Weatherlawyer wrote:
Check out the storm centre just south of Greenland slated for mid day
on the 26th (just after the new phase, the day before.)
http://www.westwind.ch/?link=ukmb,ht...racknell+13 2

Which new phase is an intense spell of wet weather for the UK: 25th
July 04:31.

Hopefully it will force the present conditions to shift into gear.

THe North Atlantic Oscillation

Atlantic Rhythms

I cut about a third of the original article as posted by
EarthObservatory on July 11, 2003
http://earthobservatory.nasa.gov/Study/NAO_200307/

Unlike El Niño, which occurs cyclically every three to seven years,
the NAO appears to fluctuate randomly on a yearly basis. There are,
however, slow variations in the NAO that point to an influence outside
the atmosphere.

A large difference in pressure between the mid-latitude and tropical
North Atlantic tends to produce more severe weather in the North
Atlantic, increased snowfall in Sweden and an early spring in
Washington, DC.

Typical weather caused by a small pressure difference between the
mid-latitude and tropical North Atlantic is fewer storms overall, clear
skies over the Normandy coast of France and rain in Spain.

"H" and "L"s on weather charts, show what air is pushing down on the
surface of the Earth at a given point. Generally, high and low pressure
systems form when air mass and temperature differences between the
surface of the Earth and the upper atmosphere create vertical currents.

The "L"s or Lows suck air from the surface of the Earth, decreasing the
air pressure. In "H"s or high-pressure systems, air is being pushed
down causing an increase in air pressure. Lows and Highs are giant
slow-moving cyclones and anti-cyclones, respectively.

The higher in pressure a high-pressure system gets or the lower in
pressure a low-pressure system gets, the more robust and larger this
spinning circulation pattern becomes.

Winds around a low spiral counter-clockwise (in the Northern
Hemisphere, clockwise in the Southern Hemisphere) and upwards towards
the center of the system.

At sea level, air is pushed away from a high pressure system. The winds
rotate clockwise (in the Northern Hemisphere, counter-clockwise in the
Southern Hemisphere) and away from the system's center.

The North Atlantic Oscillation is a variation in the semi-permanent
low-pressure system over Greenland and Iceland and a semi-permanent
high-pressure system exists over a group of islands roughly 900 miles
west of Portugal, known as the Azores.

The high and the low are usualy mild, and their influence on the
Atlantic basin climate is ideal. However, this can change. The pressure
systems grow much more intense and begin to fluctuate from week to week
between two different states.

In one state, a positive NAO, the high-pressure system grows especially
high, while the low-pressure system grows especially low, creating a
large pressure difference between the Azores and Iceland.

In the other, a negative NAO, the high-pressure system weakens and the
low becomes shallow, creating a milder pressure difference between the
two regions of the Atlantic.

During a strong positive NAO, the two pressure systems can just about
cause all the currents in the northern half of the northern Atlantic to
spin counterclockwise and all those currents in the southern half to
spin clockwise.

[Actually a major edit there, as the ideal weather conditions persist
in winter or summer whent the Lows and Highs maintain their singular
status. It is when they leave their normal stations that these so
called oscillations occur.]

Though the impact of the NAO and its phases can be felt across the
entire Atlantic and the surrounding continents, its greatest effect is
on the storms passing into Europe. Between the two swirling, clockwise
and counterclockwise circulation patterns created by the high and low,
there is an area where they come together and form a steady,
forward-moving current that channels weather systems from the United
States to Europe.

When the pressure difference between the two systems is large (a
positive NAO index), the winds along this conduit pick up, and they
push the storms north towards Scandinavia and northern France. When the
pressure difference is small (a negative NAO index), the storms take a
more direct course from the southern United States to southern Europe,
the Middle East, and northern Africa.

The direction these storms take cause remarkable changes in the
temperature and the weather over Europe from December through March. A
positive NAO on average can increase rainfall in northern Europe by a
little over an eighth of an inch per day and warm the air 3 degrees
Centigrade.

If the condition persists for most of the winter, it can lengthen the
growing season by 20 days in Sweden, lower reindeer populations in
Norway, lead to water shortages in the Fertile Crescent and provide
sunnier, drier conditions for tourists on the French Riviera.

A negative NAO, on the other hand, will bring rain to southern Europe,
drop the temperatures in northern Europe and maintain the already warm
climate across the Mediterranean. If the negative state persists, it
will increase the production of olives
and grapes in Greece, put Denmark in a deep freeze, and create ideal
skiing conditions in Austria.

The NAO's effects could also be felt to a lesser degree in the United
States. When the NAO is classically positive, the high-pressure system
residing near the Azores strengthens. The winds rotating around the
system expand and push warm air from the tropical Atlantic and the
Caribbean northward. On the west side of the Azores high you have warm
air advected to the Caribbean and up onto the East Coast, creating a
warmer winter along the mid-Atlantic States.

