I've taken the following info from an Italian website and it helps to explain in particular the Brewer Dobson Circulation. Talks about the QBO and solar influence on Strat. Please pardon some translation errors.
SOLAR ACTIVITY and QBO
With regard to solar activity, the situation is quite poised and therefore difficult to interpret. We know that the activity of our star, despite being "near" the phase of maximum, remains relatively low values, with the solar flux that oscillates almost periodically for months between 100 and 140. In an attempt to better frame the "sunshine state", let us understand some of the mechanisms by which solar activity affects the Earth's atmospheric circulation, referring to the top of world research (Durkenton, Hood, Labitzke, Salby and Callaghan etc. .. ). direction and intensity of equatorial stratospheric winds tropo-play a key role in the modulation of the Polar Vortex winter (VP), and therefore the climate at mid-latitudes.
Direct evidence of this comes from Quasi Biennial Oscillation (QBO), which corresponds precisely to a periodic oscillation of the stratospheric winds in the tropics: the evolution of these winds, as is well known, is one of the leading "regulators" intensity of the VP. Now, without going into too much detail (return to talk about these interesting topics in a more appropriate location), in the last few years has been identified a mechanism by which solar activity can influence the performance and strength of the equatorial stratospheric winds , thus interfering heavily on the strength of the VP. This mechanism is based on the variation of the emission of ultraviolet rays between maximum and minimum solar and its interaction with the production cycle of stratospheric ozone. In this regard it is necessary to clarify beforehand two fundamental concepts:
1) in relation to solar radiation, the only village that varies "heavily" between maximum and minimum solar ultraviolet light (also 7.6 percentage points), while all other fractions tend to vary significantly lower quantities of (the TSI in the cycles of the twentieth century to more than 0.1%).
This figure shows the emission intensity of ultraviolet (UV) during cycle 22 and 23. As can be seen, between maximum and minimum solar, there are significant variations (of the order of 6%).This fact led major research centers worldwide to focus on ultraviolet radiation to explain the very short-term climate change that occur in the northern hemisphere over the years characterized by low solar activity (as happened in recent years);
2) most of the ozone production occurs in tropical stratosphere, where it is stronger and is always present solar radiation. Ozone is created in this region as it is here that the sun, this all day and throughout the year, it is more intense flows solar (UV) break oxygen molecules (O2) into oxygen atoms (O), which react rapidly with other O2 molecules to form ozone (O3). All these reactions are highly exothermic, leading to radiative heating of the high tropical stratosphere, where there is the main area of training ' ozone. The main consequence of the radiative heating the thermal gradient is positive with increasing height (in contrast to what occurs in the troposphere), and therefore an increase in the stability of the stratosphere same. Ultimately, the reduction of the amount of ultraviolet radiation that occurs in the years of low solar activity, is due to a reduction in radiative heating: this makes the stratosphere colder and unstable (it reduces the thermal gradient positive with increasing height).
This fact, by binding with the thermal wind, produces a weakening of the zonal wind (U) in the mesosphere-stratosphere high tropical going to interact with the westerly regime of the SAO (semi-annual wind oscillation), which precisely in the months in which form the stratospheric polar vortex reaches its maximum value (October-November). The abnormal weakening of the SAO is very important for the following reasons: • it was noted that the weakening of the zonal stratospheric winds in 'high tropical stratosphere is associated with a weakening of the zonal wind within the Stratospheric Polar Vortex (VPS), particularly during the period in which they recorded their maximum (solstice ' winter) . In other words it has been observed that statistically, when the zonal wind in the mesosphere / high tropical stratosphere are less intense, the VPS tends to be weaker in the first part of the winter;• there is a relationship between the regime of the SAO and the QBO. Specifically, in the years of QBO negative, the weakening of the SAO produces, over the entire column stratospheric, twenty easterly of greater intensity (absolute value of the QBO higher) with a consequent increase in the duration of the phase.
It is no coincidence that all the episodes of QBO strongly negative (values below -23/-24) were recorded only in the years of low solar activity. these circumstances (increased intensity and duration) are essential given the importance that covers the QBO negative action of disturbance to the detriment of future Stratospheric Polar Vortex. In this regard it should be noted that when the wind regime is stratospheric tropical eastern tropical easterlies tend to restrict the width of the planetary wave-guide in the extratropical lower stratosphere, favoring a larger amplitude wave and a lower phase velocity.The result is an increase in wave propagation in the stratosphere, thus heating and slowing down the VPS.discovery of these dynamics, although constitutes a major step forward in the understanding of the phenomena of "transmission" of the solar signal, does not yet allow to explain fully the real mechanism of coupling between the high stratosphere tropical lower stratosphere-troposphere tropical and polar stratosphere.
It is no coincidence that the simulation models ("GCM simulations"), while showing consistent results with theoretical predictions, are still substantial differences with the experimental measurements. This suggests that there are additional retroactive phenomena can strongly amplify the response of the atmosphere with respect to the primary signal "photochemical" Variable-induced solar, thus playing an important role in coupling: one of these is definitely the Brewer-Dobson circolation (BDC).
