Two experiments with a regional Earth System Model (ESM) are performed. We discovered that in a simulation where light attenuation is calculated taking into account the water temperature-phytoplankton feedback the average sea surface temperature (SST) is lower over most of the tropical Indian ocean in comparison with the reference experiment in which a constant light attenuation coefficient equal to 0.06 m-1, typical in global ESM runs, is used. We also find that the strongest differences (more than 1 °C) in SST occur in the summer period and a cooling of subsurface layers and a rise of the thermocline are noted in the experiment with the above feedback. Thus, including the full water temperature-phytoplankton feedback with corresponding light attenuation coefficient generally lowers the SST and water temperature in subsurface layers of the Indian ocean, with strong implications for the ocean-atmosphere coupling and, therefore for the simulated regional climate.