Nonmodal growth of atmospheric disturbances relevant to the East Asian pressure surge in boreal winter

Abstract

Significant events of East Asian pressure surge are identified based on sea level pressure for boreal winters of the period 1979–2016. These events typically grow from a high pressure anomaly over Northern Europe and the northwestern Russia and attain their peak over Siberia around ten days after the initiation. The structure of the pressure surge at its peak intensity mimics that of the semi-permanent Siberian High. With a two-level quasi-geostrophic (QG) model, a nonmodal instability analysis of the winter climatological flow reveals that one of the most rapidly amplifying “optimal disturbances” with an optimization time of 2 days resembles the structure of the East Asian pressure surge. This optimal disturbance originates from small-scale precursor disturbances over Northern Europe and the Barents Sea. Such initial disturbances grow in magnitude via Siberia towards East Asia, forming a low-level ridge extending to around 50° N. The optimal disturbance successfully captures the temporal and spatial scales and the overall structure of the circulation anomaly associated with the pressure surge, albeit with varying degrees of accuracy of the location of individual trough/ridge of the surge anomaly. Additional experiments suggest that (1) adding damping effect to the two-level QG model extends the low-level ridge further south to around 40° N and (2) the upper level wave train feature associated with pressure surge can be captured even in a barotropic model by optimal disturbances with an optimization time of 4 days.