Representation of the Scandinavia-Greenland Pattern and its Relationship with the Polar Vortex in S2S Models

Abstract:

The strength of the stratospheric polar vortex is a key contributor to subseasonal prediction during boreal winter. Anomalously weak polar vortex events can be induced by enhanced vertically propagating Rossby waves from the troposphere, driven by blocking and wave breaking. Here, we analyse a tropospheric pattern – the Scandinavia-Greenland (S-G) pattern – associated with both processes. The S-G pattern is here defined as the second empirical orthogonal function (EOF) of mean sea-level pressure in the northeast Atlantic. The first EOF is a zonal pattern resembling the North Atlantic Oscillation. We show that the S-G pattern is associated with a transient amplification of planetary wavenumber-2 and meridional eddy heat flux, followed by the onset of a weakened polar vortex which persists for the next 2 months. We then analyse 10 different models from the S2S database. We find that while all models represent the structure of the S-G pattern well, some models have a zonal bias with more than the observed variability explained by their first EOFs, and correspondingly less explained by the second EOF. This bias is largest in models with the lowest resolution. Similar results are found for predictive skill of the S-G pattern, which is poorest in models with the lowest resolution though is not high beyond week 2 for any model. We find that the relationship between the S-G pattern and enhanced eddy heat flux and a weakened polar vortex is initially well-represented but significantly decays with lead time in most S2S models. Our results motivate improved representation of the S-G pattern and its stratospheric response at longer lead-times for improved subseasonal prediction of the stratospheric polar vortex.