Day‐to‐day migrating and nonmigrating tidal variability due to the six‐day planetary wave

Abstract

To investigate day‐to‐day variability in the mesosphere and lower thermosphere (MLT), an idealized simulation of a six‐day westward propagating zonal wave number‐1 planetary wave is performed using the National Center for Atmospheric Research (NCAR) Thermosphere‐Ionosphere‐Mesosphere‐Electrodynamics General Circulation Model (TIME‐GCM). The six‐day planetary wave introduces a six‐day periodicity in the zonal mean atmosphere, migrating and nonmigrating tides, as well as in secondary waves that are produced by nonlinear planetary wave‐tide interactions. We have further used the linear Global Scale Wave Model (GSWM) to isolate the effect of how the day‐to‐day changes in zonal mean zonal winds may influence tides in the MLT. The most significant changes are observed in the migrating diurnal tide (DW1), eastward propagating nonmigrating tides with zonal wave numbers‐2 and ‐3 (DE2 and DE3), and a 20 hr eastward propagating wave with zonal wave number‐2 (20E2). Because we have included the lower atmospheric source of nonmigrating tides, DE2 and DE3 are present with relatively large amplitudes in the MLT, even in the absence of planetary wave forcing. The 20E2 wave is produced by the nonlinear interaction between the DE3 and the six‐day planetary wave, and its large amplitude indicates the importance of including the realistic spectra of nonmigrating tides in numerical simulations of planetary waves. The GSWM simulations reveal that the DW1 is not significantly influenced by the changes in the zonal mean winds. We thus conclude that the DW1 changes are driven by a combination of changes due to nonlinear interaction with the six‐day planetary wave as well as changes due to zonal asymmetries that result from the six‐day planetary wave. The six‐day planetary wave induced changes in zonal mean zonal winds lead to a general reduction in the amplitude of DE2 and DE3, and introduce a slight periodic behavior in these tides. The effect of changing zonal mean zonal winds appears to be the primary driver of the changes in the DE2. However, for DE3, although the changes that can be attributed to zonal mean zonal wind variability are not insignificant, the primary driver of the DE3 perturbations appears to be the nonlinear interaction with the six‐day planetary wave. Last, we demonstrate that the day‐to‐day changes in the DE3 introduce similar day‐to‐day changes in the daytime wave number‐4 longitude structure in the low‐latitude ionosphere. These results indicate that short‐term variability in the low‐latitude ionosphere is likely to be driven by similar short‐term variability in nonmigrating tides in the MLT.