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 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 oscillations L(t)are more important than the seasonal cycle Y(t) in determining the onset and withdrawal of the summer monsoon over South Asia. 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. Thus, the correlation between the seasonal cycle Y(t) and the amplitude of low-frequency oscillations L(t) or L2(t) becomes negative in these monsoon regions and indicates a possible nonlinear interaction of the low-frequency variations with the seasonal cycle. Near the equator, low-frequency variations appear to exhibit little seasonality. An approximate inverse relationship exists between the low-frequency oscillations L(t) and the amplitude of the transient perturbations T(t) or T2(t). Namely, transient disturbances tend to be more active [large T2(t)] during the wet phase of L(t), while their activity is depressed [small T2(t)] during the dry half-cycle of L(t). Thus, the low-frequency oscillations are enhanced through mutual interaction with transient disturbances. This is particularly true over the Northern and Southern Hemisphere monsoon regions.