Causal drivers of Recurrent Rossby Wave Packets

The recurrence of upper-level synoptic-scale Rossby wave packets (RRWPs) at a location over a short period can lead to persistent surface weather that may drive extreme weather events. RRWPs were observed during several extreme weather events, for example, the 2010 heatwave over Russia, the 2004 and 2009 south-eastern Australian heatwaves, and the anomalous western European precipitation of 1983. RRWPs have been shown to lengthen hot, cold, dry and wet spells across the globe.

Motivated by the importance of RRWPs, this work investigates the causal drivers of RRWPs in the North Atlantic basin for summer and winter in a two-step approach. First, composite maps of top-30 RRWP events during 1979 – 2018 are used to study the characteristics of RRWP events and identify potential drivers using ERA-5 reanalysis data. The potential drivers are shortlisted following statistical significance in a bootstrapping approach. Subsequently, the causality of the shortlisted drivers is assessed using a Bayesian causal network (CN) framework for time series.

Composite maps reveal that RRWP events have a preferred seasonal phase configuration despite not having an explicit condition for phasing in the event selection. In winter, wavenumbers 3, 4 and 5 dominate with a hemisphere-wide wave pattern, whereas wavenumbers 5, 6 and 7 dominate in summer without a hemisphere-wide wave pattern. The causal networks reveal that local changes in atmospheric blocking and low wavenumber flow, termed background flow, are major drivers of RRWPs. RRWPs also have a feedback effect on background flow and blocks. The tropical-extratropical causal link only exists in winter and is indirect, mediated by the changes in the background flow over the Pacific. The causal drivers outlined in this study help to further the understanding of RRWPs.