Abstract 127

Abstract ID: 127

Predicting the coupled AO extremes and strong polar vortex conditions during early 2020

Lead Author: Philip Rupp
LMU Munich, Germany

Keywords: Strat-trop coupling, S2S predictability, Extreme events, Ensemble forecasts, Year 2020

Abstract: February-March 2020 was marked by highly anomalous large-scale circulations in the Northern extratropical troposphere and stratosphere. The Atlantic jet reached extreme strength, linked to some of the strongest and most persistent positive values of the Arctic Oscillation (AO) index on record, which provided conditions for extreme windstorms hitting Europe. Likewise, the stratospheric polar vortex reached extreme strength that persisted for an unusually long period. Past research indicated that such circulation extremes occurring throughout the troposphere-stratosphere system are dynamically coupled. We employ sets of numerical ensemble simulations to statistically characterize the mutual coupling of the early 2020 extremes and how this coupling potentially affected the subseasonal-to-seasonal predictability of each layer. We find the extreme vortex strength linked to the reflection of upward propagating planetary waves. The strengthened vortex, in return, had a long-lasting downward influence increasing the chance for persistent and extreme positive AO events. Our results show that capturing the details of this complex reflection process was crucial to predict the strong vortex conditions, but also underline the role of internal variability within the troposphere. The circulation extremes during early 2020 may be viewed as resulting from a fortuitous alignment of dynamical evolutions within the troposphere and stratosphere, aided by each layer’s modification of the other layer’s boundary condition.

Sheena Loeffel (DLR Oberpfaffenhofen)
Hella Garny (DLR Oberpfaffenhofen)
Xiaoyang Chen (KIT Karlsruhe)
Joaquim Pinto (KIT Karlsruhe)
Thomas Birner (LMU Munich