Observed Modal Characteristics of the Intraseasonal Oscillation

Abstract

Global datasets produced by the Geophysical Fluid Dynamics Laboratory for the 1979 summer season are used to study the observed three-dimensional structure of the tropical 30?50 day oscillation. Decomposition of them data onto horizontal and vertical functions obtained as solutions of a linearized primitive equation model reveals a dominant vertical structure with a 256 m equivalent depth, a zonal wavenumber 1, and meridional structures projecting mainly on the two Rossby modes with largest meridional length (R1 and R2) and on the Kelvin (K) model. The R1 and K modes exhibit a regular eastward propagation circling the globe 2.5 times during the 16-week period studied here. The amplitude of the K mode is smaller than that of the RI mode. Correlations with outgoing longwave radiation values are presented that show the 200 mb Kelvin westerly maximum located about 45° long east of the outgoing longwave radiation minimum. The Kelvin mode phase speed remains constant during this period. Some evidence is presented for the existence of westward propagating R1 modes, though these episodes last only up to 2 weeks. The amplitude of the Kelvin mode decreases east of the dateline during the first cycle of the oscillation. This is not the case for the R1 mode. The K and R1 modes both amplify over the central and eastern Pacific Ocean during the second cycle. Such amplifications require alternative explanations from those previously presented in the literature which predict a decay of the oscillation over regions where the atmospheric heating is cut off.