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.