OS3.1

The uptake of carbon dioxide (CO₂) from the atmosphere is changing the ocean’s chemical state. Such changes, commonly known as ocean acidification, include reduction in pH and the carbonate ion concentration ([CO₃^2-]), which in turn lowers oceanic saturation states (Ω) for calcium carbonate (CaCO₃) minerals. The Ω values for aragonite (Ω_aragonite; one of the main CaCO₃ minerals formed by marine calcifying organisms) influence the calcification rate and geographic distribution of cold-water corals (CWCs), important for biodiversity. In this work we use high-quality data of inorganic carbon measurements, collected on thirteen cruises along the same track during 1991–2018, to determine the long-term trends in Ω_aragonite in the Irminger and Iceland Basins of the North Atlantic Ocean, providing the first trends of Ω_aragonite in the deep waters of these basins. The entire water column of both basins showed significant negative Ω_aragonite trends between -0.0015 ± 0.0002 and -0.0061 ± 0.0016 per year. The decrease in Ω_aragonite in the intermediate waters, where nearly half of the CWC reefs of the study region are located, caused the Ω_aragonite isolines to migrate upwards rapidly at a rate between 6 and 34 m per year. The main driver of the observed decline in Ω_aragonite in the Irminger and Iceland Basins was the increase in anthropogenic CO₂. But this was partially offset by increases in salinity (in Subpolar Mode Water), enhanced ventilation (in upper Labrador Sea Water) and increases in alkalinity (in classical Labrador Sea Water, cLSW; and overflow waters). We also found that water mass aging reinforced the Ω_aragonite decrease in cLSW. Based on the observed Ω_aragonite trends, we project that the entire water column of the Irminger and Iceland Basins will likely be undersaturated for aragonite when in equilibrium with an atmospheric mole fraction of CO₂ (xCO₂) of ~860 ppmv, corresponding to climate model projections for the end of the century based on the highest CO₂ emission scenarios. However, intermediate waters will likely be aragonite undersaturated when in equilibrium with an atmospheric xCO₂ of ~600 ppmv, an xCO₂ level slightly above that corresponding to 2 ºC warming, thus exposing CWCs inhabiting the intermediate waters to undersaturation for aragonite.

How to cite: García-Ibáñez, M. I., Bates, N. R., Bakker, D. C. E., Fontela, M., and Velo, A.: Cold-water corals in the Subpolar North Atlantic Ocean exposed to aragonite undersaturation if Paris 2 ºC is not met, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9649, https://doi.org/10.5194/egusphere-egu21-9649, 2021.

Understanding the coastal ocean variability and quantifying its significance in the global biogeochemical cycles is crucial to our ability to project future changes. In the shallow coastal waters, the contribution of the biological activity to water chemistry can be high locally, and responsible for seasonal and diurnal variations. These variations are not yet well-understood: they are often under-estimated and the general lack of observations means that they are seldom integrated into global predictive models such as those used by the IPCC.
    In this presentation, we will present results on the natural carbonate chemistry diurnal variability in tidal rock pools in Brittany (France), during emersion times. We chose tidal rock pools as to represent "mini-coastal seas": realistic small mesocosms that simulate coastal environments with extreme variability. These have the advantage to be closed systems containing a range of calcifying organisms such as coralline encrusting and non-encrusting algae, that influence and are influenced by the carbonate chemistry. We calculated calcification of the pools community by using the alkalinity anomaly method and estimated the community photosynthesis/respiration. We also compared night-time dissolution and day-time calcification. Finally, we manipulated the pools chemistry at emersion by adding CO₂ to mimick future acidification changes, and explored the impact of seawater acidification on the calcification of the tidal pools' communities.

How to cite: Dorey, N., Martin, S., and Kwiatkowski, L.: Rocky tidal pools: carbonate chemistry, diurnal variability and calcifying organisms in future high-CO2 conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9816, https://doi.org/10.5194/egusphere-egu21-9816, 2021.

