Temporal Variations of Satellite-Observed Outgoing Longwave Radiation over the Winter Monsoon Region. Part I: Long-Period (15–30 Day) Oscillations

Abstract

Empirical orthogonal function analysis was applied to outgoing longwave radiation (OLR) data obtained from NOAA polar orbiting satellites during the winter months in 1974–77, over the monsoon region extending from 50°E to the date-line and from 30°N to 20°S. Spectral analyses of the amplitude functions for the ten largest eigenvectors exhibit marked peaks in a period range of about 15 to 30 days. Standard deviations of 15°30 day filtered OLR data am extremely large over the Arafura-Indonesian Seas region with a maximum at 10°S and 130°E. A compositing technique was applied to 15–30 day filtered OLR data to investigate the relationships between long-period changes at a reference point (10°S, 130°E) and those over other regions. Composite maps, which were constructed by considering only the first ten eigenvectors, indicate distinct southward propagation of OLR perturbations along 100–115°E from about 25°N to 20°S. These longitudes appear to correspond to a region of maximum penetration of Northern Hemisphere midlatitude effects and, perhaps, interhemispheric interactions in 15–30 day filtered OLR fields. Intensification of negative OLR perturbations over the South China Sea-Bay of Bengal region, which is presumably associated with active winter monsoonal surges, tends to occur about 5–7 days prior to a decrease in OLR values (increase in convection and/or rainfall) over the Arafura-Indonesian Seas region.