The Interaction of Multiple Time Scales in the Tropical Climate System

Abstract

The spatial distribution and evolution of variability of near-global SST and SLP data in the quasi-biennial (QB) and 3–7 year low-frequency (LF) period bands are investigated and described. The largest signals in both bands are in the tropics. The near-equatorial characteristics of the QB in the SLP field are those of a quasi-progressive wave while the LF variation in the same field is closer to the standing wave. Both bands show the traditional Southern Oscillation pattern. The SST variability in both bands is essentially that of El Niño. It is shown that ENSO is partially due to a nonlinear interaction between the two frequency bands. Both bands appear important to the ENSO cycle. The current work could not establish conclusively that if either was the fundamental mode, although there is weak evidence favoring the QB mode. The QB signal described here is essentially the ENSO signal and does not seem to be simply related to the stratospheric QBO. Calculations suggest the tropospheric QB described here is not due to a consistent interaction of the annual cycle with itself. The current results do not exclude the possibility that the QB is due to forcing processes which regularly switch sign with season.