Air-sea CO2 Research Articles

Mechanisms controlling the air–sea [formula omitted] flux in the North Sea

The mechanisms driving the air–sea exchange of carbon dioxide ( CO 2 ) in the North Sea are investigated using the three-dimensional coupled physical–biogeochemical model ECOHAM (ECOlogical-model, HAMburg). We validate our simulations using field data for the years 2001–2002 and identify the controls of the air–sea CO 2 flux for two locations representative for the North Sea's biogeochemical provinces. In the seasonally stratified northern region, net CO 2 uptake is high ( 2.06 mol m - 2 a - 1 ) due to high net community production (NCP) in the surface water. Overflow production releasing semi-labile dissolved organic carbon needs to be considered for a realistic simulation of the low dissolved inorganic carbon (DIC) concentrations observed during summer. This biologically driven carbon drawdown outcompetes the temperature-driven rise in CO 2 partial pressure ( pCO 2 ) during the productive season. In contrast, the permanently mixed southern region is a weak net CO 2 source ( 0.78 mol m - 2 a - 1 ). NCP is generally low except for the spring bloom because remineralization parallels primary production. Here, the pCO 2 appears to be controlled by temperature.

Partial pressure and air–sea CO 2 flux in the Aegean Sea during February 2006

Data on the distribution of fCO 2 were obtained during a cruise in the Aegean Sea during February 2006. The fCO 2 of surface water ( fCO 2 sw) was lower than the atmospheric fCO 2 ( fCO 2 atm) throughout the area surveyed and Δ fCO 2 values varied from −34 to −61 μatm. The observed under-saturation suggests that surface waters in the Aegean represent a sink for atmospheric CO 2 during the winter of 2006. Higher fCO 2 sw values were recorded in the ‘less warm’ and ‘less saline’ shallow northernmost part of the Aegean Sea implying that the lower seawater temperature and salinity in this area play a crucial role in the spatial distribution of fCO 2 sw. A first estimate of the magnitude of the air–sea CO 2 exchange and the potential role of the Aegean Sea in the transfer of atmospheric CO 2 was also obtained. The air–sea CO 2 fluxes calculated using different gas transfer formulations showed that during February 2006, the Aegean Sea absorbs atmospheric CO 2 at a rate ranging from −6.2 to −11.8 mmol m −2 d −1 with the shipboard recorded wind speeds and at almost half rate (−3.5 to −5.5 mmol m −2 d −1) with the monthly mean model-derived wind speed. Compared to recent observations from other temperate continental shelves during winter period, the Aegean Sea acts as a moderate to rather strong sink for atmospheric CO 2. Further investigations, including intensive spatial and temporal high-resolution observations, are necessary to elucidate the role of the Aegean Sea in the process of transfer of atmospheric CO 2 into the deep horizons of the Eastern Mediterranean.

Primary production in the Strait of Gibraltar: Carbon fixation rates in relation to hydrodynamic and phytoplankton dynamics

Primary production was studied at nine sites distributed within the Strait of Gibraltar (Southern Spain) and North-Western (NW) Alboran Sea by analyzing photosynthesis-irradiance (P–I) relationships and integrated primary production rates in relation to the different types of Deep Chlorophyll Maxima (DCM) detected in the area. The characteristics of the DCM were examined by several methods, including flow cytometry, quantification of transparent expolymer particles and fluorimetric measurements that were applied in order to assess the photo-physiological state of the phytoplankton assemblages with respect to their species composition and water column structure (hydrology). The photosynthetic parameters (derived from P–I relationships) and integrated primary production (range 6–644 mg m −2 d −1) responded greatly to the diverse DCM identified and thereby the spatial variability of the primary production observed in the region was found to depend upon the occurrence of the different types of phytoplankton accumulations, which were themselves indicative of the previous history of the water column. The net contribution of the primary production to the air–sea CO 2 exchange process was also evaluated in the area. Results indicated that this region behaved as a net sink for the atmospheric CO 2, with the intensity of the flux being strongly modulated by the wind intensity.

