Gravity wave excitation and momentum transport in the solar interior: implications for a residual circulation and lithium depletion

We present a conceptual model for the excitation, filtering, and anisotropic propagation of gravity waves in the stratified solar interior at and below the base of the convection zone. Excitation occurs via penetrative convection into the stratified and sheared interior, where gravity waves (or g-modes) are excited on spatial and temporal scales imposed by convection and the zonal shearing due to differential rotation. The resulting wave spectrum propagates into the solar interior with increasing anisotropy in the horizontal azimuth of propagation with increasing depth. This is due both to the preferential excitation of waves propagating against the mean flow in shear and to the filtering of the spectrum by the mean shear below the source depth. Anisotropic propagation into the solar interior induces momentum transports and accompanying body forces where the waves undergo dissipation. Because the radial shear of the zonal motion reverses sign at ∼ 37, these momentum fluxes and their associated body forces are prograde at lower latitudes and retrograde at high latitudes with respect to the nearly solidbody rotation of the core. The implications of this forcing in the absence of thermal diffusion on the large scale motions are an induced residual circulation providing Coriolis torques that balance the body forces and a systematic overturning at outer radii of the solar radiative interior. For plausible estimates of the relevant spatial scales and magnitudes of gravity wave forcing, we find that the induced circulation penetrates to depths at which Lithium is destroyed and occurs on time scales that are consistent with its observed depletion and the age of the Sun. Using the same estimates, we also find that these processes cannot contribute significantly to Beryllium depletion on the same time scales.