Gas Transfer Velocity Of CO2 Research Articles

Evaluation of wave-based parameterizations of air–sea CO2 gas transfer over global oceans

The parameterization of air–sea CO2 transfer velocity employed in the estimation of bulk fluxes over global ocean is typically established on wind speed but could suffer from the deviations induced by sea states. In this study, the effectiveness of wave-based formulations are substantiated by reproducing climatological air–sea CO2 flux and gas transfer velocity. Sea states play a significant role in facilitating CO2 transfer, particularly in mid to high latitude regions with high wind speeds. The variability in transfer velocity induced by sea states is estimated up to 19% at the wind speed of 15 m/s. The two wave-based formulations used in this study are combined using a critical value of the Reynolds number. The combined formulation further improves estimates of the CO2 gas transfer velocity.

A Sea State Dependent Gas Transfer Velocity for CO2 Unifying Theory, Model, and Field Data

AbstractWave breaking induced bubbles contribute a significant part of air‐sea gas fluxes. Recent modeling of the sea state dependent CO2 flux found that bubbles contribute up to ∼40% of the total CO2 air‐sea fluxes (Reichl & Deike, 2020, https://doi.org/10.1029/2020gl087267). In this study, we implement the sea state dependent bubble gas transfer formulation of Deike and Melville (2018, https://doi.org/10.1029/2018gl078758) into a spectral wave model (WAVEWATCH III) incorporating the spectral modeling of the wave breaking distribution from Romero (2019, https://doi.org/10.1029/2019gl083408). We evaluate the accuracy of the sea state dependent gas transfer parameterization against available measurements of CO2 gas transfer velocity from 9 data sets (11 research cruises, see Yang et al. (2022, https://doi.org/10.3389/fmars.2022.826421)). The sea state dependent parameterization for CO2 gas transfer velocity is consistent with observations, while the traditional wind‐only parameterization used in most global models slightly underestimates the observations of gas transfer velocity. We produce a climatology of the sea state dependent gas transfer velocity using reanalysis wind and wave data spanning 1980–2017. The climatology shows that the enhanced gas transfer velocity occurs frequently in regions with developed sea states (with strong wave breaking and high significant wave height). The present study provides a general sea state dependent parameterization for gas transfer, which can be implemented in global coupled models.

An assessment of air-sea CO2 flux parameterizations during tropical cyclones in the Bay of Bengal

The exchange of air-sea CO2 plays a significant role in regulating the Earth’s climate. The errors associated with the estimations of air-sea CO2 fluxes during extreme transient events like tropical cyclones (TCs) are important for climate research. In this study, we assess the estimates of CO2 gas transfer velocity and the corresponding air-sea flux derived by employing five wind-dependent and two wave-dependent parameterizations for eight TCs in the Bay of Bengal using mooring observations and reanalysis datasets. To start with, we analyze drag coefficient (CD) and associated frictional velocity (u*) derived from two globally very commonly used bulk flux algorithms, COARE 3.0 and the updated version COARE 3.6, with the estimates from wind-wave tank experiments for moderate and high wind speeds for all TCs. The analysis indicates that COARE 3.6 provides the best estimate of drag coefficient. Further, we find that the wave-dependent parameterization by Woolf (2005) provides the best estimates of CO2 gas transfer velocity compared to existing estimates of laboratory-based wind-wave tank experiments for high winds. Among all wind-only parameterizations, the hybrid parameterization proposed by Nightingale et al. (2000) performs best for high winds. We find that for winds < 20 m/s, the resultant fluxes of CO2 estimated using these seven parameterizations vary within 5 mmol CO2 m-2 d-1. However, for winds > 20 m/s, the difference between wind- and wave-parameterized fluxes are significant (∼50 mmol CO2 m-2 d-1). The percentage of variation in CO2 flux explained by transfer velocity (difference in sea and air pCO2) during TC conditions is nearly 78 (15)%.

