A study of the dynamics of the equatorial lower stratosphere by use of ultra-long-duration balloons 2. Gravity waves

This study is the companion paper of Vial et al. [this issue]. A campaign o f ultra-long-duration, superpressure balloons in the equatorial lower strat osphere was held in September 1998. By conception these balloons evolve on isopycnic surfaces. Pressure and position were measured every 12 min, which enable to infer the characteristics of gravity waves with periods between 1 hour and 1 day in this region of the atmosphere. The intrinsic-frequency spectra of horizontal wind fluctuations exhibit a -2 slope, while the one a ssociated with vertical-wind fluctuations is flat. Significant inhomogeneit y of the wave activity is observed, and the variance of the shortest freque ncy waves is found to be linked to the position of the balloons with respec t to the Intertropical Convergence Zone. On average, the total energy assoc iated with gravity waves in the period range studied in this paper is found to be similar to 7 J kg(-1). Calculations of momentum flux have also been undertaken. It appears that there is an approximate equipartition of flux b etween eastward and westward propagating gravity waves and that the absolut e value of the flux is 8 - 12 x 10(-3) m(2) s(-2) at 20 km. A larger flux i s also observed above convective regions. These values suggest that gravity waves may carry the largest part of the Eliassen-Palm flux required for th e driving of the quasi-biennial oscillation.