Longitudinal variations in planetary wave activity in the equatorial mesosphere

Abstract

Zonal and meridional winds in the equatorial mesosphere and lower thermosphere (65-98 km) measured at two sites separated by 94• in longitude are used to study the zonal structure of planetary-scale waves. The data were obtained with MF radars located at Pontianak (0 • N, 109• E)and Christmas Island (2 • N, 157• W). The data at Christmas Island were collected from January 1990 to December 1997 and the observations at Pontianak were made from November 1995 to July 1997. Power spectral techniques are used to study the amplitude and frequency variations of long-period oscillations as a function of height and time. A mean climatology of these variations taken from years 1990-1997 is presented. Strong peaks in zonal and meridional winds are found at tidal periods and for the quasi 2-day wave. Zonal spectra exhibit considerable power at periods of 3-10 days, with transient oscillations with periods near 3.5 day and 6.5 days being especially prominent. The 6.5-day wave is particularly strong during April and September. Examination of the phase differences obtained from cross-spectra between the two stations show that the 6.5-day wave is westward propagating with zonal wavenumber 1, while the 3.5 day wave is eastward propagating with wavenumber 1. The 6.5-day wave is identified as a manifestation of an unstable mode, while the 3.5-day wave is identified as an ultrafast Kelvin wave. There are significant longitudinal variations in the amplitudes and inferred momentum fluxes of the 3.5-day wave, amplitudes being larger in the Asian region than in the central Pacific.

The 5-8-Day Kelvin and Rossby Waves in the Tropics as Revealed by Ground and Satellite-Based Observations

“…The amplitude increase and phase decrease with increasing altitude appear to be continuous from the lower stratosphere to MLT heights. There have been several observations and theoretical studies on planetary scale waves of having periods of between five and eight days in the mesosphere and lower thermosphere (Wu et al 1994;Meyer and Forbes 1997;Kovalam et al 1999;Talaat et al 2001Talaat et al , 2002Lieberman et al 2003;Liu et al 2004). As in the present study, these waves have, in general, been identified as symmetric Rossby waves.…”

“…Previous observations of the 6.5-day wave in the MLT region show that the wave is mainly an equinoctial phenomenon (for example, Kovalam et al 1999;Talaat et al 2001). In order to determine whether similar periodicity is present during the equinoctial months of other years, the temporal variation of the W1 component of the 5-8-day wave for the years 1993-2006 is shown in Fig.…”

Ultrafast Kelvin waves in the MLT airglow and wind, and their interaction with the atmospheric tides

“…The UFKW is believed to play an important role in the well-known intraseasonal (ISO), semiannual (SAO) and quasibiennial (QBO) oscillations by depositing eastward momentum into the mean flow (e.g., Andrews, 1987;Miyoshi and Fujiwara, 2006). Deposition of momentum is found to induce zonal mean flow acceleration of up to 5 m s −1 day −1 (Riggin et al, 1997;Kovalam et al, 1999;Sridharan et al, 2002;Davis et al, 2012).…”

“…The first reports of the UFKW activity in the MLT region go back to the 1990s (Vincent and Lesicar, 1991;Vincent et al, 1993). The UFKW presents zonal phase speed ranging approximately from 100 to 130 m s −1 , and the zonal wavenumber one is dominant (e.g., Salby et al, 1984;Kovalam et al, 1999;Forbes et al, 2009). The UFKW is believed to play an important role in the well-known intraseasonal (ISO), semiannual (SAO) and quasibiennial (QBO) oscillations by depositing eastward momentum into the mean flow (e.g., Andrews, 1987;Miyoshi and Fujiwara, 2006).…”

Interhemispheric structure and variability of the 5-day planetary wave from meteor radar wind measurements

“…Kovalam et al (1999) demonstrated that the 6.5-day wave is westward propagating with zonal wavenumber 1 and has an enhanced amplitude in April and September using MF radars at the equatorial sites of Pontianak (0 • , 109 • E) and Christmas Island (2 • N, 157 • W). The 6.5-day wave at Pontianak and Christmas Island was compared with that observed at Yamagawa (32 • N, 131 • E) by Isoda et al (2002), and all yielded similar enhancements from mid-April to mid-May.…”