Relationship among seasonal cycles, low-frequency oscillations, and transient disturbances as revealed from outgoing longwave radiation data.

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 analysis 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). Over the Northern Hemisphere monsoon region, the low-frequency oscillations exhibit seasonality; namely, they are more pronounced during summer than winter. The same is also true over the Southern Hemisphere monsoon region. An approximate inverse relationship exists between the low-frequency oscillations L(t) and the amplitude of the transient perturbations T(t) or T2(t). -from Authors