Why the properties of El Niño changed during the late 1970s

Following the abrupt North Pacific climate shift in the mid-1970s, the period, amplitude, spatial structure, and temporal evolution of the El Niño notably changed. Theory is needed to explain why the coherent changes in several El Niño characteristics are nearly synchronized with the decadal climate shift. While the decadal variation in the equatorial thermocline could potentially change El Niño behavior, observation indicates that from the pre-shift (1961 - 1975) to the post-shift (1981 - 1995) period the change of equatorial eastern Pacific thermocline is insignificant. Our numerical experiments with a coupled atmosphere-ocean model illustrate that the observed changes in ENSO properties may be attributed to decadal changes in the surface winds and associated ocean surface layer dynamics without changes in the mean thermocline. A theoretical analysis is presented to elucidate the mechanisms by which the decadal variations in winds and upwelling modify the structure and propagation of the El Niño and amplify and prolong the El Niño-La Nina cycle.