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Eddy Lifetime, Number, and Diffusivity and the Suppression of Eddy Kinetic Energy in Midwinter

Schemm, Sebatian and Schneider, Tapio (2018) Eddy Lifetime, Number, and Diffusivity and the Suppression of Eddy Kinetic Energy in Midwinter. Journal of Climate, 31 (14). pp. 5649-5665. ISSN 0894-8755. doi:10.1175/JCLI-D-17-0644.1.

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The wintertime evolution of the North Pacific storm track appears to challenge classical theories of baroclinic instability, which predict deeper extratropical cyclones when baroclinicity is highest. Although the surface baroclinicity peaks during midwinter, and the jet is strongest, eddy kinetic energy (EKE) and baroclinic conversion rates have a midwinter minimum over the North Pacific. This study investigates how the reduction in EKE translates into a reduction in eddy potential vorticity (PV) and heat fluxes via changes in eddy diffusivity. Additionally, it augments previous observations of the midwinter storm-track evolution in both hemispheres using climatologies of tracked surface cyclones. In the North Pacific, the number of surface cyclones is highest during midwinter, while the mean EKE per cyclone and the eddy lifetime are reduced. The midwinter reduction in upper-level eddy activity hence is not associated with a reduction in surface cyclone numbers. North Pacific eddy diffusivities exhibit a midwinter reduction at upper levels, where the Lagrangian decorrelation time is shortest (consistent with reduced eddy lifetimes) and the meridional parcel velocity variance is reduced (consistent with reduced EKE). The resulting midwinter reduction in North Pacific eddy diffusivities translates into an eddy PV flux suppression. In contrast, in the North Atlantic, a milder reduction in the decorrelation time is offset by a maximum in velocity variance, preventing a midwinter diffusivity minimum. The results suggest that a focus on causes of the wintertime evolution of Lagrangian decorrelation times and parcel velocity variance will be fruitful for understanding causes of seasonal storm-track variations.

Item Type:Article
Related URLs:
URLURL TypeDescription
Schneider, Tapio0000-0001-5687-2287
Additional Information:© 2018 American Meteorological Society. Open Choice. (Manuscript received 26 September 2017, in final form 10 April 2018) Sebastian Schemm acknowledges funding from the Swiss National Science Foundation (Grants P300P2_167745 and P3P3P2_167747). ECMWF is acknowledged for providing the ERA-Interim dataset. We are grateful to Farid Ait-Chaalal for helpful comments during the course of the analysis. We also thank three anonymous reviewers and Hisashi Nakamura for their helpful comments during the review process.
Funding AgencyGrant Number
Swiss National Science Foundation (SNSF)300P2_167745
Swiss National Science Foundation (SNSF)P3P3P2_167747
Subject Keywords:Atmosphere; Extratropical cyclones; Lagrangian circulation/transport; Storm tracks; Kinetic energy; Seasonal cycle
Issue or Number:14
Record Number:CaltechAUTHORS:20180711-153419610
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Official Citation:Schemm, S. and T. Schneider, 2018: Eddy Lifetime, Number, and Diffusivity and the Suppression of Eddy Kinetic Energy in Midwinter. J. Climate, 31, 5649–5665,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:87776
Deposited By: George Porter
Deposited On:11 Jul 2018 23:05
Last Modified:15 Nov 2021 20:51

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