Convergence of atmospheric and North Atlantic CO2 trends on multidecadal timescales

The oceans’ carbon uptake substantially reduces the rate of anthropogenic carbon accumulation in the atmosphere1, and thus slows global climate change. Some diagnoses of trends in ocean carbon uptake have suggested a significant weakening in recent years2-8, while others conclude that decadal variability confounds detection of long-term trends9-11. Here, we study trends in observed surface ocean partial pressure of CO2 (pCO2) in three gyre-scale biomes of the North Atlantic, considering decadal to multidecadal timescales between 1981 and 2009. Trends on decadal timescales are of variable magnitudes and depend sensitively on the precise choice of years. As more years are considered, oceanic pCO2 trends begin to converge to the trend in atmospheric pCO2. North of 30oN, it takes 25 years for the influence of decadal-timescale climate variability to be overcome by a long-term trend that is consistent with the accumulation of anthropogenic carbon. In the permanently stratified subtropical gyre, warming has recently become a significant contributor to the observed increase in oceanic pCO2. This warming, previously attributed to both a multidecadal climate oscillation and anthropogenic climate forcing12,13, is beginning to reduce ocean carbon uptake

Upper Ocean Box Model which solves for the time change of Dissolved Inorganic Carbon (DIC) in single upper ocean box

Dataset: Upper Ocean Box ModelThe box model solves for the time change of Dissolved Inorganic Carbon (DIC) in single upper ocean box. The upper ocean box model is forced by observed atmospheric pCO2 and temperature. It calculates the pCO2 and air-sea CO2 flux. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/840371NSF Division of Ocean Sciences (NSF OCE) OCE-1558225, NSF Division of Ocean Sciences (NSF OCE) OCE-1558258, NSF Division of Ocean Sciences (NSF OCE) OCE-181850

Interannual variability of air-sea fluxes of carbon dioxide and oxygen

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2002.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 161-169).The currently observed increase in atmospheric CO2 due anthropogenic emissions is substantially slowed by natural processes that incorporate CO2 into the terrestrial biota and the ocean. Year-to-year changes in the CO2 growth rate that exceed variations in the fossil fuel source indicate a significant variability in these global CO2 sinks. However, the enormous complexity of the terrestrial and oceanic biogeochemical systems that absorb atmospheric CO2 makes these sinks extremely difficult to understand and precisely quantify. Many techniques, including the interpretation of the relative changes in atmospheric CO2 and O2/N2, ocean modeling, and atmospheric data inversions, have been employed to estimate the mean and variability of global CO2 sinks. However, uncertainty remains large. The goal of this thesis is to improve understanding of global CO2 sinks by considering (1) the error in the atmospheric O2/N2 partitioning method due to the neglect of interannual variability in the air-sea fluxes of 02, and (2) the interannual variability of the ocean CO2 sink.(cont.) A global, high-resolution ocean general circulation model is used to estimate the magnitude and understand the mechanisms of interannual variability in air-sea fluxes of both CO2 and 02. I find that the global variability in the fluxes of both gases are dominantly forced by large-scale physical processes governing upper ocean dynamics, particularly El Nifio / Southern Oscillation (ENSO) and, for 02, the North Atlantic Oscillation (NAO). Estimates of the extremes of CO2 and 02 flux variability for the period 1980-1998 are +/-0.5x1015 grams Carbon/yr (PgC/yr) and -70/+100x1012 mol/yr (Tmol/yr), respectively. Global 02 flux variability implies up to a 1.0 PgC/yr error in estimates of interannual variability in land and ocean CO2 sinks derived from atmospheric 02/N2 observations. This error is significant for estimates of annual sinks, but it is cumulatively negligible for estimates of mean sinks from October 1991 to April 1998. Increasing convergence of estimates of land.by Galen Anile McKinley.Ph.D

Interannual variability of the air-sea flux of oxygen in the North Atlantic

In studies using timeseries observations of atmospheric O[subscript 2]/N[subscript 2] to infer the fate of fossil fuel CO[subscript 2], it has been assumed that multi-year trends in observed O[subscript 2]/N[subscript 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[superscript −2] y[superscript −1]) that is a large fraction of the mean (0.85 mol m[superscript −2] y[subscript −1]). This is primarily a consequence of variability in winter convection in the subpolar gyre.Goddard Space Flight Center (Grants NGTS-30189 and NCC5-244

Global evaluation of particulate organic carbon flux parameterizations and implications for atmospheric pCO\u3csub\u3e2\u3c/sub\u3e

The shunt of photosynthetically derived particulate organic carbon (POC) from the euphotic zone and deep remineralization comprises the basic mechanism of the “biological carbon pump.” POC raining through the “twilight zone” (euphotic depth to 1 km) and “midnight zone” (1 km to 4 km) is remineralized back to inorganic form through respiration. Accurately modeling POC flux is critical for understanding the “biological pump” and its impacts on air‐sea CO2 exchange and, ultimately, long‐term ocean carbon sequestration. Yet commonly used parameterizations have not been tested quantitatively against global data sets using identical modeling frameworks. Here we use a single one‐dimensional physical‐biogeochemical modeling framework to assess three common POC flux parameterizations in capturing POC flux observations from moored sediment traps and thorium‐234 depletion. The exponential decay, Martin curve, and ballast model are compared to data from 11 biogeochemical provinces distributed across the globe. In each province, the model captures satellite‐based estimates of surface primary production within uncertainties. Goodness of fit is measured by how well the simulation captures the observations, quantified by bias and the root‐mean‐square error and displayed using “target diagrams.” Comparisons are presented separately for the twilight zone and midnight zone. We find that the ballast hypothesis shows no improvement over a globally or regionally parameterized Martin curve. For all provinces taken together, Martin\u27s b that best fits the data is [0.70, 0.98]; this finding reduces by at least a factor of 3 previous estimates of potential impacts on atmospheric pCO2 of uncertainty in POC export to a more modest range [−16 ppm, +12 ppm]

