On Annular Modes and Zonal Jets

Abstract

This study considers the relation of the annular mode to the kinematics of a fluctuating jet in zonal-mean zonal wind and to the zonal index, using an idealized model of fluctuations in the eddy-driven jet. When the sphericity of the domain is accounted for, observed and numerically simulated annular modes for the Southern Hemisphere summertime are found to be in excellent agreement. In particular, the annular mode and zonal index mode are shown to be related but distinct. Although the annular mode is strongly (but not identically) related to fluctuations in jet position, fluctuations in jet strength and width are shown to also be important for its simulation. When the sphericity of the domain is neglected, analytic expressions for the leading empirical orthogonal function (EOF) modes of zonal-mean geopotential for the cases of individual fluctuations in jet strength, position, and width can be obtained. None of these EOF modes have the characteristics of the annular mode. In the presence of simultaneous fluctuations in jet strength and position, the leading zonal-mean geopotential EOF mode (strongly resembling the annular mode) is shown to mix the zonal index mode of zonal-mean zonal wind with other EOF modes, demonstrating why the annular mode and zonal index mode are related but distinct. The greater sensitivity to domain size of EOF modes of geopotential relative to the EOF modes of zonal-mean zonal wind is also discussed. This study focuses on the Southern Hemisphere summertime, which is characterized by a single, eddy-driven jet; the generality of the results presented suggest that the conclusions should be qualitatively unchanged in the presence of both subtropical and eddy-driven jets.