ENSO循环的非对称性及其机制初步探讨

摘要

利用SODA(Simple Ocean Data Assimilation)资料,以Nin03.4区SSTA(sea surface temperature anomaly)指数为标准,选取典型El Ni?o和La Ni?a事件进行合成分析,观察ENSO的非对称性.通过分析ENSO事件发生过程中温跃层深度异常和斜压能量转化率的变化,研究其对ENSO循环非对称性的影响.由于与ENSO相关的海洋异常信号在南、北太平洋的传播主要是以Rossby波的形式,Rossby波可以通过不稳定机制获得能量成长和传播,斜压能量转化率越强,携带ENSO相关海洋异常信号的Rossby波就可能获得更多的能量成长,这种信号传播至赤道区域可以更快地导致ENSO事件的爆发.研究表明,在El Ni?o事件达到顶峰时,北太平洋ENSO相关的Rossby 波传播区斜压不稳定能量很强(可使Rossby波增幅)、温跃层加深(可使Rossby波加速),有利于El Ni?o相关的暖信号传出,这很好地解释了El Ni?o事件总是比La Ni?a事件振幅大以及El Ni?o事件向La Ni?a事件转化比La Ni?a事件向El Ni?o事件转化快很多的现象.因为强斜压不稳定能量和加深的温跃层促使携带异常信号的Rossby波吸收能量成长和传播,这使得El Ni?o事件时东太平洋暖信号更容易西传,El Ni?o事件更快地衰退,并迅速向La Ni?a事件转化,这样可以解释ENSO循环的不对称现象.%The Simple Ocean Data Assimilation (SODA) product is used to make composite analysis about El Nino and La Nina events discerned by the sea surface temperature anomaly (SSTA) in the Nino3.4 region. We analyze the asymmetry of the El Nino-Southern Oscillation (ENSO) events and its mechanism. The difference between the distributions of baroclinic energy conversion rate and thermocline depth anomaly of composite El Nino and La Nina events is analyzed to find their influence on the asymmetry of ENSO. Due to the fact that ENSO-related anomalous signals travel mainly in the form of Rossby waves in the North and South Pacific where the Rossby waves can get energy to grow by unstable mechanism, the larger the baroclinic energy conversion rate is, the more energy the ENSO-related Rossby waves can obtain to grow. Many researches have shown that, there is a large domain with significant unstable energy and deeper thermocline in the North Pacific in El Nino events. Larger baroclinic instable energy helps to enlarge the amplitude of Rossby wave and deeper thermocline helps to speed up Rossby waves. This explains why the amplitude of SSTA during El Nino events is usually larger than that in La Nina events and why the process that El Nino transfers into La Nina takes shorter time than the one that La Nino transfers into El Nino. Baroclinic instable energy forces the Rossby wave to absorb energy to grow, which make it easy for the warm signal propagating to the east and the El Nifio transfers into La Nino quickly. So the asymmetry of ENSO can be explained.