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 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.