The alkalinity of seawater sets the overall capacity of the ocean to hold carbon dioxide in dissolved forms. Variations in past alkalinity, related to changing weathering or carbonate compensation, may have played an important role in moderating or controlling past variations of atmospheric pCO₂.  Future manipulation of ocean alkalinity by direct addition of suitable chemicals to seawater, or through enhanced weathering on land, has also been suggested as one possible route to intentionally draw CO₂ from the modern atmosphere and mitigate the impacts of future climate change [1]. Although we know an increasing amount about how biological species and ecosystems respond to changes in pH, we know much less about their response to changes in alkalinity. Calcifying plankton play a crucial role in modulating the surface ocean carbonate system and its buffering of alkalinity perturbations [2]. Here we investigate the growth and calcification response of both coccolithophores and foraminifera to elevated ocean alkalinity and potential CO₂ limitation [3] through a series of carefully designed batch culture laboratory experiments. Alkalinity is raised by two different methods during the experiments: by (i) addition of NaHCO₃ and (ii) addition of Na₂CO₃ and CaCl₂. The reason for two differing elevated alkalinity treatments is that they allow us to constrain how physiology and calcification respond to two different modes of alkalinity manipulation; both of which provide simple laboratory analogues for probable real-world scenarios.

I will present results from experiments with two species of coccolithophores: Emiliania huxleyi and Coccolithus braarudii, as well as two species of planktonic foraminifera: Gloigerinoides ruber and Globigerinella siphonifera. We have found that the main bloom-forming coccolithophore, Emiliania huxleyi, may increase its calcification and growth rate in response to enhanced alkalinity up to Total Alkalinity (TA) = 4000µmol/kg. Whereas Coccolithus braarudii, a much larger and relatively less abundant coccolithophore, shows only a hint of increased calcification in enhanced alkalinity, with negligible changes in growth rate in enhanced alkalinity up to a threshold of Total Alkalinity (TA) = 3500µmol/kg. However, at TA = 4000µmol/kg, C. braarudii’s growth is significantly suppressed/delayed compared to control conditions. In contrast, planktonic foraminifera’s gametogenic success rate alters with enhanced alkalinity, and they may live longer in enhanced alkalinity before undergoing gametogenesis, but with no concurrent measurable increase in calcification. These results from two major groups of calcifiers have implications for future experiments on biotic response to ocean alkalinity enhancement (OAE) schemes, as well as implications for the design implementation of OAE schemes. 

[1] Renforth, P., Henderson, G., 2017. Assessing ocean alkalinity for carbon sequestration. Rev. Geophys. [2] Boudreau, B.P., Middelburg, J.J., Luo, Y., 2018. The role of calcification in carbonate compensation. Nat. Geosci. 11, 894. [3] Bach, L. T., Gill, S. J., Rickaby, R. E. M., Gore, S., Renforth, P., 2019. CO2 Removal With Enhanced Weathering and Ocean Alkalinity Enhancement: Potential Risks and Co-Benefits for Marine Pelagic Ecosystems. Frontiers in Climate 1.

How to cite: Gill, S., Rickaby, R., Erez, J., and Henderson, G.: Assessing the response of coccolithophores and foraminifera to enhanced ocean alkalinity as a CO2 sequestration technique, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5360, https://doi.org/10.5194/egusphere-egu21-5360, 2021.

Mesoscale eddies formed in Eastern boundary upwelling systems are elementary components of ocean circulation and play important roles in the offshore transport of organic carbon and nutrients. Yet, most of our knowledge about this lateral transport and its influence on biogeochemical cycles relies on modelling studies and satellite observations, while in situ measurements of biogeochemical parameters are scarce. For example, little is known about the effects of mesoscale eddies on organic carbon distribution, microbial activity, and organic matter (OM) turnover in the open oligotrophic ocean. To address this gap, we investigated the horizontal and vertical variability of phytoplankton and bacterial activity as well as dissolved organic carbon along a zonal corridor of the westward propagation of eddies between the Cape Verde Islands and Mauretania in the Eastern Tropical North Atlantic (ETNA). We additionally collected samples from a cyclonic eddy along this transect at high spatial resolution. Our results indicate a strong impact of cyclonic eddies on both microbial abundance and metabolic activity in the epipelagic layer (0–200 m). Generally, all determined parameters (bacterial abundance, heterotrophic respiration rates, bacterial biomass production, bacterial growth efficiency, bacterial carbon demand and net primary production) were higher in the eddy than in the stations along the meridional transect. Along the transect, microbial biomass and activity rates were gradually decreasing from the coast to the open ocean. We further observed high variability of biogeochemical parameters within the eddy with elevates microbial abundances as well as process rates in the south-western periphery. This can be explained by the rotational flow of the cyclonic eddy, which perturbs local OM and nutrient distribution via azimuthal advection. The local positive anomaly of microbial activity in the cyclonic eddy compared to all other stations including the near coast ones results from eddy pumping of nutrient into the epipelagic layer that promotes growth of phytoplankton. Overall, our study supports that cyclonic eddies are important vehicles for the transport of fresh OM that fuel heterotrophic activity the open ocean, highlighting the coupling between productive EBUS and the adjacent oligotrophic ETNA.

