The irregularity of El Niñ o is investigated with a 400-year simulation of a coupled ocean-atmosphere general circulation model. The model produces irregular El Niñ os with peak sea surface temperature (SST) anomalies ranging from 1 C to 4 C in the equatorial central-eastern Pacific. In the equatorial Pacific, the temporal phase relationship of the upper ocean heat content (OHC) anomaly relative to SST, and wind stress anomalies can be explained by the "recharge oscillator" mechanism. A

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... ce of the zonal mean OHC anomaly between the equator and the northern subtropics arises before the development of equatorial SST anomaly. It is found that a larger OHC anomaly is accumulated on the equator as a precursor of strong El Niñ o. The heat-budget analysis suggests that horizontal advection in the ocean interior is a major contributor to the build-up of the larger OHC anomaly during the recharge phase, which is associated with the zonal-mean wind-curl anomaly in the off-equatorial North Pacific. This also implies that the surface heating in the subtropics is a potential contributor through meridional heat transport. Besides the aspect of amplitude irregularity, the model El Niñ o shows irregularities in frequency and seasonal phase locking. Possible linkages between these irregularities are discussed.