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Abstract Midlatitude weather extremes such as blocking events and Rossby wave breaking are often related to large meridional shifts in the westerly jet stream. Numerous diagnostic methods have been developed to characterize these weather events, each emphasizing different yet interrelated aspects of circulation waviness, including identifying large-amplitude ridges or persistent anomalies in geopotential height. In this study, we introduce a new metric to quantify the circulation waviness in terms of effective time scale. This is based on the Rossby wave packet from the one-point correlation map of anomalous meridional wind, applicable to jet waviness involving multiple wavenumbers. Specifically, we estimate the intrinsic frequency of Rossby waves and decay time scale of wave amplitude in the reference frame moving at the local time mean zonal wind. The resulting effective time scale, derived from linear theory, serves as a proxy for the eddy mixing time scale in jet meandering. Remarkably, its spatial distribution roughly resembles that of circulation waviness in the Northern Hemisphere winter as depicted by local wave activity (LWA). In the high-latitude regions characterized by weak zonal winds, the long time scale in waviness aligns with large values in LWA. By contrast, short waviness time scales in subtropical jet regions correspond to the suppressed amplitude in waviness despite large values in eddy kinetic energy (EKE). Furthermore, the effective time scale in waviness largely captures the interannual variability of LWA in observations and its projected future changes in climate model simulations. Thus, this relation between the waviness time scale and zonal wind provides a physical mechanism for understanding how zonal wind changes impact regional weather patterns in a changing climate. Significance StatementThe purpose of this study is to better understand what controls weather extremes in midlatitude regions such as blocking events and Rossby wave breaking. We introduce a novel concept, the effective time scale of jet stream meandering, which sheds light on these phenomena. Through analyzing Rossby waves in the reference frame moving at the local time mean zonal wind, we derive a scaling relation between circulation waviness and eddy mixing time scale. Our findings reveal that this time scale closely mirrors the spatial distribution of circulation waviness in the Northern Hemisphere winter. Importantly, it captures interannual variability and climate change responses. These insights provide a physical basis for understanding how changes in zonal wind impact regional weather patterns in observations and climate models.
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Jet Stream Meandering in the Northern Hemisphere Winter: An Advection–Diffusion Perspective
Abstract Large meridional excursions of a jet stream are conducive to blocking and related midlatitude weather extremes, yet the physical mechanism of jet meandering is not well understood. This paper examines the mechanisms of jet meandering in boreal winter through the lens of a potential vorticity (PV)-like tracer advected by reanalysis winds in an advection–diffusion model. As the geometric structure of the tracer displays a compact relationship with PV in observations and permits a linear mapping from tracer to PV at each latitude, jet meandering can be understood by the geometric structure of tracer field that is only a function of prescribed advecting velocities. This one-way dependence of tracer field on advecting velocities provides a new modeling framework to quantify the effects of time mean flow versus transient eddies on the spatiotemporal variability of jet meandering. It is shown that the mapped tracer wave activity resembles the observed spatial pattern and magnitude of PV wave activity for the winter climatology, interannual variability, and blocking-like wave events. The anomalous increase in tracer wave activity for the composite over interannual variability or blocking-like wave events is attributed to weakened composite mean winds, indicating that the low-frequency winds are the leading factor for the overall distributions of wave activity. It is also found that the tracer model underestimates extreme wave activity, likely due to the lack of feedback mechanisms. The implications for the mechanisms of jet meandering in a changing climate are also discussed.
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- PAR ID:
- 10364176
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 35
- Issue:
- 6
- ISSN:
- 0894-8755
- Format(s):
- Medium: X Size: p. 2055-2073
- Size(s):
- p. 2055-2073
- Sponsoring Org:
- National Science Foundation
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