Eastward Propagation of 30–60 Day Perturbations as Revealed from Outgoing Longwave Radiation Data

Abstract

Some of the characteristic features of low-frequency oscillations were investigated by applying an extended empirical orthogonal function (EEOF) analysis to 30–60 day filtered outgoing longwave radiation (OLR) data during the eight years of.1975–77 and 1979–83. The first two EEOF eigenvectors indicate a systematic eastward propagation of low-frequency modes between about 60°E and the date line (Indian Ocean–western Pacific), where they are most predominant. They weaken considerably by the time they reach the eastern Pacific east of the date line, and their phase propagation becomes ill-organized over the equatorial South America–Atlantic Ocean–Africa region. This appears to be due to an out-of-phase relationship between zonal (Fourier) wavenumbers 1 and 2 over the equatorial western hemisphere. In contrast, there exists a general in-phase relationship between these two Fourier components, and, thus, the consolidated perturbations of wavenumbers 1 and 2 become very pronounced with a distinct eastward propagation over the equatorial eastern hemisphere. During the northern summer, low-frequency OLR perturbations are most pronounced over the Bay of Bengal–western North Pacific region and propagate systematically northward with an approximate phase speed of 1.2 degrees of latitude per day. In comparison, during northern winter, low-frequency OLR perturbations, which are prominent over the equatorial Indian Ocean and the equatorial western Pacific, do not exhibit any systematic meridionial propagation. Thus, there exists a distinct seasonal difference in the meridional phase propagation between summer and winter. The equatorial low-frequency OLR modes tend to propagate eastward in both summer and winter. However, the eastward propagation is slightly more clear in summer than in winter. This slight seasonal difference in the zonal phase propagation was confirmed by comparing the first two EEOF eigenvectors obtained for summer with those for winter. Occasionally, low-frequency OLR perturbations exhibit irregular movement even in summer. During such occurrences, coefficients of the first two eigenvectors become much smaller than normal. In contrast, occasions of above-normal coefficients are characterized by well-defined eastward propagation. The change in the intensity of eigenvector coefficients appear to be associated with the amplitude modulation of low-frequency oscillations that are characterized by a wide period range from about 30 to 60 days. Such amplitude modulation is perhaps responsible for the year-to-year and/or season-to-season differences in the nature (intensity and phase propagation) of low-frequency modes.