Changes in the leading ENSO modes associated with the late 1970s climate shift: Role of surface zonal current

In this study, using the nonlinear principal analysis (NLPCA) technique, we demonstrated that the leading El Nino‐Southern Oscillation (ENSO) mode in a physical basis changed since the late 1970s. The ENSO mode during the pre‐1980s resembles the destabilized ‘ocean basin mode’, while during the post‐1980s does ‘the recharge‐mode’. In particular, for the pre‐1980s, the surface zonal current mainly acts as an intensifier of the ENSO, while that during post‐1980s it plays a role in the transition of the ENSO cycle. The NLPCA results are reconfirmed by the eigen analysis of the linearized intermediated ENSO model. Over a reasonable parameter range, the leading eigen modes associated with the pre‐1980s are completely separable from those associated with the post‐1980s. The eigen structures for each decadal‐period resemble the corresponding NLPCA patterns.