TIDAL GENERATION OF THE PHASE-LOCKED 2-DAY WAVE IN THE SOUTHERN-HEMISPHERE SUMMER BY WAVE-WAVE INTERACTIONS

The quasi-2-day wave is a prominent dynamical feature of the mesopause region. The onset of the large-amplitude summertime 2-day wave in the southern hemisphere is characterized by a sudden increase in amplitud e and by phase locking so that the wind maximum occurs at nearly the s ame local time from cycle to cycle. The phase locking suggests a linka ge to a diurnally repeatable phenomenon. We suggest that the phase-loc ked 2-day wave is a nearly resonant wave forced parametrically by the zonally symmetric diurnal tides. Recent observations and modeling supp ort significant tidal forcing. It may also be possible for the migrati ng tides to generate the 2-day wave nonlinearly through a set of mutua lly reinforcing interactions. The process involves the self-excitation of the 2-day wave by an interaction with a diurnal zonal wavenumber 6 oscillation, itself generated nonlinearly in a two-step process initi ated by the interaction of the 2-day wave and the migrating tides. Sud den onset, large amplitude, and phase locking are all consistent with the instability of a wave that has become resonantly tuned to a subhar monic frequency of the tides by the background wind system.