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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 lifecycle 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 the Pacific storm track. 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 lifecycle 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 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.
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Recurrent Rossby waves during Southeast Australian heatwaves and links to quasi-resonant amplification and atmospheric blocks
In the Northern Hemisphere, recurrence of transient Rossby wave packets over periods of days to weeks, termed RRWPs, may repeatedly create similar weather conditions. This recurrence leads to persistent surface anomalies and high-impact weather events. Here, we demonstrate the significance of RRWPs for persistent heatwaves in the Southern Hemisphere (SH). We investigate the relationship between RRWPs, atmospheric blocking, and amplified quasi-stationary Rossby waves with two cases of heatwaves in Southeast Australia (SEA) in 2004 and 2009. This region has seen extraordinary heatwaves in recent years. We also investigate the importance of transient systems such as RRWPs and two other persistent dynamical drivers: atmospheric blocks and quasi-resonant amplification (QRA). We further explore the link between RRWPs, blocks, and QRA in the SH using the ERA-I reanalysis dataset (1979–2018). We find that QRA and RRWPs are strongly associated: 40% of QRA days feature RRWPs, and QRA events are 13 times more likely to occur with an RRWPs event than without it. Furthermore, days with QRA and RRWPs show high correlations in the composite mean fields of upper-level flows, indicating that both features have a similar hemispheric flow configuration. Blocking frequencies for QRA and RRWP conditions both increase over the south Pacific Ocean but differ substantially over parts of the south Atlantic and Indian Ocean.
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- Award ID(s):
- 1934358
- PAR ID:
- 10321911
- Date Published:
- Journal Name:
- Weather and climate dynamics
- ISSN:
- 2698-4016
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/wcd-2022-1