How to cite: Devresse, Q., Becker, K. W., and Engel, A.: Organic matter transport by cyclonic eddies formed in eastern boundary upwelling system supports heterotrophy in the open oligotrophic ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9818, https://doi.org/10.5194/egusphere-egu21-9818, 2021.

Aragonite is about 50% more soluble than calcite in seawater and its pelagic production is dominated by pteropods. Moreover, it could account for a large fraction of marine CaCO₃ export. The aragonite compensation depth (ACD, the depth at which accumulation is balanced by dissolution) is generally very close to the aragonite saturation depth, i.e. within a few hundred metres. Conversely, the calcite compensation depth (CCD) can be 1-2 kilometres deeper than the calcite saturation depth. That aragonite disappears shallower than calcite in marine sediments is coherent with aragonite’s greater solubility, but why is the calcite lysocline, i.e. the distance between its compensation and saturation depths, much thicker than its aragonite equivalent?

Here, we suggest that at the seafloor, the addition of a soluble CaCO₃ phase (aragonite) results in the preservation of a predeposited stable CaCO₃ phase (calcite), and term this a negative priming action. In soil science, priming action refers to the increase in soil organic matter decomposition rate that follows the addition of fresh organic matter, supposedly resulting from a globally increased microbial activity (Bingeman et al., 1953). Using a new 3D model of CaCO₃ dissolution at the grain scale, we show that a conceptually similar phenomenon could occur at the seafloor, in which the dissolution of an aragonite pteropod at the sediment-water interface buffers the porewaters and causes the preservation of surrounding calcite. Since aragonite-producing organisms are particularly vulnerable to ocean acidification, we expect an increasing calcite to aragonite ratio in the CaCO₃ flux reaching the seafloor as we go further in the Anthropocene. This could, in turn, hinder the proposed aragonite negative priming action, and favour chemical erosion of calcite sediments.

Reference: Bingeman, C.W., Varner, J.E., Martin, W.P., 1953. The Effect of the Addition of Organic Materials on the Decomposition of an Organic Soil. Soil Science Society of America Journal 17, 34-38.

How to cite: Sulpis, O., Agrawal, P., Wolthers, M., Munhoven, G., Walker, M., and Middelburg, J.: Aragonite is calcite’s best friend at the seafloor, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2660, https://doi.org/10.5194/egusphere-egu21-2660, 2021.

Marine biogeochemistry models are generally coupled to a physical ocean model. The biases in these coupled models can be attributed to simplified and empirical representation of biogeochemical processes, insufficient spatial mesh resolution which has an impact on the transport and mixing of biogeochemical substances in the ocean, and a deficit of physical parameterizations that intent to mimic unresolved processes such as eddies. Ocean Biogeochemical models based on variable mesh resolution proved to be convenient tools due to their computational efficiency and flexibility. Unlike standard structured-mesh ocean models, the mesh flexibility allows for a realistic representation of eddy dynamics in certain regions. Here, we present preliminary results of the coupling between the Finite-volumE Sea ice-Ocean Model (FESOM2.0) and the biogeochemical model REcoM2 (Regulated Ecosystem Model 2) in a coarse spatial resolution global configuration.
Surface maps of the simulated nutrients, chlorophyll a and net primary production (NPP) are comparable to available observational data sets. The control simulation forced with the JRA55-do data set reveals a realistic spatial distribution of nutrients, nanophytoplankton and diatom NPP, carbon stocks and fluxes.
FESOM2 utilizes a new dynamical core based on a finite-volume approach. The computational efficiency is about 2-3 times higher than the previous version FESOM1.4, whereas the quality of the simulated ocean and sea ice conditions and representation of biogeochemical variables are comparable in the two models. Thus, the new coupled model FESOM2- REcoM2 is very promising for ocean biogeochemical modelling applications.

