Air-sea CO2 Flux Research Articles

Seasonal variation of CO2 air-sea flux and effects of warming in the Kuroshio current of the East China Sea

The partial pressure of CO2 (pCO2) and associated CO2 air-sea flux exhibit highly heterogeneous temporal and spatial patterns in ocean margins. In this study, we analyzed a three-year time-series data sampled during 2011–2014 along the Kuroshio Current within the East China Sea (ECS) to investigate the seasonal pattern of carbonate chemistry and CO2 air-sea fluxes. Annually, the Kuroshio within the ECS operates as a net CO2 sink at approximately 1.3 mol C m−2 yr−1, less than estimates over the ECS shelf (~1.8 mol C m−2 yr−1). The thermal control of pCO2 makes the Kuroshio a strong CO2 sink in winter, with a transition to net-neutral, or a weak CO2 source in summer. On an interannual basis, however, the seasonal CO2 air-sea fluxes in the Kuroshio may undergo potential shifts if warming conditions continue.

Distribution and air-sea fluxes of CO2 in coral reefs in the Greater Bay Area, China

From May to June 2021, the sea surface partial pressure of CO2 (pCO2) and air-sea CO2 fluxes around six coral reefs in the Great Bay Area (contain Pearl River Estuary (PRE) and Daya Bay) were investigated respectively. The distribution of seawater pCO2 around different coral reefs were quite different. The seawater pCO2 of the coral reef region of Daya Bay is relatively high, ranging from 268 to 496 µatm. In contrast, the pCO2 levels in the coral reef area of the PRE are significantly lower, with measurements ranging from 84 to 374 µatm. Further analysis showed that the distribution of seawater pCO2 around the coral reefs of the PRE were mainly affected by biological metabolism and seawater mixing, while those in Daya Bay were mainly controlled by temperature and seawater mixing. During the survey, the three coral reefs in the PRE were affected by algal blooms showing strong sinks of atmospheric CO2 ranging from −12.3 to −19.2 mmol CO2 m−2 d−1. However, the difference between the seawater pCO2 and atmospheric pCO2 in the three coral reefs of Daya Bay were all very small, and they were weak sources or sinks of atmospheric CO2, respectively.

Temperature effect on seawater fCO2 revisited: theoretical basis, uncertainty analysis and implications for parameterising carbonic acid equilibrium constants

Abstract. The sensitivity of the fugacity of carbon dioxide in seawater (fCO2) to temperature (denoted υ, reported in % °C−1) is critical for the accurate fCO2 measurements needed to build global carbon budgets and for understanding the drivers of air–sea CO2 flux variability across the ocean. However, understanding and computing υ have been restricted to either using empirical functions fitted to experimental data or determining it as an emergent property of a fully resolved marine carbonate system, and these two approaches are not consistent with each other. The lack of a theoretical basis and an uncertainty estimate for υ has hindered resolving this discrepancy. Here, we develop a new approach for calculating the temperature sensitivity of fCO2 based on the equations governing the marine carbonate system and the van 't Hoff equation. This shows that, to first order, ln (fCO2) should be proportional to 1/tK (where tK is temperature in kelvin), rather than to temperature, as has previously been assumed. This new approach is, to first order, consistent with calculations from a fully resolved marine carbonate system, which we have incorporated into the PyCO2SYS software. Agreement with experimental data is less convincing but remains inconclusive due to the scarcity of direct measurements of υ, particularly above 25 °C. However, the new approach is consistent with field data, performing better than any other approach for adjusting fCO2 by up to 10 °C if spatiotemporal variability in its single fitted coefficient is accounted for. The uncertainty in υ arising from only measurement uncertainty in the main experimental dataset where υ has been directly measured is in the order of 0.04 % °C−1, which corresponds to a 0.04 % uncertainty in fCO2 adjusted by +1 °C. However, spatiotemporal variability in υ is several times greater than this, so the true uncertainty due to the temperature adjustment in fCO2 adjusted by +1 °C using the most widely used constant υ value is around 0.24 %. This can be reduced to around 0.06 % using the new approach proposed here, and this could be further reduced by more measurements. The spatiotemporal variability in υ arises mostly from the equilibrium constants for CO2 solubility and carbonic acid dissociation (K1∗ and K2∗), and its magnitude varies significantly depending on which parameterisation is used for K1∗ and K2∗. Seawater fCO2 can be measured accurately enough that additional experiments should be able to detect spatiotemporal variability in υ and distinguish between different parameterisations for K1∗ and K2∗. Because the most widely used constant υ was coincidentally measured from seawater with roughly global average υ, our results are unlikely to significantly affect global air–sea CO2 flux budgets, but they may have more important implications for regional budgets and studies that adjust by larger temperature differences.

