The very existence of El Ni¿o-the oscillatory behavior of the tropical Pacific climate-may be due to the warmth of the tropics (relative to the coldness of the high latitudes). This is elucidated by subjecting a mathematical model for the coupled tropical ocean-atmosphere system to a varying radiative heating. The temperature of the deep ocean is kept fixed. In response to an increasing radiative heating, the coupled system first experiences a pitch-fork bifurcation that breaks the zonal symmetry imposed by the solar radiation. The resulting zonal sea surface temperature (SST) gradients increase with increases in the radiative heating. When the zonal SST gradients exceed a critical value, a Hopf bifurcation takes place which brings the system to an oscillatory state, a state that closely resembles the observed tropical Pacific climate. Further increases in the radiative heating result in increases in the magnitude of the oscillation. The results shed new light on the physics of El Ni¿o and suggest that climate change due to anthropogenic forcing may occur through the same dynamic modes that sustain natural variability.¿ 1997 American Geophysical Union