New insights into the spatial variability of the surface water carbon dioxide in varying sea ice conditions in the Arctic Ocean

In the summer of 2005, continuous surface water measurements of fugacity of CO 2 ( fCO 2 sw), salinity and temperature were performed onboard the IB Oden along the Northwest Passage from Cape Farwell (South Greenland) to the Chukchi Sea. The aim was to investigate the importance of sea ice and river runoff on the spatial variability of fCO 2 and the sea–air CO 2 fluxes in the Arctic Ocean. Additional data was obtained from measurements of total alkalinity ( A T) by discrete surface water and water column sampling in the Canadian Arctic Archipelago (CAA), on the Mackenzie shelf, and in the Bering Strait. The linear relationship between A T and salinity was used to evaluate and calculate the relative fractions of sea ice melt water and river runoff along the cruise track. High-frequency fCO 2 sw data showed rapid changes, due to variable sea ice conditions, freshwater addition, physical upwelling and biological processes. The fCO 2 sw varied between 102 and 678 μatm. Under the sea ice in the CAA and the northern Chukchi Sea, fCO 2 sw were largely CO 2 undersaturated of approximately 100 μatm lower than the atmospheric level. This suggested CO 2 uptake by biological production and limited sea–air CO 2 gas exchange due to the ice cover. In open areas, such as the relatively fresh water of the Mackenzie shelf and the Bering Strait, the fCO 2 sw values were close to the atmospheric CO 2 level. Upwelling of saline and relatively warm water at the Cape Bathurst caused a dramatic fCO 2 sw increase of about 100 μatm relative to the values in the CAA. At the southern part of the Chukchi Peninsula we found the highest fCO 2 sw values and the water was CO 2 supersaturated, likely due to upwelling. In the study area, the calculated sea–air CO 2 flux varied between an oceanic CO 2 sink of 140 mmol m −2 d −1 and an oceanic source of 18 mmol m −2 d −1. However, in the CAA and the northern Chukchi Sea, the sea ice cover prevented gas exchange, and the CO 2 fluxes were probably negligible at this time of the year. Assuming that the water was exposed to the atmosphere by total melting and gas exchange would be the only process, the CO 2 undersaturated water in the ice-covered areas will not have the time to reach the atmospheric CO 2 value, before the formation of new sea ice. This study highlights the value of using high-frequency measurements to gain increased insight into the variable and complex conditions, encountered on the shelves in the Arctic Ocean.

On the seasonal variation of air – sea CO 2 fluxes in the outer Changjiang (Yangtze River) Estuary, East China Sea

Based upon seven field surveys conducted during April 2005 – April 2008, we examined the surface partial pressure of CO 2 ( pCO 2) and dissolved oxygen (DO) in the outer Changjiang (Yangtze River) Estuary, on the inner shelf of the East China Sea (ECS). This area represents a most dynamic zone of the ECS where high pCO 2 riverine water meets with highly productive shelf waters, covering a 2° × 3° area, ~ 10% of the surface area of the entire ECS. Surface pCO 2 ranged 320 – 380 µatm (average ~ 345 µatm) in winter, 180 – 450 µatm (average ~ 330 µatm) in spring, 150 – 620 µatm (average ~ 310 µatm) in summer and 120 – 540 µatm (average ~ 375 µatm) in autumn. The seasonal variation pattern of surface DO generally mirrored that of pCO 2, ranging 95% – 105% in winter, 96% – 142% (average 110%) in spring, 73% – 192% (average 118%) in summer and 81% – 178% (average 102%) in autumn. The dynamics of pCO 2 drawdown and DO enhancement in the warm seasons (from April to October) appeared to be controlled by primary productivity and air – sea exchange, while mixing dominated the aqueous pCO 2 in the cold seasons (from November to March of the following year). This study showed that the outer Changjiang Estuary served as a moderate or significant sink of atmospheric CO 2 in winter, spring and summer, while it turned to a net source in autumn. The integrated sea – air CO 2 flux in the outer Changjiang Estuary was estimated as − 1.9 ± 1.3 mol m − 2 year − 1, which is double the recent sea–air CO 2 flux estimation for the northern ECS.