Enhanced ocean CO2 uptake due to near-surface temperature gradients

The ocean annually absorbs about a quarter of all anthropogenic carbon dioxide (CO2) emissions. Global estimates of air–sea CO2 fluxes are typically based on bulk measurements of CO2 in air and seawater and neglect the effects of vertical temperature gradients near the ocean surface. Theoretical and laboratory observations indicate that these gradients alter air–sea CO2 fluxes, because the air–sea CO2 concentration difference is highly temperature sensitive. However, in situ field evidence supporting their effect is so far lacking. Here we present independent direct air–sea CO2 fluxes alongside indirect bulk fluxes collected along repeat transects in the Atlantic Ocean (50° N to 50° S) in 2018 and 2019. We find that accounting for vertical temperature gradients reduces the difference between direct and indirect fluxes from 0.19 mmol m−2 d−1 to 0.08 mmol m−2 d−1 (N = 148). This implies an increase in the Atlantic CO2 sink of ~0.03 PgC yr−1 (~7% of the Atlantic Ocean sink). These field results validate theoretical, modelling and observational-based efforts, all of which predicted that accounting for near-surface temperature gradients would increase estimates of global ocean CO2 uptake. Accounting for this increased ocean uptake will probably require some revision to how global carbon budgets are quantified.

Seasonality of pCO2 and air-sea CO2 fluxes in the Central Labrador Sea

The Labrador Sea in the subpolar North Atlantic is known for its large air-to-sea CO2 fluxes, which can be around 40% higher than in other regions of intense ocean uptake like the Eastern Pacific and within the Northwest Atlantic. This region is also a hot-spot for storage of anthropogenic CO2. Deep water is formed here, so that dissolved gas uptake by the surface ocean directly connects to deeper waters, helping to determine how much atmospheric CO2 may be sequestered (or released) by the deep ocean. Currently, the Central Labrador Sea acts as a year-round sink of atmospheric CO2, with intensification of uptake driven by biological production in spring and lasting through summer and fall. Observational estimates of air-sea CO2 fluxes in the region rely upon very limited, scattered data with a distinct lack of wintertime observations. Here, we compile surface ocean observations of pCO2 from moorings and underway measurements, including previously unreported data, between 2000 and 2020, to create a baseline seasonal climatology for the Central Labrador Sea. This is used as a reference to compare against other observational-based and statistical estimates of regional surface pCO2 and air-sea fluxes from a collection of global products. The comparison reveals systematic differences in the representation of the seasonal cycle of pCO2 and uncertainties in the magnitude of air-sea CO2 fluxes. The analysis reveals the paramount importance of long-term, seasonally-resolved data coverage in this region in order to accurately quantify the size of the present ocean sink for atmospheric CO2 and its sensitivity to climate perturbations.

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.

Impacts of shellfish and macroalgae mariculture on the seawater carbonate system and air-sea [formula omitted] flux in Haizhou Bay, China

China is the largest mariculture country, and shellfish and algae output ranks first, showing high carbon sink capacity. In recent years, the single cultivation of macroalgae (Pyropia yezoensis) has been changed to macroalgae-shellfish mariculture in Haizhou Bay to increase the yield of P. yezoensis and improve the water environment quality. In this study, four surveys were carried out in July 2022 during the monoculture period of oyster (Magallana gigas), as well as at different stages of P. yezoensis culture (head-crop period, November 2022, peak growing season, January 2023, and end of harvesting, March 2023) in the mariculture and the surrounding waters of Haizhou Bay. The effects of different stages of culture on the seawater environment and seasonal and spatial variations in the carbonate system were analyzed, and the carbon sink capacity was preliminarily estimated. The results showed that in summer, the calcification of M. gigas and the primary production process of phytoplankton effectively reduced the dissolved inorganic carbon (DIC) level in the culture area. The culture area acts as a CO2 sink, with an average air-sea CO2 flux of −4.5 mmol m−2 d−1. During the polyculture period, the P. yezoensis culture activities maintained the stability of the seawater carbonate system, and the culture area shows strong CO2 sinks, with the average air-sea CO2 flux of −24.10 mmol m−2 d−1, -37.68 mmol m−2 d−1, and -38.99 mmol m−2 d−1, respectively. The absorption of CO2 by large-scale cultured P. yezoensis through the "biological pump" effect is the main factor affecting the CO2 exchange process at the air-sea interface, and the absorption rate of CO2 by P. yezoensis at the mature stage is higher than that at the growth stage before harvesting. The study revealed that macroalgae-shellfish mariculture could promote mutual growth, alleviate environmental pressure, and enhance the carbon sink of the culture area. The relationship between mariculture and the carbon cycle of a mariculture ecosystem is very complicated, and its biochemical process should be given great attention for further study.

