Satellite estimation of coastal pCO2 and air-sea flux of carbon dioxide in the northern Gulf of Mexico

Abstract

Satellite approaches for estimation of the partial pressure of CO2 (pCO2) and air-sea flux of CO2 in coastal regions offer the potential to reduce uncertainties in coastal carbon budgets and improve understanding of spatial and temporal patterns and the factors influencing them. We used satellite-derived products in combination with an extensive data set of ship-based observations to develop an unprecedented multi-year time-series of pCO2 and air-sea flux of CO2 in the northern Gulf of Mexico for the period 2006–2010. A regression tree algorithm was used to relate satellite-derived products for chlorophyll, sea surface temperature, and dissolved and detrital organic matter to ship observations of pCO2. The resulting relationship had an r2 of 0.827 and a prediction error of 31.7μatm pCO2 (root mean-squared error of the relationship was 28.8μatm). Using a wind speed and gas exchange relationship along with satellite winds, estimates of air-sea flux of CO2 were derived yielding an average annual flux over the period 2006–2010 of −0.8 to −1.5 (annual mean=−1.1±0.3) molCm−2y−1, where the negative value indicates net ocean uptake. The estimated total annual CO2 flux for the study region was −4.3+1.1TgCy−1. Relationships of satellite-derived pCO2 with salinity were consistent with shipboard observations and exhibited a concave relationship with low values at mid- and low salinities attributed to strong biological drawdown of CO2 in the high productivity river-mixing zone. The time-series of satellite-derived pCO2 was characterized by a seasonal pattern with values lower during winter and spring, low to intermediate values during fall, and higher and more variable values during summer. These findings were similar to simulations from a coupled physical-biogeochemical model. A seasonal pattern was also evident in the air-sea flux of CO2 with generally more negative fluxes (i.e., ocean uptake) during winter and spring, and positive fluxes during summer months with fall being a period of transition. Interannual variations in annual means of both air-sea flux of CO2 and DIN loading were significant, with higher DIN loading coinciding in some cases with more negative air-sea flux of CO2 (i.e., net ocean uptake). Spatial patterns of pCO2 reflected regional environmental forcing including effects of river discharge, wind forcing, and shelf-slope circulation. Our study also illustrates the utility of satellite extrapolation for highlighting areas that may contribute significantly to regional signals and for guiding prioritization of locations for acquiring further observations. The approach should be readily applicable to other regions given adequate availability of in situ observations for algorithm development.