SEASONAL-VARIATION OF UPPER-TROPOSPHERIC AND LOWER STRATOSPHERIC EQUATORIAL WAVES OVER THE TROPICAL PACIFIC

Upper tropospheric and lower stratospheric wind data spanning 31 years from 1964 to 1994 were analyzed at rawinsonde stations in the central /western Pacific. Traditional spectral and cross-spectral analysis led to the conclusion that there is a significant signal with periods bet ween 3 and 4.5 days, which the authors link with the dominant antisymm etric waves predicted, by theory to have these periods, mixed Rossby-g ravity waves, and equatorial Rossby waves. Then the authors applied th e seasonally varying spectral analysis method developed by Madden to s tudy the average seasonal variation of these waves. The seasonally var ying analysis suggested that there are significant twice-yearly maxima in equatorial wave activity throughout the upper troposphere and lowe r stratosphere, with peaks occurring in late winter-spring and in late summer-fall. The twice-yearly signal was most prominent at the 70-hPa and 100-hPa levels. Similar and consistent results were also shown by an autoregressive cyclic spectral analysis. The cyclic spectral analy sis suggested that the frequency characteristics of the u-wind wave po wer are different during the two maxima at some stations. In addition, the seasonally varying squared coherence between the u and v winds an d the associated phase implied that there is horizontal momentum flux associated with these waves and that the sign of the flux is different during the two maxima. The differences in wave characteristics during the maxima periods may be related to different wave modes, seasonal v ariation of the basic zonal state, or possibly to different equatorial wave forcing mechanisms.(i.e., convective versus lateral excitations) .