Satellite-derived surface water pCO 2 and air–sea CO 2 fluxes in the northern South China Sea in summer

An empirical approach is presented for the estimation of the partial pressure of carbon dioxide ( pCO 2) and air–sea CO 2 fluxes in the northern South China Sea in summer using satellite-derived sea surface temperatures (SSTs), chlorophyll-a (Chl a) concentrations, and wind fields. Two algorithms were tested. The first used an SST-dependent equation, and the other involved the introduction of Chl a. Regression equations were developed for summer based on in situ data obtained in July, 2004. Using the monthly average SST and Chl a fields derived from the advanced very high resolution radiometer (AVHRR) and the SeaWiFS (sea-viewing wide field of view sensor), respectively, the monthly pCO 2 fields were computed. The derived pCO 2 was compared with the shipboard pCO 2 observations conducted in July, 2000. This resulted in a root-mean-square error of 4.6 μatm, suggesting that the satellite-derived pCO 2 was in general agreement with the in situ observations. The air–sea CO 2 flux was further computed with the aid of the monthly mean QuikSCAT wind speed. We contend that more shipboard data are necessary for refining the empirical algorithms and reducing the uncertainty in the results.

Effect of eutrophication on air–sea CO2 fluxes in the coastal Southern North Sea: a model study of the past 50 years

AbstractThe RIVERSTRAHLER model, an idealized biogeochemical model of the river system, has been coupled to MIRO‐CO2, a complex biogeochemical model describing diatom and Phaeocystis blooms and carbon and nutrient cycles in the marine domain, to assess the dual role of changing nutrient loads and increasing atmospheric CO2 as drivers of air–sea CO2 exchanges in the Southern North Sea with a focus on the Belgian coastal zone (BCZ). The whole area, submitted to the influence of two main rivers (Seine and Scheldt), is characterized by variable diatom and Phaeocystis colonies blooms which impact on the trophic status and air–sea CO2 fluxes of the coastal ecosystem. For this application, the MIRO‐CO2 model is implemented in a 0D multibox frame covering the eutrophied Eastern English Channel and Southern North Sea and receiving loads from the rivers Seine and Scheldt. Model simulations are performed for the period between 1951 and 1998 using real forcing fields for sea surface temperature, wind speed and atmospheric CO2 and RIVERSTRAHLER simulations for river carbon and nutrient loads. Model results suggest that the BCZ shifted from a source of CO2 before 1970 (low eutrophication) towards a sink during the 1970–1990 period when anthropogenic DIN and P loads increased, stimulating C fixation by autotrophs. In agreement, a shift from net annual heterotrophy towards autotrophy in BCZ is simulated from 1980. The period after 1990 is characterized by a progressive decrease of P loads concomitant with a decrease of primary production and of the CO2 sink in the BCZ. At the end of the simulation period, the BCZ ecosystem is again net heterotroph and acts as a source of CO2 to the atmosphere. R‐MIRO‐CO2 scenarios testing the relative impact of temperature, wind speed, atmospheric CO2 and river loads variability on the simulated air–sea CO2 fluxes suggest that the trend in air–sea CO2 fluxes simulated between 1951 and 1998 in the BCZ was mainly controlled by the magnitude and the ratio of inorganic nutrient river loads. Quantitative nutrient changes control the level of primary production while qualitative changes modulate the relative contribution of diatoms and Phaeocystis to this flux and hence the sequestration of atmospheric CO2.

Climatological mean and decadal change in surface ocean pCO2, and net sea–air CO2 flux over the global oceans

