Relationships Between Seasonal Cycles, Low-Frequency Oscillations, and Transient Disturbances as Revealed from Outgoing Longwave Radiation Data

Applying harmonic analyses to outgoing longwave radiation (OLR) data at a 2.5° longitude-latitude resolution over a tropical belt between 45°N and 45°S, the harmonics (m = 1–15) were computed for each year in 1975–1977 and 1970–1983. The sum of the first three harmonics (m = 1–3), which is referred to as Y(t), corresponds to the seasonal cycles. The sum of m = 4–15 represents low-frequency oscillations, denoted as L(t), with an approximate period range of 24 to 91 days.The onset and withdrawal of the Australian summer monsoon appears to be determined by the phase changes of Y(t) and L(t). In comparison, L(t) is more important than Y(t) in determining the onset and withdrawal of the summer monsoon over southern Asia. Over the Northern Hemisphere monsoon region, the low-frequency modes exhibit seasonality, namely, they are more pronounced during the summer than the winter. The same is also true over the Southern Hemisphere monsoon region.