Abstract In the oligotrophic subtropical gyre of the North Atlantic, the processes that allow for an imbalance between annual biological productivity and organic carbon export have been sought for decades. We use biogeochemical data from profiling floats and 26‐year bottle samples off Bermuda to provide the first evidence for a mechanism that allows for heterotrophy in the presence of oxygen accumulation in the lower euphotic zone (50–100 m) during the stratified season. After the spring bloom, surface waters that are enriched in oxygen and organic matter, but low in nitrate, are subducted and transported along the seasonal isopycnals that progressively displace downward. Due exclusively to this downward displacement, a positive 50‐ to 100‐m depth‐integrated O 2 anomaly appears (1,688 ± 545 mmol O 2 /m 2 ) from mid‐May to mid‐October. Neglecting this effect of isopycnal displacement would suggest an excess of biological productivity over remineralization at 50–100 m (344 ± 330 mmol O 2 /m 2 ). Yet, when these changes are differenced, significant along‐isopycnal oxygen consumption (−1,344 ± 537 mmol O 2 /m 2 ) is identified. After accounting for mixing, net biological‐driven oxygen consumption is still found (−827 ± 509 mmol O 2 /m 2 ), which indicates heterotrophy. Remineralization of sinking and suspended organic matters at 50–100 m could support 90 ± 67% of the heterotrophic demand. Our analysis also shows that the spread in the biological‐driven oxygen sink is linked to the strength of isopycnal displacement that modulates the supply of nutrients and organic matters. This along‐isopycnal transport and heterotrophy in the lower euphotic zone reduces carbon export at 100 m and helps to resolve previously noted imbalances between surface biological productivity and total organic carbon export.
Motivated by the use of atmospheric O 2 /N 2 to determine CO 2 sinks under the assumption of negligible interannual variability in air‐sea O 2 fluxes, we examine interannual fluctuations of the global air‐sea flux of O 2 during the period 1980–1998 using a global ocean circulation and biogeochemistry model along with an atmospheric transport model. It is found that both the El Niño/Southern Oscillation (ENSO) cycle and wintertime convection in the North Atlantic are primary drivers of global air‐sea oxygen flux interannual variability. Model estimated extremes of O 2 flux variability are −70/+100 × 10 12 mol/yr (Tmol/yr), where positive fluxes are to the atmosphere. O 2 /N 2 variability could cause an up to ±1.0 PgC/yr error in estimates of interannual variability in land and ocean CO 2 sinks derived from atmospheric O 2 /N 2 observations.
The importance of biology to the ocean carbon sink is often quantified in terms of export, the removal of carbon from the ocean surface layer. Satellite images of sea surface chlorophyll indicate variability in biological production, but how these variations affect export and air‐sea carbon fluxes is poorly understood. We investigate this in the North Atlantic using an ocean general circulation model coupled to a medium‐complexity ecosystem model. We find that biological CO 2 drawdown is significant on the mean and dominates the seasonal cycle of pCO 2 , but variations in the annual air‐sea CO 2 flux and export are not significantly correlated. Large year‐to‐year variability in summertime pCO 2 occurs, because of changing bloom timing, but integrated bloom strength and associated carbon uptake and export do not vary substantially. The model indicates that small biological variability, quantitatively consistent with SeaWiFS (1998–2006), is not sufficient to be a first‐order control on annual subpolar air‐sea CO 2 flux variability.
Lamont-Doherty Earth Observatory Hybrid Physics Data (LDEO-HPD) This collection contains the Lamont-Doherty Earth Observatory Hybrid Physics Data (LDEO-HPD) product. This product makes near-global monthly estimates of surface ocean pCO2 from January 1982 through December 2018 with a 1ºx1º spatial resolution. LDEO-HPD uses an eXtreme Gradient Boosting (XGB) algorithm to make estimates of the misfit between observations and mode-based estimates. These spatiotemporal misfit estimates are then added back to the model-based estimate to make the final LDEO-HPD estimate. This is done for each of nine models part of the Global Carbon Budget 2020. Air-sea CO2 flux is calculated using a bulk parameterization, which is a function of wind speed. Three wind speed products are used to estimate flux. The average across these three estimates is taken as the best estimate. Datasets in this collection:LDEO-HPD_error_v20210425_1x1_198201-201812.nc : contains estimates of the misfit from each model LDEO-HPD_spco2_v20210425_1x1_198201-201812.nc : contains estimates of surface ocean pCO2 for each model LDEO-HPD_fgco2_v20210425_1x1_198201-201812.nc : contains estimates of air-sea CO2 flux for each model and wind product
Satellite remote sensing offers one of the best spatial and temporal observational approaches. However, well‐validated satellite imagery has remained elusive for Lake Superior. Lake Superior's optical properties are highly influenced by colored dissolved organic matter (CDOM), which has hindered the retrieval of chlorophyll concentration through band‐ratio algorithms. This study evaluated seven existing inversion algorithms. The top‐performing inversion algorithm was tuned to a Lake Superior optical data set and applied to satellite imagery. The retrieval of chlorophyll concentration via inversion algorithms was not possible due to errors in derived CDOM absorption being greater than phytoplankton absorption values and the very small contribution of phytoplankton absorption to the overall absorption budget. However, the retrieval of absorption due to CDOM from satellite imagery was encouraging. To ensure that the best satellite remotely sensed reflectance estimates were used in the retrieval of absorption due to CDOM, several atmospheric correction schemes were evaluated. The absorption due to CDOM was greatest in the western arm of Lake Superior and near river mouths and decreased with distance offshore. The absorption due to CDOM had a bimodal distribution over the annual cycle with the greatest peak in fall and a smaller peak in spring.
