The tropical influence on sub-seasonal predictability of wintertime stratosphere and stratosphere-troposphere coupling

A unique set of relaxation experiments with a forecast model initialized during December-January 1999-2019 is used to explore tropical influence on the Northern Hemisphere polar stratosphere and stratosphere-troposphere coupling, to quantify predictability benefits due to the perfect knowledge of the tropical variability. On average, predictability of the polar stratosphere, as represented by the 50 hPa geopotential height anomalies north of 50 degrees N (Z50), increases from 17 days in freely running (control) forecasts to 21 days in the tropical relaxation experiments. At sub-seasonal time-scales, a statistically significant improvement in weekly mean skill scores can be demonstrated in 14%-20% of individual forecast ensembles, mostly in cases when the skill of the corresponding control forecast is worse than average. In these forecasts, root-mean-square errors and forecast spread of Z50 during forecast weeks 3-5 are decreased by 10%-15%. Stratospheric improvements are detected during periods of both vortex strengthening and vortex weakening, including most major Sudden Stratospheric Warmings that occurred during the study period, via modulation of the upward wave activity fluxes. An active Madden-Julian Oscillation (MJO) is found in most of these events with MJO phase 5-7 preceding vortex weakening and MJO phase 3-4 preceding vortex strengthening. Forecasts with improved stratospheric circulation also have improved tropospheric circulation during and after the periods when improvements are detected in the stratosphere. We attribute these improvements to both stratosphere-troposphere coupling and tropospheric tropical teleconnections. We show that average skill of extratropical 500 hPa geopotential height forecasts is improved for weeks 3-5 in tropical (TROP-S) and stratospheric (STRAT) relaxation experiments with respect to control experiment (CTRL) (solid lines). For dates when TROP-S forecasts show improved stratospheric forecasts in week 3 (dashed lines), the tropospheric forecasts in TROP-S show larger improvements during weeks 3-4 than those in STRAT, despite superior stratospheric skill in STRAT. This suggests contribution of both tropospheric teleconnections and stratosphere-troposphere coupling to these improvements.image