A climatological mean distribution for the surface water pCO2 over the global oceans in non-El Nino conditions has been constructed with spatial resolution of 4° (latitude) ×5° (longitude) for a reference year 2000 based upon about 3 million measurements of surface water pCO2 obtained from 1970 to 2007. The database used for this study is about 3 times larger than the 0.94 million used for our earlier paper [Takahashi et al., 2002. Global sea–air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects. Deep-Sea Res. II, 49, 1601–1622]. A time-trend analysis using deseasonalized surface water pCO2 data in portions of the North Atlantic, North and South Pacific and Southern Oceans (which cover about 27% of the global ocean areas) indicates that the surface water pCO2 over these oceanic areas has increased on average at a mean rate of 1.5 μatm y−1 with basin-specific rates varying between 1.2±0.5 and 2.1±0.4 μatm y−1. A global ocean database for a single reference year 2000 is assembled using this mean rate for correcting observations made in different years to the reference year. The observations made during El Nino periods in the equatorial Pacific and those made in coastal zones are excluded from the database. Seasonal changes in the surface water pCO2 and the sea-air pCO2 difference over four climatic zones in the Atlantic, Pacific, Indian and Southern Oceans are presented. Over the Southern Ocean seasonal ice zone, the seasonality is complex. Although it cannot be thoroughly documented due to the limited extent of observations, seasonal changes in pCO2 are approximated by using the data for under-ice waters during austral winter and those for the marginal ice and ice-free zones. The net air–sea CO2 flux is estimated using the sea–air pCO2 difference and the air–sea gas transfer rate that is parameterized as a function of (wind speed)2 with a scaling factor of 0.26. This is estimated by inverting the bomb 14C data using Ocean General Circulation models and the 1979–2005 NCEP-DOE AMIP-II Reanalysis (R-2) wind speed data. The equatorial Pacific (14°N–14°S) is the major source for atmospheric CO2, emitting about +0.48 Pg-C y−1, and the temperate oceans between 14° and 50° in the both hemispheres are the major sink zones with an uptake flux of −0.70 Pg-C y−1 for the northern and −1.05 Pg-C y−1 for the southern zone. The high-latitude North Atlantic, including the Nordic Seas and portion of the Arctic Sea, is the most intense CO2 sink area on the basis of per unit area, with a mean of −2.5 tons-C month−1 km−2. This is due to the combination of the low pCO2 in seawater and high gas exchange rates. In the ice-free zone of the Southern Ocean (50°–62°S), the mean annual flux is small (−0.06 Pg-C y−1) because of a cancellation of the summer uptake CO2 flux with the winter release of CO2 caused by deepwater upwelling. The annual mean for the contemporary net CO2 uptake flux over the global oceans is estimated to be −1.6±0.9 Pg-C y−1, which includes an undersampling correction to the direct estimate of −1.4±0.7 Pg-C y−1. Taking the pre-industrial steady-state ocean source of 0.4±0.2 Pg-C y−1 into account, the total ocean uptake flux including the anthropogenic CO2 is estimated to be −2.0±1.0 Pg-C y−1 in 2000.

Mechanisms governing interannual variability in upper-ocean inorganic carbon system and air–sea CO2 fluxes: Physical climate and atmospheric dust

Abstract We quantify the mechanisms governing interannual variability in the global, upper-ocean inorganic carbon system using a hindcast simulation (1979–2004) of an ecosystem-biogeochemistry model forced with time-evolving atmospheric physics and dust deposition. We analyze the variability of three key, interrelated metrics—air–sea CO 2 flux, surface-water carbon dioxide partial pressure p CO 2 , and upper-ocean dissolved inorganic carbon (DIC) inventory—presenting for each metric global spatial maps of the root mean square (rms) of anomalies from a model monthly climatology. The contribution of specific driving factors is diagnosed using Taylor expansions and linear regression analysis. The major regions of variability occur in the Southern Ocean, tropical Indo-Pacific, and Northern Hemisphere temperate and subpolar latitudes. Ocean circulation is the dominant factor driving variability over most of the ocean, modulating surface dissolved inorganic carbon that in turn alters surface-water p CO 2 and air–sea CO 2 flux variability (global integrated anomaly rms of 0.34 Pg C yr −1 ). Biological export and thermal solubility effects partially damp circulation-driven p CO 2 variability in the tropics, while in the subtropics, thermal solubility contributes positively to surface-water p CO 2 and air–sea CO 2 flux variability. Gas transfer and net freshwater inputs induce variability in the air–sea CO 2 flux in some specific regions. A component of air–sea CO 2 flux variability (global integrated anomaly rms of 0.14 Pg C yr −1 ) arises from variations in biological export production induced by variations in atmospheric iron deposition downwind of dust source regions. Beginning in the mid-1990s, reduced global dust deposition generates increased air–sea CO 2 outgassing in the Southern Ocean, consistent with trends derived from atmospheric CO 2 inversions.

