From the surface into the deep: advances in marine carbon dynamics with models and observations

The net amount of CO2 that is taken up and stored by the ocean is a major driver of the rate of climate change but also affects biogeochemical stressors such as ocean acidification. Alongside the gradual increase in the ocean’s anthropogenic carbon inventory, the uptake, storage, and transformation of carbon display a large degree of spatial and temporal variability. In this session, we wish to shine a light on such trends and variability in ocean carbon dynamics, focusing on underlying processes and the consequences for marine ecosystems in the recent past, present, and future.

We are specifically interested in temporal changes in the fluxes and inventories of natural and anthropogenic inorganic carbon, as well as other marine carbonate system parameters, such as alkalinity, pCO2, and pH. We welcome contributions with a focus on the open or coastal ocean, surface, and/or ocean interior, based on observations, models, or theory and with a global or regional focus. Observational and multi-model constraints on marine carbon dynamics are particularly welcome, as are studies based on GLODAP or SOCAT data and insights from the recent Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations.

Public information:
We are also organizing an informal get-together in the evening after our session. You'll have the opportunity to meet up and connect with new and old peers from our field, via breakout rooms on the platform GatherTown.

Please mark April 27th from 6 pm to 7 pm in your calendars for that.
We'll provide more information and the log-in details closer to the date.

Including OS Division Outstanding ECS Award Lecture 2020
Co-organized by BG4
Convener: Lydia KepplerECSECS | Co-conveners: Jens Daniel Müller, Lester Kwiatkowski
vPICO presentations
| Tue, 27 Apr, 13:30–17:00 (CEST)
Public information:
We are also organizing an informal get-together in the evening after our session. You'll have the opportunity to meet up and connect with new and old peers from our field, via breakout rooms on the platform GatherTown.

Please mark April 27th from 6 pm to 7 pm in your calendars for that.
We'll provide more information and the log-in details closer to the date.

vPICO presentations: Tue, 27 Apr

Chairpersons: Lydia Keppler, Jens Daniel Müller, Lester Kwiatkowski
Opening Talk
OS Division Outstanding ECS Award Lecture 2020
Insights into the ocean’s biological carbon pump: What makes marine snow falling into the deep ocean?
Frederic Le Moigne
Ocean acidification / vulnerability
Maribel I. García-Ibáñez, Nicholas R. Bates, Dorothee C.E. Bakker, Marcos Fontela, and Antón Velo

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 ([CO32-]), which in turn lowers oceanic saturation states (Ω) for calcium carbonate (CaCO3) minerals. The Ω values for aragonite (Ωaragonite; 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 Ωaragonite in the Irminger and Iceland Basins of the North Atlantic Ocean, providing the first trends of Ωaragonite in the deep waters of these basins. The entire water column of both basins showed significant negative Ωaragonite trends between -0.0015 ± 0.0002 and -0.0061 ± 0.0016 per year. The decrease in Ωaragonite in the intermediate waters, where nearly half of the CWC reefs of the study region are located, caused the Ωaragonite isolines to migrate upwards rapidly at a rate between 6 and 34 m per year. The main driver of the observed decline in Ωaragonite 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 Ωaragonite decrease in cLSW. Based on the observed Ωaragonite 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.

Narimane Dorey, Sophie Martin, and Lester Kwiatkowski

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.

Kunal Madkaiker and Vinu Valsala

The Indian Ocean (IO) is witnessing acidification of its surface waters as a consequence of the continuous rising of atmospheric CO2 concentration thus disrupting the biological and chemical balance of the ecosystem in the region. The basin wide spatial variability of biogeochemical properties induces spatial variability of surface water pH. This study investigates the seasonality and trends of surface pH over the IO bioprovinces and regionally assesses the individual contribution of the factors affecting its variability. Simulations from global ocean models (OTTM and ROMS) coupled with suitable biogeochemical modules were validated with pH observations over the basin, and used to discern the regional response of pH seasonality (1990-2010) and trend (1961-2010) to changes in ocean temperature (SST), Dissolved Inorganic Carbon (DIC), Total Alkalinity (ALK) and Salinity (S). DIC and SST are the major contributors to the seasonal variability of pH in almost all bioprovinces consistent in both model simulations. The acidification in IO basin of 0.0675 units during 1961-2010 is attributed to 69.28% contribution of DIC followed by 13.82% contribution of SST. For most of the regions DIC remains a dominant contributor to changing trend in pH except for the Northern Bay of Bengal and Around India (NBoB-AI) region, wherein pH trend is dominated by ALK (55.6%) and SST (16.8%). The interdependence of SST and S over ALK is significant in modifying the carbonate chemistry and biogeochemical dynamics of NBoB-AI and a part of tropical, subtropical IO. The strong negative correlation between SST and pH infers the increasing risk of acidification in the bioprovinces with the rising SST.

