Guide To.....teleconnections

[Paul]

The NAO-- whats it all about and why is it important.......

For those who don't know what those "Highs" and "Lows" stand for on the charts here is a brief description of them..

Air pressure is a measure of how much 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. In a low-pressure system, these vertical winds travel upwards and suck air away from the surface of the Earth like a giant vacuum cleaner, decreasing the air pressure above the ground or sea. This decrease in surface air pressure in turn causes atmospheric currents moving parallel to the surface of the Earth near the base of the low to spin counter clockwise (clockwise in the Southern Hemisphere). Conversely, in a high-pressure system, air is being pushed down on the ground like a vacuum put in reverse. The downward vertical winds cause an increase in air pressure on the ground and force atmospheric currents to spin clockwise (counter clockwise in the Southern Hemisphere). Both lows and highs function like giant slow-moving hurricanes 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.

A low pressure system will pull in air from the surrounding area. Winds around a low spiral counter-clockwise (in the Northern Hemisphere, clockwise in the Southern Hemisphere) and upwards towards the centre of the system

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 centre

In terms of the NAO

A permanent low-pressure system exists over Greenland and Iceland, and a permanent high-pressure system exists over a group of islands roughly 900 miles (1400 kilometers) west of Portugal, known as the Azores. For most of the year, the high and the low are mild, and their influence on the Atlantic basin climate is minimal. When winter hits, however, all of this begins to change. Both pressure systems grow much more intense and begin to fluctuate from week to week between two different states. In one state, which scientists call 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 state, known as 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. As the low and high intensify and relax, the winds revolving around their centers increase and decrease in both strength and in extent. 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.

Here Illustrated below is the 2 MAIN states of the NAO ( The third being neutral..)

What it all means to us......

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 there by roughly 5 degrees Fahrenheit (2.8 degrees Celsius). 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.

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, , and create ideal skiing conditions in Austria.

The PNA or Pacific North American Pattern is characterized by atmospheric flow in which the west coast of North America is out of phase with the Eastern Pacific and Southeast United States. It tends to be most pronounced in the winter months.

There can be 3 defined states of the PNA- rather like the NAO- We have Positive, Negative and Neutral.

When we are in a neutral phase or a 'High index' Phase of the jet stream the typical west to east flow of Low pressure systems remains largly Un-interupted- ( this pattern is generally I 'Think' indicitive of a global high index phase)..

When in either a Strong positive or negative phase this has dramatic effects on the weather type distribution of the continent of the USA,and ultimately A FINGER in the 'Pie' of the weather type over the British Aisles....

The 2 main states of the PNA here demonstrated below-

In a positive phase ( Demonstarted in the image below) we have a mild western ridge and a cold eastern trough, the net result is the whole of the Eastern Seaboard is plunged into a colder winter with Artic air being dragged south and then south east out of Central Canada-

Above we have the October to January departures from normal of temperatures. Temperatures were as much as 3 degrees above normal near the ridge in parts of the west and 3 degrees below normal in the trough in parts of the east.......

Another Sypnotic illustration of the Positive PNA pattern here just shows how much of an influence over the Eastern Seaboard the PNA is...

ABOVE This pattern is delivering a series of bitter arctic cold air masses, with the greatest impact in the north central and northeast states. Meanwhile, the subtropical jet stream is again increasing. The increased subtropical jet stream will be the conveyor belt for the southern low-pressure systems that can deliver heavy precipitation (rains and snows).

The total Link between the PNA and NAO / our weather cannot be summed up as When the PNA is positive the UK is mild etc as- as we have all found there can still be inconsistencies...

We do however know that in a Positive Phase the energy that is thrown into the jet stream from the extreme cold streaming off the eastern seaboard does encourage 'DEEP" cyclogenesis- and I 'Think' this is where we can begin to make a few tentative links-

From my own observations-

* When we have a positive Phase of the PNA and the NAO stays positive a VERY strong period of Zonal weather is Likely for the british Aisles- this pattern is common for the formation of the dreaded 'Bartlett' high as we have a powerful jetstream across the Atlantic and a strong icelandic low-

* When the PNA is positive and the NAO stays Negative which is a relatice 'Rarety' there is an opportunity for the UK to sucumb to some severe weather- because the Cyclogenesis still begins to occur in the atlantic but the Low pressure systems are 'steered' either to the North up around the Western side of greenland or south into Spain and italy- again I Think there were similar type patterns in the Winters of 47,63,79- However I cant be sure.

* When the PNA is Negative mild weather prevails across the Eastern side of the US with 'Less' energy being thrown into the jet- leaving it substantually weakened and probably more scope for Blocking across Europe- Again the NAO will still need to be Negative (as does any situation regarding Snow for the UK-)

As I have already mentioned just because these patterns occur it doesnt Guarentee snow as we have to consider other important factors like 'Exact' Location of any blocking within a Negative NAO- Also Current SST's determining the exact location of the polar front. As well as for the British Aisles one other Teleconnection Pattern called the POL ( the Poalr/Eurasia Pattern)is very important and in January is more Prominent for Us than the NAO- and Of which I have the least information on, only to say though again the more negative this indices is the less 'low' anomalies

are observed in the regions where we want High pressure to build.

Finally, The blocking models- Which are an experimental output by NOAA -

The series of maps show the 500 hPa height field along with the corresponding anomalies (colored) from the 22 year climatology (1979-2000) for their current 00Z MRF forecast verifying at the indicated time. At the top of each panel the blocking index strength is shown in units of (m/deg lat).

Here is the current outlook for the next 5 days-

The information that can be extracted form these is very useful in determining the strength and activity of the jet stream over the period.. If you combine this with the teleconnection pattern outlook you can begin to build up a very good picture of whats likely to happen in the 'bigger' picture.

These images are really No good for painting detail for localised events but will show more vividly the general trends better than say the Euro model outputs.....

I hope this can shed some light on this subject.......

Steve