Air-Sea CO₂ fluxes on the Scotian Shelf: seasonal to multi-annual variability

Abstract. We develop an algorithm to compute pCO₂ in the Scotian Shelf region (NW Atlantic) from satellite-based estimates of chlorophyll-a concentration, sea-surface temperature, and observed wind speed. This algorithm is based on a high-resolution time-series of pCO₂ observations from an autonomous mooring. At the mooring location (44.3° N and 63.3° W), the surface waters act as a source of CO₂ to the atmosphere over the annual scale, with an outgassing of −1.1 mol C m⁻² yr⁻¹ in 2007/2008. A hindcast of air-sea CO₂ fluxes from 1999 to 2008 reveals significant variability both spatially and from year to year. Over the decade, the shelf-wide annual air-sea fluxes range from an outgassing of −1.70 mol C m⁻² yr⁻¹ in 2002, to −0.02 mol C m⁻² yr⁻¹ in 2006. There is a gradient in the air-sea CO₂ flux between the northeastern Cabot Strait region which acts as a net sink of CO₂ with an annual uptake of 0.50 to 1.00 mol C m⁻² yr⁻¹, and the southwestern Gulf of Maine region which acts as a source ranging from −0.80 to −2.50 mol C m⁻² yr⁻¹. There is a decline, or a negative trend, in the air-sea pCO₂ gradient of 23 μatm over the decade, which can be explained by a cooling of 1.3 °C over the same period. Regional conditions govern spatial, seasonal, and interannual variability on the Scotian Shelf, while multi-annual trends appear to be influenced by larger scale processes.