Abstract 253

Abstract ID: 253

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

Lead Author: Alexey Karpechko
FMI, Finland

Keywords: Stratosphere, Madden-Julian Oscillation, Stratosphere-troposphere coupling, Relaxation experiments, Subseasonal forecasts

Abstract: 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 (Z50), increases from 17 days in freely running (control) forecasts to 21 days in the tropical relaxation experiments. At sub-seasonal timescales, 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 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. However, stratosphere-troposphere coupling is limited to the Atlantic sector and overall plays only a secondary role in these tropospheric improvements. More likely, these improvements are due to tropospheric tropical teleconnections.

Frederic Vitart (European Centre for Medium-range Weather Forecasts),
Irina Statnaia (Finnish Meteorological Institute)
Inna Polistchouk (European Centre for Medium-range Weather Forecasts)
Magdalena Balmaseda-Alonso (European Centre for Medium-range Weather Forecasts)