Tropical Indo-Pacific Ocean chlorophyll response to MJO forcing

Abstract

Previous studies that analyzed ocean color satellite data have suggested that the primary mechanism of surface chlorophyll (Chl) response to the MJO is wind-induced turbulent mixing and the corresponding mixed layer entrainment. In this study, this notion is examined with an ocean biophysical model in an ensemble framework, focusing on upper ocean processes (z < 200 m). As a whole, the model's mean Chl state is lower than observations except in the tropical Pacific basin, but its seasonal variation is acceptable, particularly in the tropical Indian Ocean. In this basin, the model can simulate surface Chl responses to the MJO consistent with the observations in terms of the phase-by-phase anomaly evolution patterns. These Chl responses are mostly induced by surface wind forcing, which is consistent with previous studies. Further investigation of subsurface variations is performed at select grid points, and it is revealed that (1) entrained nutrients are the primary source of enhancement for surface Chl concentration and detrainment blooms are relatively less common; (2) in limited regions, Ekman pumping can effectively reduce Chl concentration; and (3) both entrainment/detrainment and Ekman pumping mechanisms rely on background states of nutrient availability, so the same forcing can result in completely different Chl responses depending on the background state.