Seasonal Forecasts of Winter Temperature Improved by Higher-Order Modes of Mean Sea Level Pressure Variability in the North Atlantic Sector

The variability of the sea level pressure in the North Atlantic sector is the most important driver of weather and climate in Europe. The main mode of this variability, the North Atlantic Oscillation (NAO), explains up to 50% of the total variance. Other modes, known as the Scandinavian index, East Atlantic and East Atlantic/West Russian pattern, complement the variability of the sea level pressure, thereby influencing the European climate.

Current seasonal forecasts of European winter climate, though highly desirable for society and economy, are as yet not fully reliable. There exist a number of autumn predictors, such as sea surface and stratospheric temperature, Eurasian snow depth, and Arctic sea ice extension, that impact on the upcoming pressure regimes in a predictable way. The present dynamical seasonal forecast systems respond still too weakly to these known seasonal predictors. But the relationship is reproduced quite well by means of statistics.

In combination, statistical and dynamical forecasts have the potential to improve forecasts of the North Atlantic pressure conditions and thereby affected variables like temperature and precipitation in Europe considerably. A seasonal prediction system with enhanced winter NAO skill due to ensemble subsampling w.r.t. a statistical estimate of the NAO index entails an improved prediction of the surface climate variables as well. Here, we show that a refined subselection procedure that accounts both for the NAO index and for the three additional modes of sea level pressure variability, is able to further increase the prediction skill of the operational seasonal forecast model of the German Meteorological Service GCFS of wintertime mean sea level pressure, near-surface temperature and precipitation across Europe.