Water-air Carbon Dioxide Exchange and Nutritional Controls in a Typical Karst River

To explore water-air carbon dioxide (CO2) exchange and its nutritional controls in karst rivers, water sampling and analyses were conducted in the Furong River as a representative karst river system. Regional hydrometeorological characteristics, carbonate system parameters, total organic carbon (TOC), nutrients and their stoichiometric ratios, and the riverine pressure of aqueous CO2 (pCO2) and its couplings to trophic status were analyzed. The results showed that gas transfer velocity of CO2 (k) varied between 2.71 and 13.0 m·d-1. Riverine pCO2 varied from 78.5 Pa to 21491.2 Pa and was significantly higher in the tributaries than the main stem. Riverine TOC, total nitrogen (TN), and total phosphorus (TP) concentrations were (302.8±50.1), (128.9±67.9), and (0.65±0.98) μmol·L-1, respectively, demonstrating the trophic status of P-limitation. Riverine pCO2 was tightly linked to P-related parameters, suggesting that in-situ metabolism triggered aquatic CO2 supersaturation. Water-air CO2 flux (F) averaged (534.5±801.4) mmol·(m2·d)-1 in the Furong River, which is higher than most of the world's rivers, and thus indicates high potential CO2 emissions.

Wind, Convection and Fetch Dependence of Gas Transfer Velocity in an Arctic Sea‐Ice Lead Determined From Eddy Covariance CO2 Flux Measurements

AbstractThe air‐water exchange of trace gases such as CO2 is usually parameterized in terms of a gas transfer velocity, which can be derived from direct measurements of the air‐sea gas flux. The transfer velocity of poorly soluble gases is driven by near‐surface ocean turbulence, which may be enhanced or suppressed by the presence of sea ice. A lack of measurements means that air‐sea fluxes in polar regions, where the oceanic sink of CO2 is poorly known, are generally estimated using open‐ocean transfer velocities scaled by ice fraction. Here, we describe direct determinations of CO2 gas transfer velocity from eddy covariance flux measurements from a mast fixed to ice adjacent to a sea‐ice lead during the summer‐autumn transition in the central Arctic Ocean. Lead water CO2 uptake is determined using flux footprint analysis of water‐atmosphere and ice‐atmosphere flux measurements made under conditions (low humidity and high CO2 signal) that minimize errors due to humidity cross‐talk. The mean gas transfer velocity is found to have a quadratic dependence on wind speed: k660 = 0.179 U102, which is 30% lower than commonly used open‐ocean parameterizations. As such, current estimates of polar ocean carbon uptake likely overestimate gas exchange rates in typical summertime conditions of weak convective turbulence. Depending on the footprint model chosen, the gas transfer velocities also exhibit a dependence on the dimension of the lead, via its impact on fetch length and hence sea state. Scaling transfer velocity parameterizations for regional gas exchange estimates may therefore require incorporating lead width data.

Carbon dioxide flux in the Java Sea estimated from satellite measurements

The oceans play a pivotal role in the carbon cycle due to its dynamics to act as a carbon source or sink to the atmosphere. Previous studies showed that the Java Sea acts as a carbon dioxide (CO2) source. However, their analysis was only based on the observation data which was limited in time and coverage. In the present study, we utilize the advantage of satellite measurements which have wide coverage and continues monitoring. Sea surface temperature, chlorophyll-a, sea surface salinity, surface wind speed, tropospheric mole fraction of CO2 and sea level pressure were analyzed to calculate CO2 gas transfer velocity, CO2 solubility, CO2 pressure in the sea water and atmosphere to estimate CO2 flux in the Java Sea. We found that the Java Sea acts as a CO2 source. However, during May 2015 and August 2016 small areas along the southern Borneo Island become the sink areas of CO2. The influence of CO2 gas transfer velocity and CO2 solubility is stronger than the difference of CO2 pressure between sea and atmosphere to determine CO2 flux in the Java Sea. Therefore, wind speed and sea surface salinity play important role to determine the variability of CO2 flux in the Java Sea. El Niño condition tends to amplify the release of CO2 to the atmosphere since CO2 gas transfer velocity, CO2 solubility and the difference of CO2 pressure between sea and atmosphere increases during El Niño.