Sea-air CO2 Flux in the South Yellow Sea Based on Seasonal Underway Observations

Sea-air CO2 Flux in the South Yellow Sea Based on Seasonal Underway Observations

Spatiotemporal evolution of air-sea CO2 flux in the Northwest Pacific and its response to ENSO

Global warming, driven by human activities since the Industrial Revolution, has significantly elevated atmospheric carbon dioxide (CO2) levels, leading to higher global temperatures and a rise in extreme weather events. The ocean, as a major carbon sink, has absorbed about 30% of human-induced carbon emissions, helping mitigate global warming’s impacts. This study examines the spatiotemporal distribution of air-sea CO2 flux in the Northwest Pacific from 1982 to 2021 and its response to El Niño-Southern Oscillation, using Empirical Orthogonal Function and composite analysis. The seasonal patterns of air-sea CO2 flux and the influence of environmental factors were further evaluated. The results show that air-sea CO2 flux in the Northwest Pacific exhibits clear seasonal fluctuations. In winter, high-latitude areas act as significant carbon sources. Strong winds deepen the mixed layer, promoting CO2 release from the ocean into the atmosphere. In contrast, in summer, longer daylight hours, rising SST, and melting sea ice lead to upwelling, which brings nutrients to the surface and stimulates phytoplankton growth. This process turns the region into a carbon sink as phytoplankton growth, driven by intense sunlight, enhances the ocean’s CO2 absorption. The mid-latitude region consistently acts as a carbon sink year-round. During El Niño events, more negative air-sea CO2 flux anomalies appear in the eastern Northwest Pacific, enhancing carbon uptake. La Niña events have the opposite effect in the eastern regions. These findings highlight the Northwest Pacific’s critical role in modulating regional and global carbon cycles under varying climatic conditions. Understanding these dynamics is crucial for improving predictions of future climate impacts and for developing effective strategies to mitigate global warming.

Surface carbonate dynamics in a temperate coastal system in the Northern Yellow Sea, China

The carbon sink/source in estuaries and coastal seas may have significant implications for regional climate change. This study investigated seawater carbon dioxide partial pressure (pCO2), aragonite saturation state (Ωarag), and related carbonate parameters in the Yalu River estuary and the adjacent Northern Yellow Sea, China, in September 2023. The results indicate that seawater pCO2 ranged from 261 to 1121 μatm, while Ωarag ranged from 0.49 to 3.34 in the study area. High pCO2 were observed in the estuarine and northwestern areas, whereas low pCO2 were found in the southern offshore area. In the estuarine area, high pCO2 levels were attributed to the input of diluted water from the Yalu River. In the southern areas, low pCO2 were a result of primary production, while community respiration played an important role in increasing seawater pCO2 in the northwestern area. On the other hand, seawater Ωarag was extremely low in the estuarine area, and greater than 1.5 in the southern offshore area. Ωarag in the study area was mainly controlled by water mixing and temperature. The study area acted as a source of atmospheric CO2 with an average air-sea CO2 flux of 6.2 mmol m−2d−1 in autumn, which was relatively high compared with other marginal seas. The high pCO2 and low Ωarag associated with diluted water inputs and community respiration could weaken the ability of coastal oceans to uptake CO2. Therefore, it is crucial to explore ways to reduce CO2 levels in riverine and adjacent coastal seas.