Strategies for high-latitude northern hemisphere CO2 sampling now and in the future

Abstract A sampling strategy to determine the annual mean air–sea CO 2 fluxes in the high-latitude North Pacific and North Atlantic was developed using a combination of signal-to-noise ratios and 2D Fourier transforms in conjunction with a coupled climate carbon model. To account for unresolved mesoscale variability in our simulations, we used summer and winter cruises to determine the magnitude of this variability and estimate its impact on the air–sea fluxes. From our analysis, we propose a regular sampling strategy of every 6° in latitude and 10° in longitude every 3 months for the North Pacific and North Atlantic. Applying this sampling strategy to the simulation returned an annual mean air–sea flux value within ±15% of the total simulated value, when applied now and into the future. Our study highlights several key points for measuring annual mean basin-scale CO 2 fluxes: (1) the combination of temporal and spatial sampling dramatically reduces the noise in the observations and provides a good representation of the fluxes with far fewer measurements than required for resolving the spatial or temporal signals independently; (2) that sampling at higher than recommended frequencies in time and space provides little improvement in the estimated annual mean flux; (3) the uncertainty in the decadal annual mean uptake is limited by interannual variability and not by sampling error or unresolved mesoscale variability; and (4) our high-latitude sampling strategies remain valid until at least the end of this century.

Estimation of air–sea CO 2 fluxes in the Bay of Biscay based on empirical relationships and remotely sensed observations

An empirical algorithm has been developed to compute the sea surface CO 2 fugacity (fCO 2 sw) in the Bay of Biscay from remotely sensed sea surface temperature (SST RS) and chlorophyll a (chl a RS) retrieved from AVHRR and SeaWiFS sensors, respectively. Underway fCO 2 sw measurements recorded during 2003 were correlated with SST RS and chl a RS data yielding a regression error of 0.1 ± 7.5 µatm (mean ± standard deviation). The spatial and temporal variability of air–sea fCO 2 gradient (ΔfCO 2) and air–sea CO 2 flux (FCO 2) was analyzed using remotely sensed images from September 1997 to December 2004. An average FCO 2 of − 1.9 ± 0.1 mol m − 2 yr − 1 characterized the Bay of Biscay as a CO 2 sink that is suffering a significant long-term decrease of 0.08 ± 0.05 mol m − 2 yr − 2 in its capacity to store atmospheric CO 2. The main parameter controlling the long-term variability of the CO 2 uptake from the atmosphere was the air–sea CO 2 transfer velocity (57%), followed by the SST RS (10%) and the chl a RS (2%).

Air–sea carbon dioxide fluxes in the coastal southeastern tropical Pacific

Comprehensive sea surface surveys of the partial pressure of carbon dioxide (pCO 2) have been made in the upwelling system of the coastal (0–200 km from shore) southeastern tropical Pacific since 2004. The shipboard data have been supplemented by mooring and drifter based observations. Air–sea flux estimates were made by combining satellite derived wind fields with the direct sea surface pCO 2 measurements. While there was considerable spatial heterogeneity, there was a significant flux of CO 2 from the ocean to the atmosphere during all survey periods in the region between 4° and 20° south latitude. During periods of strong upwelling the average flux out of the ocean exceeded 10 moles of CO 2 per square meter per year. During periods of weaker upwelling and high productivity the CO 2 evasion rate was near 2.5 mol/m 2/yr. The average annual fluxes exceed 5 mol/m 2/yr. These findings are in sharp contrast to results obtained in mid-latitude upwelling systems along the west coast of North America where the average air–sea CO 2 flux is low and can often be from the atmosphere into the ocean. In the Peruvian upwelling system there are several likely factors that contribute to sea surface pCO 2 levels that are well above those of the atmosphere in spite of elevated primary productivity: (1) the upwelling source waters contain little pre-formed nitrate and are affected by denitrification, (2) iron limitation of primary production enhanced by offshore upwelling driven by the curl of the wind stress and (3) rapid sea surface warming. The combined carbon, nutrient and oxygen dynamics of this region make it a candidate site for studies of global change.

Effects of storms on primary productivity and air-sea CO<sub>2</sub> exchange in the subarctic western North Pacific: a modeling study

Abstract. Biogeochemical responses of the open ocean to storms and their feedback to climate are still poorly understood. Using a marine ecosystem model, we examined biogeochemical responses to the storms in the subarctic western North Pacific. The storms in summer through early autumn enhance net community production by wind-induced nutrient injections into the surface waters while the storms in the other seasons reduce net community production by intensifying light limitation on the phytoplankton growth due to vertical dilution of the phytoplankton. The two compensating effects diminish the storm-induced annual change of net community production to only 1%. On the contrary, the storms reduce the annual oceanic uptake of the atmospheric CO2 by 3%, resulting from storm-induced strong winds. Our results suggest that previous studies using climatological wind, sea level pressure, and CO2 data probably overestimated the air-to-sea CO2 influx during storms in the subarctic western North Pacific, and therefore, continuous high-frequent observations of these variables are required to reduce uncertainties in the global oceanic CO2 uptake.

