With winter not too far ahead, it is worth keeping an eye on the stratosphere once again to see how the conditions there may influence the tropospheric conditions below.
As usual, it is worth giving a brief explanation of the stratosphere and how it influences the troposphere for those who are not familiar with the concept.
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 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.
Last year the start of the stratospheric winter saw the stratosphere cool down far less than recent years. This was probably due to an enhanced Brewer-Dobson Circulation (BDC) which resulted in more ozone being transported from the tropical stratosphere to the polar stratosphere and a warmer polar stratosphere as a result.
One of the reasons the BDC was enhanced was because the solar minimum coincided with an easterly or negative QBO. The Quasi-Biennial Oscillation (QBO) is a major influence on the stratosphere. This is a tropical stratospheric wind or gravity pulse that has a rough two year cycle. This wind descends from the top of the stratosphere towards the troposphere in an either westerly or easterly direction. This year the QBO is in a westerly phase. The westerly phase flows in the same direction as the polar vortex circulation which does nothing to inhibit the flow, and the BDC is also reduced in a westerly phase. Therefore this year the polar vortex is likely to be stronger than last and as a result high latitude blocking is less likely.
We are already entering the start of the stratospheric winter and the temperature of the polar stratosphere will need careful monitoring in the coming weeks.
We are at a junction that can be seen from the following link that could already influence December. Looking at last year we see that the stratosphere was at record warm levels for the month of November which set the pattern of winter. We are already cooler than at any point during last November.
My guess is that we will see the stratosphere cool more in line with average this November, and it will be difficult to get any sustained northern blocking this December.
One thing that we can hope for though is a Sudden Stratospheric Warming (SSW) or also known as Major Midwinter Warmings (MMW).These are 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 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. In January 2009 we experienced a record breaking splitting SSW that was responsible for the pulse of easterly snow some areas received in February (directly from the split) but it did not lead to any major northern blocking. Last year we received much northern blocking and split polar vortices as a result of the warmer early season stratosphere. This created numerous splits in the lower stratospheric and tropospheric vortex, which displaced the vortex off the North Pole allowing pressure to build in this region. I remember Steve Murr commenting how we never see pressure rises over Svalbard in early December, only for one such split to occur and create these conditions soon after.
For those interested in further reading on the stratospheric polar vortex, here is some excellent further reading which is very comprehensive.
Other essential sites
ECM (from 1/11 hopefully) - http://strat-www.met...n/diagnostics?1
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
My main hope for the stratosphere this winter is that we may get a SSW that may lead to sufficient vortex disruption such as we saw in Feb 2009.
Edited by chionomaniac, 29 October 2010 - 21:58 .