Mesoscale variability of sea surface pCO 2 : What does it respond to?

We examine the impact of mesoscale and submesoscale oceanic processes on the distribution of sea surface pCO 2 to explain variability observed at length scales of order 10 km. We ask whether the large pCO 2 excursions (50–150 μ‐atm) that occur over 10–25 km of the sea surface could be induced by vertical advection associated with fronts and eddies in the pelagic ocean. A numerical model of a highly resolved, but idealized, mesoscale flow field is used to model the surface pCO 2 response to submesoscale upwelling, taking into account the effect of wind, heat flux, and phytoplankton production. The effect of upwelled DIC on surface pCO 2 is largely offset by the lower temperature of the upwelled water and the consumption of DIC by phytoplankton that respond to the simultaneously upwelled nitrate in a nutrient‐limited setting. Since an upwelling‐induced change in surface temperature, DIC, or nitrate is proportional to the vertical gradient of these properties beneath the mixed layer, the relative change in surface pCO 2 is also dependent on the relative strengths of these gradients. We find that only small variations in surface pCO 2 (∼10 μ‐atm) are induced by submesoscale upwelling. The larger (50–150 μ atm) variations observed at small scales (∼10 km) are therefore not a direct consequence of submesoscale upwelling. Our results suggest that these surface pCO 2 differences are likely to have been generated at larger scales (by differential properties and levels of biological productivity) and cascaded to smaller scales by horizontal advection at the sea surface.