into consideration. Moreover, our research is based on another fundamental fact: *In the case where a current flows from warm area A to cold area B, heat will be transferred to the latter and the corresponding temperature in B will increase.* A comprehensive consideration of the current changes under a background temperature field yields a new generation mechanism for El Niño and La Niña.

This paper is arranged as follows: The next section describes our data and method. In Section 3, the relationship between current changes and SST anomalies are investigated; meanwhile, the sensitive current passages for El Niño and La Niña are also clarified. In Sections 4 and 5, the occurrence mechanisms for El Niño and La Niña are illustrated separately. In the final section our results are summarized.

Data and the ESMD Method
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The observed $2^{\circ}\times 2^{\circ}$ monthly SST data during 1950–2017 were obtained from the improved Extended Reconstructed Sea Surface Temperature version 4 from the National Climate Data Center, USA. The Niño-3.4 index was provided by the Climate Prediction Center. The observed $1^{\circ}\times 1^{\circ}$ 5-day absolute geostrophic velocities during 1993–2016 were obtained from the AVISO website, France.

With regards to the occurrences of El Niño and 