The impact of diurnal variability in sea surface temperature on the central Atlantic air-sea CO2 flux

Abstract

The effect of diurnal variations in sea surface temperature (SST) on the air-sea flux of CO2 over the central Atlantic ocean and Mediterranean Sea (60 S-60 N, 60W-45 E) is evaluated for 2005-2006. We use high spatial resolution hourly satellite ocean skin temperature data to determine the diurnal warming (1SST). The CO2 flux is then computed using three different temperature fields-a foundation temperature (Tf, measured at a depth where there is no diurnal variation), Tf plus the hourly ΔSST and Tf plus the monthly average of the ΔSSTs. This is done in conjunction with a physically-based parameterisation for the gas transfer velocity (NOAA-COARE). The differences between the fluxes evaluated for these three different temperature fields quantify the effects of both diurnal warming and diurnal covariations. We find that including diurnal warming increases the CO2 flux out of this region of the Atlantic for 2005-2006 from 9.6 Tg C a -1 to 30.4 Tg Ca-1 (hourly 1SST) and 31.2 TgC a -1 (monthly average of 1SST measurements). Diurnal warming in this region, therefore, has a large impact on the annual net CO2 flux but diurnal covariations are negligible. However, in this region of the Atlantic the uptake and outgassing of CO2 is approximately balanced over the annual cycle, so although we find diurnal warming has a very large effect here, the Atlantic as a whole is a very strong carbon sink (e.g.-920 TgC a-1 Takahashi et al., 2002) making this is a small contribution to the Atlantic carbon budget.