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.
In studies using timeseries observations of atmospheric O 2 /N 2 to infer the fate of fossil fuel CO 2 , it has been assumed that multi‐year trends in observed O 2 /N 2 are insensitive to interannual variability in air‐sea fluxes of oxygen. We begin to address the validity of this assumption by investigating the magnitude and mechanisms of interannual variability in the flux of oxygen across the sea surface using a North Atlantic biogeochemical model. The model, based on the MIT ocean general circulation model, captures the gross patterns and seasonal cycle of nutrients and oxygen within the basin. The air‐sea oxygen flux exhibits significant interannual variability in the North Atlantic, with a standard deviation (0.36 mol m −2 y −1 ) that is a large fraction of the mean (0.85 mol m −2 y −1 ). This is primarily a consequence of variability in winter convection in the subpolar gyre.
Abstract. Nearly every nation has signed the UNFCC Paris Agreement, committing to mitigate anthropogenic carbon emissions so as to limit the global mean temperature increase above pre-industrial levels to well below 2 ∘C, and ideally to no more than 1.5 ∘C. A consequence of emission mitigation that has received limited attention is a reduced efficiency of the ocean carbon sink. Historically, the roughly exponential increase in atmospheric CO2 has resulted in a proportional increase in anthropogenic carbon uptake by the ocean. We define growth of the ocean carbon sink exactly proportional to the atmospheric growth rate to be 100 % efficient. Using a model hierarchy consisting of a common reduced-form ocean carbon cycle model and the Community Earth System Model (CESM), we assess the mechanisms of future change in the efficiency of the ocean carbon sink under three emission scenarios: aggressive mitigation (1.5 ∘C), intermediate mitigation (RCP4.5), and high emissions (RCP8.5). The reduced-form ocean carbon cycle model is tuned to emulate the global-mean behavior of the CESM and then allows for mechanistic decomposition. With intermediate or no mitigation (RCP4.5, RCP8.5), changes in efficiency through 2080 are almost entirely the result of future reductions in the carbonate buffer capacity of the ocean. Under the 1.5 ∘C scenario, the dominant driver of efficiency decline is the ocean's reduced ability to transport anthropogenic carbon from surface to depth. As the global-mean upper-ocean gradient of anthropogenic carbon reverses sign, carbon can be re-entrained in surface waters where it slows further removal from the atmosphere. Reducing uncertainty in ocean circulation is critical to better understanding the transport of anthropogenic carbon from surface to depth and to improving quantification of its role in the future ocean carbon sink.
Abstract Large volcanic eruptions drive significant climate perturbations through major anomalies in radiative fluxes and the resulting widespread cooling of the surface and upper ocean. Recent studies suggest that these eruptions also drive important variability in air‐sea carbon and oxygen fluxes. By simulating the Earth system using two initial‐condition large ensembles, with and without the aerosol forcing associated with the Mt. Pinatubo eruption in June 1991, we isolate the impact of this volcanic event on physical and biogeochemical properties of the ocean. The Mt. Pinatubo eruption forced significant anomalies in surface fluxes and the ocean interior inventories of heat, oxygen, and carbon. Pinatubo‐driven changes persist for multiple years in the upper ocean and permanently modify the ocean's heat, oxygen, and carbon inventories. Positive anomalies in oxygen concentrations emerge immediately post‐eruption and penetrate into the deep ocean. In contrast, carbon anomalies intensify in the upper ocean over several years post‐eruption, and are largely confined to the upper 150 m. In the tropics and northern high latitudes, the change in oxygen is dominated by surface cooling and subsequent ventilation to mid‐depths, while the carbon anomaly is associated with solubility changes and eruption‐generated El Niño—Southern Oscillation variability. We do not find significant impact of Pinatubo on oxygen or carbon fluxes in the Southern Ocean; but this may be due to Southern Hemisphere aerosol forcing being underestimated in Community Earth System Model 1 simulations.
