ENSO事件非对称性成因研究

利用NCEP、SODA等再分析资料,对东太平洋上层海洋的热量收支进行了计算,研究了产生ENSO冷暖事件强度非对称的可能原因。对海表温度异常(SSTA)的分析发现,在东太平洋SSTA存在明显的正偏,即El Nio事件中正异常的幅度大于La Nia事件中负异常的幅度,体现出ENSO事件的非对称性。通过对上层海洋热量收支的计算发现,造成ENSO事件非对称性的可能原因有3个:(1) 非线性温度平流,水平非线性温度平流在ENSO冷暖事件中均为正值,因此增强El Nio事件而减弱La Nia事件;(2) 次表层温度异常对温跃层深度异常的非线性响应,由于东太平洋温度剖面的特性,使得次表层温度异常对El Nio期间正的温跃层深度变化更为敏感,造成次表层温度异常幅度在El Nio期间比La Nia期间大,从而通过－wT′_z项引起上层海温的非对称性;(3) 赤道太平洋的纬向风异常的正偏:由于赤道太平洋存在较强的纬向西风,导致东太平洋温跃层深度异常正偏,进而造成次表层温度异常的非对称－wT′_z,并通过项影响上层海温的非对称性。

A study on the causes of the ENSO asymmetry

In this paper, a mixed layer heat budget analysis is used to investigate the possible causes of ENSO asymmetry by analyzing the HadISST data, the SODA and NCEP reanalysis data. The significant positive skewness of SSTA appears in the eastern Pacific by analyzing the SSTA data. This feature indicates the amplitude of El Nio is larger than that of La Nia. A mixed layer heat budget analysis indicates that there may be 3 possible reasons for the asymmetry of ENSO events. The first is attributed to the nonlinear temperature advection. The horizonal nonlinear advection tends to enhance the mixed layer temperature tendency during the developing phase both in El Nio and La Nia, and contribute to the positive skewness in the eastern Pacific. The second possible reason is attributed to the nonlinear response of the subsurface temperature anomaly to the variations of the thermocline depth anomaly. Because of the structure of vertical temperature distribution in the eastern Pacific, the subsurface temperature is more sensitive to the positive thermocline depth anomaly than the negative thermocline depth anomaly. This can cause the subsurface temperature anomaly larger in El Nio events than in La Nia events, and enhance the upper ocean temperature skewness through the term -wT′_z. The third is attributed to the positive skewness of equatorial zonal wind stress in the central Pacific, which may cause the positive SSTA skewness in the eastern Pacific. The positive skewed zonal wind anomaly in the central Pacific creates the positive skewness of the thermocline depth anomaly in the eastern Pacific, make the asymmetry of the subsurface temperature anomaly, and influence the skewness of the SSTA through the term -wT′_z.