In the past we have already had occasion to speak of this "fascinating" meridian circulation. Briefly recall that the BDC, so named for its discoverers Brewer and Dobson, is a slow circulation in the stratosphere and hemispheric agent arranged along the meridians. This circulation is responsible for the movement of air particles from equatorial regions up to the polar regions and is most active in the northern hemisphere. In particular, this movement is characterized by ascending motions in the equatorial regions and descending motions in extratropical areas (especially in the northern hemisphere polar). The action of the BDC produces some basic effects: • thanks to the vertical transport of chemical species and southern, mainly including ozone, BDC greatly influences the chemistry of the polar bear.transport of ozone to the north pole ricompre, including other things, a great importance for the fate of the second half of winter (mid-January onwards), since, with the arrival on the pole of the first solar radiation, ozone absorbs most of the solar ultraviolet radiation and the returns in the form of heat, favoring the development of phenomena of stratwarming and making the VPS weaker; • the vertical motions associated with the BDC have important consequences for the distribution of temperatures in the stratosphere.
Indeed, due to the action of the BDC, the tropical tropopause is the coldest region in the troposphere and stratosphere. This is because the air in the tropics lifts cools to adiabatic expansion, bringing the temperatures of the tropical lower stratosphere well below the equilibrium temperature radiative local. In this regard, since the BDC is stronger during the northern winter, the strength of the air lift (upwelling) in the tropics, and then the low temperature of the tropopause trope-equatorial, presents an annual cycle, with values records during the 'northern winter.
On the contrary, in the polar region, the descending air is heated by adiabatic compression, bringing the temperatures in the stratosphere polar to several tens of degrees above the local radiative equilibrium. The latter circumstance favors the heating and greater "instability" of the stratosphere polar also in the early stages of winter
Figure constitutes a schematization of the BDC. diagram of operation of the BDC is quite complex.
In the first analysis we might expect a mechanism type Hadley cell, in which the circulation originates from the tropics solar heating and cooling in the polar region and is characterized by a large transport of warm air ascending (tropical) towards the colder regions (where the air goes down).
Actually BDC been closely related to the action of planetary waves (Rossby waves) in the extratropical stratosphere. Indeed, when stationary planetary wave reaches the stratosphere, deposits its time Esterly, decelerating the jet stream which is westerly stratospheric winter. On these occasions the polar vortex slows down and can also be moved. The deposition of momentum in the east polar stratosphere and the resulting slowdown in the winter polar jet is known as "breaking wave". This condition produces friction the stratospheric sudden warming phenomenon. The result is a situation that is thermodynamically unbalanced.
At this point, to restore the balance radiative, starting from high stratosphere quickly starts a process of cooling. The cooling air is accompanied by movements within, as the cooler air is denser and sinks. And it is precisely this movement that determines the movement of air along the meridian from the equator to the pole hemisphere winter . In fact the air descending in the polar region must be balanced by a flow of air towards the moving toward the poles. Requirements for earth continuity, this air must come from the tropics. The BDC is therefore that cell movement in which the tropical air moves towards the poles to replace air descending to the poles.
Now that we have more clearly one of the key mechanisms of solar activity, along with the QBO modulates the intensity of the polar jet, we are certainly capable of better managing the current situation. Indeed, we understand that, to be able to decipher ' current "solar puzzle", the parameter that should be monitored with particular attention to the intensity of ultraviolet radiation arriving on earth. To do this we consider the flows at wavelengths equal to 205 nm, as are those which can penetrate up to an altitude of 30 km, or up to the border area between the mesosphere and the high stratosphere tropical (this is the altitude where there is maximum production of ozone).
Below is a graph then from which you can infer the performance of UV rays on the last year: To be able to "quantify" the intensity of the current flow, we use as comparing the performance of UV rays recorded during the solar minimum in between cycles 22 and 23 (1995-1996): As you can see we are practically at the same level, so we can say with reasonable confidence that the current situation may be considered more from minimum to solar maximum.Responding to this trend of UV rays, the QBO is clocking negative values of respect. Specifically, in reference to the proportion of 30 hPa, for two consecutive months (July and August) was marked by an almost record (-28), while in the next phase (September-October), despite the passing of the peak, the QBO remained still at very low levels (around -25). E 'likely that the current cycle of the QBO negative is particularly long. In fact, in reference to the amount of 30 hPa, if the change of sign you will have in February (as one might expect), the current cycle will be composed of no less than 18 consecutive months of easterly regime (QBO-). As regards instead the QBO to share 50hPa, what is practically certain that the change of sign will occur to winter completed. So we can conclude that, even as regards the QBO, the situation in view of the winter is decidedly positive.
Finally, Always with reference to the QBO, we wanted to make you note that the current situation is very similar to the one it had in the period 1984-1985. Even at that moment the QBO in 30hpa did mark a significant peak (still -28) at the end of the summer (in that case in September).Obviously we can not use this information as an indication of the progress of the coming winter, but surely it is a "coincidence" to watch, especially considering that in that period the solar activity appeared weak.