NSO CYCLE IN A TROPICAL COUPLED OCEAN-ATMOSPHERE ANOMALY MODEL AND ITS FORMATION MECHANISM

Yang Xiuqun(Dept. of Atmospheric Sciences. Nanjing University. Nanjing 210008)Xie Qian(Dept. of Meteorology. Air Force Institute of Meteorology. Nanjing 211101)Huang Shiong(Dept. of Atmospherc Sciences. Nanjing University. Nanjing 210008)  

A tropical Pacific coupled ocean -atmosphere anomaly model was developed for simulat ing ENSO cycle. The coupled model was run 30 years. The key features of the ENSO cycle were successfully simulated both in its horizontal spatial structure and in its temporal variabil. Based on analyses of the simulated results, a new possible negative feedback mechanism responsible for the ENSO cycle was proposed, and further a nonlinear analog model for explaining the time scale selection of the model ENSO variability was also presented. The main results show. that the model ENSO varibility involves a process with three time scales,i. e. the 3-4 year lower-frequency(LF) principle period. quasi- biennial (QB) oscillation and annual cycle (AC), that is qualitatively in agreement with the observational fact. The negative feeback mechanism that checks the unstable air - sea interaction in ENSO cycle warm phase obviously differs form that in cold phase. During the damping period of cold state, the mechanism is related to the delayed effect produced by the wave reflection at the western boundary. However, during the damping period of warm state, instead of reflection effect, the anomalous upwelling of cold water of f the equator accompanied with the strong development of unstable air-sea interaction at the equator and the nonlinear damping effect jointly restrain the instability and finally draw the system from the mature warm phase to the weak cold phase. The 3-4 year principle period oscillation (LF) is a self-exciting oscillation of nonlinear system. The time scale selection results from frequency decreased process of the QB mode that may be the intrinsic mode of the coupled system. via the certain nonlinear mechanism, in which the single-way atmospheric convergence feedback heating process appears mostly important. Although the annual cycle process can not fundamentally influence the formation of ENSO cycle, it can notably fix the phase and modulate the amplitude.Resultantly, the individual warm/cold events in ENSO cycle appear to be partially locked to the annual cycle, and also exhibit obvious irregularity both in amplitude and in duration time- Therefore, ENSO variability of ENSO cycle may greatly due to a nonlinear interaction of multiple time scales such as the LF,QB and AC processes,while the QB process is the fundamental mode.

【Fund】： 国家基础研究重大关键项目!“气候动力学和气候预测理论研究”;;国家自然科学基金
【CateGory Index】： P732

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