Representation of the Scandinavia–Greenland pattern and its relationship with the polar vortex in S2S forecast 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 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 two months. We then analyse 10 different models from the S2S database, finding 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 in their first EOF, and accordingly less in their second EOF. This bias is largest in the models with the lowest resolution. Skill in predicting the S-G pattern is not high beyond week 2 in any model, in contrast to the zonal pattern. 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 decays significantly 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.

Seasonal prediction of the boreal winter stratosphere

“…Analogous results seem to hold at shorter time scales, e.g. Lee et al ( 2020a ); Lehtonen and Karpechko ( 2016 ); Domeisen et al ( 2020b ).…”

“…This bias is consistent with findings by Garfinkel et al ( 2019 ), who revealed a recent weakening of the ENSO–SPV teleconnection that is not captured by climate models. The weakening is thought to be induced by the increasing importance of the Eurasian eddy heat flux (Garfinkel et al 2019 ; Cohen et al 2019 ; Peings 2019 ; White et al 2018 ; Lee et al 2020a ; Domeisen et al 2020b ). We also report that a weak connection of the QBO with the SPV in the C3S multi-model may provide some vortex predictability, but a comparison with the strong observed connection confirms issues in the modelling of the Holton–Tan mechanism (Garfinkel et al 2018 ; Butler et al 2016 ).…”

“…Note that process-oriented assessment (e.g. Lee et al 2020a ) is not a common practice in forecast verification, which is usually focused on skill scores of direct outputs. Recent efforts have been made to investigate the dynamics of tropospheric teleconnections (e.g.…”

Quantifying stratospheric biases and identifying their potential sources in subseasonal forecast systems

“…We first examine and compare biases in the NH eddy heat fluxes among the subseasonal forecast systems. The representation of heat fluxes in the Scandinavian/Siberian region has been shown in previous work using S2S models to be deficient (Garfinkel et al, 2019;Lee et al, 2020), and thus we show the evolution of the biases in this region in Figure 5d. In most models the week-1 Scandinavian/Siberian heat flux biases are small in magnitude, especially for the high-top models.…”

Long-range prediction and the stratosphere

“…The timescale of weeks for the predictability of sudden warmings is limited by the predictability of weather patterns in the troposphere which might trigger SSW events (e.g. Mukougawa et al, 2005;Taguchi, 2016;Garfinkel and Schwarz, 2017;Jucker and Reichler, 2018;Lee et al, 2020a). However, if we add this timescale to the timescale of a month or more for the persistence of lower-stratospheric anomalies and their surface effects (e.g.…”