Analysis of a better-resolved stratosphere on seasonal forecasts of the Northern Hemisphere Winter

Sudden stratospheric warmings (SSWs) have been shown to influence the Northern Hemispheric surface weather patterns from a few days up to 2 months after the events. Here we further explore the role of a better-resolved stratosphere on Northern Hemisphere winter forecasts. We use two configurations of the Community Earth System Model (CESM): 1) the default configuration with 30 vertical layers (30L) in which the stratosphere is poorly resolved, and 2) a new configuration with 46 vertical layers (46L) in which the stratosphere is better resolved.

The forecast skill of both models is investigated by conducting 50 ensembles of each forecast to obtain enough statistics to draw statistically significant conclusions about winters with and without SSWs. We create a simulation matrix with several different simulation start dates and initial conditions with different phases of the Quasi-Biennial Oscillation (QBO) and the El Nino Southern Oscillation (ENSO). The impact of these factors on the forecast skill is examined by focusing on sea level pressure and temperature patterns, as well as temperature extremes. We find that the largest differences between forecasted surface pressure and temperature are over Europe and Eurasia, and occur between the 46L ensembles with and without SSWs. Forecast skill varies, but the 46L model tends to perform better when a SSW event is present.