Air-Sea CO2-Exchange in a Large Annular Wind-Wave Tank and the Effects of Surfactants

Wind, chemical enhancement, phytoplankton activity, and surfactants are potential factors driving the air-sea gas exchange of carbon dioxide (CO2). We investigated their effects on the gas transfer velocity of CO2 in a large annular wind-wave tank filled with natural seawater from the North Atlantic Ocean. Experiments were run under 11 different wind speed conditions (ranging from 1.5 m s-1 to 22.8 m s-1), and we increased the water pCO2 concentration twice by more than 1000 µatm for two of the seven experimental days. We develop a conceptual box model that incorporated the thermodynamics of the marine CO2 system. Surfactant concentrations in the sea surface microlayer (SML) ranged from 301 µg L-1 to 1015 µg L-1 (as Triton X-100 equivalents) with enrichments ranged from 1.0 to 5.7 in comparison to the samples from the underlying bulk water. With wind speeds up to 8.5 m s-1, surfactants in the SML can reduce the gas transfer velocity by 54%. Wind-wave tank experiments in combination with modelling are useful tools for obtaining a better understanding of the gas transfer velocities of CO2 across the air-sea boundary. The tank allowed for measuring the gas exchange velocity under extreme low and high wind speeds; in contrast, most previous parametrizations have fallen short because measurements of gas exchange velocities in the field are challenging, especially at low wind conditions. High variability in the CO2 transfer velocities suggests that gas exchange is a complex process not solely controlled by wind forces, especially in low wind conditions.

Gas Transfer by Breaking Waves

AbstractThe transfer of gases at the ocean‐atmosphere interface impacts weather and climate from local to global scales, with carbon dioxide (CO2) key to marine life and ecosystems, and dimethyl sulfide affecting aerosols and atmospheric processes. However, the bubble‐mediated gas transfer, associated with breaking waves has remained poorly constrained. We present a spectral framework for bubble‐mediated gas transfer, computed from a mechanistic model for air bubble entrainment at the breaking wave scale combined with a chemical model. Gas transfer is upscaled to the ocean scale by evaluating air entrainment at all wave scales using wave and wave breaking statistics. The obtained CO2 gas transfer velocity reproduces the variability of historical parameterizations and recent field measurements, with very good accuracy. We propose a wind‐wave parameterization that collapses all available data, which can be directly implemented in ocean‐wave models or used with remote sensing of the ocean surface to infer gas transfer.

Using eddy covariance to measure the dependence of air–sea CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; exchange rate on friction velocity

Abstract. Parameterisation of the air–sea gas transfer velocity of CO2 and other trace gases under open-ocean conditions has been a focus of air–sea interaction research and is required for accurately determining ocean carbon uptake. Ships are the most widely used platform for air–sea flux measurements but the quality of the data can be compromised by airflow distortion and sensor cross-sensitivity effects. Recent improvements in the understanding of these effects have led to enhanced corrections to the shipboard eddy covariance (EC) measurements.Here, we present a revised analysis of eddy covariance measurements of air–sea CO2 and momentum fluxes from the Southern Ocean Surface Ocean Aerosol Production (SOAP) study. We show that it is possible to significantly reduce the scatter in the EC data and achieve consistency between measurements taken on station and with the ship underway. The gas transfer velocities from the EC measurements correlate better with the EC friction velocity (u*) than with mean wind speeds derived from shipboard measurements corrected with an airflow distortion model. For the observed range of wind speeds (u10 N = 3–23 m s−1), the transfer velocities can be parameterised with a linear fit to u*. The SOAP data are compared to previous gas transfer parameterisations using u10 N computed from the EC friction velocity with the drag coefficient from the Coupled Ocean–Atmosphere Response Experiment (COARE) model version 3.5. The SOAP results are consistent with previous gas transfer studies, but at high wind speeds they do not support the sharp increase in gas transfer associated with bubble-mediated transfer predicted by physically based models.

