Two Sides of the Same Coin: Unified Understanding of Atmospheric Blocking and Periodic Variability as Evolutions of Rossby Wave Packets
Abstract
Large-scale weather systems are building blocks of intraseasonal atmospheric variability in the mid-latitudes. While synoptic weather systems typically last 3-5 days, atmospheric blocking (the stagnation of the weather systems) can last for 10-20 days, and the storm tracks wave activity has a periodic behavior of 20-30 days. Both the atmospheric blocking and periodic behaviors often cause local extreme weather events in the mid-latitudes. Because of their distinct timescales and features, atmospheric blocking and periodic variability have always been studied separately as two different topics. Yet, since their first-order dynamics remain enigmatic, what if the same dynamics govern them and just appear to be two different phenomena? To test this hypothesis, I adopt a two-layer quasi-geostrophic model which adequately captures both the blocking dynamics and the periodic behavior. Inspired by our observational evidence, I add a new ingredient: a zonal asymmetry in the background state. This is critical to both variabilities: the spatial distributions of blocking frequency and periodic variability turn out to match each other. In other words, within the same region, a blocking episode would occur whenever the 20-30 day periodic variability reaches its peak of wave activity evolutions. In the downstream's difluent area, a noticeable optimal growing can extract energy from the mean flow to the transient perturbations. This non-modal growing is rapid and can give rise to the entire lifecycle of the nonlinear evolving and slow propagating Rossby wave packets. Reanalysis products also support this hypothesis: observed atmospheric blocking and periodic variability share spatial distribution and temporal evolution among each other. Thus, the first-order dynamics of atmospheric blocking and periodic behavior are a coherent evolution of Rossby wave packets where constructive and destructive interference happens in both cases but play out in different fashions. As two sides of the same coin, localized storm tracks are characterized by atmospheric blocking and periodic behavior governed by shared first-order dynamics.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFM.A52M1150W