Predictability of Stratospheric Sudden Warming: A Case Study for 1998/99 Winter

Predictability of a zonal-wavenumber (WN) 1 stratospheric sudden warming (SSW) event occurring in December 1998 is examined using the operational 1-month forecast dataset based upon a numerical weather prediction model (NWPM) of the Japan Meteorological Agency (JMA). The stratospheric warming in the polar region of this event is predictable by NWPM from 1 month in advance. The amplification of tropospheric planetary waves triggering the onset of the SSW is closely connected with weak vertical westerly shear near the surface around 60°N, in association with rather strong synoptic wave activity. Thus, the fair reproduction of the interaction between zonal-mean flows and synoptic waves in the troposphere for promoting the generation of WN 1 planetary waves is found to be essential for the successful prediction of this event. The authors also point out prolonged predictability for the onset of a tropospheric blocking event occurring over Alaska just after the warming peak. The blocking is formed because of the poleward propagation of planetary waves causing the SSW event, which suggests a predictable downward control of the tropospheric circulation by the stratosphere after the SSW.