The Latitude-Height Structure of 40–50 Day Variations in Atmospheric Angular Momentum

Abstract

Quasi-periodic variations in the relative angular momentum of the atmosphere on time scales of around 40?50 days have been observed by Langley et al. (1981). A description of the two-dimensional (latitude-height) structure of the winds responsible for these changes is constructed here from five years of NMC twice-daily global analyses. Using cross-spectral and amplitude-phase eigenvector techniques, we find these variations are associated with wavelike motions in the tropical upper troposphere that propagate and downward in phase within the tropics. A coherently connected midlatitude Northern Hemisphere component is also present whose phase is essentially independent of height. We believe the tropical component to be the zonally averaged part of the motions described by Madden and Julian (1971, 1972). The Northern Hemisphere midlatitude component may be a direct response to the tropical motions or both motions may be the common response to an as yet unidentified tropical forcing.