A new perspective on the stratospheric polar vortex, sudden warmings and the ENSO modulation

Circulation anomalies in the stratosphere, and particularly sudden stratospheric warmings (SSWs), have a strong impact on climate predictability at subseasonal-to-seasonal timescales. Understanding the effects of El Niño-Southern Oscillation on the occurrence and typology of SSWs remains a challenge. Here, we re-diagnose the three-dimensional structure of the stratospheric polar vortex by analysing relative vorticity – the local spinning motion of the flow – and show that it is characterised by strong positive (cyclonic) vorticity over the pole with two regions of maximum rotation that can be traced as two columns down to the troposphere, one over northern Canada and the other over Eurasia. During El Niño, enhanced wave activity over the eastern North Pacific acts as a source of negative (anticyclonic) vorticity in the upper troposphere, weakening the Canadian maximum; hence, the polar vortex appears displaced over Eurasia, favouring displacement-type SSWs. During La Niña, strengthened positive (cyclonic) vorticity over northern Canada, reinforcing the Canadian column of rotation, results in an elongated polar vortex with two well-separated centres of similar amplitude, facilitating split-type SSWs. Our results offer a novel framework for interpreting troposphere-stratosphere interaction and enhancing prediction skill of SSW events.