Abstract The current coarse-resolution version of the Community Climate System Model is used to assess the impact of phytoplankton on El Niño–Southern Oscillation (ENSO). The experimental setup allows for the separation of the effects of climatological annual cycle of chlorophyll distribution from its interannually varying part. The main finding is that the chlorophyll production by phytoplankton is important beyond modifying the mean and seasonal cycle of shortwave absorption; interannual modifications to the absorption have an impact as well, and they dampen ENSO variability by 9%. The magnitude of damping is the same in the experiment with smaller-than-observed, and in the experiment with larger-than-observed, chlorophyll distribution. This result suggests that to accurately represent ENSO in GCMs, it is not sufficient to use a prescribed chlorophyll climatology. Instead, some form of an ecosystem model will be necessary to capture the effects of phytoplankton coupling and feedback.