Simulating the tropical Atlantic air-sea CO2 exchange with a Regional high resolution ocean modeling system

Marcus Silva
Centro de Estudos e Ensaios em Risco e Modelagem Ambiental - CEERMA, Departamento de Oceanografia - DOCEAN, Universidade Federal de Pernambuco - UFPE, Recife-PE, Brazil (moa@ufpe.br, +558121268225)

Fabiana Soares Leite
Centro de Estudos e Ensaios em Risco e Modelagem Ambiental - CEERMA, Universidade Federal de Pernambuco - UFPE, Recife-PE, Brazil (fasoaresl@yahoo.ca, +558121268225)

Carlos Noriega
Centro de Estudos e Ensaios em Risco e Modelagem Ambiental - CEERMA, Universidade Federal de Pernambuco - UFPE, Recife-PE, Brazil (carlos.delnor@gmail.com, +558121268225)

Nathalie Lefèvre
Laboratoire d'Océanographie et du Climat: Expérimentations et approches numériques. UMR 7159 CNRS / IRD / Université Pierre et Marie Curie/MNHN. Institut Pierre Simon Laplace.Boîte 100 - 4, place Jussieu 75252 PARIS Cedex 05, France (nathalie.lefevre@locean-ipsl.upmc.fr, +33144277159)

Moacyr Araujo
Centro de Estudos e Ensaios em Risco e Modelagem Ambiental - CEERMA, Departamento de Oceanografia - DOCEAN, Universidade Federal de Pernambuco - UFPE, Recife-PE, Brazil (marcus@ufpe.br, +558121268225)

Recent assessments indicate that the oceans are responsible for the absorption of approximately 30% to 40% of excess CO2 emitted by anthropogenic sources since the onset of the industrial revolution (Canadell et al., 2007; UNEP, 2009). If the current rates of emission are maintained, it is estimated that the concentrations of CO2 in the atmosphere will increase from 385 ppm in 2008 to 450-650 ppm by 2060, which would increase the average acidity of the ocean surface from 8.1 to 7.9 – 7.8 pH units (UNEP, 2009). As a result of this process, a rapid modification of the global ocean is currently in progress. Fundamentally, this modification is generated by the acidification of the top 2000m of the water column. The main consequences of these changes are associated with the reduced number of habitats where the organisms that incorporate calcium carbonate (CaCO3) into their shells and skeletons can thrive. Thus, undermining a whole range of marine organisms and food chains that depend on them. Although scientists know that the tropical Atlantic is a source of CO2 to the atmosphere, very little is known about the spatial and seasonal-interannual variability in the CO2 flux along the air-sea interface in this oceanic region. In this work ROMS is coupled to the Pelagic Interaction Scheme for Carbon and Ecosystem Studies (PISCES) biogeochemical routines and used to simulate the interannual cycle (1995-2012) of the tropical Atlantic ocean (20°N-30°S) circulation/biogeochemistry with an isotropic horizontal grid resolution of 1/12° and 40 terrain-following layers. Initially, two scenarios were simulated; one with and the other without river discharges. These results show that the runoff of main rivers in the tropical Atlantic play an important role in the salinity budget and nutrients cycles in the south tropical Atlantic. Model results show good agreement with the observational Brazilian REVIZEE program. The horizontal and vertical comparisons at different seasons inside the REVIZEE region (0°30’N-14°00’ S; 31°24’-41°48’ W) show that the coupled model can represent seasonal nutrient cycles along water depth. Even though this region can be considered an oligotrophic zone. Model CO2 results are compared with the oceanic and atmospheric pCO2 data obtained from the underway ship measurements along the 38°W longitude (4°S-15°N) and from the CARIOCA sensors installed in two ATLAS buoys that are part of the Prediction and Research moored Array in the Tropical Atlantic - PIRATA network (6°S-10°W and 8°N-38°W). Air-sea CO2 fluxes are calculated using Sweeney et al. (2007)´s formulas for estimating gas transfer velocities. Results illustrate the complexity of the space-time variability of the surface CO2 exchanges in the tropical Atlantic, evidencing the need for the expansion of the observational pCO2 array system in that region. The authors thank the Brazilian National Council of Scientific and Technological Development - CNPq under the scope of the Project BIO-NE (Grant 558143/2009-1).