The result is typically less snowfall for the Washington-New York
corridor. During a negative NAO, the high-pressure system grows weak
and winter storms, and cold weather heads south.

If the NAO was strongly positive the warm temperatures from the Azores
high counteracted the colder temperatures

From week to week, the NAO flip-flops between positive and negative
phases seemingly at random, sending good and bad weather intermittently
to both southern and northern Europe. Yet, each winter the NAO almost
always shows a predominantly negative or
positive average for the year. When these yearly averages are put into
an index and plotted next to one another, a clear pattern emerges.
Since the 1960s, the entire index has overall been growing more
positive. Despite its regular appearance, the NAO is still too erratic
to predict by looking at a chart of its history.

The only way scientists could forecast the dips and peaks of the NAO is
if they first understood exactly what was causing the two pressure
systems to vary relative to one another.

[I rather think I have identified the main factor, thank you.]

The irregular sinusoidal pattern exhibited by the NAO requires some
type of climatic memory. For the NAO yearly averages to climb upward or
downward over several consecutive winters, there would have to be some
mechanism in the atmosphere or the ocean that keeps track of where the
Azores high and the Icelandic low were the year before. But atmospheric
currents change in temperature and density so rapidly over time that
there is no way they could maintain a pattern into the spring and
summer months after the low- and high-pressure systems break up.

The current thinking is the NAO's variation must be tied to the land
or the ocean.

[D..OH! And they almost had it in their hands.]

Several years ago scientists made a breakthrough when they confirmed
through the use of computer models that part of this climatic memory
driving the NAO lies in the deeper ocean temperatures of the Atlantic
and changes in these temperatures are largely responsible for
variations in the NAO.

Mark Rodwell, a climate researcher at the Met Office in the United
Kingdom, was one of the researchers who made the connection. Based on
this earlier work, he is now using similar models to make forecasts on
the sign of NAO nearly one year in advance.

[Sorry Mark but it has to go pear shaped and join its ancestors as is
the way of all flesh ATM. But the following fits nicely with what I
found in the Shipping Forecast:]

Although the pressure difference between Iceland and Lisbon (known as
the NAO Index) varies from day to-day, the average index for a single
winter is generally positive or negative. From December 2002 through
March 2003 the average for the past winter was -0.56.

[But loses it he]

The NAO is responsible for the path of strong storms that pass across
the Atlantic, and these strong storms influence the temperatures of the
ocean. By the spring of each year, the NAO has left a deep mark on the
temperatures of the Atlantic.

During the summer, these ocean temperatures are largely preserved
because a relatively thin layer of water heated by the sun covers the
ocean beneath like a thermal blanket. When the following winter rolls
around, the warm layer is removed, revealing the sea temperatures from
the previous spring, which in turn affect air pressure over the
Atlantic and the next NAO.

Rodwell obtains the average sea surface temperatures for the current
May from satellite readings, as winter storms have dwindled and the
thin layer of summer water has yet to cover the ocean and runs the
simulation forward through the next winter to obtain a forecast of the
average values for the NAO for January through March of the following
year.

"Using the forecast, we'd expect to get the sign of the NAO with
66 percent accuracy. This is better than the 50 percent chance you'd
have without any forecast at all," Unfortunately, this year, the
model wasn't quite on target. It predicted a slightly positive NAO to
occur over the winter 2003.

[Or to put it another way a 10% improvement is not quite a third
instead of an half but it can also mean that the people relying on it
can be caught with their trousers down.]

Researchers predict the state of the NAO by looking at the North
Atlantic's sea surface temperature in the spring. Prior to 1999, the
"predictions" were based on historical data. These predictions are
correct about two thirds of the time.

For over 100 years, since reliable pressure readings of the Atlantic
began, the yearly average NAO values remained within a set range -the
peak negative and positive NAO values never went far above or below
what they had been in years past.

Then about 30 to 40 years ago the entire NAO index started to become
more positive. The peak NAO years began growing more positive and the
negative NAO years began growing less negative.

The overall increase in the index has corresponded to a noticeably
longer growing season in Europe and milder winters in the mid-Atlantic
region of the United States. This past winter has been one of the few
exceptions.

Lately, Hurrell's research into the NAO has focused on uncovering the
cause of the upward trend. Such a trend would suggest that something
bigger than ocean temperatures or currents in the Atlantic, something
acting on a global scale, was pushing at the NAO.

For now, the prime suspect is global warming.

[AAAAARRRRRGH!]

Over the past 30 years a correlation has existed between the rise in
the North Atlantic Oscillation and an increase in temperatures in the
Indian Ocean. Within the climatology community, it's fairly well
established that the increased temperatures and rainfall in the Indian
Ocean is due directly to global warming produced by greenhouse gases.

[Rubbish in Rubbish out removed.]

There are simply too many random variables that influence the NAO on a
yearly basis.