Abstract: 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.

Key words: ENSO, asymmetry, nonlinear advection, subsurface temperature