QUASI-QUADRENNIAL AND QUASI-BIENNIAL VARIABILITY IN THE EQUATORIAL PACIFIC

Evaluation of competing El Nino/Southern Oscillation (ENSO) theories r equires one to identify separate spectral peaks in equatorial wind and sea-surface temperature (SST) time series. To sharpen this identifica tion, we examine the seasonal-to-interannual variability of these fiel ds by the data-adaptive method of multi-channel singular spectrum anal ysis (M-SSA). M-SSA is applied to the equatorial band (4 degrees N-4 d egrees S), using 1950-1990 data from the Comprehensive Ocean and Atmos phere Data Set. Two major interannual oscillations are found in the eq uatorial SST and surface zonal wind fields, U. The main peak is center ed at about 52-months; we refer to it as the quasi-qundrennial (QQ) mo de. Quasi-biennial (QB) variability is split between two modes, with p eriods near 28 months and 24 months. A faster, 15-month oscillation ha s smaller amplitude. The QQ mode dominates the variance and has the mo st distinct spectral peak. In time-longitude reconstructions of this m ode, the SST has the form of a standing oscillation in the eastern equ atorial Pacific, while the U-field is dominated by a standing oscillat ion pattern in the western Pacific and exhibits also slight eastward p ropagation in the central and western Pacific. The locations of maximu m anomalies in both QB modes are similar to those of the QQ mode, Slig ht westward migration in SST, across the eastern and central, and east ward propagation of U, across the western and central Pacific, are fou nd. The significant wind anomaly covers a smaller region than for the QQ. The QQ and QB modes together represent the ENSO variability well a nd interfere constructively during major events. The sharper definitio n of the QQ spectral peak and its dominance are consistent with the '' devil's staircase'' interaction mechanism between the annual cycle and ENSO.