PCO2 and carbon fluxes across sea-air interface in the Changjiang Estuary and Hangzhou Bay

Partial pressure of CO(2) (pCO(2)) was investigated in the Changjiang (Yangtze River) Estuary, Hangzhou Bay and their adjacent areas during a cruise in August 2004, China. The data show that pCO(2) in surface waters of the studied area was higher than that in the atmosphere with only exception of a patch east of Zhoushan Archipelago. The pCO(2) varied from 168 to 2 264 mu atm, which fell in the low range compared with those of other estuaries in the world. The calculated sea-air CO(2) fluxes decreased offshore and varied from -10.0 to 88.1 mmol m(-2) d(-1) in average of 24.4 +/- 16.5 mmol m(-2) d(-1). Although the area studied was estimated only 2 x 10(4) km(2), it emitted (5.9 +/- 4.0) x 10(3) tons of carbon to the atmosphere every day. The estuaries and their plumes must be further studied for better understanding the role of coastal seas playing in the global oceanic carbon cycle.

Estimate of global sea-air CO2 flux with sea-state-dependent parameterization

Although the annual global sea-air CO2 flux has been estimated extensively with various wind-dependent-k parameterizations, uncertainty still exists in the estimates. The sea-state-dependent-k parameterization is expected to improve the uncertainty existing in these estimates. In the present study, the annual global sea-air CO2 flux is estimated with the sea-state-dependent-k parameterization proposed by Woolf (2005), using NOAA/NCEP reanalysis wind speed and hindcast wave data from 1998 to 2006, and a new estimate, −2.18 Gt C year−1, is obtained, which is comparable with previous estimates with biochemical methods. It is interesting to note that the averaged value of previous estimates with various wind-dependent-k parameterizations is almost identical to that of previous estimates with biochemical methods by various authors, and that the new estimate is quite consistent with these averaged estimates.

Detection of regional scale sea-to-air oxygen emission related to spring bloom near Japan by using in-situ measurements of the atmospheric oxygen/nitrogen ratio

Abstract. We have been carrying out in-situ monitoring of atmospheric O2/N2 ratio at Cape Ochi-ishi (COI; 43°10' N, 145°30' E) in the northern part of Japan since March 2005 by using a modified gas chromatography/thermal conductivity detector (GC/TCD). The standard deviation of the O2/N2 ratio is estimated to be about ±14 per meg (≈3 ppm) with intervals of 10 minutes. Thus, the in-situ measurement system has a 1σ precision of ± 6 per meg (≈1.2 ppm) for one-hour mean O2/N2 ratio. Atmospheric potential oxygen (APO≈O2+1.1 CO2), which is conserved with respect to terrestrial photosynthesis and respiration but reflects changes in air-sea O2 and CO2 fluxes, shows large variabilities from April to early July 2005. Distribution of satellite-derived marine primary production indicates occurrences of strong bloom in the Japan Sea and the latitudinal band between 30° and 40° N in the western North Pacific in April and in the Okhotsk Sea and northeastern region near Hokkaido Island in the North Pacific in June. Back trajectory analysis of air masses indicates that high values of APO, which last for several hours or several days, can be attributed to the oxygen emission associated with the spring bloom of active primary production.

Application of satellite remote sensing techniques for estimating air–sea CO 2 fluxes in Hudson Bay, Canada during the ice-free season

The role of coastal seas as either a sink or a source of CO 2 is subject to a great deal of uncertainty. This uncertainty largely arises from a lack of observations in the coastal zones. Remote sensing offers an avenue for expanding these observations by allowing for the extrapolation of relatively limited data sets of dissolved CO 2 ( pCO 2sw). In this paper, predictive algorithms for pCO 2sw that could be applied to remote sensing products were created from a field data set collected from September–October, 2005 in Hudson Bay, Canada. The field data showed that an effective pCO 2sw interpolation algorithm could be created using sea surface temperature (SST) as a predictor, and that a slight improvement of the algorithm could be achieved if measurements of absorption due to coloured dissolved organic material ( a CDOM) were included. Unfortunately, satellite retrievals of a CDOM did not match well with in situ observations, and so only SST (obtained from the MODIS Aqua sensor) was used to create monthly maps of pCO 2sw for the period of August–October. To estimate fluxes of CO 2, constructed surfaces of pCO 2sw were combined with estimates of gas transfer velocity derived from QuikSCAT wind retrievals, and pCO 2air based on field observations. The results of these calculations revealed that Hudson Bay acts as a source of CO 2 during August and September, but reverts to a sink of CO 2 in October as the water temperature decreases. Overall, a positive flux of 1.60 TgC was estimated for the region during the ice-free season. This result is in contrast to most Arctic or sub-Arctic continental shelf seas, where usually strong absorptions of CO 2 are observed.

