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

热带西太平洋对流活动是控制全球气候系统的大气能量的主要来源。本文研究了热带对流活动的季节变化、季节变化和年际变化对亚洲季风活动和热带太平洋海气相互作用的影响。研究结果表明：1)活动对流区的突变伴随着异常的季节变化。这种跃变与亚洲季风环流的爆发和日本白云的爆发有关。2)活动对流区的跃变与海表温度的季节变化有关，并通过海陆相互作用受对流活动强度的控制。3)这种年际变化是由与活动对流区跳跃相关的季节间变化的异常行为的强度引起的。另一方面，通过一个简单的正压模式实验来考察季节间和年际变化的原因。预计强迫年周期通过内部的非线性动态调制来激发其谐波和次谐波，表现为季节间和年际变化。然而，结果表明，除了与年周期强迫相关的孤立谱峰外，有或没有强迫年周期的频谱特征并没有改变。从这个数值试验的结果可以看出，诸如半年和两年一次的振荡等谐波和亚谐并不是由强迫年周期的非线性动态调制所激发的。也就是说，低频变化，如半年和两年一次的振荡，必须由大气的外部强迫而不是内部动力来激发。

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.