Abstract
Midlatitude atmospheric variability is dominated by the dynamics of the baroclinically unstable jet stream, which meanders and sheds eddies at the scale of the Rossby deformation radius. The eddies interact with each other and with the jet, affecting the variability on a wide range of scales, but the mechanisms of planetary-scale fluctuations of the jet are not well understood. Here, we develop a theoretical framework to explore the stability of planetary-scale motions in an idealized two-layer model of the atmosphere. The model is based on a combination of vertical shear and the Sverdrup relation, providing the dynamic link between the two layers, with meridional eddy heat fluxes parameterized as a diffusive process with the memory of past baroclinicity of the jet. We find that a planetary-scale instability exists if the vertical shear of the jet does not exceed a particular threshold. The inclusion of the eddy-memory effect enables westward or eastward propagation of planetary waves relative to the barotropic mean flow. Importantly, we find growing planetary waves that propagate slowly westward or are stationary, which could have important implications for the formation of atmospheric blocking events. Our theoretical results suggest that, with ongoing polar amplification due to global warming and the corresponding reduction of the vertical shear of the mean wind, the background conditions for the growth of planetary-scale waves via planetary-scale baroclinic instability are becoming more favorable.
Original language | English |
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Pages (from-to) | 809-825 |
Number of pages | 17 |
Journal | Quarterly Journal of the Royal Meteorological Society |
Volume | 148 |
Issue number | 743 |
Early online date | Jan 2022 |
DOIs | |
Publication status | Published - Jan 2022 |
Bibliographical note
Funding Information:W.M. acknowledges the support of Swedish Research Council Grant No. 638‐2013‐9243. G.E.M. acknowledges support from the United States Office of Naval Research award N00014‐19‐1‐2421. H.D. acknowledges support by the Netherlands Earth System Science Centre (NESSC), financially supported by the Ministry of Education, Culture and Science (OCW), Grant No. 024.002.001.
Funding Information:
Office of Naval Research Global, N00014‐19‐1‐2421; the Ministry of Education, Culture and Science, the Netherlands, 024.002.001; Vetenskapsrådet, 638‐2013‐9243 Funding information
Publisher Copyright:
© 2021 The Authors. Quarterly Journal of the Royal Meteorological Society published by John Wiley & Sons Ltd on behalf of Royal Meteorological Society.
Keywords
- Southern Hemisphere baroclinic annular mode (BAM)
- eddy heat fluxes
- eddy memory
- low-frequency variability
- quasiperiodic oscillation
- zonal index