Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 140: 1 – 21, January 2014 A Review Article High-latitude inﬂuence of the quasi-biennial oscillation James A. Anstey a,b * and Theodore G. Shepherd a,c a Department of Physics, University of Toronto, Ontario, Canada b Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, UK c Department of Meteorology, University of Reading, UK *Correspondence to: J. A. Anstey, Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK. E-mail: anstey@atm.ox.ac.uk The interannual variability of the stratospheric winter polar vortex is correlated with the phase of the quasi-biennial oscillation (QBO) of tropical stratospheric winds. This dynamical coupling between high and low latitudes, often referred to as the Holton – Tan effect, has been the subject of numerous observational and modelling studies, yet important questions regarding its mechanism remain unanswered. In particular it remains unclear which vertical levels of the QBO exert the strongest inﬂuence on the winter polar vortex, and how QBO – vortex coupling interacts with the effects of other sources of atmospheric interannual variability such as the 11-year solar cycle or the El Ni˜ no Southern Oscillation. As stratosphere-resolving general circulation models begin to resolve the QBO and represent its teleconnections with other parts of the climate system, it seems timely to summarize what is currently known about the QBO’s high-latitude inﬂuence. In this review article, we offer a synthesis of the modelling and observational analyses of QBO – vortex coupling that have appeared in the literature, and update the observational record. Key Words: Holton – Tan effect; polar vortex; interannual variability; decadal variability; stratosphere – troposphere coupling; seasonal predictability; middle atmosphere; annular modes Received 17 September 2012; Revised 1 February 2013; Accepted 5 February 2013; Published online in Wiley Online Library 28 March 2013 Citation: Anstey JA, Shepherd TG. 2014. High-latitude inﬂuence of the quasi-biennial oscillation. Q. J. R. Meteorol. Soc. 140: 1 – 21. DOI:10.1002/qj.2132 1. Introduction The quasi-biennial oscillation (QBO) consists of alternating westerly and easterly zonal wind regimes that descend from the tropical upper stratosphere to the tropical tropopause at a rate of ≈1 km per month, with ≈28 months being required for a complete cycle (Figure 1(a)). The QBO is well known to inﬂuence the atmospheric circulation at altitudes and latitudes outside the tropical stratosphere; an extensive review of the QBO, including its manifestations outside the Tropics, has been given by Baldwin et al. (2001). In this article we provide an expanded and updated review of one particular area of interest related to the QBO: its impact on the atmospheric circulation at high latitudes during winter, often referred to as the Holton–Tan effect. Dynamical coupling between the QBO and the winter stratospheric polar vortices of both hemispheres has been extensively studied using both observations and models (albeit with the Northern Hemisphere – NH – having received more attention than the Southern – SH), yet the mechanism of coupling between low and high latitudes is still not well understood. Here we attempt a summary of the relevant literature on this subject by collecting in one place a useful list of references, an update of some key diagnostics, a synthesis of current understanding, and some discussion of open questions. The wintertime variability of the stratospheric polar vortex, particularly the Stratospheric Sudden Warmings (SSWs) that occur roughly every two years in the NH (Charlton and Polvani, 2007), has been a topic of interest for more than ﬁfty years. An understanding of polar vortex variability on intraseasonal and interannual c  2013 Royal Meteorological Society