High initial‐time sensitivity of medium‐range forecasting observed for a stratospheric sudden warming

Abstract

High initial‐time sensitivity for the predictable period of the tropospheric Northern Annular Mode (NAM) around the stratospheric sudden warming (SSW) in the winter of 2003–4 is examined using ensemble simulations of a general circulation model. It is found that the predictable period tends to become very long and reaches to a few months when a forecast is performed before the occurrence of the SSW. When the initial time is set after the SSW, however, the predictable period turns out to be very short. As a result, the predictable limit of time tends to become far longer when forecasts are performed before the occurrence of the SSW compared with those performed after. Comparison is made with winter of 2005–6 with a major SSW and prominent stratospheric variability. The reason for the appearance of such high initial‐time sensitivity is discussed.