Once again we shall be looking at the polar stratosphere this coming season, to help give us guidance to how this may influence tropospheric conditions. As always a brief description of why this is important is provided below.
The stratosphere is the layer of the atmosphere situated between 10km and 50km above the earth. It is situated directly above the troposphere, the first layer of the atmosphere that is directly responsible for the weather that we receive. The boundary between the stratosphere and the troposphere is known as the tropopause. The air pressure ranges from around 100hPa at the lower levels of the stratosphere to around 1hPa at the upper levels. The middle stratosphere is often considered to be around the 30hPa level.
Every winter the stratosphere cools down dramatically as less solar UV radiation is absorbed by the ozone content in the stratosphere. The difference in the temperature between the North Pole and the latitudes further south creates a strong vortex – the wintertime stratospheric polar vortex. The colder the stratosphere, the stronger this vortex becomes. The stratospheric vortex has a strong relationship with the tropospheric vortex below. The stronger the stratospheric vortex, the stronger the tropospheric vortex becomes.
The strength and position of the tropospheric vortex influences the type of weather that we are likely to experience. A strong polar vortex is more likely to herald a positive AO with the resultant jet stream track bringing warmer wet southwesterly winds. A weaker polar vortex can contribute to a negative AO with the resultant mild wet weather tracking further south.
The stratosphere is a far more stable environment then the troposphere below it. However, there are certain influences that can bring about changes - the stratospheric ozone content, the phase of the solar cycle, the Quasi Biennial Oscillation ( the QBO), wave breaking events from the troposphere and the autumnal Eurasion/Siberian snow cover to name but a few.
The ozone content in the polar stratosphere has been shown to be destroyed by CFC's permeating to the stratosphere from the troposphere but there can be other influences as well. One such influences is the ozone transport mechanism from the tropical stratosphere to the polar stratosphere, known as the Brewer Dobson Circulation (BDC). The strength of this circulation varies from year to year and can in turn be dictated by other influences.
One of these influences is the QBO. This is a tropical stratospheric wind that descends in an easterly then westerly direction over a period of around 28 months. This can have a direct influence on the strength of the polar vortex in itself. The easterly (negative ) phase is though to contribute to a weakening of the stratospheric polar vortex, whilst a westerly (positive) phase is thought to increase the strength of the stratospheric vortex. However, in reality the exact timing and positioning of the QBO is not precise and the timing of the descending wave is critical throughout the winter.
The direction of the QBO when combined with the level of solar flux has been shown to influence the BDC. When the QBO is in a west phase during solar maximum there are more warming events (increased strength BDC) in the stratosphere as there is during an easterly phase QBO during solar minimum. ( http://strat-www.met...-et-al-2006.pdf )
This winter we are around the mid solar cycle and easterly QBO in the mid stratosphere - I think it is difficult to draw any firm conclusions from this.
One warming event that can occur in the stratospheric winter is a Sudden Stratospheric Warming ( SSW) or also known as a Major Midwinter Warming (MMW). This as the name suggests a rather dramatic event. Normally the polar night jet at the boundary of the polar vortex demarcates the boundary between warmer tropical and cooler polar stratospheric air (and ozone levels) and is very difficult to penetrate. SSWs can be caused by large-scale planetary waves being deflected up into the stratosphere and towards the North Pole, often after a strong mountain torque event. These waves can introduce warmer temperatures into the polar stratosphere and can seriously disrupt the stratospheric vortex, leading to a slowing or even reversal of the vortex. This can occur by the vortex being displaced off the pole – a displacement SSW, or by the vortex being split in two – a splitting SSW.
The effects of a SSW can be transmitted into the troposphere as the propagation of the SSW occurs and this can have a number of consequences. There is a higher incidence of northern blocking after SSW’s but we are all aware that not every SSW leads to northern blocking. Any northern blocking can lead to cold air from the tropospheric Arctic flooding south and colder conditions to latitudes further south can ensue. There is often thought to be a time lag between a SSW and northern blocking from any downward propagation of negative mean zonal winds from the stratosphere. This has been quoted as up to 6 weeks though it can be a lot quicker if the polar vortex is ripped in two following a split SSW.
Here are a list of sites for data on the latest state of the stratosphere:
Other essential sites
ECM (from 1/11 hopefully) - http://wekuw.met.fu-.../wdiag/diag.php
JMA - http://ds.data.jma.g...x.html#monit_nh
NCEP data- http://acdb-ext.gsfc...t/ann_data.html
The sudden stratospheric warming site - http://www.appmath.c.../ssws/index.php
So that brings us neatly on to conditions so far for this year.
Recently we have seen a period of decreased mean zonal winds at 30 hPa but this is now changing:
A cooling of the stratosphere is currently occurring with no forecast warming and ozone forecasts look low. So I would expect an increase in both the stratospheric and tropospheric polar vortices with no major blocking to occur from stratospheric influences for the next four weeks.
For the rest of winter, hopefully the stratosphere will gives us an early warning, but a SSW in January and a cold February is what I would be looking out for... we can but hope.
Edited by chionomaniac, 24 October 2011 - 16:50 .