Can Background Circulation Facilitate Intraseasonal Mixed Rossby-Gravity Waves over theCentral-Eastern Pacific?

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. Furthermore, the intraseasonal disturbances are confined to the upper troposphere.  The spatial structure and dynamical characteristics of these disturbances are consistent with those of mixed Rossby-gravity waves (MRGWs), indicating the presence of intraseasonal MRGWs in the atmosphere. 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 not only explain the emergence of intraseasonal MRGWs but also the overall distribution of 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.