How to cite: Gurses, O., Hauck, J., Zeising, M., and Oziel, L.: Global ocean biogeochemical modelling with FESOM2-REcoM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14980, https://doi.org/10.5194/egusphere-egu21-14980, 2021.

The ocean has played a key role in mitigating the impact of climate change by taking up excess anthropogenic heat and CO₂ leading to warming and increased ocean acidity, which goes in hand with a reduction of the saturation state of seawater with regard to the mineral carbonate aragonite, i.e., Ω_AR. While the threats posed by these long-term changes to marine organisms and ecosystems are well recognized, only more recently has the community realized that these threats might be much more imminent owing to extreme events. This is the result of these extremes exposing vulnerable ecosystems already today to conditions that lie in the far future when considering only the changes in the mean conditions. Of particular concern are so-called compound events, i.e., conditions when both temperatures are extremely hot and the saturation states extremely low, as this compounding might be particularly threatening for marine ecosystems, especially for warm water coral reefs.

Here we use satellite records of sea surface temperature (SST) and satellite Ω_AR to map globally the occurrence of marine heat waves (MHW) and low saturation state extreme events and their compounding for the period 1985 and 2018. We use SSTs from the OSTIA product, while we take Ω_AR from the newly developed OceanSODA-ETHZ (monthly 1°x1°) observation-based product that extrapolates ship observations with satellite data. Our study focuses on the Pacific Ocean between 25°S and 25°N, a region with more than 1000 identified coral reefs. We define extremes using the approach of Hobday et al. (2018) with a fixed baseline determined from the entire record (1985-2018) and where extremes are below/above the 10^th/90^th percentiles for Ω/SST respectively.

The majority of the compound extreme events (too hot and too low saturation state) occur in the western tropical Pacific, with 757 of the 1206 reefs in the Pacific experiencing at least three months of compound extreme events over the entire period. The average duration of these compound extremes was 3.6 months, and the average area was 247 600 km² (roughly the size of the United Kingdom). The compound events had an average intensity of –0.13 for Ω_AR and 0.71°C, where the intensity is the anomaly from the climatology. The largest and longest lasting extreme event started in 2016 and lasted nearly three years, coinciding with the El Niño event over the same period, covering an area equivalent to Australia. These findings suggest that more than 60% of coral reefs in the Pacific Ocean are located in regions where heating events may have been compounded by decreased potential for calcification. Given the continuing increase in atmospheric CO₂, the severity of this type of compound events is bound to increase in the future. 

How to cite: Gregor, L. and Gruber, N.: The growing exposure of Pacific coral reefs to compound extremes caused by marine heatwaves coalescing with low saturation state extremes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15096, https://doi.org/10.5194/egusphere-egu21-15096, 2021.

As the climate warms due to greenhouse gas emissions, the ocean absorbs excess heat and carbon. The patterns of ocean excess heat and carbon storage appear tightly linked when the large-scale circulation is fixed. This unique link is not shared with any other ocean tracer, such as Chlorofluorocarbons (CFCs). At the same time, ocean excess carbon storage patterns are mostly unchanged whether the large-scale circulation is free to evolve, or fixed to the pre-industrial circulation pattern, as the climate warms. Here, we interpret the reason for this behavior by breaking ocean carbon storage into two parts: uptake of atmospheric anomalies by the surface ocean, and subsequent internal storage by the ocean’s circulation. We show that the patterns of surface ocean carbon anomalies are dictated by mean state biogeochemical properties and therefore mostly unchanged by circulation changes. Furthermore, surface biogeochemical properties are strongly shaped by the ocean temperature, providing a link between ocean heat and carbon uptake. CFCs on the hand, lack chemical buffering and therefore the patterns of CFC storage do not correlate with heat as much as carbon patterns do. The patterns of surface anomalies ultimately explain most of the differences in how temperature, carbon and CFCs are stored by the ocean, while changes in internal pathways are of secondary importance. Furthermore, the ratio of total ocean carbon and heat storage is roughly constant across warming scenarios and climate models, which might have further implications for relating ocean carbon storage to important climate metrics, such as the transient response to cumulative emissions.

How to cite: Bronselaer, B. and Zanna, L.: Ocean carbon storage uniquely linked to ocean heat, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13595, https://doi.org/10.5194/egusphere-egu21-13595, 2021.