Temporal variability of air-sea CO2 exchange in a low-emission estuary

There is the need for further study of whether global estimates of air-sea CO2 exchange in estuarine systems capture the relevant temporal variability and, as such, the temporal variability of bulk parameterized and directly measured CO2 fluxes was investigated in the Danish estuary, Roskilde Fjord. The air-sea CO2 fluxes showed large temporal variability across seasons and between days and that more than 30% of the net CO2 emission in 2013 was a result of two large fall and winter storms. The diurnal variability of ΔpCO2 was up to 400 during summer changing the estuary from a source to a sink of CO2 within the day. Across seasons the system was suggested to change from a sink of atmospheric CO2 during spring to near neutral during summer and later to a source of atmospheric CO2 during fall. Results indicated that Roskilde Fjord was an annual low-emission estuary, with an estimated bulk parameterized release of 3.9 ± 8.7 mol CO2 m−2 y−1 during 2012–2013. It was suggested that the production-respiration balance leading to the low annual emission in Roskilde Fjord, was caused by the shallow depth, long residence time and high water quality in the estuary. In the data analysis the eddy covariance CO2 flux samples were filtered according to the H2OCO2 cross-sensitivity assessment suggested by Landwehr et al. (2014). This filtering reduced episodes of contradicting directions between measured and bulk parameterized air-sea CO2 exchanges and changed the net air-sea CO2 exchange from an uptake to a release. The CO2 gas transfer velocity was calculated from directly measured CO2 fluxes and ΔpCO2 and agreed to previous observations and parameterizations.

Carbon-14 in streams as a tracer of discharging groundwater

Summary Quantification of the volume of groundwater discharge to streams, and the source aquifer of that discharge, is required to adequately manage the impacts of groundwater use on stream ecosystems. This has been achieved through longitudinal surveys of gaseous tracers, but their effectiveness can be limited by rapid equilibration between the stream and the atmosphere. Here we develop the use of carbon-14 in dissolved inorganic carbon (DIC) in a stream as a tracer of groundwater discharge. A controlled equilibration experiment was conducted, during which groundwater with an initial 14C activity of 5.5 pMC was allowed to equilibrate with the atmosphere over 72 days. The effective transfer velocity for 14C was measured as 0.013 m d−1. The method was then tested at an artificial groundwater discharge location, where the effective transfer velocity was measured as 0.025 m d−1. In these simple systems, the ratio of the effective 14C transfer velocity to the CO2 gas transfer velocity is a function of pH, and proportional to the fraction of DIC present as CO2. The method was then applied along a reach of the Daly River, Australia, where groundwater discharge is known to occur. A decrease of 7 pMC was observed across the major spring discharge zone, with subsequent equilibration with the atmosphere occurring over at least tens of kilometres. This allowed for the effective transfer velocity to be estimated at between 0.09 and 0.15 m d−1, and for the 14C activity of groundwater discharge to be estimated at between 60 and 66 pMC. The equilibration of 14C in stream DIC is in the order of 10 times slower than for gas tracers, which may allow for the detection of smaller groundwater discharge fluxes than is possible with gas tracers. If the total groundwater discharge flux is known, measurements of 14C in stream DIC can also be used to infer the 14C activity of discharging groundwater. This method may be a useful alternative to direct groundwater sampling, particularly in remote basins with few groundwater wells.

The effect of wind speed products and wind speed-gas exchange relationships on interannual variability of the air-sea CO2 gas transfer velocity

