Abstract
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, 60 W–45 E) is evaluated for 2005–2006. We use high spatial resolution hourly satellite ocean skin temperature data to determine the diurnal warming (ΔSST). 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 C a−1 (hourly ΔSST) and 31.2 Tg C a−1 (monthly average of ΔSST 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 Tg C a−1 Takahashi et al., 2002) making this is a small contribution to the Atlantic carbon budget.
Highlights
During the day, the upper 2 m of the ocean typically absorbs about 50% of the solar radiation reaching its surface
When satellite-measured diurnal variations are included (Fdv, Scenario 2) this outgassing is increased to 30.4 Tg C a−1, and when diurnal warming is represented by a monthly-averaged value
Since the effect of diurnal covariability is small, in the rest of this section we focus on the differences between using Fdv and Ff
Summary
The upper 2 m of the ocean typically absorbs about 50% of the solar radiation reaching its surface. At night this layer cools, losing heat to the atmosphere through radiative latent and sensible heat fluxes. This diurnal heating and cooling can lead to significant variations in the sea surface temperature (SST) (e.g., Stuart-Menteth et al, 2003; Gentemann et al, 2003). Diurnal changes in SST ( SST) shown in Fig. 1 are only up to 1.5 K but on individual days localised warming can be as much as 6 K within a shallow warm layer at the sea surface (Merchant et al, 2008; Stramma et al, 1986)
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