Posted on October 8th, 2016 in Isaac Held's Blog
Monthly mean equatorial zonal winds in the stratosphere as a function of time and height. Eastward winds are shaded. 10m/s contour interval. This figure is updated monthly here, thanks to Marcus Kunze. (Original version created by Christian Marquardt (Marquardt, C. (1998): Die tropische QBO und dynamische Prozesse in der Stratosphäre. PhD Thesis, Met. Abh. FU-Berlin, Serie A, Band 9/Heft 4, Verlag Dietrich Reimer Berlin, 260 S.) I have highlighted the last two years.
The quasi-biennial oscillation (QBO) in the equatorial stratosphere is one of the more remarkable phenomena in our atmosphere. In a region of about 15 degrees north and south of the equator, the east-west winds change directions, from about 20 m/s eastward to 30m/s westward and back again with roughly a 27 month period. As seen in the height/time plot shown above, these alternating winds first appear with a given sign at upper levels and then descend more or less regularly before stalling and decaying near the tropopause. There is generally one eastward wind layer and one westward layer at any given time: a new layer of eastward winds appears at upper levels once the previous eastward layer near the tropopause has been squeezed away by descending westward winds. Baldwin et al. 2001 is a classic review of both observational and theoretical aspects of the QBO.
In its most recent evolution, the QBO has exhibited some strange behavior, as seen in the plot shown above — in the past year westward winds unexpectedly appeared at about 40 mb, interrupting the eastward winds in their familiar steady descent. This unusual behavior, evidently with no good analog over the period of our observations of the QBO, is discussed in two recent papers: Newman et al 2016 and Osprey et al 2016.
(Note on confusing terminology: meteorologists typically speak of easterly and westerly flow, emphasizing where the air is coming from, with easterly = westward and westerly = eastward. I am using the the eastward-westward terminology in this post.)
The theory for the QBO is one of the triumphs of atmospheric fluid dynamics. The starting point is two papers by Dick Lindzen and Jim Holton (1968 and 1972), The theory describes the evolution of a system that consists of two interacting components, a zonally symmetric jet, and vertically propagating waves generated in the troposphere by tropical moist convection, which then propagate into the stratosphere. The theory falls into the class that we call “wave-mean flow interaction theory”– the waves are assumed to be linear; the only nonlinearity is the interaction of the waves with the mean flow (the zonal winds). I usually recommend Plumb 1977 to students as the best point of entry into the theory, the essence of which goes something like this:
Some of the waves propagating upwards through the stratosphere have phase speeds that are eastward and some have phase speeds that are westward with respect to the ground. These waves have temperature perturbations associated with them and are radiatively damped as they propagate upwards through the stratosphere. The two keys to the theory, neither of which is very intuitive but follow straightforwardly from the linear wave dynamics, are —
- Where a wave is dissipated, the zonal mean flow is accelerated towards the phase speed of the wave.
- The closer the zonal wind to the phase speed of the wave, the more slowly the wave propagates vertically. In the presence of damping, this means that the waves do not propagate as far, confining their effect on the zonal mean flow to lower levels in the atmosphere.
Think of two waves with identical amplitudes at the tropopause, one with westward phase speed of 20m/s and one with eastward phase speed 20m/s, propagating upwards. And think of the zonal winds as being zero initially. Each wave accelerates the zonal winds towards it own phase speed over the region over which it is dissipated, but these two forces balance initially — in this rather artificial symmetric state. Now suppose you break this symmetry by making the zonal winds slightly eastward say. The wave with eastward phase speed now has slower vertical propagation and transfers eastward momentum to the winds in a shallower layer. The wave with westward phase speed propagates further into the stratosphere and deposits momentum through a deeper layer. The result is an eastward push to the winds in the lower layers and a westward push in the upper layers. You can iterate this kind of argument to follow the evolution as these new zonal winds modify the wave propagation and dissipation, to show that these winds will strengthen, saturating at the phase speed of the waves, while propagating downwards.
An important detail in generating oscillatory behavior is the necessity for a mechanism that mixes away the zonal winds near the tropopause — if there are eastward winds near the lower boundary of the stratosphere, while westward winds are descending and confining these eastwards winds to a shallower and shallower layer, a mechanism for destroying this shallow jet is needed to allow eastward propagating waves to bust through and propagate deeply to generate a new eastward wind layer at upper levels.See Alan Plumb’s paper linked above if interested in pursuing this.
The QBO evolves very slowly by atmospheric standards. For this picture to make sense the equatorial stratosphere has to be very well protected from waves and mixing initiated from midlatitudes. The wintertime stratosphere in particular is full of Rossby waves and turbulence that have the potential to disrupt the stately progression of the QBO if they can mange to penetrate close to the equator and mix things up. It seems plausible that the unusual evolution of the winds in this past year was the result of this protection breaking down, allowing some extratropical influence to penetrate to the equator. It is hard to construct an explanation of the zonal wind evolution over the past year, with westward winds appearing sandwiched between eastward layers,if you confine yourself to the classic picture of vertical redistribution of momentum.
Was the strong El Nino in part responsible for the unusual QBO behavior this past year? Was this behavior predictable, say, a few months in advance? Could it be telling us something about subtle trends in the stratospheric circulation that allow more extratropical influence on the equatorial winds?
The stratospheric QBO missed a beat last year.
Is this the end of civilization as we know it?
Or is it simply that the stratospheric QBO missed a beat?
(with apologies to Alan Bennett)
[The views expressed on this blog are in no sense official positions of the Geophysical Fluid Dynamics Laboratory, the National Oceanic and Atmospheric Administration, or the Department of Commerce.]