Seasonal and interannual variability of gravity wave activity revealed by long-term lidar observations over Mauna Loa Observatory, Hawaii

The seasonal and interannual variability of gravity wave (GW) variance in the upper stratosphere (35-50 km) and lower mesosphere (48-63 km) has been studied using 10.5 years (January 1997 to June 2007) of temperature profile results obtained with the Jet Propulsion Laboratory Rayleigh lidar at Mauna Loa Observatory, Hawaii (19.5 degrees N, 155.6 degrees W). Seasonal variability with a maximum in winter and a minimum in summer was observed in the upper stratosphere, suggesting dominance of the annual oscillation. In the lower mesosphere the seasonal oscillations of GW variance were dominated by a semiannual oscillation (SAO), likely due to the selective filtering of GWs by the tropical upper stratospheric SAO wind. Modulation of GW variance by the quasi-biennial oscillation was clearly present only for the long vertical wavelength band in the upper stratosphere, and not in the lower mesosphere. The United Kingdom Met Office zonal mean zonal wind further supports that enhanced GW activity in the upper stratosphere corresponds to the westerly shear phase of the zonal wind at 10 hPa (similar to 30 km), and suppressed activity corresponds to the easterly shear phase. During the strong El Nino event in the winter of 1997-1998, enhanced GW activity was observed only in the lower mesosphere, and not in the upper stratosphere. Additional enhancement of GW variance, especially clear in the upper stratosphere, was also found during 2001-2002 and winter 2005-2006.