Annular modes in multiple migrating zonal jet regime

Abstract

Recent studies have linked hemispheric climate variability to annular modes, zonally symmetric structures that describe the horizontal redistribution of atmospheric mass. The resulting changes in the pressure patterns consequently alter the atmospheric circulation, including the movement of zonal jets in the atmosphere. While the literature contains much observational evidence describing these annular modes, the fundamental dynamics in the perpetuation of the annular modes still remains poorly understood. We investigate the dynamics of the annular modes using the MITGCM, a semi-hemispheric ocean model. The forcings imposed in the model are an atmospheric wind stress and relaxation to a latitudinal temperature profile, which induces a baroclinically unstable flow. Despite such an idealized setup, the model output shows striking similarities to the observed atmospheric annular modes, where the leading mode of variability is associated with the primary zonal jet's meridional displacement. By convention, when the zonal jet is poleward (equatorward) of its time-mean position, the principal component (PC) of the first empirical orthogonal function (EOF) is positive (negative) and is referred to as the high (low) zonal index.

(cont.) In the model, systematic secondary (weaker) jets migrate equatorward into the primary jet. The total eddy forcing associated with the migrating jets aids in sustaining the primary jet in the presence of frictional forces. Plots of the anomalous eddy fields for both indexes show that the strongest eddy activity in the main jet is associated with the high zonal index. The zonal flow anomalies, which systematically migrate into the poleward flank of the main jet, are largely responsible for causing this positively anomalous eddy forcing. This asymmetrical forcing to the primary jet results in the zonal index variability. In this thesis, the dynamics associated with the secondary jets and its equatorward migration will be examined. We will show that when (1) the sphericity of the earth is accounted for, (2) the interior PV is homogenized, and (3) the width of the baroclinically unstable region exceeds the Rhines scale by several factors, multiple zonal jets emerge and migrate equatorward.