the importance of the anomaly in the region south of the
Grand Banks was explained as being the main spawning ground of Atlantic
depressions. It is the area where the North Atlantic Drift and Labrador
current meet, providing a source of warm water and strong thermal contrast.
If this area is warmer than normal, Atlantic depressions become more
vigorous, cooler and they are weaker than normal.

The SST anomalies for many years were analysed and grouped into several
types. The pressure patterns for subsequent months were then analysed and
anomalies produced. Charts of the mean anomalies for each SST pattern were
produced. These included isopleths indicating the areas of highest
statistical significance. At the long-range-forecast conference the SST
anomaly matching the current month's anomaly was selected and the next
month's associated pressure anomaly was used as one of the guides to the
monthly forecast.

Here are a few patterns I remember:

Warm pool:
As mentioned above, this produces depressions more vigorous than usual. The
shape of the pool is important. If elliptical with an E-W long axis, there
will be more westerlies than usual. This can be similar to a zonal pattern.
Without this E-W long axis, the low pressure tends to be further to the NW,
over the Norwegian Sea, and the UK is affected by NW'lies. Both cases would
give a positive NAO but the latter can give cold winters - at least it did
in the sixties but I'm not sure this is possible any more.

Cold pool:
Atlantic lows are weaker. A positive pressure anomaly is situated over
Iceland with low over the Azores. 1962-3 is the classic case of this
pattern. If the pool has an E-W long axis, the high anomaly becomes a belt
from Greenland into northern Europe and Russia. This may give cold
easterlies over the UK but variations could bring mild air from the
Mediterranean.

Zonal:
A belt of warm water from the south of the Grand Banks eastward with a belt
of cold water from Labrador to the UK. Leads to an E-W belt of low pressure
between Iceland and Scotland with strong, cyclonic, westerlies over the UK.
May be expected to give wet weather but the colder water to the west of the
UK can limit precipitation. This sort of pattern would become a permanent
feature if the NAD were to close down so bitterly cold easterly winters
would be even more unlikely than they are now!

As I said, these descriptions are based on memories over 30 years old so
should be taken with a pinch of salt. Also, the patterns vary depending on
the time of year due to the changing wavelengths of the upper-air patterns
- most of the descriptions apply to winter though those patterns with E-W
long axes should more consistent through the year. Also, the SST regime and
atmosphere have changed over the past 30 years and I don't see the patterns
working as well as they used to.


--
Graham Davis
Bracknell