Nonlinear interactions in the upper atmosphere: The s = 1 and s = 3 nonmigrating semidiurnal tides

Abstract

A semidiurnal oscillation with zonal wave number s = 1 has been observed over the South Pole, and longitudinal variations of the semidiurnal tide at northern midlatitudes have been ascribed to the presence of nonmigrating tides. A three‐dimensional nonlinear spectral model is used herein to study nonlinear wave‐wave interaction as a possible mechanism for the generation of nonmigrating tides in the mesosphere and lower thermosphere. In particular, the s = 1 and s = 3 nonmigrating semidiurnal tides are studied as a by‐product of the nonlinear interaction between the semidiurnal s = 2 migrating tide and a stationary s = 1 planetary wave in the lower and middle atmosphere. This process is found to yield significant amplitudes (∼10–30 m s−1) of the nonmigrating semidiurnal components with s = 1 and s = 3 in the upper atmosphere. Comparisons with monthly nonmigrating tidal amplitudes and phases at 95 km derived from High Resolution Doppler Imager and Wind Imaging Interferometer wind measurements from UARS show good agreement with the model simulations. The effect of the dissipating nonmigrating tides on the zonal mean circulation is found to reinforce that of the migrating component, contributing ∼10–20 m s−1 to the zonal wind field and ∼5–10 m s−1 to the mean meridional wind field. Several other wave components are generated through nonlinear interactions, including 6‐hour tides with zonal wave numbers s = 0, 1, 4 and a semidiurnal oscillation with s = 6. These wave components have amplitudes in the range 6–30 m s−1 in the 110–150 km height regime. The combination of all nonmigrating tides together with the migrating component suggests significant longitudinal variations in the semidiurnal tide in the E region which may be important for studies of the quiet‐time generation of ionospheric electric fields by the dynamo mechanism.