Exact Reason(s) for Cold Snap

slayer91

I have been reading the threads here and find them very interesting. However, I don't see any threads discussing the reasons for the unusal length and serverity of the cold snap.
I read it is due to the shift of the Gulf stream and Altantic current away from Ireland allowing much colder air streams from the North and East.

Any of the experts care to expand???

Regards

nacho libre

Arctic oscillation. This is a measure of north-south differences in air pressure between the northern mid-latitudes and polar regions. When the Arctic oscillation is positive pressures are unusually high to the south and low to the north. This helps shuttle weather systems quickly across the Atlantic, often bringing warm, wet conditions to Europe.

however, the Arctic oscillation has dipped to astoundingly low levels - among the lowest observed in the past 60 years. This has fupped up the hemisphere's usual west-to-east flow with huge ''blocking highs'' that route frigid air southward and as a result our airflow is coming from the east and north east whereas in normal situations it's a prevailing south westerly airflow over.

k123456

I'm prob way off the mark here but interesting doc on BBC a couple of years ago

As the ice melts in sends lots of cool water currents, these travel down the Atlantic and block the effect of Gulf Stream.

In their sceanario, it was a regular climatic occururence

mike65

Its the North Atlanic Oscillation which is our version of el Nino, it pushes the jet stream south so the med and even North Africa gets the fronts we usually get.

d'Oracle

mike65 wrote: »

Its the North Atlanic Oscillation which is our version of el Nino, it pushes the jet stream south so the med and even North Africa gets the fronts we usually get.

We need a cool name like "el nino".

How bout, d'Young fella.

snow ghost

slayer, as well as the qualified responses above check out this from the UK Met Office:

http://www.metoffice.gov.uk/corporate/pressoffice/2010/pr20100106b.html

"What’s causing the cold weather?

In most winters, and certainly those in the last 20 years or so, our winds normally come from the south-west. This means air travels over the relatively warm Atlantic and we get mild conditions in the UK. However, over the past three weeks the Atlantic air has been ‘blocked’ and cold air has been flowing down from the Arctic or the cold winter landmass of Europe.


The low temperatures in the UK have also been accompanied by snow. This is because areas of low pressure have been running in from the north-east, tracking across the North Sea and picking up moisture along the way, which falls as snow.

However, it is not cold everywhere in the world. North-east America, Canada, North Africa, the Mediterranean, and south-west Asia have all seen temperatures above normal – in many places by more than 5 °C, and in parts of northern Canada, by more than 10 °C.




Fig 1. The map shows that while it has been cold in Northern Europe, other parts of the world have seen above average temperatures.

Is it colder than average?

The mean UK temperature for December was 2.1 °C, making it the coldest for 14 years and colder than the long-term average for December of 4.2 °C. However, December was one of only two months in 2009 which had a below-average mean temperature.
What does this say about climate change?

Climate change is taking place as the earth continues to warm up.
In the UK, 2009 as a whole was the 14th-warmest on record (since 1914). This above-average temperature trend was reflected globally, with 2009 being the fifth-warmest year on the global record (since 1850).
The current cold weather in the UK is part of the normal regional variations that take place in the winter season. It doesn’t tell us anything about climate change, which has to be looked at in a global context and over longer periods of time."

