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Abstract The annular modes of the extratropical atmosphere have received much attention for quantifying variability of the jet streams and storm tracks, despite the fact that the midlatitude circulation itself does not vary uniformly with longitude. While tropical fluctuations in geopotential height have lower amplitude than in the extratropics, they exhibit stronger zonal coherence, or dynamical annularity. A simple index is developed to characterize zonal‐mean anomalies of the tropical circulation. It reveals that anomalies in geopotential height and zonal wind migrate downward from the upper troposphere to the surface on a time scale of about 10 days. These features are distinguishable from known modes of tropical variability, the Madden‐Julian Oscillation in particular. Evidence from reanalysis and idealized model experiments confirms that this downward migration is quite generic and driven by mechanically forced variations in the strength of the Hadley circulation on subseasonal time scales.
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Frequency‐Dependent Behavior of Zonal Jet Variability
Abstract Recent work suggests that storm track diagnostics such as eddy heat fluxes and eddy kinetic energies have very small signatures in the first annular mode of zonal mean zonal wind, suggesting a lack of co‐variability between the locations of the extratropical jet and storm tracks. The frequency‐dependence of this apparent decoupling is explored in ERA‐Interim reanalysis data. The annular modes show similar spatial characteristics in the different frequency ranges considered. Cancellation between the signatures of storm track diagnostics in the leading low‐pass and high‐pass filtered annular modes is evident, partly explaining their small signature in the total. It is shown that at timescales greater than 30 days, the first zonal wind mode describes latitudinal shifts of both the midlatitude jet and its associated storm tracks, and it appears that the persistence of zonal wind anomalies is sustained primarily by a baroclinic feedback.
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- Award ID(s):
- 1921409
- PAR ID:
- 10453560
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 47
- Issue:
- 6
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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