We have recently shown the neglect of small temperature differences in the ocean mixed layer has led to substantial underestimates in the ocean sink for atmospheric CO₂ as calculated from surface pCO₂ observations, which we find should be increased by ~0.8 Pg Cyr-1 when globally integrated. Surface observations of ocean pCO₂ such as those in the SOCAT (Surface Ocean CO₂ Atlas, www.socat.info) are reported at a temperature typically  measured at several metres depth, but co-location of satellite estimates of the subskin surface temperature (at a few centimetres depth) differ from this, and are on average lower. In addition the top millimetre or so of the ocean is cooler than the underlying subskin because the ocean is a source of radiative and latent heat to the atmosphere. These two temperature deviations have subtly different effects on the air-sea flux of CO₂ as calculated by the gas exchange equation, but both result in an increase in the flux into the ocean and the combined effect is large. We are making available several datasets enabling calculation of these effects, including the regular provision of SOCAT data corrected to the subskin temperature, a climatology of the skin temperature deviation, and corrected ocean-atmosphere CO₂ flux estimates for the period since 1985.

How to cite: Watson, A. J., Shutler, J. D., Landschützer, P., Woolf, D. K., Holding, T., Goddijn-Murphy, L., Schuster, U., and Ashton, I. G. C.: Correcting Net Ocean-Atmosphere CO2 Fluxes for Near-surface Temperature Deviations., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4768, https://doi.org/10.5194/egusphere-egu21-4768, 2021.

Deep oceanic convection occurs in few locations around the globe. One such location is found in the Labrador Sea where dense waters can subside to depths in excess of 2km below the surface. The weak stratification preconditions the water column for deep convection, triggered by wintertime surface cooling associated with high wind speed events. The convected water brings with it dissolved gases, such as Carbon Dioxide, which are in constant flux between ocean and atmosphere. It is thought that this process of turbulent boundary layer interactions coupled with deep convection is responsible for mixing these gases into the deep ocean, making the ocean the largest sink of anthropogenic carbon.

The convective overturning process depends on the temperature and salinity profiles which, together dictate density and thus the static stability of the water column. We have adapted a widely used one-dimensional mixed-layer model, referred to as PWP, to include a parameterization of the air-sea flux of gases such as Oxygen and Carbon Dioxide.  The model is forced with surface meteorological fields from the ERA5 reanalysis as well as the higher resolution operational reanalysis from the ECMWF.

With the model, we investigate the sensitivity of deep-water formation and the vertical profile of these gases to various atmospheric forcing scenarios. Overturning in the Labrador Sea is most active during the winter months when heat flux out of the ocean is at its maximum. It is found that overturning is far more sensitive to thermal forcing than it is to freshwater forcing within the range of forcings typical to the Labrador Sea. We explore the impact of this sensitivity, including the dependence of the atmospheric forcing on modes of climate variability such as the NAO,  has on the role that the Labrador Sea plays as a marine sink for anthropogenic carbon.

How to cite: Piunno, R. and Moore, K.: Sensitivity of convective overturning and turbulent mixing of dissolved gases in the Labrador Sea to atmospheric forcing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13461, https://doi.org/10.5194/egusphere-egu21-13461, 2021.

Understanding the variability and drivers of air-sea CO₂ fluxes on seasonal timescales is critical for resolving the ocean carbon sink's evolution and variability. Here, we investigate whether discrepancies in the representation of air-sea CO₂ fluxes on a seasonal timescale accumulate to influence the representation of CO₂ fluxes on an interannual timescale in two important ocean CO₂ sink regions – the North Atlantic basin and the Southern Ocean. Using an observation-based product (SOM-FFN) as a reference, we investigate the representation of air-sea CO₂ fluxes in the Max Planck Institute's Earth System Model Grand Ensemble (MPI-ESM GE). Additionally, we include a simulation based on the same model configuration, where observational data from the atmosphere and ocean components is assimilated (EnKF assimilation) to verify if the inclusion of observational data alters the model state significantly and if the updated modelled CO₂ flux values better represent observations.

We find agreement between all three observation-based and model products on an interannual timescale for the North Atlantic basin. However, the agreement on a seasonal timescale is inconsistent with discrepancies as large as 0.26 PgC/yr in boreal autumn in the North Atlantic. In the Southern Ocean, we find little agreement between the three products on an interannual basis with significant seasonal discrepancies as large as 1.71 PgC/yr in austral winter. However, while we identify regional patterns of dominating seasonal variability in MPI-GE and EnKF, we find that the SOM-FFN cannot demonstrate robust conclusions on the relevance of seasonal variability in the Southern Ocean. In turn, we cannot pin down the problems for this region.