The hidden acceleration pump uncovers the role of shellfish in oceanic carbon sequestration

Whether shellfish mariculture should be included in the blue carbon profile as a strategy to combat climate change has been controversial. It is highly demanding not only to provide calibration quantitation, but also to provide an ecosystem-based mechanism. In this study, we chose mussel farms as a case study to evaluate their contributions to carbon sinks and their responses to sedimentary carbon fixation and sequestration. First, we quantified the air–sea CO2 flux in the mussel aquacultural zone and observed a weak carbon sink (−0.15 ± 0.07 mmol·m−2·d−1) during spring. Next, by analyzing the carbon composition in the sediment, we recorded a noticeable and unexpected increase in the sedimentary recalcitrant carbon (RC) content in the mussel farming case. To address this surprising sedimentary phenomenon, a long-term indoor experimental test was conducted to distinguish the consequences of mussel engagement with sedimentary RC. Our observational data support the idea that mussel engagement promotes accumulation of RC in sediments by 2.5-fold. Furthermore, the relative intensity of carboxylic-rich alicyclic molecule (CRAM)-like compounds (recalcitrant dissolved organic matter (RDOM)) increased by 451.4 % in the mussel-engaged sedimentary dissolved organic matter (DOM) in comparison to the initial state. Mussel engagement had a significantly positive effect on the abundance of sedimentary carbon-fixing genes. Therefore, we definitively conclude that mussel farming is blue carbon positive and propose a new alternative theory that mussel farming areas may have high carbon sequestration potential via an ecologically integrated “3 M” (microalgae–mussel–microbiota) consortium. The “mussel pump” accelerates carbon sequestration and enhances climate-related ecosystem services.

Sea surface carbon dioxide during early summer at the Tuandao nearshore time series site

To investigate air-sea CO2 flux at the Qingdao nearshore site and its temporal variations, a high-resolution continuous observation of surface carbon dioxide partial pressure (pCO2) was carried out at Zhongyuan Pier near Tuandao from May 25 to July 8, 2019. It was observed that during this period, surface pCO2 varied between ∼490 and ∼690 μatm, mainly associated with sea surface temperature. Surface pCO2 also displayed substantial diurnal variations, with an average amplitude of 64 ± 21 μatm, largely dominated by biological activities. During the observational period, this site acted as a source of atmospheric CO2, releasing 361 mmol CO2 m−2. The notable diurnal variations in air-sea CO2 flux, such as the observed average amplitude of 10.9 mmol m−2 d−1 in this study, pose a challenge for accurately estimating the air-sea CO2 flux in coastal regions without high-resolution observations.

Implementation and assessment of a model including mixotrophs and the carbonate cycle (Eco3M_MIX-CarbOx v1.0) in a highly dynamic Mediterranean coastal environment (Bay of Marseille, France) – Part 2: Towards a better representation of total alkalinity when modeling the carbonate system and air–sea CO2 fluxes

Abstract. The Bay of Marseille (BoM), located in the northwestern Mediterranean Sea, is affected by various hydrodynamic processes (e.g., Rhône River intrusion and upwelling events) that result in a highly complex local carbonate system. In any complex environment, the use of models is advantageous since it allows us to identify the different environmental forcings, thereby facilitating a better understanding. By combining approaches from two biogeochemical ocean models and improving the formulation of total alkalinity, we develop a more realistic representation of the carbonate system variables at high temporal resolution, which enables us to study air–sea CO2 fluxes and seawater pCO2 variations more reliably. We apply this new formulation to two particular scenarios that are typical for the BoM: (i) summer upwelling and (ii) Rhône River intrusion events. In both scenarios, our model was able to correctly reproduce the observed patterns of pCO2 variability. Summer upwelling events are typically associated with a pCO2 decrease that mainly results from decreasing near-surface temperatures. Furthermore, Rhône River intrusion events are typically associated with a pCO2 decrease, although, in this case, the pCO2 decrease results from a decrease in salinity and an overall increase in total alkalinity. While we were able to correctly represent the daily range of air–sea CO2 fluxes, the present configuration of Eco3M_MIX-CarbOx does not allow us to correctly reproduce the annual cycle of air–sea CO2 fluxes observed in the area. This pattern directly impacts our estimates of the overall yearly air–sea CO2 flux as, even if the model clearly identifies the bay as a CO2 sink, its magnitude was underestimated, which may be an indication of the limitations inherent in dimensionless models for representing air–sea CO2 fluxes.

Direct observational evidence of strong CO2 uptake in the Southern Ocean.

