Quasi-Biennial Oscillation

The quasi-biennial oscillation [QBO] is a quasiperiodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere with a mean period of 28 to 29 months. The alternating wind regimes develop at the top of the lower stratosphere and propagate downwards at about 1 km (0.6 mi) per month until they are dissipated at the tropical tropopause. Downward motion of the easterlies is usually more irregular than that of the westerlies. The amplitude of the easterly phase is about twice as strong as that of the westerly phase. At the top of the vertical QBO domain, easterlies dominate, while at the bottom, westerlies are more likely to be found. At the 30mb level, with regards to monthly mean zonal winds, the strongest recorded easterly was 29.55 m/s in November 2005, while the strongest recorded westerly was only 15.62 m/s in June 1995.



The QBO what happens now?

The QBO went wonky last year if you remember, and this is the first time I’ve thought to have a look and see if and how it’s recovering. Cycle 30 should have finished 18 or 29 months after it started in April 2014, that would be around the start of Autumn last year as far as I can make out. The cycle took a very early nose dive last Spring, but since then has recovered to a maximum of +15.09 mps in December 2016. It now looks like it’s on the decline again, but of course the longer it takes to right itself, the longer cycle 30 will be extended (fig 1). At the moment Cycle 30 has lasted 36 months, and will last a few more months before the equatorial winds flip and enter the negative easterly phase. Please feel free to correct me on any of this, I am no expert on the QBO, and there seems to be little news about it that I can pick up about it out on the internet. I don’t know what caused what once was a very regular cycle to become as disrupted. I think it maybe a symptom, or maybe it’s the cause, of the unusual mild winter last year across North America, but I’ll freely admit that I’m guessing.

Figure 1 – Data courtesy of ESRL/NOAA PSD

Figure 2 – Data courtesy of ESRL/NOAA PSD

Met Office see a cold start to the winter…

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Courtesy of Met Office

I don’t know why I’m getting carried away with the latest news from the Met Office about a cold start to the beginning of winter 2016/17, but I must admit I am a little. It must be the little boy in me that just wants to see the world turn white, at least for a few days or so, preferably over the Christmas holidays. I love the infographic they use in their long-range forecasting it says very little in effect, but apparently because the QBO is in an easterly phase and totally out of phase, this is expected to weaken the westerlies over the North Atlantic and allow blocked conditions to occur more and hence more winds from the north and east. One thing about the Met Office is it knows how to take advantage of social media in all it’s forms, I can just see them getting warmed up down at the Daily Express at this very moment!

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Courtesy of the Met Office

 

The curious case of QBO cycle 30

qbo-jul-2011-sep-2016

Last month I wrote a simple application to download and graph monthly quasi-biennial oscillations [QBO] values from Physical Sciences Division [PSD]. I wrote an article about it, and that has more details about what it the QBO is and why it’s important. The values for September 2016 have just been released , and cycle thirty of the QBO (as far as my list of cycles is concerned) is still stuck in its westerly phase that started in June 2015. The problem with that is that the QBO should have flipped and be well into it’s E’ly phase by now. The problem started in April 2016 when the QBO index suddenly increased when it should have been decreasing and about to enter its E’ly phase, and that increase has continued till now September 2016, although it does look like the levels of increase has flattened out. Although the QBO has usually behaved itself since it was identified with a quite regular period of 28 or 29 months, this last cycle has gone a little haywire, and it’s now in its thirtieth month.

Quasi-Biennial Oscillation

There a number of quasi-biennial oscillations, but the best known is probably the one that measures the average zonal wind [QBO]. I can’t do any better in describing what the QBO is more than the Wikipedia article does, so here it is in a nutshell:

The quasi-biennial oscillation is a quasiperiodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere with a mean period of 28 to 29 months. The alternating wind regimes develop at the top of the lower stratosphere and propagate downwards at about 1 km (0.6 mi) per month until they are dissipated at the tropical tropopause. Downward motion of the easterlies is usually more irregular than that of the westerlies. The amplitude of the easterly phase is about twice as strong as that of the westerly phase. At the top of the vertical QBO domain, easterlies dominate, while at the bottom, westerlies are more likely to be found.

I’ve downloaded the data file of monthly QBO values from Physical Sciences Division [PSD] who compute it from the National Centers for Environmental Prediction [NCEP] and National Center for Atmospheric Research [NCAR] reanalysis data. The series starts in 1948, and is the average 30mb zonal wind at the equator. Negative values indicate an easterly component, and positive values a westerly component, the higher or lower the value the stronger the component. The easterly phase is usually stronger than the westerly phase, and last a little longer. In the graph below the red area indicates the positive westerly phase of the QBO, and the blue the negative easterly phase.

quasi-biennial-oscillation-qbo

I have written code to find the start and end of a complete east-west cycle. I have counted 30 complete cycles since 1948. The first one started mid way into the easterly phase so I didn’t count that, in fact the early fifties looks a little odd with very weak westerly events. Cycle eight was very short, and because it did fit the rules of the code that I use to identify the cycles I have counted it, even though it’s less than six months long.

Curious cycle thirty

Cycle thirty at the top of the list is the westerly phase of the QBO that we are currently in and is also behaving rather strangely, even though this cycle hasn’t finished quite yet.

quasi-biennial-zonal-wind-oscillation-qbo-january-1948-august-2016

Here’s a close up of the last few years QBO and you will immediately see how cycle 30 took an early nose dive in April of 2016, but since then has staged a comeback, when in reality it should be just about to switch to the easterly phase of the QBO. This is the subject of a lot of debate at the moment especially when the QBO has been relied on in the past for indicating how severe winters in Europe will be, and the frequency of North Atlantic hurricanes.

quasi-biennial-oscillation-qbo-january-2007-september-2016

Cycle length

I thought I would generate a graph of the length of the past 30 cycles just to see how much they vary. I have omitted out of this chart so that the linear trend  I have added isn’t too badly skewed. As you can see from the chart below the QBO is far from clockwork, and varies from 20 to 36 months over the last 68 years, but averages around 27 to 29 months as far as I can make out.

qbo-cycle-length-january-1948-august-2016

QBO and central England temperatures

One final chart I thought I would generate was a correlation between the monthly QBO and the monthly central England temperature [CET] anomalies. I’ve included the chart even though I can’t say that it proves very much at all!

qbo-cet-correlation-january-1948-august-2016