The lack of a firm relationship between wind speed (U 10) and gas transfer velocity (k )i sconsidered to be one of the factors that hinders accurate quantification of interannual variations of ocean‐atmosphere CO2 fluxes. In this paper the interannual variations of k of using four different k‐U 10 parametrizations are examined using wind speed data from the NCEP/NCAR reanalysis project. The extent to which interannual variations are faithfully reproduced in the NCEP/NCAR data is also investigated. This is carried out through comparison with QuikSCAT data. Compared with 4y ro fQuikSCAT data, NCEP/NCAR data reproduce interannual k variations, although the absolute magnitude of k is underestimated. Interannual k variation shows great sensitivity to selection of k‐U 10 parametrization, and in the Westerlies it changes by a factor of three depending on k‐U 10 parametrization. Use of monthly mean winds speeds leads to overestimation of interannual k variations compared with k variations computed using 6-hourly wind speeds and the appropriate k‐U 10 parametrization. Even though the effect of changing k‐U 10 parametrization is large enough to be an issue that needs to be considered when computing interannual air‐sea CO2 flux variations through combining estimates of k with data for the air‐sea CO2 gradient, it is not sufficient to bridge the gap between such estimates and estimates based on analyses of atmospheric oxygen, CO2 and δ 13 C data. Finally it is shown that the ambiguity in the relationship between wind speed and k introduces an uncertainty in interannual flux variations comparable to a bias of

The effect of wind speed products and wind speed—gas exchange relationships on interannual variability of the air—sea CO2 gas transfer velocity

The lack of a firm relationship between wind speed (U10) and gas transfer velocity (k) is considered to be one of the factors that hinders accurate quantification of interannual variations of ocean—atmosphere CO2 fluxes. In this paper the interannual variations of k of using four different k—U10 parametrizations are examined using wind speed data from the NCEP/NCAR reanalysis project. The extent to which interannual variations are faithfully reproduced in the NCEP/NCAR data is also investigated. This is carried out through comparison with QuikSCAT data. Compared with 4 yr of QuikSCAT data, NCEP/NCAR data reproduce interannual k variations, although the absolute magnitude of k is underestimated. Interannual k variation shows great sensitivity to selection of k—U10parametrization, and in the Westerlies it changes by a factor of three depending on k—U10 parametrization. Use of monthly mean winds speeds leads to overestimation of interannual k variations compared with k variations computed using 6-hourly wind speeds and the appropriate k—U10 parametrization. Even though the effect of changing k—U10 parametrization is large enough to be an issue that needs to be considered when computing interannual air—sea CO2 flux variations through combining estimates of k with data for the air—sea CO2 gradient, it is not sufficient to bridge the gap between such estimates and estimates based on analyses of atmospheric oxygen, CO2 and δ13C data. Finally it is shown that the ambiguity in the relationship between wind speed and k introduces an uncertainty in interannual flux variations comparable to a bias of interannual ΔpCO2 variations of at most ±5 µatm.

Gas transfer velocities of CO2 in three European estuaries (Randers Fjord,Scheldt, and Thames)

We measured the flux of CO2 across the air‐water interface using the floating chamber method in three European estuaries with contrasting physical characteristics (Randers Fjord, Scheldt, and Thames). We computed the gas transfer velocity of CO2 (k) from the CO2 flux and concomitant measurements of the air‐water gradient of the partial pressure of CO2 (pCO2). There was a significant linear relationship between k and wind speed for each of the three estuaries. The differences of the y‐intercept and the slope between the three sites are related to differences in the contribution of tidal currents to water turbulence at the interface and fetch limitation. The contribution to k from turbulence generated by tidal currents is negligible in microtidal estuaries such as Randers Fjord but is substantial, at low to moderate wind speeds, in macrotidal estuaries such as the Scheldt and the Thames. Our results clearly show that in estuaries a simple parameterization of k as a function of wind speed is site specific and strongly suggest that the y‐intercept of the linear relationship is mostly influenced by the contribution of tidal currents, whereas the slope is influenced by fetch limitation. This implies that substantial errors in flux computations are incurred if generic relationships of the gas transfer velocity as a function of wind speed are employed in estuarine environments for the purpose of biogas air‐water flux budgets and ecosystem metabolic studies.

