Medium‐frequency radar studies of gravity‐wave seasonal variations over Hawaii (22°N, 160°W)

Using simple numerical filters, estimates of wind variances with period bands of 0.1 to 1 hour and 1 to 5 hours have been derived from data taken with the medium‐frequency radar on the island of Kauai, Hawaii (22°N, 160°W). The observations cover altitudes of 70 to 90 km and extend over the years 1990–2000. The results show seasonal and interannual variations of the wind variances, which can be attributed to atmospheric gravity waves. The mean zonal wind has mainly an annual variation below an altitude of ∼83 km and a semiannual variation above. The gravity waves have maximum intensity at the solstices below ∼83 km, but at higher altitudes the times of the maxima shift to the equinoxes. Numerical simulations suggest this behavior results from the dependence of gravity‐wave generation and propagation on the background wind and temperature.