Optimal forcing of ENSO either side of the 1970’s climate shift and its implications for predictability

is a skillful ENSO predictor on 7 month lead times, afterwards Indian and south Atlantic sea surface temperature anomalies are inferred to have become important. Optimal ENSO growth over 15 months also contains a significant extra-Pacific contribution, and it is possible to skillfully hindcast some (but not all) ENSO events in both periods over 15 month lead times. Of the four considered linear precursors, only the 7 month optimal perturbation corresponding to the period 1959–1978 is able to skillfully hindcast ENSO amplitude from 1998 to the present, correctly predicting the development of El Niño conditions since February 2014. As such the optimal precursor structure appears to be related to the phase of the IPO, and we conjecture that extra-Pacific teleconnections may gain importance during a positive phase of the IPO.