Improvement in the decadal prediction skill of the North Atlantic extratropical winter circulation through increased model resolution

As the scientific and societal interest in skillful decadal predictions grows, a lot of effort is currently put into the development and advancement of such prediction systems worldwide. Studies evaluating the skill of basic atmospheric quantities, such as e.g. surface temperatures, in those prediction systems are numerous. However, dynamical quantities are discussed only rarely. Also, there is a lack of investigations which assess the exclusive impact of the model’s resolution on the forecast skill. 

In this study, we address both these issues: we analyse a set of four quantities of the extratropical circulation (storm track, blocking frequencies, cyclone frequencies, windstorm frequencies) and compare the deterministic forecast skill for lead winters 2-5 within the German MiKlip prediction system of two different spatial resolutions. While the lower resolution (LR, atm: T63L47, ocean: 1.5° L40) shows common deficits in the climatological representation, e.g. an overly zonal extratropical storm track and a deficit in blocking frequencies over the North Atlantic and Europe, the higher resolution version (HR, atm: T127L95, ocean: 0.4° L40) counteracts these biases. In return, the deterministic decadal prediction skill, which is measured in terms of anomaly correlation, increases (statistically significant) with the increase in resolution for all four quantities. 

The improvements found in our study for the different metrics follow a physically consistent line of argument, and the areas of improved forecast skill are crucial regions for the genesis and intensification of synoptic weather systems over the North Atlantic and for their impact on Europe. Thus, we identified a significant improvement of the storm track skill along the North Atlantic Current (i.e., the source region of synoptic eddies), a downstream improvement of the cyclone frequency skill over the central North Atlantic (where the synoptic systems intensify), and finally improved skill of the cyclone, windstorm and blocking frequencies over the European continent (i.e., the impact area).

Not only is the skill improved with the increase in resolution (HR vs. LR), but also the HR system itself offers significant deterministic decadal forecast skill for the extratropical circulation metrics in large regions over the North Atlantic and Europe (HR vs. ERA-Interim) for the considered lead time of two to five winters. 

Our results are encouraging for the advancement of decadal prediction systems as they document that even small improvements in the bias of the model, through an increased spatial resolution and possibly a better representation of smaller scales, can have a substantial effect on the representation of dynamical processes and can ultimately lead to a significant improvement of the decadal prediction skill for extratropical features and extreme events.