-> Mbt het afkoelen van de stratosfeer. Heeft Chio vandaag wat geschreven.
I have just been checking out some figures for the latest stratospheric temperatures and mean zonal wind profiles from the NASA site:
http://acdb-ext.gsfc.nasa.gov/Data_services/met/ann_data.html
So when looking at the temperatures the disappointing news is that at almost every level of the polar stratosphere the temperature is below average and the forecasts suggest that even further cooling is likely to take place.
So one would expect that the mean zonal winds for each level would show above average - evidence of the strengthening vortex. But alas that is not so! The mean zonal wind speed is pretty much also below average as well. So why is that?
Well my guess for this is two fold:
One and probably the main reason is due to the QBO. I suspect that we are seeing the perfect timing this winter for the easterly QBO to create resistance to the increasing polar vortex. To compare - a spinning top is less likely to gain as much speed if for every three pushes one way there is another push in the other direction - and I guess the eQBO must have some effect in creating this effect. In a westerly QBO regime we see the spinning top only spun the one way.
The second I would expect is due to the temperature contrast between the tropical and polar stratosphere. The less that this is, the less strength we see in the polar vortex.
We have still yet to see any evidence of an increased BDC this autumn - though it is early days for this. When this kicks in and we see the feedback from the Eurasion snow cover then prospects will increase.
In the meantime, looking at the surprising mean zonal wind speed values so far this autumn it demonstrates how many variables we have to take into consideration and how complex the stratospheric forecasting can be.
I think we will need to monitor the zonal mean wind speed at 60ºN very carefully alongside the temperatures, because evidence suggests so far that we can get away with a cooler polar stratosphere so far when other factors are in our favour.
Stewart voegt daar nog aan toe:
We know to our costs that not all upper events downwell, indeed,are we upwelling a negative wind anomaly from the lower stratosphere ? This would be consistent with the hard time that any cooling signal at 10 and 30 hpa is having right now.
I think the longer this goes on, with the stratospheric temperatures being prevented from free fall, the deeper into the boreal winter season we get and the greater the chance of a favourable tropospheric pattern leading to wave breaking. The pattern over the next week or so looks favourably aligned for Wave 1 activity with that deep North Pacific Low, so 20+ days to some form of disturbance to the PV ?
I think I mentioned this the other day, but I would almost want to see an upper level +ve zonal wind anomaly attempt to downwell, as it would meet a stiff resisting force with the resultant torque likely to generate some exciting activity around the mid stratosphere.
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