Attribution of NAO Predictability beyond Two Weeks

There is increasing evidence that stratospheric processes and variability provide a source of predictive skill on subseasonal time scale. Specifically, the knowledge of extreme stratospheric polar vortex conditions has been shown to exhibit enhanced predictive skill of tropospheric circulation related to the North Atlantic Oscillation (NAO) up to four weeks. In this study, we explore the source of NAO predictability beyond two weeks. We utilize 45-day reforecasts (20 member ensembles) over the period 1999-2015 with NCAR’s CESM1 and its corresponding AMIP style atmospheric model simulations that are un-initialized but forced with prescribed observational SST and sea ice conditions. We find that initialized forecasts have superior skill compared to AMIP simulations and persistence in predicting the NAO for the one-month period from week 3 to week 6. Consistent with other studies we find that following extreme stratospheric polar vortex conditions (both strong and weak) the NAO predictive skill is generally higher compared to stratospheric neutral vortex states. For the period studied (1999-2015) we also show that the higher NAO predictive skill persists longer (up to week 6) for weak than for strong stratospheric vortex events (only up to week 4). The enhanced NAO predictive skill for weak vortex events is related to stratospheric downward coupling, while in the case of strong vortex events the skill is partly related to persistence and lower boundary forcing.