Speaker: Daniela I.V. Domeisen (ETH Zurich)
Topic: Stratosphere-troposphere coupling: Implications for sub-seasonal prediction

Historically, stratosphere-troposphere coupling was thought of as a one-way upward process. With the realization that there exists a downward influence from the stratosphere onto surface weather came the expectation for the improvement of long-range weather forecasts in the aftermath of stratospheric extreme events. Indeed, the stratosphere has been found to be a valuable predictor of tropospheric weather on weekly to monthly timescales. For example, forecast systems on average predict a more negative North Atlantic Oscillation following sudden stratospheric warming events, as is expected from observations. However, not all stratospheric events have a downward impact, and the evolution and timing of the downward impact is highly variable between events. Prediction models struggle to reproduce this inter-event variability, in fact, models are often not able to anticipate the existence, timing, or evolution of surface weather after a stratospheric event. A better understanding of stratosphere-troposphere coupling will therefore likely advance long-range prediction in winter and spring. This presentation will provide an overview of the current state and the challenges of long-range prediction using the stratosphere.