TOPEX/POSEIDON and ERS-2 (T/ERS) sea surface height altimeter observations and the Naval Research Laboratory Layered Ocean Model (NLOM) are used to study the circulation along the southwest coast of Mexico. The results of this research indicate that strong El Ni¿o/Southern Oscillation (ENSO) warm phase Kelvin waves (KW) destabilize the upper ocean circulation. The effect of ENSO appears as three distinct stages. First, a coastal jet characterized by strong vertical shear flow develops. Second, the shear flow strengthens, increasing its horizontal dimension and the amplitude of its oscillations. Finally, the jet becomes unstable and breaks into anticyclonic eddies, which separate from the coast and drift southwestward. The genesis and strengthening of the jet is due to the simultaneous occurrence of the poleward-flowing currents along the southwest coast of Mexico and the poleward circulation associated with ENSO downwelling KW. ¿ 2001 American Geophysical Union