2016 Volume 12 Pages 121-126
Persistent and/or recurring large-/synoptic-scale atmospheric flow patterns can cause severe weather events in surrounding areas. This study first classify the large-/synoptic-scale patterns in the ERA-Interim 500 hPa geopotential height over East Asia in extended winters of 1979/80-2013/14 and then assess their predictability on medium-range timescales for the extended winters of 2006/07-2013/14 and 1985/86-2013/14, using operational ensemble forecasts and ensemble reforecasts, respectively. The winter monsoon, western Pacific (WP), high and low pressure, and southerly flow (SF) patterns are detected as dominant patterns. Some transitions among these patterns occur more frequently, leading to preferred winter circuits of patterns. The occurrence of El Niño/La Niña can also significantly increase or decrease the frequency of patterns. Models have broadly similar biases in the transitions and frequencies of patterns, but some models show different biases with lead time. Verification of probabilistic flow-pattern forecasts reveals that the forecasts made by state-of-the-art models are useful up to a lead time of 14 days on average, and that forecasts related to WP (SF) tend to show higher (lower) skills than the other forecasts. This suggests that models find it difficult to predict the movement of low pressure systems south of Japan.