Propagating Annular Modes: Empirical Orthogonal Functions, Principal Oscillation Patterns, and Time Scales
Author(s)
Sheshadri, Aditi; Plumb, R. Alan
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The two leading empirical orthogonal functions (EOFs) of zonal-mean zonal wind describe north-south fluctuations, and intensification and narrowing, respectively, of the midlatitude jet. Under certain circumstances, these two leading EOFs cannot be regarded as independent but are in fact manifestations of a single, coupled, underlying mode of the dynamical system describing the evolution in time of zonal wind anomalies. The true modes are revealed by the principal oscillation patterns (POPs). The leading mode and its associated eigenvalue are complex, its structure involves at least two EOFs, and it describes poleward (or equatorward) propagation of zonal-mean zonal wind anomalies. In this propagating regime, the principal component (PC) time series associated with the two leading EOFs decay nonexponentially, and the response of the system to external forcing in a given EOF does not depend solely on the PC decorrelation time nor on the projection of the forcing onto that EOF. These considerations are illustrated using results from an idealized dynamical core model. Results from Southern Hemisphere ERA-Interim data are partly consistent with the behavior of the model's propagating regime. Among other things, these results imply that the time scale that determines the sensitivity of a model to external forcing might be different from the decorrelation time of the leading PC and involves both the rate of decay of the dynamical mode and the period associated with propagation. Keywords: Annular mode; Atmospheric circulation; Dynamics
Date issued
2017-04Department
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary SciencesJournal
Journal of the Atmospheric Sciences
Publisher
American Meteorological Society
Citation
Sheshadri, Aditi, and R. Alan Plumb. “Propagating Annular Modes: Empirical Orthogonal Functions, Principal Oscillation Patterns, and Time Scales.” Journal of the Atmospheric Sciences 74, 5 (May 2017): 1345–1361 © 2017 American Meteorological Society
Version: Final published version
ISSN
0022-4928
1520-0469