The interannual variations of tropical instability waves (TIWs) in the eastern Pacific Ocean and their relationships with ENSO intensity are studied using a 28-year long coupled atmosphere-ocean general circulation model (CGCM) simulation. The activity of TIWs is measured by the root-mean-square value of the 50-day high-pass filtered sea surface temperature (SST) perturbations produced in the simulation. The CGCM realistically produces two branches of large TIW activity near the equator: one at 2¿N and the other at 2¿S. It is found that along both branches there are linear relationships between the year-to-year variations of TIW activity and those of ENSO intensity. TIW activity is enhanced or reduced in proportion to the NINO3 SST anomalies, with larger activity in La Ni¿a years and smaller activity in El Ni¿o years. It is found that ENSO modulates TIW activity in both branches primarily by changing the latitudinal SST gradient associated with the SST front immediately north of the equator. Weaker correlations are found between the year-to-year variations of TIW activity and those of the ocean current shears between the south equatorial current and the equatorial undercurrent or the north equatorial countercurrent. This CGCM study suggests that the baroclinic instability associated with the northern SST front is a major generation mechanism for both the northern and southern TIW branches in the eastern Pacific Ocean.