Variability of the Tropical Atlantic Ocean Simulated by a General Circulation Model with Two Different Mixed-Layer Physics

Abstract The impact of ocean–atmosphere interactions on summer rainfall over the South Atlantic Ocean is explored through the use of coupled ocean–atmosphere models. The Brazilian Center for Weather Forecast and Climate Studies (CPTEC) coupled ocean–atmosphere general circulation model (CGCM) and its atmospheric general circulation model (AGCM) are used to gauge the role of coupled modes of variability of the climate system over the South Atlantic at seasonal time scales. Twenty-six years of summer [December–February (DJF)] simulations were done with the CGCM in ensemble mode and the AGCM forced with both observed sea surface temperature (SST) and SST generated by the CGCM forecasts to investigate the dynamics/thermodynamics of the two major convergence zones in the tropical Atlantic: the intertropical convergence zone (ITCZ) and the South Atlantic convergence zone (SACZ). The results present both numerical model and observational evidence supporting the hypothesis that the ITCZ is a thermally direct, SST-driven atmospheric circulation, while the SACZ is a thermally indirect atmospheric circulation controlling SST variability underneath—a consequence of ocean–atmosphere interactions not captured by the atmospheric model forced by prescribed ocean temperatures. Six CGCM model results of the Ensemble-based Predictions of Climate Changes and their Impacts (ENSEMBLES) project, NCEP–NCAR reanalysis data, and oceanic and atmospheric data from buoys of the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) Project over the tropical Atlantic are used to validate CPTEC’s coupled and uncoupled model simulations.

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

Biogeosciences ◽

10.5194/bg-6-535-2009 ◽

2009 ◽

Vol 6 (4) ◽

pp. 535-544 ◽

Cited By ~ 27

Author(s):

I. Hense ◽

B. Quack

Keyword(s):

Atlantic Ocean ◽

Water Column ◽

Mixed Layer ◽

Surface Mixed Layer ◽

Tropical Atlantic ◽

Tropical Atlantic Ocean ◽

Subsurface Maximum ◽

The Vertical Distribution ◽

Production Layer ◽

Inverse Proportionality

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.

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