Long-term observations of the quasi two-day wave by Hawaii MF radar

A mesospheric horizontal wind data set measured during 1991-2006 by the medium frequency (MF) radar at Kauai, Hawaii (22 degrees N, 160 degrees W) is analyzed to examine the long-term variability of the quasi two-day wave (QTDW). The QTDW over Hawaii is amplified twice a year, with the January and July events most likely being the representation of zonal wave numbers 3 and 4 modes, respectively. The amplitudes of the January monthly mean QTDW in both meridional and zonal winds and the July monthly mean QTDW in meridional component are nearly in phase with the solar cycle but with the solar maxima leading the QTDW maxima by 1 or 2 years. However, the July monthly mean QTDW in zonal wind is more antiphase with the solar cycle. Enhanced QTDW oscillations are evident in both wind components in January 1998, which is likely related to the strong El Nino event during the winter of 1997/1998. The enhanced gravity wave activity and the increased barotropic/baroclinic/inertial instability related to the strengthened stratosphere summer easterly jet might provide additional forcing to amplify the QTDW. Moreover, the enhanced migrating diurnal tide during warm El Nino-Southern Oscillation events could also contribute to the abnormally strong QTDW by increasing the refractive index and thus the growth rate of the QTDW. Additional enhancement of the QTDW with a short period of similar to 43h is observed during the major sudden stratospheric warming in January 2006.