Ronald, zoals vaker, weer erg interessant om je verhaal te lezen. Je verwachting deed me denken aan wat ik vanmiddag op de onderstaande site vond. Wat mij betreft zeer de moeite waard om te lezen. Waar het op neer komt dat het meest gerespecteerde forumlid er van uitgaat dat we in de herfst veelvuldig te maken gaan krijgen met hogedruk op het midden van de oceaan, waarbij de hogedruk richting november en december meer richting Groenland gaat. Hieronder heb ik wat citaten neergezet. Beter lijkt me om op de onderstaande site te kijken. Staan nl. nogal wat afbeeldingen bij zijn voornaamste bijdrage.
The relationship between QBO state and polar stratosphere during the winter is modulated by the solar cycle. Low solar activity and east QBOs are good for blocking (as per last year). Conversely, west QBOs during low solar favour no blocking. The reverse is true during high solar conditions. The mechanism is hugely complex and not one we fully understand however the Brewer Dobson Circulation (BDC)(general meridional hemipsheric circulatiuon in the stratosphere) is thought to be a key process. East QBOs under low solar conditions tend to intensify the BDC transporting more ozone to the pole. Ozone formation and presence leads to warming of the upper atmosphere through exothermic reaction. The opposite is true for the westerly QBO under low solar conditions.
A key measure of the BDC is the temperature differential between the stratospheric equator and pole during the autumn. Anomalous warm air over the equator signals desceding air which warms. Upwelling air cools. Hence, a warmer than normal stratosphere over the equator and cooling polar stratosphere signals a weakened BDC.
The west QBO / solar combination will I think bring about a slow sink of the ridge to a mid latitude feature in January and February although the later we go the more vulnerable we become to a Major Mid Winter Warming of the stratosphere, much like 2008/9. Its worth pointing out the equatorial westerly winds in the stratosphere have increased and we may now be looking at values of +10 to +20 m/s this winter. This is already having an impact on stratospheric ozone concentrations and the Brewer Dobson Circulation which would appear to be weak, even at this early stage.
Last winter we were fortunate that the stratospheric conditions were favourable right from the start of winter, with the polar vortex decreased in strength due to the enhanced BDC and -QBO having a knock on effect. When this was aligned with the 'La Nino' and the excellent NH snowcover a great winter ensued in terms of snowfall.
However, this winter I believe that we are going to start off on the wrong foot with the stratosphere. Presently, the polar stratosphere is cooling rapidly as it does every autumn. More of concern, though, is the temperature of the tropical stratosphere which is showing signs of warming and is warmer than it has been at any time last year in the run up to winter.
If this continues, then the differential between the polar and tropical stratosphere would be increased leading to a stronger vortex.
Whether or not this could disrupt the setup that you suggest that the strong La Nina will bring will be interesting to see.
Curiously, there is a paradox that as a prelude to cold winter stratospheres, we tend to see a lot of blocking early doors - almost as the upper level weakness (upwelling) leads to an increase in tropopausal height and weakened polar westerlies. Hence my thinking that the blocking ridge will migrate poleward during November and December before it falls back given an increase in the polar westerlies during January.
Yes, very much the ecxpectation that the low angular momentum base state, aided and abetted by the developing profile of SSTAs in the North Atlantic will lead to strong forcing for a mid Atlantic blocking ridge becoming shifted further northwards during November and December. It's also worth noting that analogues for tropical convection centred west of Indonesia (MJO phase 4-5) also support this solution.
http://forum.netweather.tv/topic/58354-in-depth-technical-model-discussion/page__st__391
Quote selectie