Three‐dimensional structures of tropical nonmigrating tides in a high‐vertical‐resolution general circulation model

Abstract

This paper investigates nonmigrating tides from the ground to the lower mesosphere using data from a high-resolution general circulation model (KANTO GCM), as well as observational data from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on board the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics satellite and from GPS radio occultation measurements obtained with the COSMIC/FORMOSAT-3 mission. We extract nonmigrating tides using a composite as a function of universal time in physical space, without performing a zonal wave number decomposition. The KANTO GCM clearly demonstrates that tropical nonmigrating tides are regarded as gravity waves excited by diabatic heating enhanced over two major continents, specifically Africa and South America. They propagate zonally, in a direction away from their sources; that is, west and eastward propagating waves are dominant on the western and eastern sides of the continents, respectively. These characteristics are observed in two satellite data sets as well, except that the amplitudes in the KANTO GCM are larger than those in the observations. Seasonal variations of nonmigrating tides are also investigated. It is suggested that filtering owing to the stratopause semiannual oscillation, as well as diabatic heating in the troposphere, is important for the seasonal variations of nonmigrating tides in the stratosphere and the lower mesosphere.

Representation of solar tides in the stratosphere and lower mesosphere in state-of-the-art reanalyses and in satellite observations

“…All data sets show that zonal wavenumber 0 (socalled D0; particularly for antisymmetric components as seen in Fig. 14), westward zonal wavenumbers 5 and 2 (DW5 and DW2), and eastward zonal wavenumber 3 (DE3) are dominant, which is consistent with previous studies (Forbes and Wu., 2006;Zhang et al, 2006;Sakazaki et al, 2015b). Particularly, DW5 in the stratosphere corresponds to the clear westward-tilting waves in Fig.…”

“…The biggest outliers are JRA-55C and JRA-55AMIP, which seem to display somewhat larger amplitudes than the full reanalyses. It may be worth mentioning that Sakazaki et al (2015b) also noted that the amplitude in their model (a free-running model) was significantly larger than that for SABER and COSMIC.…”

“…As described by Sakazaki et al (2015b), before further analysis, data were averaged in bins of 15 • longitude, 5 • latitude, and 2 km in log-pressure vertical coordinates for each day and for each ascending and descending node. We emphasize that SABER data were not assimilated into any of the reanalyses used in this study.…”

“…The analysis procedure to extract these three components basically follows the method proposed by Sakazaki et al (2015b) as briefly explained below.…”

“…Such asymmetry may be explained by two factors. First, the major excitation regions are confined around −60 to +20 • E (see also Sakazaki et al, 2015b); because waves are subject to dissipation during the horizontal propagation, westward waves are likely dominant to the west of −60 • E and eastward waves are dominant to the east of 20 • E. Secondly, westward signals are clearer between −60 and +20 • E, even though in this region both westward waves (from Africa) and eastward waves (from South America) might be equally important. This asymmetry is likely because westward waves (mainly wavenumber 5) are more efficiently excited by tropospheric heating than eastward waves (mainly wavenumber 3; see also Fig.…”