Sub-seasonal predictability of the stratosphere

References: Tripathi et al. 2014 for extreme polar vortex events; Christiansen 2005, Charlton and Polvani 2007, and Yoden et al. 2014Marshall et al. 2009;Stan and Straus 2009, Li and Ding 2011, and Zhang et al. 2013Cai et al. 2015