This study is an attempt to identify the regional influencers of pH variability so that adequate mitigation action can be planned and the acidification can be decelerated in near future.

How to cite: Madkaiker, K. and Valsala, V.: Understanding the trends and controlling factors of Indian Ocean acidification, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14298, https://doi.org/10.5194/egusphere-egu21-14298, 2021.

Zouhair Lachkar, Michael Mehari, Alain De Verneil, Marina Lévy, and Shafer Smith

Recent observations and modeling evidence indicate that the Arabian Sea (AS) is a net source of carbon to the atmosphere. Yet, the interannual variability modulating the air-sea CO2 fluxes in the region, as well as their long-term trends, remain poorly known. Furthermore, while the rising atmospheric concentration of CO2 is causing surface ocean pH to drop globally, little is known about local and regional acidification trends in the AS, a region hosting a major coastal upwelling system naturally prone to relatively low surface pH. Here, we simulate the evolution of air-sea CO2 fluxes and reconstruct the progression of ocean acidification in the AS from 1982 through 2019 using an eddy-resolving ocean biogeochemical model covering the full Indian Ocean and forced with observation-based winds and heat and freshwater fluxes. Additionally, using a set of sensitivity simulations that vary in terms of atmospheric CO2 levels and physical forcing we quantify the variability of fluxes associated with both natural and anthropogenic CO2 and disentangle the contributions of climate variability and that of atmospheric CO2 concentrations to the long-term trends in air-sea CO2 fluxes and acidification. Our analysis reveals a strong variability in the air-sea CO2 fluxes and pH on a multitude of timescales ranging from the intra-seasonal to the decadal. Furthermore, a strong progression of ocean acidification with an important penetration into the thermocline is simulated locally near the upwelling regions. Our analysis also indicates that in addition to the increasing anthropogenic CO2 concentrations in the atmosphere, recent warming and monsoon wind changes have substantially modulated these trends regionally.

How to cite: Lachkar, Z., Mehari, M., De Verneil, A., Lévy, M., and Smith, S.: Recent trends in air-sea CO2 fluxes and ocean acidification in the Arabian Sea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14228, https://doi.org/10.5194/egusphere-egu21-14228, 2021.

Siv K Lauvset and Nadine Goris

Ocean acidification is a process driven by the ocean uptake of anthropogenic CO2 emissions. Because this uptake happens at the ocean-atmosphere interphase, ocean acidification is presently foremost a surface ocean phenomenon. A recent study (Lauvset et al., 2020) shows that away from the surface ocean pH changes primarily due to organic matter remineralization, and in ocean depths between 500–1500 m this process enhances ocean acidification by on average 28 ± 15%. Presently, this signal is very weak, and not detectable outside calculation uncertainties. However, as time passes the ocean overturning circulation will transport all carbon chemistry perturbations on and near the surface into the interior ocean, which can already be seen in the deep North Atlantic. Our hypothesis is that if CO2 emissions, and thus ocean acidification, continue in the future then this remineralization enhancement will become significant and lead to some regions and habitats being more vulnerable to continued ocean acidification than others. Here we evaluate this enhancement over the 21st century using the Norwegian Earth System Model (NorESM), to assess which oceanic regions are made more vulnerable to future ocean acidification from this enhancement, and at what timescales the enhancement becomes important.

How to cite: Lauvset, S. K. and Goris, N.: Anthropogenic ocean acidification below the surface: does organic matter cycling result in enhanced vulnerability?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1953, https://doi.org/10.5194/egusphere-egu21-1953, 2021.

Sophie Gill, Rosalind Rickaby, Jonathan Erez, and Gideon Henderson

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.

