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

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Abstract
Applying harmonic analyses to outgoing longwave radiation (OLR) data, harmonics m = 1 to 15 were computed for each year in 1975–77 and 1979–83. The sum of the first three harmonics (m = 1 to 3), referred to as Y(t), corresponds primarily to the seasonal cycle. The sum of m = 4 to 15 represents low-frequency oscillations, denoted as L(t), with an approximate period range of 24–91 days. The residual in the harmonic analyses is specified as T(t), which corresponds to transient components with periods shorter than 24 days. The onset and withdrawal of the Australian summer monsoon appears to be determined by the phase changes of the seasonal cycle Y(t) and the low-frequency oscillations L(t). When the phase shift from the winter [dry; Y(t) > 0] to summer [wet; Y(t) < 0] occurs earlier than normal, the Australian monsoon also commences earlier than usual. The onset generally coincides with the wet phase of the first L(t) cycle after the transition [Y(t) = 0]. In comparison, the low-frequency oscillatio... Abstract Applying harmonic analyses to outgoing longwave radiation (OLR) data, harmonics m = 1 to 15 were computed for each year in 1975–77 and 1979–83. The sum of the first three harmonics (m = 1 to 3), referred to as Y(t), corresponds primarily to the seasonal cycle. The sum of m = 4 to 15 represents low-frequency oscillations, denoted as L(t), with an approximate period range of 24–91 days. The residual in the harmonic analyses is specified as T(t), which corresponds to transient components with periods shorter than 24 days. The onset and withdrawal of the Australian summer monsoon appears to be determined by the phase changes of the seasonal cycle Y(t) and the low-frequency oscillations L(t). When the phase shift from the winter [dry; Y(t) > 0] to summer [wet; Y(t) < 0] occurs earlier than normal, the Australian monsoon also commences earlier than usual. The onset generally coincides with the wet phase of the first L(t) cycle after the transition [Y(t) = 0]. In comparison, the low-frequency oscillatio...

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