The 11 year solar cycle signature on wave-driven dynamics in WACCM

This study describes the influence of the 11 year solar cycle on gravity waves and the wave-driven circulation, using an ensemble of six simulations of the period from 1955 to 2005 along with fixed solar maximum and minimum simulations of the Whole Atmospheric Community Climate Model (WACCM). Solar cycle signals are estimated by calculating the difference between solar maximum and minimum conditions. Simulations under both time-varying and fixed solar inputs show statistically significant responses in temperatures and winds in the Southern Hemisphere (SH) during austral winter and spring. At solar maximum, the monthly mean, zonal mean temperature in the SH from July to October is cooler (~1-3 K) in the stratosphere and warmer (~1-4 K) in the mesosphere and the lower thermosphere (MLT). In solar maximum years, the SH polar vortex is more stable and its eastward speed is about 5-8 m s⁻¹ greater than during solar minimum. The increase in the eastward wind propagates downward and poleward from July to October in the SH. Because of increase in the eastward wind, the propagation of eastward gravity waves to the MLT is reduced. This results in a net westward response in gravity wave drag, peaking at ~10 m s⁻¹ d⁻¹ in the SH high-latitude MLT. These changes in gravity wave drag modify the wave-induced residual circulation, and this contributes to the warming of ~1-4 K in the MLT.