How to cite: Rustogi, P., Landschuetzer, P., Brune, S., and Baehr, J.: Seasonal Analysis of Air-Sea CO2 Flux Variability in the North Atlantic and the Southern Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13637, https://doi.org/10.5194/egusphere-egu21-13637, 2021.

Elucidating the coherent spatio-temporal patterns of historical ocean CO₂ fluxes is an essential step to understand the dominant drivers of their variability and predict how they may be altered by future climate change. Here, we applied an unsupervised classification of SST to tease out and assess the spatial and temporal variability of marine CO₂ uptake in the Pacific basin. The classification is performed using a Gaussian Mixture Model (GMM) that decomposes the Probability Density Function of a dataset into a weighted sum of Gaussian distribution. Classification is performed on monthly SST anomalies from the JRA-55 reanalysis and CMIP6 historical simulations. The associated patterns of CO₂ fluxes anomalies in both observations and models are evaluated for consistencies. Our objective is to determine the ability of the GMM-based clustering method, applied on surface temperature, to retrieve relevant physical mechanisms that predominantly explain the observed spatial and temporal CO₂ fluxes patterns. The evolution of these clustering-based patterns, as projected by the models, under future scenario will also be presented.

How to cite: Vaittinada Ayar, P. and Tjiputra, J.: Pacific CO2 fluxes pattern analysis through SST clustering, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8286, https://doi.org/10.5194/egusphere-egu21-8286, 2021.

Air-sea CO₂ fluxes display large temporal fluctuations on seasonal to interannual timescales, both at global and regional scales. These fluctuations in the oceanic carbon uptake suggest that the interior dissolved inorganic carbon (DIC) is equally highly variable, driven by changes in this uptake, but also by changes in circulation and biological activity. In turn, fluctuations in DIC affect the air-sea CO₂ exchange, thus altering the amount of CO₂ in the atmosphere. However, most studies at global scale have focused on the anthropogenic increase in oceanic carbon and have done so at decadal mean time scales. Consequently, to date, the seasonal and interannual variability (IAV) of the contemporary DIC (natural + anthropogenic) in the water column has not been quantitatively mapped from observations at a global scale. Here, we fill this gap by using our newly developed global ocean DIC map product “Mapped Observation-Based Oceanic DIC” (MOBO-DIC) which is based on DIC measurements from GLODAPv2.2019 and a 2-step neural network method to gap-fill and map the measurements globally until 2000 m. Its seasonal climatology (Keppler et al., 2020a) reveals that the seasonal surface DIC amplitudes range from 0 to more than 50 μmol kg⁻¹. The seasonal variations mostly stem from high DIC concentrations in winter, when mixed layers are deep, and low DIC concentrations in summer, when enhanced net community production (NCP) removes large amounts of DIC. We estimate a spring-to-fall NCP in the euphotic zone of the mid-latitudes of 3.9±2.7 Pg C yr^-1, which corresponds to 8.2±5.6 Pg C yr^-1 when upscaling globally (Keppler et al., 2020b). The monthly fields of MOBO-DIC from 2004 through 2018 reveals that the largest interannual variability of DIC is found in the tropical Pacific, strongly driven by the El Niño Southern Oscillation. The DIC trend suggests that in the upper 500 m, the DIC concentration has increased by ~21 Pg C from 2004 through 2018 (i.e., ~14 Pg C decade^-1) in our study domain.

References

Keppler, L., Landschützer, P., Gruber, N., Lauvset, S. K. & Stemmler, I. (2020a). Mapped Observation-Based Oceanic Dissolved Inorganic Carbon (DIC), monthly climatology from January to December (based on observations between 2004 and 2017), from the Max-Planck-Institute for Meteorology (MOBO-DIC_MPIM) (NCEI Accession 0221526). NOAA National Centers for Environmental Information. Dataset. https://doi.org/10.25921/yvzj-zx46.

Keppler, L., Landschützer, P., Gruber, N., Lauvset, S. K., & Stemmler, I. (2020b). Seasonal carbon dynamics in the near-global ocean. Global Biogeochemical Cycles, 34, e2020GB006571. https://doi.org/10.1029/2020GB006571.