Moritz Baumann, Jan Taucher, Allanah Joy Paul, Malte Heinemann, Mari Vanharanta, Lennart Thomas Bach, Kristian Spilling, Joaquin Ortiz-Cortes, Javier Arístegui, and Ulf Riebesell

To counteract climate change, measures to actively remove carbon dioxide from the atmosphere are required, since the reduction of global CO2 emissions alone will not suffice to meet the 1.5 °C goal of the Paris agreement. Artificial upwelling in the ocean has been discussed as one such carbon dioxide removal technique, by fueling primary production in the surface ocean with nutrient-rich deep water and thereby potentially enhancing downward fluxes of organic matter and carbon sequestration. In this study we tested the effect of different rates and modes of artificial upwelling on carbon export and its potential attenuation with depth in a five-week mesocosm experiment in the subtropical Northeast Atlantic. We fertilized oligotrophic surface waters with different amounts of deep water in a pulsed (deep water fertilization once at the beginning) and a continuous manner (deep water fertilization every four days) and measured the resulting export flux as well as sinking velocities and respiration rates of sinking particles. Based on this, we applied a simple one-dimensional model to calculate flux attenuation. We found that the export flux more than doubled when fertilizing with deep water, while the C:N ratios of produced organic matter increased from values around Redfield (6.6) to ~8-13. The pulsed form of upwelling resulted in a single export event, while the continuous mode led to a persistently elevated export flux. Particle sinking velocity and remineralization rates were highly variable over time and showed differences between upwelling modes. We stress the importance of experiments with a prolonged application of artificial upwelling and studies including real world open water application to validate the CO2 sequestration potential of artificial upwelling.

How to cite: Baumann, M., Taucher, J., Paul, A. J., Heinemann, M., Vanharanta, M., Bach, L. T., Spilling, K., Ortiz-Cortes, J., Arístegui, J., and Riebesell, U.: Effect of different rates and modes of artificial upwelling on particle flux and potential POC deep export, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6330, https://doi.org/10.5194/egusphere-egu21-6330, 2021.

Ocean biogeochemistry
Quentin Devresse, Kevin W Becker, and Anja Engel

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.

Himanshu Saxena, Deepika Sahoo, Sipai Nazirahmed, Deepak Kumar Rai, Mohammad Atif Khan, Niharika Sharma, Sanjeev Kumar, Athiyarath K Sudheer, and Arvind Singh

The twilight zone of the oceans layering between the bottom of the sunlit ocean and 1000 m depth, is one of the largest continuous ecosystems on the Earth, yet remains least explored. While the sunlit ocean is well-studied for its major role in sequestering CO2 from the atmosphere, the role of twilight zone in CO2 sequestration remains a mystery. The twilight zone of the Arabian Sea, north-western part of the Indian Ocean inarguably possesses an active nitrogen‐cycle owing to abundant chemoautotrophic (anammox, nitrite oxidising, nitrifying) microorganisms and heterotrophic (denitrifying) microorganisms. However, these microorganisms with ramifications for the nitrogen cycle, incentivize the carbon cycle. Since chemoautotrophy is a light-independent autotrophic process, a significant amount of dissolved CO2 may be assimilated rather than released in the Arabian Sea twilight zone by these organisms. With this supposition, we commenced the expedition in the off-shore and the central Arabian Sea during winter monsoon (Dec-Jan 2019) to measure carbon fixation rates in its sunlit and twilight zone using 13C tracer incubation technique. The sunlit zone and twilight zone carbon fixation rates ranged from 6.8 to 40 mmol C m-2 d-1 and 0.4 to1.6 mmol C m-2 d-1, respectively. The twilight zone carbon fixation did not vary spatially much, unlike sunlit zone which showed a sharp decreasing trend of carbon fixation from northern to the southern Arabian Sea. Notably, the twilight zone contribution to water column carbon fixation ranged from 2 to 10% during the study period. This study corroborates that the twilight zone forms an integral component of the carbon cycle; implying, the overlooked twilight zone can significantly contribute CO2 drawdown. Therefore, the role of twilight zone towards climate buffering is bigger than previously assumed, demanding a review of its role in the current paradigm of the Earth’s climate.

How to cite: Saxena, H., Sahoo, D., Nazirahmed, S., Kumar Rai, D., Khan, M. A., Sharma, N., Kumar, S., K Sudheer, A., and Singh, A.: Key role of overlooked twilight zone towards climate buffering, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10643, https://doi.org/10.5194/egusphere-egu21-10643, 2021.

Olivier Sulpis, Priyanka Agrawal, Mariette Wolthers, Guy Munhoven, Matthew Walker, and Jack Middelburg

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.

