风应力的旋转和辐散分量在ENSO循环中作用的数值试验研究

将 1 960～ 1 991年的月平均 FSU风应力资料分解为旋转部分和辐散辐合部分 ,分别用以强迫模式海洋。所用的模式为一个 2层热带太平洋区域海洋模式。结果表明 ,就季节变化而言 ,不论是用旋转分量还是散合分量强迫海洋 ,都不能产生合理的冷舌 ,哪怕将旋转或散合分量放大一倍或缩小二分之一 ,也不能使冷舌的强度和分布得到合理的改善。若采用气候平均的含有季节变化的风应力 (未对旋转和辐散分量进行分离 ) ,则可产生与实际相符的海表温度分布。在此基础上 ,分别叠加旋转和辐散分量的年际异常部分 ,通过对海洋的强迫 ,可产生海表温度异常。在年际异常旋转分量的强迫下 ,可产生较强的 SSTA振荡且具有明显的 ENSO周期 ;而在辐散辐合年际异常风应力的强迫下 ,则产生较弱的 SSTA,且振荡频率较高 ,ENSO周期不很明显。这些结果说明 ,风应力的涡旋和辐散辐合分量在海温季节变化和年际变化的形成中具有不同的作用 ,即合理的冷舌分布需要风应力旋转分量和散合分量同时作用于海洋方可产生 ,而仅有异常风应力的旋转强迫就可产生合理的 EL Nino/ La Nina现象。同时 ,风应力的辐散辐合分量在海洋平均状态的形成过程中是重要的 ,但在 EN-SO过程中就对海洋的作用而言则不如旋转分量重要。

NUMERICAL STUDY ON THE ROLE OF VORTICAL AND DIVERGENT COMPONENTS OF WIND STRESS IN ENSO CYCLE

The monthly mean FSU (Florida State University) wind stress data for the period 1960-1991 are decomposed into a vortical and a divergent component with each of which to force the model ocean in the context of a 2 layer tropical Pacific model. Evidence suggests that for the seasonal variation the ocean forcing does not produce a realistic cold tongue using either of the components and the tongue will not be effectively improved in its intensity and pattern even if the components of the wind stress are doubled or halved. The utilization of climatic mean wind stress (no decomposition is done of the wind stress) that contains its seasonal variation will lead to a realistic SST distribution. The interannual anomaly of the vortical (divergent) component superimposed on the mean climatological part tends to force the ocean to intense SSTA oscillation marked by noticeable ENSO periods (feeble SSTA with higher oscillation frequency for obscure ENSO periods). These results indicate that the roles of the two components of wind stress do differ from each other in the genesis of SST variation on a seasonal and an interannual basis. That is,a realistic cold tongue pattern follows under the joint effects on the model ocean of the two components of wind stress while rational El Nino/La Nina phenomena result from the forcing of an anomalous vortical component of the wind stress. Moreover,the divergent component is innegligible in generating a mean climatic condition of the ocean sector but of less importance compared to the vortical component in the ENSO development.