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A three-dimensional coupled physical--biogeochemical model was used to simulate and examine temporal and spatial variability of sea surface pCO2 in the Gulf of Mexico (GoM). The model was driven by realistic atmospheric forcing, open boundary conditions from a data-assimilative global ocean circulation model, and observed freshwater and terrestrial nutrient and carbon input from major rivers. A 7-year model hindcast (2004-2010) was performed and validated against ship measurements. Model results revealed clear seasonality in surface pCO2 and were used to estimate carbon budgets in the Gulf. Based on the average of model simulations, the GoM was a net CO2 sink with a flux of 1.11±0.84x1012 mol C yr-1, which, together with the enormous fluvial inorganic carbon input, was comparable to the inorganic carbon export through the Loop Current. Two model sensitivity experiments were performed: one without biological sources and sinks and the other using river input from the 1904-1910 period as simulated by the Dynamic Land Ecosystem Model (DLEM). It was found that biological uptake was the primary driver making GoM an overall CO2 sink and that the carbon flux in the northern GoM was very susceptible to changes in river forcing. Large uncertainties in model simulations warrant further process-based investigations. [ABSTRACT FROM AUTHOR]

The Gulf of Mexico (GOM) is facing large pressures from environmental changes since the beginning of the last century. However, the magnitude and long-term trend of total water discharge to the GOM and the underlying processes are not well understood. In this study, the dynamic land ecosystem model (DLEM) has been improved and applied to investigate spatial and temporal variations of evapotranspiration (ET) and runoff (R) over drainage basins of the GOM during 1901-2008. Modeled ET and discharge were evaluated against upscaled data sets and gauge observations. Simulated results demonstrated a significant decrease in ET at a rate of 15 mm yr-1 century-1 and an insignificant trend in runoff/precipitation (R/P) and river discharge over the whole region during 1901-2008. However, the trends in estimated water fluxes show substantial spatial and temporal heterogeneities across the study region. Generally, in the west arid area, ET, R, and R/P decreased; while they increased in the eastern part of the study area during the last 108 years. In the recent 30 years, this region experienced a substantial decrease in R. Factorial simulation experiments indicate that climate change, particularly P, was the dominant factor controlling interannual variations of ET and R; while land use change had the same magnitude of effects on long-term trends in water fluxes as climate change did. To eliminate modeling uncertainties, high-resolution historical meteorological data sets and model parameterizations on anthropogenic effects, such as water use and dam constructions, should be developed. [ABSTRACT FROM AUTHOR]