Fig. 2: Circulation anomalies and eddy activity by polar vortex state.
From: Stratospheric impact on subseasonal forecast uncertainty in the northern extratropics
Weeks 3–5 composite-mean of weak (a, b; 169 cases) and strong (c, d; 169 cases) polar vortex forecasts. 850 hPa horizontal wind diagnostics a, c for Mid-November to End-March climatology (purple streamlines; larger values marked darker), anomalies of ensemble mean (black arrows) and anomalies of ensemble variance (shading). Upper troposphere diagnostics b, d in terms of 200 hPa horizontal wind anomalies (black arrows), anomalies of eddy activity at 320 Kelvin isentrope as a measure for synoptic-scale Rossby waves (shading; see text for details) and Mid-November to End-March climatological latitude of the maximum meridional potential vorticity gradient at 320 Kelvin isentrope.