Variability of the gas transfer velocity of CO2 in a macrotidal estuary (the Scheldt)

We report a large set of 295 interfacial carbon dioxide (CO2) flux measurements obtained in the Scheldt estuary in November 2002 and April 2003, using the floating chamber method. From concomitant measurements of the air-water CO2 gradient, we computed the gas transfer velocity of CO2. The gas transfer velocity is well correlated to wind speed and a simple linear regression function gives the most consistent fit to the data. Based on water current measurements, we estimated the contribution of water current induced turbulence to the gas transfer velocity, using the conceptual relationship of O'Connor and Dobbins (1958). This allowed us to construct an empirical relationship to compute the gas transfer velocity of CO2 that accounts for the contribution of wind and water current. Based on this relationship, the spatial and temporal variability of the gas transfer velocity in the Scheldt estuary was investigated. Water currents contribute significantly to the gas transfer velocity, but the spatial and temporal variability (from daily to seasonal scales) is mainly related to wind speed variability.

PARAMETERIZATION FOR THE LOCAL GAS TRANSFER VELOCITY OF CARBON DIOXIDE AT THE SURFACE OF WIND WAVES

A laboratory experiment was made to parameterize the gas transfer velocity of CO2 at the surface of wind waves. The local flux of CO2 was estimated at several fetches by fitting a logarithmic law to the vertical profile of the concentration of CO2 in the air. The dissolved CO2 concentration was measured by using a gas-liquid equilibrator made of hydrophobic porous tube. The local gas transfer velocity was obtained form the results of the flux and dissolved CO2 measurements. For the parameterization, two dimensionless parameters, gx/μ*w2 and μ*w3/gvw were derived from a dimensional analysis, where μ*w is the friction velocity on the water side, g the gravitational acceleration, x the fetch and Vw the kinematic viscosity for the water. On the basis of the experimental results and the dimensional analysis, we proposed a new empirical formul a for the gas transfer velocity of CO2 in consideration of the fetch dependence.

The Western North Pacific Playing a Key Role in Global Biogeochemical Fluxes

The oceanic biogeochemical fluxes in the North Pacific, especially its northwestern part, are discussed to prove their importance on a global scale. First, the air-sea exchange processes of chemical substances are considered quantitatively. The topics discussed are sea salt particles transported to land, sporadic transport of soil dust to the ocean and its role in the marine ecosystem, the larger gas transfer velocity of CO2 indicating the effect of bubbles, and DMS and greenhouse gases other than CO2. Next, chemical tracers are utilized to reveal the water circulation systems in the region, which are the Pacific Deep Water including its vertical eddy diffusivity, the North Pacific Intermediate Water and the Japan Sea Deep Water. Thirdly, the particulate transport process of chemical substances through the water column is clarified by analyzing the distribution of insoluble radionuclides and the results obtained from sediment trap experiments. Fourthly, the northern North Pacific is characterized by stating the site decomposing organic matter and Si playing a key role in the marine ecosystem. Both are induced by the upwelled Pacific Deep Water. Fifthly, the oceanic CO2 system related to global warming is presented by clarifying the distribution of anthropogenic CO2 in the western North Pacific, and roles of the upwelled Pacific Deep Water and the continental shelf zone in the absorption of atmospheric CO2. Finally, Mn and other chemical substances in sediments are discussed as recorders of the early diagenesis and indicators of low biological productivity during glacial ages in the northwestern North Pacific. It is concluded that the western North Pacific is characterized mainly by the Pacific Deep Water bringing nutrients to the northern North Pacific, located at the exit of the global deep water circulation and, therefore, the region plays a key role in the global biogeochemical fluxes.

