Seasonal variation of planetary wave momentum flux and the forcing towards mean flow acceleration in the MLT region

Monthly and seasonal variations of vertical flux of zonal and meridional momentum (u′w′ and v′w′) for the prominently present planetary waves (2–4 day and 6–8 day period) in the Mesospheric Lower Thermospheric (MLT) region are estimated by making use of the horizontal and vertical wind information obtained from SKiYMET Meteor Wind radar observations at Trivandrum (8.5°N, 77°E), during the period April 2006–April 2007. Substantial transport of momentum is observed during equinox and winter season over the station. The momentum flux values estimated from three dimensional wind velocities obtained from SKiYMET radar are compared with that calculated by the indirect method of cross-spectra from radar measured zonal winds and temperature from the SABER onboard TIMED satellite and found to be in good agreement. An attempt is made to calculate the forcing towards the mean flow by the 2–4 day and 6–8 day planetary waves in the 82–98 km height region. The forcing by 6–8 day wave towards the mean flow acceleration is found to be more during winter and vernal equinox and by 2–4 day is more during summer and both the equinoxes. The quantitative estimation of forcing towards the mean flow acceleration show generally 30–80% contribution from both 2–4 day wave and 6–8 day wave, which varies with season also. The study presented in this paper is the first of its kind, quantitatively estimating the momentum flux values of short period planetary waves in the MLT region at an equatorial station and their forcing towards the mean flow.

► Momentum flux of 2–8 day period waves in MLT region from radar measured 3D winds.
► Momentum flux from cross-spectra of SABER measured and radar measured zonal winds.
► Momentum flux from two methods are compared and found to be in good agreement.
► Mean flow acceleration estimated from momentum flux divergence and from zonal winds.
► Quantitative estimation of the forcing from planetary waves towards the mean flow.