Monday, 28 October 2013

Stratospheric Ramblings

The current state of the atmosphere is such that we saw a pretty robust MJO wave over the west pacific. This, combined with building pressure over asia, has helped produce a burst of west pacific typhoons over the last two weeks. This has triggered a positive feedback loop as these typhoons have distributed latent heat and negative potential vorticity anomalies downstream. This has helped pump the west pacific ridge and contributed to a strong bering sea trough( -EPO/WPO pattern). So why does this have any significance in relation to the strat? Stagnant bering sea troughiness and a very potent EAMT event(trigerred by tropical forcing over the W Pac) has really helped trigger a wave 1 response over NE siberia. Mean H5 pattern in the last 7 days shows the -WPO/-EPO setup.

Currently, the stratosphere is in a strengthening state which it should be at this time of year. As the heat source(EAMT + W PAC typhoons) for this warming fades, the vortex should rebuild stronger then it was before. -AAM anoms propagating equatorward will also help rebuild the vortex, with a potentially long period of a +AO coming up. In any case, it is a good sign to see such a strong fight against the vortex in a west QBO year. I suspect this won't be your typical +QBO year as we are in a westerly shear phase which promotes more pertubations to the vortex. In addition, we have a more poleward the normal 0 u line, with a greater then normal build up of ozone over the north pacific. These factors should make a major warming easier this winter.

The analog years I have been using for the stratosphere are 85-86, 01-02, 80-81, 90-91 and 08-09 which all have a neutral ENSO state(with the exception of 08-09), a +QBO shear phase and low solar activity. All these wintera had some form of a SSW event with the exception of perhaps 85-86 which didn't make as much of an impact at the surface. I'll give 2001 and 2009 as examples, with a 2-3 wk lag for the peak H5 response.
2009: compday.Ck0LXBcuJI.gif   H5 Reflection:compday.it2zFiklwJ.gif
Notice how the cold signal is muted across NA while their is a very strong -VE anomaly across western europe.
2001 had the opposite pattern with warmth in W europe and cold anoms across NA:
500mb setup 2-3 weeks later


If we look at all +QBO shear years, without isolating for solar or ENSO we get the following H5 setup. The important feature to note is the strong East PAC ridging and bering sea trough which would help promote a mid/late winter wave 1 response as well. The euro monthlies have also been hinting at strong GOA ridging, which combined with aforementioned factors make a good case for a SSW this winter.

So the point being made here is that you can see significant SSW's in downwelling +QBO years, especially when other factors are favourable for development. Although significant SSW's can occur, placement of the coldest anomalies at 500mb will only favour some locations across the N Hem, whether its europe, asia or NA.

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