This study explores an optimal inversion strategy for assimilating the Orbiting Carbon Observatory-2 (OCO-2) column-averaged atmospheric CO2 concentration (XCO2) observations to constrain air-sea CO2 fluxes. The performance of different inversion set-ups is evaluated through Observing System Simulation Experiments (OSSEs) by comparing the optimized fluxes with assumed true fluxes. The results indicate that the conventional inversion, simultaneously optimizing terrestrial biosphere and air-sea fluxes, reduces root mean square errors (RMSEs) in regional monthly air-sea fluxes by up to 22–24% and 6–10% in the low (<40°) and high (>40°) latitudes, respectively, with up to 22% error reduction in global annual air-sea fluxes. These limited adjustments are associated with an order of magnitude higher variability of terrestrial biosphere fluxes compared to the air-sea fluxes. To isolate ocean signals within XCO2 variations, we employ a sequential inversion, first optimizing terrestrial biosphere fluxes with land XCO2 data and then optimizing air-sea fluxes with ocean XCO2 data while prescribing the optimized terrestrial biosphere fluxes. This approach achieves an 11% additional error reduction in global annual air-sea fluxes and a 33% further RMSE reduction in monthly air-sea fluxes in the southern high latitudes. However, we find that potential biases (+0.2 ppm) in ocean XCO2 measurements over this region could induce a 24% RMSE increase despite the application of sequential inversion. Our results show that sequential inversion is a promising technique for improving seasonal air-sea flux estimates in the Southern Ocean but mitigation of OCO-2 measurement biases is required for practical applications.

Nitrous oxide (N2O) is a greenhouse gas and an ozone-depleting agent with large and growing anthropogenic emissions. Previous studies identified the influx of N2O-depleted air from the stratosphere to partly cause the seasonality in tropospheric N2O (aN2O), but other contributions remain unclear. Here we combine surface fluxes from eight land and four ocean models from phase 2 of the Nitrogen/N2O Model Intercomparison Project with tropospheric transport modeling to simulate aN2O at the air sampling sites: Alert, Barrow, Ragged Point, Samoa, Ascension Island, and Cape Grim for the modern and preindustrial periods. Models show general agreement on the seasonal phasing of zonal-average N2O fluxes for most sites, but, seasonal peak-to-peak amplitudes differ severalfold across models. After transport, the seasonal amplitude of surface aN2O ranges from 0.25 to 0.80 ppb (interquartile ranges 21-52% of median) for land, 0.14 to 0.25 ppb (19-42%) for ocean, and 0.13 to 0.76 ppb (26-52%) for combined flux contributions. The observed range is 0.53 to 1.08 ppb. The stratospheric contributions to aN2O, inferred by the difference between surface-troposphere model and observations, show 36-126% larger amplitudes and minima delayed by ~1 month compared to Northern Hemisphere site observations. Our results demonstrate an increasing importance of land fluxes for aN2O seasonality, with land fluxes and their seasonal amplitude increasing since the preindustrial era and are projected to grow under anthropogenic activities. In situ aN2O observations and atmospheric transport-chemistry models will provide opportunities for constraining terrestrial and oceanic biosphere models, critical for projecting surface N2O sources under ongoing global warming.

Climate change reduces ocean oxygen levels, posing a serious threat to marine ecosystems and their benefits to society. State-of-the-art Earth System Models (ESMs) project an intensification of global oxygen loss in the future, but poorly constrain its patterns and magnitude, with contradictory oxygen gain or loss projected in tropical oceans. We introduce an oxygen water mass framework– grouping waters with similar oxygen concentrations from lowest to highest levels– and separate oxygen changes into two components: the transformation of oxygen in water masses by biological, chemical or physical processes along their pathways in ’ventilation-space’, and the redistribution of these water masses in ’geographic-space’. The redistribution of water masses explains the large projection uncertainties in the tropics. ESMs with more realistic representations of water masses provide tighter constraints on future redistribution than less skilled ESMs, leading to over a third more of tropical area exhibiting consistent oxygen projections (58% vs 22%), and a 30% reduction in model spread for tropical oxygen projections. These higher-skilled ESMs also project weaker global deoxygenation than less skilled models (median of -6.5 vs -9.5 Pmol O2 per °C of surface warming) controlled by an increase in global water residence times, and they project a stronger increase in oxygen minimum zone ventilation by ocean mixing. These tighter constraints on future oxygen changes are critical to anticipate and mitigate impacts for ecosystems, and inform management and conservation strategies of marine resources.

High-frequency wind speed and wave variability influence the air-sea CO2 flux by modulating the gas transfer velocity. Traditional gas transfer velocity formulations scale solely with wind speed and ignore wave activity, including wave breaking and bubble-mediated transfers. In this study, we quantify the effects of wave-induced spatiotemporal variability on the CO2 flux and the ocean carbon storage using a wind-wave-dependent gas transfer velocity formulation in an ocean general circulation model (MOM6-COBALTv2). We find that wave activity introduces a hemispheric asymmetry in ocean carbon storage, with gain in the southern hemisphere where wave activity is robust year-round and loss in the northern hemisphere where continental sheltering reduces carbon uptake. Compared to a traditional wind-dependent formulation, the wind-wave-dependent formulation yields a modest global increase in ocean carbon storage of 4.3 PgC over 1959-2018 (~4%), but on average, enhances the CO2 gas transfer velocity and flux variability by 5-30% on high-frequency and seasonal timescales in the extratropics and up to 200-300% during storms (>15 m s-1 wind speed). This wave-induced spatiotemporal variability in CO2 flux is comparable to the flux expected from marine carbon dioxide removal (mCDR) techniques, such that neglecting wind-wave variability in modeled CO2 fluxes could hinder distinguishing between natural variability and human-induced changes, undermining mCDR verification and monitoring efforts.

