Dynamical and radiative forcing of the summer mesopause circulation and thermal structure: 2. Seasonal variations

Abstract

We present in this paper an extension of the steady dynamical/radiative summer mesopause model by Fritts and Luo [this issue] to examine the variability of wave forcing required to account for the observed seasonal changes of temperature. The model employs dynamical forcing provided by gravity wave energy and momentum fluxes and divergence, including wave and turbulence effects, and radiative forcing describing solar heating of O2 and O3, local thermodynamic equilibrium (LTE) and non‐LTE cooling of O3 and CO2 and chemical heating due to Oχ and HOχ chemistry parameterized as Newtonian cooling. Strong sensitivity to dynamical and radiative forcing under steady conditions provides an ability to diagnose the temporal variability of dynamical forcing required to account for the observed departures from radiative and chemical equilibrium. The occurrence of minimum mesopause temperatures somewhat after summer solstice implies a similar lag in the maximum wave forcing and is consistent with the available momentum flux measurements and inferences based on the seasonal variations of the zonal mean winds. In particular, the rapid increase in mesopause temperature during August implies a rapid cessation of wave forcing accompanying the strong eastward acceleration of the mean flow during this interval.