Local Wave Activity Analysis of Atmospheric Blocks in the Northern Hemisphere Winter

Abstract

Atmospheric blocking entails a persistent, anomalous meandering of the jet stream that disrupts the eastward migration of transient eddies in the midlatitudes. Here, we analyze a large number of blocking (and blocking-like) events in the Northern Hemisphere winter with the ERA5 reanalysis through the lens of vertically averaged wave activity budget. By applying a feature-tracking algorithm, large-valued wave activity anomalies that persist for 4 days or longer at a given location are identified as blocks, and block-centered composites are constructed for the wave activity budget through the life cycle of blocks. The identified events share commonly recognized features of blocking. The majority of the persistent events occur in clusters collocated with the quasi-stationary ridge associated with the Atlantic and Pacific storm tracks. The frequency of persistent blocks is higher (lower) in regions where the “carrying capacity” of the jet stream is lower (higher). A very low carrying capacity for the transient waves leads to a large population of blocks over Europe. The composite life cycle of persistent blocks shows that convergence (divergence) of the zonal flux of wave activity dominates the budget during the onset (decay) phase of the block, while the eddy-induced wind plays a crucial role of suppressing the zonal flux during the maturation period. Our finding broadly supports the “traffic jam” hypothesis of Nakamura and Huang as a common mechanism of block formation, although there is a vast diversity in the actual manifestation of individual blocks. It is argued that carrying capacity is suited for estimating blocking probability rather than for making deterministic forecasts of blocking events.