Biogeochemical modelling
Fatemeh Chegini, Lennart Ramme, Jöran März, Katharina Six, Daniel Burt, and Tatiana Ilyna
Ocean biogeochemistry as part of the Earth system impacts the uptake of atmospheric CO2 and storage of carbon in the ocean. In the ICON-O (Icosahedral non-hydrostatic general circulation model) ocean model, ocean biogeochemistry is represented by the HAMburg Ocean Carbon Cycle model (HAMOCC; Ilyina et al. 2013, Mauritsen et al. 2019, Maerz et al. 2020). Here, we present the results of an ongoing effort to tune HAMOCC (i.e. adapt parameters within the uncertainty range) to accommodate the ocean circulation simulated by ICON-O.
The tuning of biogeochemical models, including HAMOCC, has previously been an iterative, and a rather random process combining expert knowledge and a suite of parameter testings. A documented, systematic procedure, describing how to tune these models is lacking. Therefore, while tuning HAMOCC in ICON-O, we aim at filling this gap by structuring the process and documenting the steps taken to tune a biogeochemistry model in a global general ocean circulation model.
The ocean circulation has a large impact on the distribution of biogeochemical tracers, as biases in the circulation will, for example, impact the upwelling of nutrients or the CO2 exchange with the atmosphere. We investigate the impact of physical parameterization such as the Gent-McWilliam eddy parameterization and the vertical mixing scheme on the choice of HAMOCC tuning parameters. We then compare the spatial distribution of major state variables such as nutrients and alkalinity to observational data ( WOA; Garcia et al 2013, GLODAP; Key et al 2004) and evaluate the key tendencies such as CO2 surface fluxes and attenuation of particulate organic matter fluxes. Furthermore, we discuss the tuning steps, choices of the tuning parameters and their impact on the simulated biogeochemistry. The envisioned outcome of this work is a tuned ocean biogeochemistry component for the here used ICON-O model and a more generalized tuning procedure that can be applied to other models or HAMOCC in different model configurations (coupled runs, different resolution).

Garcia, H. E., et al. 2014: World Ocean Atlas 2013, NOAA Atlas NESDIS 76, Volume4: Dissolved Inorganic Nutrients (phosphate, nitrate, silicate), 25pp.

lyina, T., et al. 2013: Global ocean biogeochemistry model HAMOCC: Model architecture and performance as component of the MPI-Earth system model in different CMIP5 experimental realizations, J. Adv. Model. Earth Sy., 5, .

Key, R., et al. 2004: A global ocean carbon climatology: Results from Global Data Analysis Project, Global Biogeochem. Cycles, 18, 4, https://doi.org/10.1029/2004GB002247.

Maerz et al. 2020: Microstructure and composition of marine aggregates as co-determinants for vertical particulate organic carbon transfer in the global ocean, Biogeosciences, 17, 7, https://doi.org/10.5194/bg-17-1765-2020.

Mauritsen, T., et al. 2019: Developments in the MPI-M Earth System Model version 1.2 (MPI-ESM1.2) and Its Response to Increasing CO2, J. Adv. Model. Earth Sy., 11, https://doi.org/10.1029/2018MS001400.


How to cite: Chegini, F., Ramme, L., März, J., Six, K., Burt, D., and Ilyna, T.: Tuning ocean biogeochemistry in the Earth system — insights from the HAMburg Ocean Carbon Cycle model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14984, https://doi.org/10.5194/egusphere-egu21-14984, 2021.

Ozgur Gurses, Judith Hauck, Moritz Zeising, and Laurent Oziel

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.

Compound extremes
Luke Gregor and Nicolas Gruber

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., ΩAR. 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 ΩAR 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 ΩAR 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 10th/90th 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 ΩAR 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.

Friederike Fröb and Tatiana Ilyina

Long-term changes in ocean biogeochemistry that are projected under an evolving climate in the 21st century are superimposed by short-term extreme events. Of particular interest are compound events, where such extreme events occur successively or simultaneously, combining or amplifying the impact of multiple stressors on ocean ecosystems. The resilience of marine species to the simultaneous exposure of extremely high temperature, low pH and low oxygen concentration presumably depends on the magnitude and variability of the perturbation, which is likely to increase and intensify in response to rising global mean temperatures. However, changes in marine heat waves, ocean acidification and deoxygenation extremes, remain to be detected, in order to quantify their combined impact. Here, we use the Grand Ensemble of the fully coupled Max Planck Institute Earth System Model (MPI-GE) that consists of 100 members forced by historical CO2 emissions and those according to the Representative Concentration Pathway 4.5 (RCP4.5). The daily frequency of the simulation output for sea surface temperature, hydrogen ion concentration and oxygen concentration allows analysing spatio-temporal changes of marine extreme events between 1850 and 2100. We assess the number, duration, and intensity of extreme states using a moving threshold criterion, and aim to identify concurrent and consecutive driving mechanisms for such events in the surface ocean in order to evaluate potential risks for the marine ecosystem.

