Comparison of Subseasonal‐to‐Seasonal Model Forecasts for Major Stratospheric Sudden Warmings. Issue 18 (22nd September 2018) - Taguchi, M.
Abstract: This study explores forecasts for five distinctive major stratospheric sudden warmings (MSSWs) using subseasonal‐to‐seasonal prediction data with lead times up to about 2 weeks. Results reveal model‐to‐model variability of the forecasts, as some models, such as the European Centre for Medium‐Range Weather Forecasts and National Centers for Environmental Prediction models, forecast the MSSWs better. Results also demonstrate greater difficulty of forecasting the vortex split MSSWs (three of the five) than the vortex displacement MSSWs (the other two). It is also shown that the variable forecasts among different lead times and models for each MSSW are characterized by how well the models reproduce the weakenings and deformations of the stratospheric polar vortex around the MSSWs and the intensifications of tropospheric planetary waves before them. The variability is well captured by planetary wave activity, or poleward heat flux, in the lower stratosphere. This underlines the importance of enhanced planetary waves in the troposphere and lower stratosphere for successful forecasts of the forecasting MSSWs, although the causality between the MSSWs and enhanced planetary waves will deserve further investigation. Plain Language Summary: This study investigates subseasonal forecasts of nine models (with time scales up to about 2 weeks) in terms of five distinctive major stratospheric sudden warmings or breakups of the polar vortex in the winter stratosphere. Results reveal variations in the forecasts among the different models and stratospheric warmings. In particular, the stratospheric sudden warmings in which the vortex is split are more difficult to forecast than the others in which the vortex is displaced away from the pole. The forecasts from the different models and lead times for each warming are predominantly different in weakenings and deformations of the polar vortex. These differences are associated with intensified planetary waves in the troposphere before the stratospheric warmings, underlining the importance of the stratosphere‐troposphere dynamical coupling. Key Points: Some of the S2S models of interest forecast the five target MSSWs better than others The vortex split MSSWs are more difficult to forecast than the vortex displacement MSSWs The forecasts vary with lead times and models in stratospheric polar vortex and tropospheric planetary waves