Abstract 151

Abstract ID: 151

Stratospheric influenece on North Atlantic storm track predictability in subseasonal-to-seasonal reforecasts

Lead Author: Hilla Afargan-Gerstman
ETH Zürich, Switzerland

Keywords: subseasonal-to-seasonal predictability , extratropical cyclones , stratosphere-troposphere coupling, storm track, North Atlantic variability

Abstract: Intense winter storms are a major natural hazard in the extratropics. These storms can cause substantial economic damages, lead to flooding and heavy snowfall, and impact electricity and transportation networks. Skilful predictions of winter storms, on timescales of several weeks to months, rely on the connection between the storm tracks and large-scale atmospheric variability modes. One important factor influencing winter storms is the stratosphere, where extreme stratospheric polar vortex events, such as sudden stratospheric warmings (SSW) or extremely strong polar vortex (SPV) states, can have a downward impact on surface weather for several weeks to months. Thus, identifying forecast biases in the downward impact of stratospheric polar vortex events has the potential to improve the predictability of winter storms on subseasonal-to-seasonal timescales.
Using European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data and ECMWF reforecasts from the Subseasonal to Seasonal (S2S) Prediction Project database, we investigate the predictability of extratropical cyclones following stratospheric polar vortex events. Cyclone forecasts following SPV events are found to be generally more successful relative to forecasts after SSW events. We further use the Brier score to assess skill, and discuss the differences in predictability of extratropical cyclones between the two types of stratospheric extremes. The results provide new insights on the role of the stratosphere in subseasonal predictability of winter storms that can be used for forecasting their frequency and surface impacts.

Dominik Büeler (ETH Zürich), C. Ole Wulff (NORCE Norwegian Research Centre, Bjerknes Centre, Bergen, Norway), Michael Sprenger (ETH Zürich), and Daniela I.V. Domeisen (University of Lausanne, ETH Zürich)