Temporal variability of the 40–50-day oscillation in tropical convection

Abstract

The temporal variability of the structure and two-dimensional phase propagation of the 40–50-day oscillation in tropical convection are diagnosed using complex empirical orthogonal function (CEOF) analysis of the 1974–1988 outgoing longwave radiation (OLR) data.

Results from autoregressive spectrum analysis of the OLR time series indicate that apart from the well-known 40–50-day peak, there are other significant spectral peaks near 20–30 and 17 days. In much of the tropics, excluding the equatorial Indian and western Pacific Oceans, these higher frequency peaks appear to be distinct from the 40–50 day spectral peak.

The dominant spatial mode of the 40–50-day oscillation consists of the well-known equatorial dipole and associated eastward propagation. Interactions between this mode and the extratropical 40–50-day oscillation are manifested through the propagation of convection anomalies to and from the tropics via the North Pacific, north-east Atlantic and north-east African subtropics. The sense of this propagation varies from season to season and year to year.

Analysis of the oscillation during individual summers and winters and each of the 1976–1977, 1982–1983 and 1986–1987 El Niño-Southern Oscillation (ENSO) events revealed marked variability in the oscillation's characteristics. The characteristic dipole and associated eastward propagation were suppressed during the winter of 1980–1981 and the entire 1982–1983 ENSO episode. In 1975–1976 and 1983–1984, convection anomalies were confined to the Indian Ocean and Australia sector and the spatial pattern differed from the characteristic equatorial dipole. The equatorial dipole was most pronounced in February-May 1981, December 1984 to May 1985 and during the 1976–1977 ENSO. During certain occasions, such as the 1981–1982 and 1983–1984 winters, there was westward as well as eastward propagation from an apparent heating source in the east Indian Ocean.