The coastal ocean contributes to regulating atmospheric greenhouse gas concentrations by taking up carbon dioxide (CO2) and releasing nitrous oxide (N2O) and methane (CH4). Major advances have improved our understanding of the coastal air-sea exchanges of these three gasses since the first phase of the Regional Carbon Cycle Assessment and Processes (RECCAP in 2013), but a comprehensive view that integrates the three gasses at the global scale is still lacking. In this second phase (RECCAP2), we quantify global coastal ocean fluxes of CO2, N2O and CH4 using an ensemble of global gap-filled observation-based products and ocean biogeochemical models. The global coastal ocean is a net sink of CO2 in both observational products and models, but the magnitude of the median net global coastal uptake is ~60% larger in models (-0.72 vs. -0.44 PgC/yr, 1998-2018, coastal ocean area of 77 million km2). We attribute most of this model-product difference to the seasonality in sea surface CO2 partial pressure at mid- and high-latitudes, where models simulate stronger winter CO2 uptake. The global coastal ocean is a major source of N2O (+0.70 PgCO2-e /yr in observational product and +0.54 PgCO2-e /yr in model median) and of CH4 (+0.21 PgCO2-e /yr in observational product), which offsets a substantial proportion of the net radiative effect of coastal \co uptake (35-58% in CO2-equivalents). Data products and models need improvement to better resolve the spatio-temporal variability and long term trends in CO2, N2O and CH4 in the global coastal ocean.

Observations suggest that the tropical Pacific Ocean has lost oxygen since the 1960s leading to the expansion of its oxygen minimum zone (OMZ). Attribution to anthropogenic forcing is, however, difficult because of limited data availability and the large natural variability introduced by the Pacific Decadal Oscillation (PDO). Here, we evaluate the PDO influence on oxygen dynamics and OMZ extent using observations and hindcast simulations from two global ocean circulation models (NEMO-PISCES, MOM6-COBALT). In both models, the tropical Pacific oxygen content decreases by about 30 Tmol.decade$^{-1}$ and the OMZ volume expands by $1.3\times10^5$ km$^3$.decade$^{-1}$ during PDO positive phases, while variations of similar magnitude but opposite sign are simulated during negative phases. Changes in equatorial advective oxygen supply, partially offset by biological demand, control the oxygen response to PDO. Observations which cover 39\% of the tropical Pacific volume only partially capture spatio-temporal variability, hindering the separation of anthropogenic trend from natural variations.

The geographic rearrangement of the tropical oceanic and atmospheric circulation during an El Niño event is associated with a well-understood strong surface warming of the climatologically cold eastern equatorial Pacific. However, the concomitant warming of the warmest waters where deep convection occurs - responsible for the tropics-wide free tropospheric warming- is less well understood. Here, we show that in both a coupled atmosphere-ocean climate model and in reanalysis data, El Niño is associated with an increase in evaporation over the colder ~70%, but with a decrease in evaporation over the warmest ~30% of the tropical oceans where atmospheric deep convection connects the surface with the free troposphere. The reduction in evaporation is driven by a weakening of the near-surface winds. We propose that the prominent tropics-wide warming during El Niño is a consequence of the reduction of near-surface winds in regions of deep convection due to the anomalous large-scale circulation.

Global ocean oxygen loss - deoxygenation - is projected to persist in the future. Previous generations of Earth system models (ESMs) have, however, failed to provide a consistent picture of how deoxygenation will influence oxygen minimum zones (OMZs; O2<= 80 μmol/kg), in particular the largest OMZ in the tropical Pacific Ocean. The expansion of the Pacific OMZ would threaten marine ecosystems and ecosystem services such as fisheries and could amplify climate change by emitting greenhouse gases. Here, we use the latest generation of ESMs (CMIP6) and a density framework that isolates oxygen changes in the thermocline and intermediate waters. We show that the Pacific OMZ expands by the end of the century in response to high anthropogenic emissions (multi-ESM median expansion of 2.4 * 10^15 m^3m, about 4% of the observed OMZ volume). The expansion is driven by a reduction of the shallow overturning circulation in the thermocline and a robust weakening of the oxygen supply to the upper OMZ in all ESMs. The magnitude of this expansion is, however, uncertain due to the less constrained balance between physical and biological changes in the lower OMZ. Despite uncertainties in the biological response, our results suggest that models with more complex biogeochemistry project weaker changes in the lower OMZ, and therefore stronger overall OMZ expansion. The fact that the OMZ largely expands in the upper ocean maximizes its ecological, economic, and climatic impacts (release of greenhouse gases).