How to cite: Keppler, L., Landschützer, P., Gruber, N., and Lauvset, S. K.: Temporal Variability in Interior Dissolved Inorganic Carbon, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8745, https://doi.org/10.5194/egusphere-egu21-8745, 2021.

The North Atlantic and Southern Oceans are major sinks of anthropogenic carbon and excess heat. The Earth system model projections of these sinks provided by the CMIP5 and CMIP6 scenario experiments remain highly uncertain, hindering an effective development of climate mitigation policies for meeting the ambitious climate targets laid down in the Paris agreement. A recent study identified an emergent coupling between anthropogenic carbon and excess heat uptake, highlighting the dominant passive-tracer behavior of these two quantities under high-emission scenarios. This coupling potentially allows for the use of a single observational constraint to reduce these projection uncertainties. As a first step, we investigate the causes of these uncertainties in the Southern Ocean (30°S-55°S) by looking regionally at different contemporary physical and biogeochemical quantities. We find that the variations in model´s contemporary water-column stability over the first 2000 m is highly correlated to both its future anthropogenic carbon uptake and excess heat uptake efficiency. Using an observation-based estimate of contemporary water-column stability, this allows us to reduce the uncertainty of future estimates of (1) the cumulative anthropogenic carbon uptake by up to 50% and (2) the excess heat uptake efficiency by 23%. Our results show that improving representation of water-column stratification in Earth system models should be prioritized to constrain future carbon budget and climate change projections.

How to cite: Bourgeois, T., Goris, N., Schwinger, J., and Tjiputra, J.: Constraining anthropogenic carbon and excess heat uptake in climate projections, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7654, https://doi.org/10.5194/egusphere-egu21-7654, 2021.

The uptake of anthropogenic carbon (C_ant) by the ocean leads to ocean acidification, causing the reduction of pH and the calcium carbonate saturation states of aragonite (Ω_arag) and calcite (Ω_calc). The Arctic Ocean is particularly vulnerable to ocean acidification due to its naturally low pH and saturation states and due to ongoing freshening and the concurrent reduction in alkalinity in this region. Here, we present projections of  C_ant and ocean acidification in the Arctic Ocean over the 21^st century across Earth System Models (ESMs) from the latest Coupled Model Intercomparison Project Phase 6 (CMIP6). Compared to the previous model generation (CMIP5), the inter-model uncertainty of projected end-of-century Arctic Ocean Ω_arag/calc is reduced by 44–64 %. The strong reduction in projection uncertainties of Ω_arag/calc can be attributed to compensation between C_ant uptake and alkalinity reduction in the latest models. Specifically, ESMs with a large increase in Arctic Ocean C_ant over the 21^st century tend to simulate a relatively weak concurrent freshening and alkalinity reduction, while ESMs with a small increase in C_ant simulate a relatively strong freshening and concurrent alkalinity reduction. Although both mechanisms contribute to Arctic Ocean acidification over the 21^st century, the increase in C_ant remains the dominant driver. Even under the low-emissions shared socioeconomic pathway SSP1-2.6, basin-wide averaged aragonite undersaturation occurs before the end of the century. While under the high-emissions pathway SSP5-8.5, the Arctic Ocean mesopelagic is projected to even become undersaturated with respect to calcite. An emergent constraint, identified in CMIP5, which relates present-day maximum sea surface densities in the Arctic Ocean to the projected end-of-century Arctic Ocean C_ant inventory, is found to generally hold in CMIP6. However, a coincident constraint on Arctic declines in Ω_arag/calc is not apparent in the new generation of models. This is due to both the reduction in Ω_arag/calc projection uncertainty and the weaker direct relationship between projected changes in Arctic Ocean C_ant and Ω_arag/calc. In CMIP6, models generally better simulate maximum sea surface densities in the Arctic Ocean and consequently the transport of C_ant into the Arctic Ocean interior, with simulated historical increases in C_ant in improved agreement with observational products.

How to cite: Terhaar, J., Torres, O., Bourgeois, T., and Kwiatkowski, L.: Arctic Ocean acidification over the 21st century co-driven by anthropogenic carbon increases and freshening in the CMIP6 model ensemble, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7937, https://doi.org/10.5194/egusphere-egu21-7937, 2021.