Modelling the vertical distribution of bromoform in the upper water column of the tropical Atlantic Ocean

Abstract

Abstract. The relative importance of potential source and sink terms for bromoform (CHBr3) in the tropical Atlantic Ocean is investigated with a coupled physical-biogeochemical water column model. Bromoform production is either assumed to be linked to primary production or to phytoplankton losses; bromoform decay is treated as light dependent (photolysis), and in addition either vertically uniform, proportional to remineralisation or to nitrification. All experiments lead to the observed subsurface maximum of bromoform, corresponding to the subsurface phytoplankton biomass maximum. In the surface mixed layer, the concentration is set by entrainment from below, photolysis in the upper few meters and the outgassing to the atmosphere. The assumed bromoform production mechanism has only minor effects on the solution, but the various loss terms lead to significantly different bromoform concentrations below 200 m depth. The best agreement with observations is obtained when the bromoform decay is coupled to nitrification (parameterised by an inverse proportionality to the light field). Our model results reveal a pronounced seasonal cycle of bromoform outgassing, with a minimum in summer and a maximum in early winter, when the deepening surface mixed layer reaches down into the bromoform production layer.