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.