Sources of nonmigrating tides in the tropical middle atmosphere

Abstract

Temperature measurements from the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment taken during the U.S. Space Shuttle mission STS‐66 in November 1994 provide a variety of zonally symmetric and asymmetric tidal signatures in the altitude range from 40–90 km. The most prominent zonally asymmetric tidal pattern is identified to be the westward propagating nonmigrating diurnal tide of zonal wavenumber 2 (s = −2). This mode has an amplitude of approximately half the amplitude of the migrating (s = −1) component of the diurnal tide. The observational findings are compared to model results from the Global Scale Wave Model (GSWM) and from the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME‐GCM). GSWM and TIME‐GCM account for different tidal sources and therefore allow an analysis of the tidal forcing responsible for the observed signatures. The GSWM tidal forcing in the model version used is based upon latent heat release associated with tropical deep convection while nonmigrating tides in the TIME‐GCM are predominantly forced by nonlinear interactions between the migrating diurnal tide and stationary planetary waves. It is shown that the latter mechanism forces a large part of the s = −2 mode while the stationary, zonally uniform (s = 0) diurnal nonmigrating tide is mainly driven by latent heat release. This result suggests that dynamical models for the mesosphere/lower thermosphere (MLT) region need to account for the tropospheric latent heat source and stationary planetary wave activity in order to predict realistic longitudinal diurnal tidal variability.