How to cite: Fröb, F. and Ilyina, T.: Towards detecting biogeochemical compound extremes in the surface ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15506, https://doi.org/10.5194/egusphere-egu21-15506, 2021.

Chairpersons: Lester Kwiatkowski, Lydia Keppler, Jens Daniel Müller
Opening Talk 2
Ben Bronselaer and Laure Zanna

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.

Global carbon fluxes
Galen A. McKinley, Jessica Cross, Timothy DeVries, Judith Hauck, Amanda Fay, Peter Landschützer, Goulven G. Laruelle, Nicole Lovenduski, Pedro Monteiro, Ray Najjar, Laure Resplandy, Christian Rödenbeck, Christopher Sabine, Rik Wanninkhof, and Nancy Williams

By means of a variety of international observing and modeling efforts, the ocean carbon community has developed numerous estimates for ocean carbon uptake. In this presentation, we report on the synthesis effort we are undertaking under the auspices of an Ocean Carbon and Biogeochemistry Working Group.  Our initial goal for this working group is to determine the best estimate for the net and anthropogenic carbon sink from 1994-2007 based on three approaches that independently use interior data, surface data or hindcast ocean models. Combining two approaches that use interior ocean data to estimate anthropogenic carbon, Fant = -2.40+-0.21 PgC/yr (2 sigma uncertainty). Estimates for the net, or contemporary, ocean carbon uptake come from 6 products that interpolate surface ocean pCO2 data to global coverage: Fnet = -1.58+-0.19  PgC/yr for 1994-2007. Uncertain closure terms for naturally-outgassed river-derived carbon and non-steady state natural carbon fluxes in the open ocean are then added to derive Fant from surface observation-based Fnet. Ocean models do not include river-derived carbon, but do include non-steady state natural carbon fluxes, and thus a third estimate for Fant is derived. The combined best-estimate is Fant = -2.35+-0.53 PgC/yr.  We detail the uncertainties and assumptions made in deriving these estimates, and suggest paths forward to further reduce uncertainties.

How to cite: McKinley, G. A., Cross, J., DeVries, T., Hauck, J., Fay, A., Landschützer, P., Laruelle, G. G., Lovenduski, N., Monteiro, P., Najjar, R., Resplandy, L., Rödenbeck, C., Sabine, C., Wanninkhof, R., and Williams, N.: Quantifying the ocean carbon sink for 1994-2007: Combined evidence from multiple methods, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6704, https://doi.org/10.5194/egusphere-egu21-6704, 2021.

Alizee Roobaert, Goulven Laruelle, Laure Resplandy, Peter Landschützer, Nicolas Gruber, Enhui Liao, Lei Chou, and Pierre Regnier

The spatio-temporal variability and the underlying drivers of the carbon dioxide (CO2) exchange at the air-water interface (FCO2) of the global coastal ocean are still poorly understood and their quantification remains highly uncertain. Here, we present an analysis of the spatial and seasonal variability of FCO2 using a high-resolution (0.25 degree) monthly climatology (1998-2015 period) for coastal sea surface partial pressure in CO2 (pCO2), globally.

Overall, a clear latitudinal pattern emerges from our analysis regarding sources/sinks distribution of atmospheric CO2 and we find that in most regions, annual mean CO2 flux densities are comparable in sign and magnitude to those of the adjacent open ocean except for river dominated systems. Globally, coastal regions act as a CO2 sink with a more intense uptake occurring in summer because of the disproportionate influence of high latitude coastal seas in the Northern Hemisphere. The majority of the coastal seasonal FCO2 variations stems from the air-sea pCO2 gradient, although changes in wind speed and sea-ice cover can also be significant regionally. To investigate further the drivers of the spatio-seasonal variability, our observation-based pCO2 climatology is used in conjunction with global ocean biogeochemistry model MOM6-COBALT. The model outputs allow us to quantify the respective contributions of thermal effects, biology, and non-thermal physical processes (circulation and freshwater inputs) to seasonal variations in coastal pCO2. Generally, biological activity is the dominant driver of the pCO2 seasonal variability in temperate and high latitudes while thermal and non-thermal physical processes dominate in low latitudes.

