ENSO循环的数值模拟(Ⅱ)──变化性及时间尺度选择机制

NUMERICAL SIMULATION OF THE ENSO CYCLE PART Ⅱ:──VARIABILITY AND TIME SCALE SELECTION MECHANISM

中文关键词:  ENSO循环  时间尺度  形成机制

英文关键词:ENSO cycle  Time scale  Formation mechanism

中文摘要:

      利用热带太平洋海气耦合异常模式的30a模拟结果,对模式ENSO的变化性及多重时间尺度过程进行了细致分析,建立了一ENSO循环多重时间尺度过程相互作用的非线性相似(Analog)模型,并提出了ENSO循环主周期形成的一种可能机制。指出:和观测事实类似,模式ENSO过程确实涉及到三种时间尺度,即3-4a主周期振荡(LF)、准两年振荡(QB)和年循环(AC);其中,QB过程是线性海气耦合系统的本征模态,年循环(AC)对其形成没有本质的影响;3-4a主周期振荡(LF)是一非线性系统的自激振荡现象,其形成是线性系统的本征模即QB过程通过非线性机制尤其是通过大气辐合反馈加热的“单向性”过程在QB的暖态产生的减频增幅所致;平均年循环(AC)虽然不能对ENSO循环形成有本质影响,但它可明显影响ENSO循环的具体振幅和位相,使得ENSO循环具有明显的不规则性并对季节循环具有明显的“锁相”特征;ENSO变化性确是LF、QB以及AC多重时间尺度相互作用形成的。本文提出的ENSO循环时间尺度选择机制不仅解释了主周期振荡的形成过程,而且也较好地解释了ENSO变化的谱,因此,这一机制更接近于观测事实。

英文摘要:

      By using a tropical Pacific coupled air-sea model with an oceanic surface boundary layer and atmospheric internal convergence feedback effect,after a long time integration of 30 years,key features of the observed multiple time scale variability of ENSO cycle have been successfully simulated.Based on the simulated results a nonlinear analog model for understanding the formation of the principal period of ENSO cycle has been established and a possible time scale selection mechanism for ENSO cycle has also been proposed.It is reported that,1)similar to the observational facts,model ENSO cycle is truely a three time scale processes,i.e.the principal 3-4 year lower-frequency(LF) oscillation,quasi-biennial (QB) oscillation explaining the variance of 10%-200% and annual cycle (AC);2)the LF mode with 3-4 year principal period is a self-sustaining oscillation of nonlinear system.Its time scale selection results from a process of decreasing frequency and intensifying amplitude in the warm state of the QB mode,i.e.the fundamental intrinsic mode of the linear system,via the nonlinear mechanism,in which the single-way atmospheric convergence feedback appears to be mostly important to the process;3)although the annual cycle process can not essentially 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 partly locked to the annual cycle,and also exhibit obvious irregularity both in amplitude and in lasting time.Therefore,ENSO variability or ENSO cycle is truely due to a nonlinear interaction of multiple time scales among the LF,QB and Ac modes,in which the QB mode is a fundamental mode.The time scale selection mechanism presented in this paper is more objective to approach real world ENSO process,because it can explain not only formation of the ENSO principal period but also the spectrum of ENSO variability.