Global sea–air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects

Based on about 940,000 measurements of surface-water pCO2 obtained since the International Geophysical Year of 1956–59, the climatological, monthly distribution of pCO2 in the global surface waters representing mean non-El Nino conditions has been obtained with a spatial resolution of 4°×5° for a reference year 1995. The monthly and annual net sea–air CO2 flux has been computed using the NCEP/NCAR 41-year mean monthly wind speeds. An annual net uptake flux of CO2 by the global oceans has been estimated to be 2.2 (+22% or ?19%) Pg C yr?1 using the (wind speed)2 dependence of the CO2 gas transfer velocity of Wanninkhof (J. Geophys. Res. 97 (1992) 7373). The errors associated with the wind-speed variation have been estimated using one standard deviation (about±2 m s?1) from the mean monthly wind speed observed over each 4°×5° pixel area of the global oceans. The new global uptake flux obtained with the Wanninkhof (wind speed)2 dependence is compared with those obtained previously using a smaller number of measurements, about 250,000 and 550,000, respectively, and are found to be consistent within±0.2 Pg C yr?1. This estimate for the global ocean uptake flux is consistent with the values of 2.0±0.6 Pg C yr?1 estimated on the basis of the observed changes in the atmospheric CO2 and oxygen concentrations during the 1990s (Nature 381 (1996) 218; Science 287 (2000) 2467). However, if the (wind speed)3 dependence of Wanninkhof and McGillis (Res. Lett. 26 (1999) 1889) is used instead, the annual ocean uptake as well as the sensitivity to wind-speed variability is increased by about 70%. A zone between 40° and 60° latitudes in both the northern and southern hemispheres is found to be a major sink for atmospheric CO2. In these areas, poleward-flowing warm waters meet and mix with the cold subpolar waters rich in nutrients. The pCO2 in the surface water is decreased by the cooling effect on warm waters and by the biological drawdown of pCO2 in subpolar waters. High wind speeds over these low pCO2 waters increase the CO2 uptake rate by the ocean waters. The pCO2 in surface waters of the global oceans varies seasonally over a wide range of about 60% above and below the current atmospheric pCO2 level of about 360 ?atm. A global map showing the seasonal amplitude of surface-water pCO2 is presented. The effect of biological utilization of CO2 is differentiated from that of seasonal temperature changes using seasonal temperature data. The seasonal amplitude of surface-water pCO2 in high-latitude waters located poleward of about 40° latitude and in the equatorial zone is dominated by the biology effect, whereas that in the temperate gyre regions is dominated by the temperature effect. These effects are about 6 months out of phase. Accordingly, along the boundaries between these two regimes, they tend to cancel each other, forming a zone of small pCO2 amplitude. In the oligotrophic waters of the northern and southern temperate gyres, the biology effect is about 35 ?atm on average. This is consistent with the biological export flux estimated by Laws et al. (Glob. Biogeochem. Cycles 14 (2000) 1231). Small areas such as the northwestern Arabian Sea and the eastern equatorial Pacific, where seasonal upwelling occurs, exhibit intense seasonal changes in pCO2 due to the biological drawdown of CO2.

Difference in O2 and CO2 gas transfer velocities in Funka Bay

At a station in Funka Bay (92 m deep), regular observations were made almost once a month for 16 months; and the water column concentrations of dissolved oxygen (DO), phosphate, total carbonate and the δ13C of CO2, as well as some auxiliary components, were measured. Funka Bay has a characteristic water exchange which occurs twice a year and is otherwise limited. This enabled us to apply a closed-system model for the water, except during the flushing periods, to obtain gas transfer velocities from the changes in concentrations. The TC budget corrected for biological production and decomposition showed an increase in water column content, due to atmospheric CO2 absorbed into the bay water at the rate of 7 mol m−2/year or more. This result was in agreement with the observation that the fugacity, f(CO2), of surface seawater was always lower than the atmospheric f(CO2), by about 200 ppm, especially in April. By applying the closed-system model, we obtained a mean gas transfer velocity for O2 of 3.3±1.6 m/day in summer and 7.4±4.3 m/day in winter. The gas transfer velocity of CO2 from carbonate was larger than that of CO2 converted from O2. The mean ratio of the gas transfer velocity of CO2 obtained from carbonate to that of CO2 from O2 was 1.76±0.34. The transfer velocity of CO2 estimated from the δ13C budget was even larger; its ratio to that of CO2 from O2 was 2.8±1.3, although the velocity was obtained for only one period in winter. This order of increase in the gas transfer velocities coincides with the increase in the times necessary for gas exchange equilibration at the surface. This suggests the importance of bubbles or air masses being taken into deeper water.