An improved comparison of atmospheric Ar/N2 time series and paired ocean-atmosphere model predictions

Ar/N2 variations in the atmosphere reflect ocean heat fluxes, air-sea gas exchange, and atmospheric dynamics. Here atmospheric Ar/N2 time series are compared to paired ocean-atmosphere model predictions. Agreement between Ar/N2 observations and simulations has improved in comparison to a previous study because of longer time series and the introduction of automated samplers at several of the atmospheric stations, as well as the refinement of the paired ocean-atmosphere models by inclusion of Ar and N2 as active tracers in the ocean component. Although analytical uncertainties and collection artifacts are likely to be mainly responsible for observed Ar/N2 outliers, air parcel back-trajectory analysis suggests that some of the variability in Ar/N2 measurements could be due to the low-altitude history of the air mass collected and, by extension, the local oceanic Ar/N2 signal. Although the simulated climatological seasonal cycle can currently be evaluated with Ar/N2 observations, longer time series and additional improvements in the signal-to-noise ratio will be required to test other model predictions such as interannual variability, latitudinal gradients, and the secular increase in atmospheric Ar/N2 expected to result from ocean warming. Copyright 2008 by the American Geophysical Union

Global ocean particulate organic carbon flux merged with satellite parameters

Particulate organic carbon (POC) flux estimated from POC concentration observations from sediment traps and 234 Th are compiled across the global ocean. The compilation includes six time series locations: CARIACO, K2, OSP, BATS, OFP, and HOT. Efficiency of the biological pump of carbon to the deep ocean depends largely on biologically mediated export of carbon from the surface ocean and its remineralization with depth; thus biologically related parameters able to be estimated from satellite observations were merged at the POC observation sites. Satellite parameters include net primary production, percent microplankton, sea surface temperature, photosynthetically active radiation, diffuse attenuation coefficient at 490 nm, euphotic zone depth, and climatological mixed layer depth. Of the observations across the globe, 85% are concentrated in the Northern Hemisphere with 44% of the data record overlapping the satellite record. Time series sites accounted for 36% of the data, while 71% of the data are measured at≥ 500m with the most common deployment depths between 1000 and 1500m. This data set is valuable for investigations of CO2 drawdown, carbon export, remineralization, and sequestration. The compiled data can be freely accessed at doi.org/10.1594/PANGAEA.855600

Mechanisms of northern North Atlantic biomass variability

In the North Atlantic Ocean north of 40&deg;&thinsp;N, intense biological productivity occurs to form the base of a highly productive marine food web. SeaWiFS satellite observations indicate trends of biomass in this region over 1998&ndash;2007. Significant biomass increases occur in the northwest subpolar gyre and there are simultaneous significant declines to the east of 30&ndash;35&deg;&thinsp;W. These short-term changes, attributable to internal variability, offer an opportunity to explore the mechanisms of the coupled physical&ndash;biogeochemical system. We use a regional biogeochemical model that captures the observed changes for this exploration. Biomass increases in the northwest are due to a weakening of the subpolar gyre and associated shoaling of mixed layers that relieves light limitation. Biomass declines to the east of 30&ndash;35&deg;&thinsp;W are due to reduced horizontal convergence of phosphate. This reduced convergence is attributable to declines in vertical phosphate supply in the regions of deepest winter mixing that lie to the west of 30&ndash;35&deg;&thinsp;W. Over the full time frame of the model experiment, 1949&ndash;2009, variability of both horizontal and vertical phosphate supply drive variability in biomass on the northeastern flank of the subtropical gyre. In the northeast subpolar gyre horizontal fluxes drive biomass variability for both time frames. Though physically driven changes in nutrient supply or light availability are the ultimate drivers of biomass changes, clear mechanistic links between biomass and standard physical variables or climate indices remain largely elusive.</p

Equatorial Pacific pCO2 Interannual Variability in CMIP6 Models

The El Niño-Southern Oscillation (ENSO) in the equatorial Pacific is the dominant mode of global air-sea carbon dioxide (CO2) flux interannual variability (IAV). Air-sea CO2 fluxes are driven by the difference between atmospheric and surface ocean pCO2, with variability of the latter driving flux variability. Previous studies found that models in Coupled Model Intercomparison Project Phase 5 (CMIP5) failed to reproduce the observed ENSO-related pattern of CO2 fluxes and had weak pCO2 IAV, which were explained by both weak upwelling IAV and weak mean vertical dissolved inorganic carbon (DIC) gradients. We assess whether the latest generation of CMIP6 models can reproduce equatorial Pacific pCO2 IAV by validating models against observations-based data products. We decompose pCO2 IAV into thermally and non-thermally driven anomalies to examine the balance between these competing anomalies, which explain the total pCO2 IAV. The majority of CMIP6 models underestimate pCO2 IAV, while they overestimate sea surface temperature IAV. Insufficient compensation of non-thermal pCO2 to thermal pCO2 IAV in models results in weak total pCO2 IAV. We compare the relative strengths of the vertical transport of temperature and DIC and evaluate their contributions to thermal and non-thermal pCO2 anomalies. Model-to-observations-based product comparisons reveal that modeled mean vertical DIC gradients are biased weak relative to their mean vertical temperature gradients, but upwelling acting on these gradients is insufficient to explain the relative magnitudes of thermal and non-thermal pCO2 anomalies