Attribution of NAO Predictive Skill Beyond 2 Weeks in Boreal Winter
We investigate the role of the stratosphere and El Nino Southern Oscillation (ENSO) on the predictive skill of the North Atlantic Oscillation (NAO) on the subseasonal, weeks 3 to 6, timescale. We use a 20-member ensemble of 45-day initialized reforecasts over the period from 1999-2015 together with uninitialized simulations forced with observed radiative and lower boundary conditions with the Community Earth System Model, version 1 (CESM1). We find that CESM1 is able to better predict the NAO up to 3 weeks following extreme weak or strong states of the stratospheric polar vortex compared to stratospheric neutral vortex states. Enhanced weeks 3-6 NAO predictive skill for weak vortex events results primarily from
Winter climate over Europe and eastern North America is significantly affected by variability of the NAO. This study demonstrates that the NAO can be predicted up to 6 weeks ahead during strong and weak stratospheric vortex events, as compared to only 3 weeks ahead during neutral stratospheric vortex states.
In this study, we quantify the NAO predictive skill for lead times of 3–6 weeks and attribute it to two main sources: the stratosphere and ENSO. We find that the model is able to better predict the NAO up to 3 weeks following extreme weak or strong states of the stratospheric polar vortex compared to stratospheric neutral vortex states. Enhanced weeks 3–6 NAO predictive skill for weak vortex events results primarily from stratospheric coupling to the troposphere, while enhanced skill for strong vortex events can be attributed in part to lower boundary forcing related to ENSO.