Nonlinear singular spectrum analysis of the tropical stratospheric wind

The neural-network-based nonlinear singular spectrum analysis (NLSSA) is applied to the zonal winds in the 70-10 hPa region (roughly 20-30 km altitude) measured at near-equatorial stations during 1956-2000. The data are pre-filtered by the linear singular spectrum analysis (SSA), with the leading eight SSA principal components (PCs) used as inputs for the NLSSA. The NLSSA fits a curve to the data in the eight-dimensional PC space. This NLSSA curve, when projected onto the two-dimensional plane spanned by any two PCs, shows the relation between the two SSA PCs. As different SSA modes are associated with different time-scales, the relations found by the NLSSA reveal the time-scales between which there are interactions-interactions between the dominant quasi-biennial oscillation (QBO) time-scale of about 28 months and the first harmonic at 14 months, and between 28 months and 12 months are found. The anharmonic nature of the QBO is well represented by the NLSSA mode 1, but not by individual SSA modes. The NLSSA is also applied to the time series of the zonal-wind acceleration.