Equatorial Inertia-Gravity Waves in the Lower Stratosphere Revealed by TOGA-COARE TOP Data

Abstract

Short-period disturbances in the equatorial lower stratosphere are analyzed by using radiosonde data of horizontal winds and temperature at 10 stations during the TOGA-COARE intensive observational period (IOP). The analysis is focused on disturbances whose periods are shorter than about 3 days.
Time-height cross sections of both horizontal winds and temperature show the existence of disturbances with periods of about two days and vertical wavelengths of 3-5km. First, we made power and cross-spectral analyses. Dominant disturbances in northern [7°N-8°N] and southern [9°S-11°S] areas of the TOGA-COARE Large-scale Soundings Array (LSA) regions, propagate eastward and have similar structure in which vertical and horizontal wavelengths are 3-4km and several to ten thousand kilometers, respectively. The phase differences between the zonal and meridional wind components at one location, and the phase differences in wind components between the northen and southern regions, exhibit the typical structure of equatorially-trapped inertia-gravity waves. In the TOGA-COARE LSA equatorial area, dominant disturbances have vertical wavelengths of 4-5km which are a little longer than those observed in the northern and southern areas. Phase differences are clear for meridional wind components between zonally separated stations, and show that the horizontal wavelengths are three to four thousand kilometers, and that the disturbances propagate eastward. On the other hand, phase differences between tonally separated stations for zonal wind components are not significant.
Next, a composite analysis was made with a reference of convective activity in the troposphere. Short period disturbances with 3-5km vertical wavelengths, which are consistent with the results of the spectral analysis, appear in the composite of lag-time-height sections in the lower stratosphere, indicating that these disturbances be strongly linked to the convective activity. Vertical phase propagation is downward in the upper troposphere and lower stratosphere, showing that these disturbances propagate energy upward there.
Possible equatorially-trapped modes having the wave structures revealed by the above spectral and composite analysis are estimated using the theoretical dispersion relation. It is likely that the disturbances in the northern and southern areas correspond to n=1 eastward inertia-gravity waves. The possible modes in the equatorial regions are different for zonal and meridional wind disturbances, because odd (even) modes have zonal (meridional) wind component only.
The estimated wave parameters for meridional wind disturbances are consistent with n=0 inertia-gravity waves propagating eastward. The disturbances dominant in the zonal wind component in the equatorial areas which do not show clear coherent zonal structure are probably due to a mixture of n=-1 Kelvin waves, n=1 eastward and westward inertia-gravity waves.