Pa ElGrande

The North Atlantic Oscillation : El Nino's Young Cousin

Prof. Martin Beniston

Director
Department of Geography
University of Fribourg
Switzerland


The North Atlantic Oscillation (NAO) is one of the large-scale modes of variability in the climate system. As its name indicates, the NAO is centred on the North Atlantic Ocean basin. Here the atmospheric circulation normally displays a strong meridional (north-south) pressure contrast, with low pressure in the northern edge of the basin, centred close to Iceland, and high pressure in the subtropics, centred near the Azores. This pressure contrast drives the mean surface winds and the wintertime mid-latitude storms from west to east across the North Atlantic, bringing warm moist air to the European continent. It has long been observed that the monthly and seasonal (particularly wintertime) averaged sea level pressure in stations in Iceland and the Azores, display an out-of-phase relationship with one another. More precisely, there is a tendency for sea level pressure to be lower than normal in the Icelandic low pressure centre when it is higher than normal near the Azores and vice versa. This fluctuation is referred to as the NAO. It is related to noticeable changes in monthly and seasonal averaged wind speed and direction over the ocean, and concomitant changes in the paths of wintertime storms and their effect over the ocean and Europe. The NAO is the dominant mode of atmospheric variability in the North Atlantic sector throughout the year, but it is most pronounced during the winter season.
<snip>
The NAO seems not to be a stationary stochastic (or deterministic) process in the time scales that are common in climate research. Appenzeller et al. (1998) showed by means of wavelet analysis, that in a 1400-year simulation of the ECHAM3 General Circulation Model (GCM) developed at the Max-Planck-Institute in Hamburg, as well as in ice-core data, the dominant frequencies of the NAO-index changes in time. One frequency in the NAO-index of ECHAM3 could be attributed to a coupled ocean-atmosphere mode which projects into the NAO-index (Timmermann, 1998).

Another indication that the NAO can change its regime is the strong positive trend of the index since the late 1960s. During this latter part of the record, an 8-year oscillation may be observed

This trend in the index may come from a very significant mechanism in a changing climate which GCMs must reproduce if climate projections in the North Atlantic region are to have any meaning. (see also section 2.2.1.3)

M.T. Cranium

In "climate science" sometimes "reasons" for things are actually just different descriptions of things. Example, somebody says "it has been cold because the jet stream took a southward turn." These are just two different ways of describing the same thing. And to some extent, all of the reasons offered so far in this thread are like that. An "arctic oscillation" may be a real physical process but it does not explain cold, it is the cold. You now have to explain the arctic oscillation to get a real, total cause and effect explanation.

In my long-range forecast (7 Nov, forecast thread) I talked about this being a colder than average winter and even gave some fairly accurate guidance on when (late December, mid-January was how it read there, should have been late December, first half January perhaps). I think there may be a few more cold days to come, 31 Jan to 3 Feb is the most likely time for them especially if we see strong storms developing 27-30 Jan as expected by me.

But since I did make this not-inaccurate prediction of lengthy cold, I'll mention my reasons for it, whether you accept them or not, these are the reasons why I thought it might be a cold winter.

Atmospheric ridges and troughs, in my research, reflect the positions in the solar system magnetic field, of "sectors" of somewhat different solar flux. There is (again according to my research) a direct correlation between eastern North American temperature and strength of field sectors, as if our magnetic field is processing this input and delivering it to the strongest sector of the magnetic field (the current position of the zero line for the magnetic compass is similar to what I call timing line one or the mean position of the cause and effect of the warming.)

In this system, western Europe represents conditions in the solar system magnetic field about 30% of the orbital circumference behind the earth, in other words, your set-up will represent what the earth passed a few months previously, but also what is catching up to the earth in terms of faster-moving sectors. These correlate with retrograde activity, highs moving westward against the general west to east flow. For these reasons, given a high retrograde index expected in late December and early January, I went with the idea of strong blocking in the North Atlantic sector followed by a second wave after a brief breakdown. The brief breakdown turned out to be almost nothing, it was that period from about 26 to 30 December when another component of this model, lunar declination, tried to lift the block and run low pressure across the sector. That worked out to be a brief milder spell and rainstorm that turned back to sleet and snow (30-31 Dec), and my original idea of about 3-4 more mild days was crushed by the continuing block. Probably the second wave of blocking is better represented now by the developing high over Russia, but other factors in the model correlated with mild zonal flow have come into play and we have the standoff situation where the retrograde cold cannot make it this far west.

So in other words, the cause and effect behind the cold is blocking, and the cause and effect behind blocking is (in my opinion) an inter-related response of atmosphere to magnetosphere, and magnetosphere to solar system magnetic field sectors. All of the other ways of describing this fail to identify a predictable external factor that could be used in the future to make other predictions.

The retrograde signal comes earlier next winter so we'll get to test this out with what will likely be a prediction of cold weather in early to mid December. I have not really looked into this in any detail so will promise to make a prediction around mid-September for the winter season to follow.