Air‐sea CO₂ exchange in the equatorial Pacific

Abstract

[1] GasEx-2001, a 15-day air-sea carbon dioxide (CO 2 ) exchange study conducted in the equatorial Pacific, used a combination of ships, buoys, and drifters equipped with ocean and atmospheric sensors to assess variability and surface mechanisms controlling air-sea CO 2 fluxes. Direct covariance and profile method air-sea CO 2 fluxes were measured together with the surface ocean and marine boundary layer processes. The study took place in February 2001 near 125°W, 3°S in a region of high CO 2 . The diurnal variation in the air-sea CO 2 difference was 2.5%, driven predominantly by temperature effects on surface solubility. The wind speed was 6.0 ± 1.3 m s À1 , and the atmospheric boundary layer was unstable with conditions over the range À1 < z/L < 0. Diurnal heat fluxes generated daytime surface ocean stratification and subsequent large nighttime buoyancy fluxes. The average CO 2 flux from the ocean to the atmosphere was determined to be 3.9 mol m À2 yr À1 , with nighttime CO 2 fluxes increasing by 40% over daytime values because of a strong nighttime increase in (vertical) convective velocities. The 15 days of air-sea flux measurements taken during GasEx-2001 demonstrate some of the systematic environmental trends of the eastern equatorial Pacific Ocean. The fact that other physical processes, in addition to wind, were observed to control the rate of CO 2 transfer from the ocean to the atmosphere indicates that these processes need to be taken into account in local and global biogeochemical models. These local processes can vary on regional and global scales. The GasEx-2001 results show a weak wind dependence but a strong variability in processes governed by the diurnal heating cycle. This implies that any changes in the incident radiation, including atmospheric cloud dynamics, phytoplankton biomass, and surface ocean stratification may have significant feedbacks on the amount and variability of air-sea gas exchange. This is in sharp contrast with previous field studies of air-sea gas exchange, which showed that wind was the dominating forcing function. The results suggest that gas transfer parameterizations that rely solely on wind will be insufficient for regions with low to intermediate winds and strong insolation.

Open ocean DMS air/sea fluxes over the eastern South Pacific Ocean

“…the warmest CO 2 data set has little wind speed dependence but the warmest DMS data has the highest wind speed dependence). McGillis et al (2004) hypothesized that in certain oceanic regions the diurnal heat budget may be the primary physical forcing on gas exchange. Diurnal variations in solar insolation drive stratification and buoyancy effects at the sea surface, both of which enhance gas exchange.…”

“…1). Data are reported for 10 to 24 January 2006, corresponding with latitudes 0 to 55 • S. These data are compared to the existing database of open ocean DMS and CO 2 flux measurements from both the Atlantic and Pacific oceans (Blomquist et al, 2006;Huebert et al, 2004;Marandino et al, 2007;McGillis et al, 2001McGillis et al, , 2004. The solid black line is the cospectrum corrected for ship motion, solid grey is the motion uncorrected cospectrum, and the dashed line is the idealized scalar cospectrum from Kaimal et al (1972).…”

Determining the sea-air flux of dimethylsulfide by eddy correlation using mass spectrometry

“…The first field deployments were aircraft-based studies of DMS in the marine boundary layer (Bandy et al, 2002). Ship-based studies are a logical application and considerable progress has been made in the correction of wind data for motion artifacts (Edson et al, 1998) such that EC is now frequently employed for CO 2 flux measurements on ships (Fairall et al, 2000;McGillis et al, 2001McGillis et al, , 2004Miller et al, 2009;Taddei et al, 2009). Several studies of DMS sea-air exchange have been published to-date, yielding a huge increase in the catalog of DMS flux measurements and derived exchange velocities (Huebert et al, 2004;Blomquist et al, 2006;Marandino et al, 2007Marandino et al, , 2008Marandino et al, , 2009.…”

The impact of diurnal variability in sea surface temperature on the central Atlantic air-sea CO&lt;sub&gt;2&lt;/sub&gt; flux

“…Thus pCO 2w is always computed using the foundation temperature but pCO 2a varies between the three scenarios (following the method used by McGillis et al, 2004 andHare et al, 2004). Figure 4 shows seasonal averages of the CO 2 flux field for 2005-2006 computed using the analysed foundation SST (T f ).…”

“…This is because averaging eliminates covariations between variables and wind-based transfer velocities predict no gas flux when there is no wind, which are the conditions under which large SSTs may occur. Moreover, a field experiment has shown that it is possible for CO 2 fluxes to have only a weak dependence on wind speed but a strong dependence on the diurnal heating cycle (e.g., GasEx-2001 in the Equatorial Pacific; McGillis et al, 2004). Therefore, in this study we use a more complex physically-based parameterisation that includes buoyancy driven, as well as wind driven, gas transfer, by Fairall et al (2000) with modifications by Jeffery et al (2007); along with a slightly different formulation for the flux (Eq.…”