Interaction of the 2-day wave with solar tides

Nonlinear integrations with a three-dimensional model of the middle and upper atmosphere are used to study how the 2-day wave interacts with solar tides. At small 2-day wave amplitude, the modulation by tides is linear and, therefore, largely reversible. At large 2-day wave amplitude, however, the modulation leads to a nonlinear interaction that is irreversible. Local reinforcement, chiefly by the diurnal tide, introduces local instability and wave breaking. This nonlinear behavior results in a cascade of variance to small scales, at the expense of large-scale wave activity. By accelerating wave damping, this process yields slower amplification and the 2-day wave being limited to amplitudes at which nonlinear interaction prevails. At altitudes of strong tidal amplitude, regular propagation is disrupted, replaced by broadband behavior associated with secondary scales that are generated nonlinearly through eddy mixing. The same process influences solar tides. However, because they are continually forced, tidal amplitudes are reduced only modestly, by 10-20%. This contrasts with the 2-day wave, which, for sufficiently strong tides, is limited to small amplitude.