The quasi‐two‐day wave observed in the equatorial middle atmosphere

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Abstract
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°N, 157° 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.