Editorial Type:
Article
Article Type:
Research Article

Forced Annular Mode Patterns in a Simple Atmospheric General Circulation Model

Michael J. Ring Program in Atmospheres, Oceans, and Climate, Massachusetts Institute of Technology, Cambridge, Massachusetts

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R. Alan Plumb Program in Atmospheres, Oceans, and Climate, Massachusetts Institute of Technology, Cambridge, Massachusetts

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Print Publication:
01 Oct 2007
Collections:
Jets and Annular Structures in Geophysical Fluids (Jets)
DOI:
https://doi.org/10.1175/JAS4031.1
Page(s):
3611–3626
Restricted access

Abstract

Previous studies using simplified general circulation models have shown that “annular modes” arise as the dominant mode of variability. A simple GCM is used here to explore to what extent these modes are also the preferred response of the system to generic forcing.

A number of trials are conducted under which the model is subjected to an artificial, zonally symmetric angular momentum forcing, and the climatologies of these trials are compared to that of the control. The forcing location is varied among the several trials. It is found that the changes in the model’s climatology are generally annular mode–like, as long as the imposed forcing projects strongly upon the annular modes of the unforced model.

The role of changes to the eddy–zonal flow feedback versus the action of direct forcing is also considered through the use of a zonally symmetric version of the model. It is found that the direct responses to forcing are insufficient to capture either the strength or the structure of the annular mode responses. Instead, the changes in eddy fluxes are needed to produce the correct responses.

Corresponding author address: Michael J. Ring, Program in Atmospheres, Oceans, and Climate, 77 Massachusetts Avenue, Room 54-1715, Cambridge, MA 02139. Email: mring@mit.edu

This article included in the Jets and Annular Structures in Geophysical Fluids (Jets) special collection.

Abstract

Previous studies using simplified general circulation models have shown that “annular modes” arise as the dominant mode of variability. A simple GCM is used here to explore to what extent these modes are also the preferred response of the system to generic forcing.

A number of trials are conducted under which the model is subjected to an artificial, zonally symmetric angular momentum forcing, and the climatologies of these trials are compared to that of the control. The forcing location is varied among the several trials. It is found that the changes in the model’s climatology are generally annular mode–like, as long as the imposed forcing projects strongly upon the annular modes of the unforced model.

The role of changes to the eddy–zonal flow feedback versus the action of direct forcing is also considered through the use of a zonally symmetric version of the model. It is found that the direct responses to forcing are insufficient to capture either the strength or the structure of the annular mode responses. Instead, the changes in eddy fluxes are needed to produce the correct responses.

Corresponding author address: Michael J. Ring, Program in Atmospheres, Oceans, and Climate, 77 Massachusetts Avenue, Room 54-1715, Cambridge, MA 02139. Email: mring@mit.edu

This article included in the Jets and Annular Structures in Geophysical Fluids (Jets) special collection.

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