Impact of the Deep Chlorophyll Maximum in the Equatorial Pacific as Revealed in a Coupled Ocean GCM-Ecosystem Model

A coupled ocean general circulation model (OGCM)-ocean ecosystem model is used to investigate the effects of the deep chlorophyll maximum (DCM) on the ocean state in the equatorial Pacific Ocean. Climatological and interannual three-dimensional (3-D) chlorophyll (CHL) fields are captured well in control runs using the coupled ocean physics-ecosystem model forced by prescribed atmospheric fields. Further sensitivity experiments are performed to assess the effects of 3-D CHL structure using the OGCM with the prescribed CHL fields taken from the control runs: CHLclim and CHLclimsurf are runs in which climatological DCM effect is included or only surface CHL effect is included; CHLinter and CHLintersurf are runs with interannually varying DCM effects included or not. The differences in the simulated ocean conditions are analyzed to explore DCM effects on sea surface temperature (SST) and amplitude of El Nino-Southern Oscillation (ENSO). Two competing mechanisms responsible for the DCM effects are revealed: an ocean biology-induced direct heating (OBH) effect, and an indirect cooling effect due to dynamic processes associated with vertical mixing and shallow meridional overturning circulation. There are three major findings: (a) DCM acts to reduce mean SST by around 0.2 degrees C in the eastern equatorial Pacific, being larger than the surface CHL effects. (b) DCM interannual variability increases the ENSO amplitude to a comparable degree as the surface CHL effects. (c) The total net impact of vertical mixing, currents, and net surface heat flux makes SST drop more under the DCM effect than the surface CHL effect in the eastern Pacific. These findings provide a new insight into the feedback mechanisms for the bioclimate interactions. Plain Language Summary Ocean chlorophyll (CHL) concentration is influenced by physical conditions in the ocean, such as El Nino-Southern Oscillation (ENSO), and has a feedback effect on ENSO by absorbing solar radiation in the upper ocean. CHL is usually seen to have a maximum below the mixed layer due to the vertical distribution of light and nutrients, forming a deep chlorophyll maximum (DCM). However, the specific effects of DCM on physical properties and the related mechanisms in the equatorial Pacific have not been understood well. In this study, we examine the impact of DCM on the physical oceanography in the tropical Pacific, using a coupled ocean general circulation model (OGCM)-ocean ecosystem model. By performing sensitivity experiments with the DCM effects included or not, it is found that the existence of DCM effects acts to reduce climatological SST in the eastern equatorial Pacific and increase ENSO amplitude. Two competing mechanisms are identified that can be responsible for the DCM effects: one is a direct warming due to increased absorption of solar radiation in the mixed layer, and the other is an indirect cooling due to processes induced by the former.

Automated summary

This study investigates the effects of the deep chlorophyll maximum (DCM) on the ocean state in the equatorial Pacific Ocean using a coupled ocean general circulation model (OGCM)-ocean ecosystem model. The results show that DCM acts to reduce mean sea surface temperature (SST) in the eastern equatorial Pacific and increase the amplitude of El Nino-Southern Oscillation (ENSO). Two competing mechanisms, including direct warming and indirect cooling, are identified to be responsible for the DCM effects.