Can Background Circulation Facilitate Intraseasonal Mixed Rossby–Gravity Waves over the Central–Eastern Pacific?

Abstract

This study is motivated by the observation of a unique intraseasonal power in the upper-tropospheric equatorial meridional winds over the Western Hemisphere during boreal winter. The presence of intraseasonal power at both westward and eastward wavenumbers is unusual and intriguing, as the dominant tropical modes of intraseasonal variability typically exhibit little amplitude in equatorial meridional winds and are instead characterized by strong zonal wind perturbations. Diagnostic analyses provide observational evidence of intraseasonal disturbances displaying spatial structures and dynamical characteristics consistent with mixed Rossby–gravity waves (MRGWs). Additionally, these MRGWs are confined to the upper troposphere. A systematic relationship between the location and amplitude of the intraseasonal MRGWs and the upper-tropospheric westerlies suggests that background circulation is fundamental to their existence. This hypothesis is investigated through a set of diagnostic analyses guided by the dispersion relation of a linear shallow-water model incorporating a homogeneous background flow. The results explain not only the emergence of intraseasonal MRGWs but also the overall distribution of equatorial meridional wind power throughout the troposphere. The strength and direction of the background flow govern the observed spatial and temporal characteristics of MRGWs via Doppler shifting of intrinsic MRGWs. Consequently, the spectral power distribution of equatorial meridional wind perturbations across pressure levels represents a composite of MRGWs that have been Doppler shifted by a range of background flow regimes.