1991～1994年El Niño的异常特征的诊断研究

1991～1994年热带中东太平洋海温持续4a多出现正距平。Niño3指数一直为正值。在此正距平背景下,产生了两次振荡和3次ElNiño暖期。在此期间赤道东太平洋海温在±5°纬度范围内发生了两次负距平的变化,形成了一个狭窄的温度梯度很大的“冷核”,而赤道外的中纬度海洋则持续维持两个正海温距平。这一时期海表温度资料EOF分析的结果进一步表明,第一特征向量的空间分布实际上反映了上述冷核特征。1991～1994年的ENSO事件主要是低频分量发生了较大异常,赤道低层纬向风和高层西风在前期减弱(1991-01-1992-05),后期加强(1992-06-1994-011).无论是海温还是风场的低频分量都表现出一次ENSO循环的特征。因此作者认为,虽然Niño3指数等在这4a多期间均为正值,但是大气和海洋耦合系统的低频变化部分只发生了一次完整的ENSO循环过程。1991年至1992年上半年对应于ElNiño暖位相,1992年下半年至1994年底对应于LaNina冷位相。但是这个冷位相没有能够得到充分发展,只出现两次极狭窄的冷核。冷位相的明显“夭折”,而代之以出现两次较弱的增暖,可能与季节内尺度的大气强迫与低频变化部分的相互作用有关。

A STUDY ABOUT FEATURES OF EL NIÑO EVENTS DURING 1991-1994

The anomalies of the sea surface tem perature in the central and eastern tropical Pacific kept the positive values for more than 4 years from 1991 to 1994. On this background, the Ninño3 index assumed two oscillations. As the result, there were three warm episodes during this period. In this paper, a diagnostic study of the basic features of the sea surface temperature and wind fields in tropical Pacific has been done. It is found that two anomalous processes of negative SST anomalies in the eastern equatorial Pacific between 5°S-5°N existed. The narrow cold water tongue in the equatorial zone has strong temperature gradient and surrounded by the off-equatorial warm waters, with one positive SSTA area located in the middle-latitude of the Northeast Pacific and the other in the Southeast Pacific. The cold water looks like a cold core. This anomalous features of the SST A evolution during 1991 to 1994 is different from those observed in the history. Based on a EOF analysis, the distribution of the first eigenvalue (EOF1) just reflects the cold core pattern. Using the low-fre-quency filter, the LF component of the Ninño3 index showed the positive anomaly for 4 years, but the cross-section of the SSTA along the equator has two periods of negative SSTA near the equatorial East Pacific. However, the low-frequency componentof the zonal wind along the equator only demonstrates one cycle. There are the westerly zonal wind anomaly over the tropical central Pacific from 1991 to June of 1992, and easterly wind anomaly from July 1992 to the end of 1994. It is noticeable that the easterly anomaly of zonal wind did not extend to the tropical eastern Pacific. It can be found the strong westerly zonal wind anomaly at 850 hPa off the coast of tropical South America. These results suggest that the coupled system of the Pacific Ocean and the at mosphere has evolved as a whole ENSO cycle during this 4 years. The warm phase spanned from Jan., 1991 to June, 1992, and the cold phase from July, 1992 to the end of 1994. But, this La Nina phase has been aborted by the positive anomalous SSTA in the middle-latitude off East Pacific, which were linked with the adjustment of the Walker cell and Asian monsoon.