Temporal Variations of Satellite-Observed Outgoing Longwave Radiation over the Winter Monsoon Region. Part II: Short-Period (4–6 Day) Oscillations

Abstract

Empirical orthogonal function analysis was applied to outgoing longwave radiation (OLR) data over three limited regions during the three winters of 1974?75, 1975?76 and 1976?77. Regional composite maps were constructed by truncating the eigenvector series of 4?6 day OLR anomalies for each winter at 70% representation. Based on these composite charts, an attempt was made to investigate the phase relationships between changes in truncated 4?6 day filtered OLR data at a selected reference point, and those in other areas within the same region. Over region 1(25°N?O°, 95?160°E), the winters of 1974?75 and 1976?77 have similar features in 4?6 day OLR disturbances, with systematic westward phase propagation in equatorial latitudes over the western North Pacific (?8 m s?1) and the South China Sea (?5 m s?1). In contrast, the 1975?76 winter is of a different character with eastward (westward) phase propagation to the west (cast) of about 130°E at equatorial latitudes. These large year-to-year changes in phase propagation appear to be related to inter-annual differences in monsoonal surge activity near the East China Sea-Japan region. Over region 2(5°N?20°S, 90?155°E), OLR perturbations are most active over the Arafura Sea at around 10°S, 130°E. The existence of cross-equatorial, southward propagation of OLR perturbations along about 120?130°E is a common feature in all three winters. Intensification of negative OLR perturbations (increase in convection or rainfall) over the Arafura Sea region takes place approximately 3 days after the occurrence of large negative OLR anomalies near the Philippines. Over region 3 (10°N?15°8, 50?115°E), the equatorial latitudes in the Southern Hemisphere are characterized by westward phase propagation in each of the three winters. OLR perturbations intensify as they approach 70°E and weaken after passing through 60°E. They also tend to move northward along the 60?70°E meridians from about 15°S to 10°N. These longitudes appear to correspond to a region of maximum penetration of Southern Hemisphere tropical effects and, perhaps, interhemispheric interactions in 4?6 day filtered OLR perturbations.