Rol of the seasonal cycle and interseasonal variability on the interannual variability of the climate system.

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The convective activities at the tropical western Pacific is the major source of atmospheric energy which controls the global climate system. This study has investigated the role of the interseasonal variability, seasonal cycle, and interannual variability of the convective activities there upon the activities in Asian monsoon and the atmosphere-ocean interactions at the tropical Pacific.The result of this study may be summarized as follows :1) An abrupt shift of the active convection area in conjunction with the anomalous interseasonal variability is found. This jump is related to the onset of monsoonal circulation in Asia and the onset of Baiu in Japan.2) The jump of the active convection area is related to the seasonal change of sea surface temperature and is controlled by the intensity of the convection activities through the land-sea interactions.3) The interannual variability is resulted from the intensity of the unusual behavior of the interseasonal variability associated with the jump of the active convection area.On the other hand, a simple barotropic model experiment is carried out to examine the cause of the interseasonal and interannual variabilities. The forced annual cycle is expected to excite its harmonics and subharmonics by the internal nonlinear dynamic modulationwhich tern out to be interseasonal and internnual variabilities. The results, however, show that the spectral features with and without the forced annual cycle are not altered by the inclusion of the annual cycle, except for the isolated spectral peak associated with the annual cycle forcing.We suggest from the results of this numerical experiment that the harmonics and subharmonics, such as semiannual and biennial oscillations, are not excited by the nonlinear dynamic modulation of the forced annual cycle. Namely, the low-frequency variabilities, such as the semiannual and biennial oscillations, must be excited by the external forcing of the atmosphere rather than the internal dynamics.