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

Following the abrupt North Pacific climate shift in the mid-1970s, the peri od, amplitude, spatial structure, and temporal evolution of the El Nino not ably changed. Theory is needed to explain why the coherent changes in sever al El Nino characteristics are nearly synchronized with the decadal climate shift. While the decadal variation in the equatorial thermocline could pot entially change El Nino behavior, observation indicates that from the pre-s hift (1961 - 1975) to the post-shift (1981 - 1995) period the change of equ atorial eastern Pacific thermocline is insignificant. Our numerical experim ents with a coupled atmosphere-ocean model illustrate that the observed cha nges in ENSO properties may be attributed to decadal changes in the surface winds and associated ocean surface layer dynamics without changes in the m ean thermocline. A theoretical analysis is presented to elucidate the mecha nisms by which the decadal variations in winds and upwelling modify the str ucture and propagation of the El Nino and amplify and prolong the El Nino-L a Nina cycle.