The Southern Ocean is the primary region for the uptake of anthropogenic carbon dioxide (CO2) and is, therefore, crucial for Earth's climate. However, the Southern Ocean CO2 flux estimates reveal substantial uncertainties and lack direct validation. Using seven independent and directly measured air-sea CO2 flux datasets, we identify a 25% stronger CO2 uptake in the Southern Ocean than shipboard dataset-based flux estimates. Accounting for upper ocean temperature gradients and insufficient temporal resolution of flux products can bridge this flux gap. The gas transfer velocity parameterization is not the main reason for the flux disagreement. The profiling float data-based flux products and biogeochemistry models considerably underestimate the observed CO2 uptake, which may be due to the lack of representation of small-scale high-flux events. Our study suggests that the Southern Ocean may take up more CO2 than previously recognized, and that temperature corrections should be considered, and a higher resolution is needed in data-based bulk flux estimates.

The Southern Ocean carbon sink has been overestimated in the past three decades

Employing machine learning methods for mapping surface ocean pCO2 has reduced the uncertainty in estimating sea-air CO2 flux. However, a general discrepancy exists between the Southern Ocean carbon sinks derived from pCO2 products and those from biogeochemistry models. Here, by performing a boosting ensemble learning feed-forward neural networks method, we have identified an underestimation of the surface Southern Ocean pCO2 due to notably uneven density of pCO2 measurements between summer and winter, which resulted in about 16% overestimating of Southern Ocean carbon sink over the past three decades. In particular, the Southern Ocean carbon sink since 2010 was notably overestimated by approximately 29%. This overestimation can be mitigated by a winter correction in algorithms, with the average Southern Ocean carbon sink during 1992-2021 corrected to −0.87 PgC yr−1 from the original −1.01 PgC yr−1. Furthermore, the most notable underestimation of surface ocean pCO2 mainly occurred in regions south of 60°S and was hiding under ice cover. As the surface ocean pCO2 under sea ice coverage in the winter is much higher than the atmosphere, if sea ice melts completely, there could be a further reduction of about 0.14 PgC yr−1 in the Southern Ocean carbon sink.

PCO2 variation in ice‐covered regions of the Arctic Ocean from the summer 2022 observation

AbstractTo enhance our understanding of the carbon cycle in the Arctic Ocean, comprehensive observational data are crucial, including measurements from the underlying ice water. This study proposed a practical method for calibrating pCO2 sensor using measured dissolved inorganic carbon and total alkalinity. Our findings suggested the minimum number of bottle samples needed for calibration to ensure 1% accuracy. Additionally, we identified the significant role of a decrease in dissolved inorganic carbon due to photosynthesis and the increase in buffer capacity of the seawater from the release of excess alkalinity by sea ice in regulating pCO2. The mean air–sea CO2 fluxes were −48.9 ± 44.6, −7.3 ± 14.6, and −1.4 ± 2.8 mmol m−2 d−1 in the southern Chukchi Sea, northern Chukchi Sea, and northern East Siberian Sea, respectively. We found a robust negative correlation between the flux and sea ice concentration in the Arctic Sea ice regions.

Anthropogenic CO2, air–sea CO2 fluxes, and acidification in the Southern Ocean: results from a time-series analysis at station OISO-KERFIX (51° S–68° E)

Abstract. The temporal variation of the carbonate system, air–sea CO2 fluxes, and pH is analyzed in the southern Indian Ocean, south of the polar front, based on in situ data obtained from 1985 to 2021 at a fixed station (50°40′ S–68°25′ E) and results from a neural network model that reconstructs the fugacity of CO2 (fCO2) and fluxes at monthly scale. Anthropogenic CO2 (Cant) is estimated in the water column and is detected down to the bottom (1600 m) in 1985, resulting in an aragonite saturation horizon at 600 m that migrated up to 400 m in 2021 due to the accumulation of Cant. At the subsurface, the trend of Cant is estimated at +0.53±0.01 µmol kg−1 yr−1 with a detectable increase in the trend in recent years. At the surface during austral winter the oceanic fCO2 increased at a rate close to or slightly lower than in the atmosphere. To the contrary, in summer, we observed contrasting fCO2 and dissolved inorganic carbon (CT) trends depending on the decade and emphasizing the role of biological drivers on air–sea CO2 fluxes and pH inter-annual variability. The regional air–sea CO2 fluxes evolved from an annual source to the atmosphere of 0.8 molC m−2 yr−1 in 1985 to a sink of −0.5 molC m−2 yr−1 in 2020. Over 1985–2020, the annual pH trend in surface waters of -0.0165±0.0040 per decade was mainly controlled by the accumulation of anthropogenic CO2, but the summer pH trends were modulated by natural processes that reduced the acidification rate in the last decade. Using historical data from November 1962, we estimated the long-term trend for fCO2, CT, and pH, confirming that the progressive acidification was driven by the atmospheric CO2 increase. In 59 years this led to a diminution of 11 % for both aragonite and calcite saturation state. As atmospheric CO2 is expected to increase in the future, the pH and carbonate saturation state will decrease at a faster rate than observed in recent years. A projection of future CT concentrations for a high emission scenario (SSP5-8.5) indicates that the surface pH in 2100 would decrease to 7.32 in winter. This is up to −0.86 lower than pre-industrial pH and −0.71 lower than pH observed in 2020. The aragonite undersaturation in surface waters would be reached as soon as 2050 (scenario SSP5-8.5) and 20 years later for a stabilization scenario (SSP2-4.5) with potential impacts on phytoplankton species and higher trophic levels in the rich ecosystems of the Kerguelen Islands area.

