Wave activity budget through persistent anomalies in wintertime jet stream

Abstract:

Atmospheric blocking represents persistent meandering of the jet stream that disrupt the eastward migration of extratropical transient eddies. Recently, Nakamura and Huang (2018) proposed a semi-empirical theory for block formation which suggests that atmospheric blockings are similar to `traffic-jam' on a highway. Here we use the finite-amplitude local wave activity (LWA) framework with reanalysis dataset to identify signatures of traffic-jam mechanism during block formation. Using LWA budget and a feature tracking algorithm, we perform a careful analysis of major persistent anomalies in the jet streams during the Northern Hemisphere winter. We show that most of the persistent anomalies occur in clusters collocated with the quasi-stationary ridge over the Euro-Atlantic sector and the Pacific sector. In the Atlantic sector, the zonal LWA flux dominates for longer timescale (up to ~20 days), whereas in the Pacific sector, both diabatic heating and zonal LWA flux play equally important role in the wave activity budget. We show that most persistent events are associated with stronger nonlinear modification of large-amplitude Rossby waves. In such cases the total LWA flux is effectively suppressed. In contrast, the short-lived anomalous events are associated with weaker nonlinear component of LWA flux which results in ineffective flux suppression. For both persistent and short-lived events the zonal LWA flux reaches a tipping point for spontaneous accumulation of LWA. Overall, we show that despite substantial variations among individual events most of the long-lived events fall into the canonical description of the traffic-jam behavior.

References: Nakamura, Noboru, & Huang, Clare S. Y. 2018. Atmospheric blocking as a traffic jam in the jet stream. Science, 361(6397), 42-47.