Possible causes of the spring persistence barrier in ENSO sea surface temperature anomalies are examined using a coupled atmosphere-ocean general circulation model (CGCM). Our study indicates that the persistence barrier is significantly enhanced when both Pacific and Indian Ocean couplings are included in the CGCM, compared to the simulation that includes only the Pacific Ocean coupling. ENSO's variance, phase locking to the annual cycle, and biennial variability are also increased in the Indo-Pacific Run. Further analysis reveals that the overall amplitude of ENSO is not a primary factor in determining the strength of the persistence barrier, rather, it is the amplitude of the biennial component of ENSO affecting the barrier the most. The persistence barrier is consistently strong (weak) when biennial ENSO variability is large (small). No such a clear relationship is found between the strength of the barrier and the amplitude of the low-frequency (3--5 years) component of ENSO. This modeling study demonstrates that the biennial component of ENSO is one major mechanism responsible for the spring persistence barrier and that interactions between the tropical Pacific Ocean and Indian Ocean-monsoon could enhance the biennial component of ENSO.