Skill metrics for confronting global upper ocean ecosystem-biogeochemistry models against field and remote sensing data

We present a generalized framework for assessing the skill of global upper ocean ecosystem-biogeochemical models against in-situ field data and satellite observations. We illustrate the approach utilizing a multi-decade (1979–2004) hindcast experiment conducted with the Community Climate System Model (CCSM- 3) ocean carbon model. The CCSM-3 ocean carbon model incorporates a multi-nutrient, multi-phytoplankton functional group ecosystem module coupled with a carbon, oxygen, nitrogen, phosphorus, silicon, and iron biogeochemistry module embedded in a global, three-dimensional ocean general circulation model. The model is forced with physical climate forcing from atmospheric reanalysis and satellite data products and time-varying atmospheric dust deposition. Data-based skill metrics are used to evaluate the simulated time-mean spatial patterns, seasonal cycle amplitude and phase, and subannual to interannual variability. Evaluation data include: sea surface temperature and mixed layer depth; satellite-derived surface ocean chlorophyll, primary productivity, phytoplankton growth rate and carbon biomass; large-scale climatologies of surface nutrients, pCO 2, and air–sea CO 2 and O 2 flux; and time-series data from the Joint Global Ocean Flux Study (JGOFS). Where the data is sufficient, we construct quantitative skill metrics using: model–data residuals, time–space correlation, root mean square error, and Taylor diagrams.

Daily variability of dissolved inorganic radiocarbon at three sites in the surface ocean

We report radiocarbon measurements of dissolved inorganic carbon (DIC) in surface water samples collected daily during cruises to the central North Pacific, the Sargasso Sea and the Southern Ocean. The ranges of Δ 14C measurements for each cruise (11–30‰) were larger than the total uncertainty (7.8‰, 2-sigma) of the measurements. The variability is attributed to changes in the upper water mass that took place at each site over a two to four week period. These results indicate that variability of surface Δ 14C values is larger than the analytical precision, because of patchiness that exists in the DIC Δ 14C signature of the surface ocean. This additional variability can affect estimates of geochemical parameters such as the air–sea CO 2 exchange rate using radiocarbon.

Effects of an estuarine plume-associated bloom on the carbonate system in the lower reaches of the Pearl River estuary and the coastal zone of the northern South China Sea

We observed a phytoplankton bloom downstream of a large estuarine plume induced by heavy precipitation during a cruise conducted in the Pearl River estuary and the northern South China Sea in May–June 2001. The plume delivered a significant amount of nutrients into the estuary and the adjacent coastal region, and enhanced stratification stimulating a phytoplankton bloom in the region near and offshore of Hong Kong. A several fold increase (0.2–1.8 μg Chl L −1) in biomass (Chl a) was observed during the bloom. During the bloom event, the surface water phytoplankton community structure significantly shifted from a pico-phytoplankton dominated community to one dominated by micro-phytoplankton (>20 μm). In addition to increased Chl a, we observed a significant drawdown of pCO 2, biological uptake of dissolved inorganic carbon (DIC) and an associated enhancement of dissolved oxygen and pH, demonstrating enhanced photosynthesis during the bloom. During the bloom, we estimated a net DIC drawdown of 100–150 μmol kg −1 and a TAlk increase of 0–50 μmol kg −1. The mean sea–air CO 2 flux at the peak of the bloom was estimated to be as high as ∼−18 mmol m −2 d −1. For an average surface water depth of 5 m, a very high apparent biological CO 2 consumption rate of 70–110 mmol m −2 d −1 was estimated. This value is 2–6 times higher than the estimated air–sea exchange rate.