A 40 year integration of an eddy resolving numerical model of the tropical Pacific ocean is analyzed to quantify the interannual variability caused by internal variability of ocean dynamics. It is found that along the Pacific cold tongue internal variability contributes a significant amount to the observed interannual variability. This suggests that in this location the predictability of SST is limited to the persistence time of SST anomalies which is approximately 100 days. Furthermore, a comparison with other sources of variability suggests that internal variability may play an important role in modifying or setting up El Ni¿o.