In Search of Winter Blocking in the Western North Pacific Ocean

Abstract

We investigate winter blocking activity over the western North Pacific (WNP) through an inter-comparison of four detection methods, which are categorized into two types (anomaly and reversal). Most of the blocking events in the anomaly-based methods are initiated from the eastern North Pacific. Reversal-based methods capture the blocking occurrence associated with Rossby wave breaking, especially cyclonic breaking. As the two types capture different aspects of WNP blocking, the resultant regional impact is also largely different. At the onset, anomaly-based methods show a strong cold anomaly downstream of the blocking in conjunction with a Pacific/North America (PNA)-like pattern. Reversal-based methods indicate a weak cold anomaly in company with a western Pacific (WP)-like pattern, which appears downstream and upstream of the blocking in the local reversal and large-scale reversal methods, respectively. At the end of WNP blocking, all methods show strong cold anomalies both upstream and downstream of the blocking.