Abstract 240

Abstract ID: 240

Understanding the intermittency of the wintertime North Atlantic Oscillation and East Atlantic Pattern seasonal forecast skill in the Copernicus C3S multi-model ensemble

Lead Author: Laura Baker
NCAS, University of Reading, United Kingdom


Keywords: seasonal, North Atlantic, skill, model evaluation

Abstract: The wintertime North Atlantic Oscillation (NAO) and the East Atlantic Pattern (EA) are the two leading modes of pressure variability in the North Atlantic, and thus have a substantial impact on the winter weather in Europe. Over the last decade, several studies have demonstrated significant skill in winter NAO seasonal forecasts (e.g. Scaife et al., 2014; Kang et al., 2014; Stockdale et al., 2015; Baker et al., 2018). However, seasonal NAO forecasts have been shown to suffer from a `signal-to-noise’ issue, where the predictable signal in seasonal forecasts are of smaller magnitude than expected (Eade et al., 2014). In contrast to this promising NAO forecast skill, there is typically no significant skill for forecasting the EA.
Here we analyse hindcasts from seven seasonal forecast systems in the Copernicus C3S archive, focussing on the hindcast period 1993/4—2016/17. Using a diagnostic developed by Weisheimer et al (2019), we assess the year-to-year contributions to seasonal forecast skill for the wintertime NAO and EA. We use these results to assess the drivers of skill during years of forecast busts (poor forecasts) and in the most well-forecast years. We find that the presence of strong tropical forcing is a key driver of predictability in the well-forecast years, while large perturbations in the northern hemisphere stratosphere are present in the forecast bust years.

Len Shaffrey (NCAS, University of Reading)
Antje Weisheimer (NCAS, University of Oxford, ECMWF)
Stephanie Johnson (ECMWF)