Estimation of Air-Sea Carbon Dioxide Flux at the Southwestern Atlantic Ocean using a circulation and biogeochemical model

The Southwestern region of the Atlantic Ocean is where occurs the meeting of the Brazil and the Malvinas currents. This region is known as the Brazil-Malvinas Confluence (BMC). The BCM is an area of intense mixture of subantarctic and subtropical waters, high horizontal and vertical gradients of temperature, salinity and nutrients. The formation of a deep mixed layer in association with high biological productivity makes the extra-tropical region of the Atlantic Ocean an important component of the carbon cycle. Even though the Atlantic Ocean only accounts for 23% of the global oceanic surface, it is responsible for 41% of the annual carbon flux budget. In this sense this research comes along with the Brazilian research project Atlantic Carbon Experiment (ACEx), which has on its main objectives to do in situ CO2 fluxes measurements and try to better understand the air-sea fluxes exchange in this region.

The model used was the Regional Ocean Modelling System (ROMS), a primitive equation circulation model. It was also used the Fennel biogeochemical mode, that resolves the nitrogen and carbon cycles. Dissolved inorganic carbon species are only specified at the surface, and the flux is calculated using the quadratic relationship of Wanninkhof (1992). The simulations were done for 14 years, and the last 10 years, corresponding to 1987-1996 were analyzed. The latitudinal variation of the BMC position was well represented and the chlorophyll concentration was in general well calculated. There was, however, a delay in the spring bloom at the Patagonian Continental Shelf. During the spring, ΔpCO2 and carbon fluxes alternated between positive and negative values, and showed to good extent the influence of the biota and BMC eddies in the air-sea gas exchanges. On the northern part of the grid, between 20-30ºS the ΔpCO2 were slightly positive, close to zero µatm due to the warm water. The influence of the La Plata River was felt on the Continental Shelf up to 25ºS. The Southern part of the grid was, during most of the time, a sink of carbon dioxide.

The link between El Niño- Southern Oscillation and the area simulated was analyzed. During a La Niña event, ΔpCO2 anomalies were negative during the period between march and July. During an El Niño year, the negative ΔpCO2 anomalies were found during February through april while it intensifies. Positive anomalies, however, were found during the spring, while the El Niño event weakens. These ΔpCO2 anomalies are linked to SST and La Plata runoff anomalies.