This paper provides new evidence and proposes a new dynamical mechanism for the teleconnection between the two largest jet streams in the northern winter stratosphere – the tropical wind system known as the Quasi-Biennial Oscillation and the high-latitude Polar Vortex.
The Northern Polar Vortex – a strong westerly jet in the winter stratosphere, which circumnavigates the North Pole – can have an important downward influence on the layer in which we live, the troposphere. Severe cold weather conditions, such as those experienced in Northern Europe, including the UK, during the winter of 2013-14, can be closely linked to Polar Vortex variability. As the downward influence of the Polar Vortex can typically take a few weeks to be realised, understanding the causes of its variability can provide aid in improving seasonal forecasting.
The Polar Vortex can be influenced by a range of factors including the El Niño–Southern Oscillation, the 11-year solar cycle and major volcanic eruptions. An additional cause of Polar Vortex variability is related to the alternating pattern of easterly and westerly winds in the tropical stratosphere, known as the Quasi-Biennial Oscillation (QBO). In particular, when the QBO winds in the tropical lower stratosphere are easterly, the Polar Vortex becomes weaker and warmer, and vice versa. Although this teleconnection between the QBO and the Polar Vortex has been well-observed and modelled since its initial discovery in the 1980s, the mechanisms by which it occurs are not so well understood.
Our study provides a dynamical explanation for this QBO-Polar-Vortex teleconnection. It is found that changes in the propagation, growth and breaking of large-scale oscillations known as planetary waves play a crucial role. The evidence further indicates that previously suggested mechanisms cannot be held fully accountable and thus, a new, additional mechanism is proposed. The proposed mechanism suggests that the QBO induces changes in the upper-tropospheric-lower-stratospheric background circulation. In particular, when the QBO is in its easterly phase, the associated circulation changes encourage enhanced upward-propagating planetary waves, which subsequently act to weaken the stratospheric Polar Vortex.
Dynamical Response to the QBO in the Northern Winter Stratosphere: Signatures in Wave Forcing and Eddy Fluxes of Potential Vorticity
White, Ian P.; Lu, Hua; Mitchell, Nicholas J.; Phillips, Tony. 2015
Journal of the Atmospheric Sciences, 72 (12). 4487-4507