Extratropical storms induce carbon outgassing over the Southern Ocean

The strength and variability of the Southern Ocean carbon sink is a significant source of uncertainty in the global carbon budget. One barrier to reconciling observations and models is understanding how synoptic weather patterns modulate air-sea carbon exchange. Here, we identify and track storms using atmospheric sea level pressure fields from reanalysis data to assess the role that storms play in driving air-sea CO2 exchange. We examine the main drivers of CO2 fluxes under storm forcing and quantify their contribution to Southern Ocean annual air-sea CO2 fluxes. Our analysis relies on a forced ocean-ice simulation from the Community Earth System Model, as well as CO2 fluxes estimated from Biogeochemical Argo floats. We find that extratropical storms in the Southern Hemisphere induce CO2 outgassing, driven by CO2 disequilibrium. However, this effect is an order of magnitude larger in observations compared to the model and caused by different reasons. Despite large uncertainties in CO2 fluxes and storm statistics, observations suggest a pivotal role of storms in driving Southern Ocean air-sea CO2 outgassing that remains to be well represented in climate models, and needs to be further investigated in observations.

Regional differences in the air–sea CO2 flux between 3 and 14°S in the south-western tropical Atlantic

Context The fugacity of surface-seawater CO2 (fCO2sw) and the sea–air CO2 fluxes in the south-western tropical Atlantic (SWTA) were studied to increase the knowledge about the carbon cycle in this region. Aims This paper aims to describe the distribution of fCO2sw in SWTA. Methods The fCO2sw was measured from 2008 to 2020 by volunteer merchant ships with an onboard system that measures pCO2 while the vessels were underway. Key results Higher values occurred north of 8°S than in the region south of 8°S. The north is a strong source of CO2 for the atmosphere, with an annual mean value of 3.14 ± 0.52 mmol m−2 day−1. The south is a weaker source of CO2, with an annual average of 0.93 ± 0.90 mmol m−2 day−1. In the months of July and August, a weak sink of CO2 was observed, with a mean of −0.55 mmol m−2 day−1. Conclusions and implications The differences between these two regions are explained by the origin of the surface-water masses encountered along the ship track. The central branch of the South Equatorial Current (SEC) transports surface water, with a higher CO2 concentration and lower salinity, north of 8°S, whereas the surface waters between 8 and 14°S come from the southern branch of the SEC. The intertropical convergence zone is another physical process influencing the region north of 8°S.

Updated climatological mean ΔfCO2 and net sea–air CO2 flux over the global open ocean regions

Abstract. The late Taro Takahashi (Lamont-Doherty Earth Observatory (LDEO), Columbia University) and colleagues provided the first near-global monthly air–sea CO2 flux climatology in Takahashi et al. (1997), based on available surface water partial pressure of CO2 measurements. This product has been a benchmark for uptake of CO2 in the ocean. Several versions have been provided since, with improvements in procedures and large increases in observations, culminating in the authoritative assessment in Takahashi et al. (2009a, b). Here we provide and document the last iteration using a greatly increased dataset (SOCATv2022) and determining fluxes using air–sea partial pressure differences as a climatological reference for the period 1980–2021 (Fay et al., 2023, https://doi.org/10.25921/295g-sn13). The resulting net flux for the open ocean region is estimated as -1.79±0.7 Pg C yr−1, which compares well with other global mean flux estimates. While global flux results are consistent, differences in regional means and seasonal amplitudes are discussed. Consistent with other studies, we find the largest differences in the data-sparse southeast Pacific and Southern Ocean.