[A-AS05] Stratosphere-troposphere Processes And their Role in Climate

[AAS05-04] Stratospheric initial conditions provide seasonal predictability of the North Atlantic and Arctic Oscillations

*Yu Nie¹, Adam Scaife^2,3, Hong-Li Ren^1,4, Ruth Comer², Martin Andrews² (1.Laboratory for Climate Studies, National Climate Center, China Meteorological Administration, Beijing, China, 2.Met Office Hadley Centre, Exeter, UK, 3.College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, UK, 4.Department of Atmospheric Science, School of Environmental Studies, China University of Geoscience, Wuhan, China)

Keywords:Seasonal Prediction, Stratosphere, NAO/AO

The North Atlantic Oscillation (NAO), the regional manifestation of the Arctic Oscillation (AO), dominates winter climate variability in Europe and North America. Skilful seasonal forecasting of the winter NAO/AO has been demonstrated recently by dynamical prediction systems. However, the role of initial conditions in this predictability remains unknown. Using a latest generation seasonal forecasting system and reanalysis data, we show that the initial upper stratospheric zonal wind anomaly contributes to winter NAO/AO predictability through downward propagation of initial conditions. An initial polar westerly/easterly anomaly in the upper stratosphere propagates down to the troposphere in early winter, favoring a poleward/equatorward shift of the tropospheric mid-latitude jet. This tropospheric anomaly persists well into the late winter and induces the positive/negative phase of NAO/AO in the troposphere. Our results imply that good representation of stratospheric initial condition and stratosphere-troposphere coupling in models is important for winter climate prediction.