THE QUASI-2-DAY WAVE OBSERVED IN THE EQUATORIAL MIDDLE ATMOSPHERE

An oscillation with a period near 2 days is found in radar wind measurements made in the upper mesosphere and lower thermosphere at Christmas Island (2-degrees-N, 157-degrees-W) in the central Pacific. The oscillation is particularly strong in the meridional wind component, and seems to be present almost continuously ir the 80-100 km height region, although some intermittency is evident. Wave amplitudes are especially large about one month after the solstices, in July/August and in January/February, at times coincident with large 2-day wave events in the summer mesosphere at extratropical latitudes, Power spectral and complex demodulation techniques are used to study the amplitude and frequency variations of the oscillation as a function of height and time. Downward phase propagation is found, consistent with upward energy propagation, and with a vertical wavelength of about 70 km. An unusual feature is a shift in wave period from near 50 hours in July/August to 48 hours in January/February, when the largest amplitudes (up to 45 m s-1 ) are observed. A 16-hour oscillation is also found at times when the 2-day wave amplitudes are largest. The observations are consistent with the 2-day wave being a manifestation of the (3, 0) Rossby-gravity normal mode. Another, weaker oscillation, is also found in the meridional wind field at a period near 44 hours. This oscillation may be due to the (2, 0) normal mode.