How to cite: Roobaert, A., Laruelle, G., Resplandy, L., Landschützer, P., Gruber, N., Liao, E., Chou, L., and Regnier, P.: The spatiotemporal dynamics of the sources and sinks of CO2 in the global coastal ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10048, https://doi.org/10.5194/egusphere-egu21-10048, 2021.

Andrew J. Watson, Jamie D. Shutler, Peter Landschützer, David K. Woolf, Thomas Holding, Lonneke Goddijn-Murphy, Ute Schuster, and Ian G. C. Ashton

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.

Regional carbon fluxes
Romina Piunno and Kent Moore

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.

David Curbelo Hernández, Melchor González Dávila, Aridane González González, David González Santana, and Juana Magdalena Santana Casiano

The seasonal and spatial variability of the CO2 system parameters and CO2 air-sea exchange was studied in the Northeast Atlantic Ocean between the northwest African coastal upwelling and the oligotrophic open-ocean waters of the North Atlantic subtropical gyre. Data was collected aboard a volunteer observing ship (VOS) from February 2019 to February 2020. The seasonal and spatial variability of CO2 fugacity in seawater (fCO2,sw) was strongly driven by the seasonal temperature variation, which increased with latitude and was lower throughout the year in coastal regions where the upwelling and offshore transport was more intense. The thermal to biological effect ratio (T/B) was approximately 2, with minimum values along the African coastline related to higher biological activity in the upwelled waters. The fCO2,sw increased from winter to summer by 11.84 ± 0.28 µatmºC-1 on the inter-island routes and by 11.71 ± 0.25 µatmºC-1 along the northwest African continental shelf. The seasonality of total inorganic carbon (CT) normalized to constant salinity of 36.7 (NCT) was studied throughout the region. The effect of biological processes and calcification/dissolution on NCT between February and October represented >90% of the reduction of inorganic carbon while air-sea exchange described <6%. The seasonality of air-sea CO2 exchange was controlled by temperature. The surface waters of the entire region acted as a CO2 sink during the cold months and as a CO2 source during the warm months. The Canary basin acted as a net sink of -0.26 ± 0.04 molC m-2 yr-1. The northwest African continental shelf behaved as a stronger sink at -0.48 ± 0.09 molC m-2 yr-1. The calculated average CO2 flux for the entire area was -2.65 ± 0.44 TgCO2 yr-1 (-0.72 ± 0.12 TgC yr-1).

How to cite: Curbelo Hernández, D., González Dávila, M., González González, A., González Santana, D., and Santana Casiano, J. M.: Seasonal and spatial variability of the CO2 system parameters in the Northeast Atlantic based on measurements from a surface ocean observation platform., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12710, https://doi.org/10.5194/egusphere-egu21-12710, 2021.

Paridhi Rustogi, Peter Landschuetzer, Sebastian Brune, and Johanna Baehr

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.

Coraline Leseurre, Claire Lo Monaco, Gilles Reverdin, Nicolas Metzl, Jonathan Fin, and Claude Mignon

The Southern Ocean is recognized as a major player in the sequestration of anthropogenic carbon. As pH is naturally low at high latitudes, the increase in oceanic CO2 raises particular concerns in this region were surface waters could become rapidly under-saturated with respect to carbonate. We used repeated observations collected by the French monitoring program OISO (Ocean Indien Service d’Observation) in the surface ocean and the mixed layer over the last two decades (1998-2018), conducted on board the Marion Dufresne (IPEV/IFREMER). We used complementary data, available in SOCAT, to expand the study area, in order to investigate the evolution of CO2 and ocean acidification in the Southern Indian Ocean (45°S-57°S). South of the polar front in the High Nutrients Low Chlorophyll (HNLC) region, our results show an increase in the fugacity of CO2 (fCO2) in surface waters during summer, close to the increase in the atmosphere (on the order of +2 µatm yr-1) associated with a decrease in pH in the range of the mean global ocean trend (on the order of -0.0020 yr-1). However much larger changes are found in the phytoplankton blooms in the vicinity of Crozet and Kerguelen Islands for both fCO2 (between +3.0 µatm yr-1 and +5.0 µatm yr-1) and pH (ranging from -0.0033 yr-1 to -0.0059 yr-1). In all regions, the trends observed during summer are mainly driven by an increase in total carbon that is consistent with the accumulation of anthropogenic carbon evaluated below the summer mixed layer.

How to cite: Leseurre, C., Lo Monaco, C., Reverdin, G., Metzl, N., Fin, J., and Mignon, C.: CO2 increase and ocean acidification in the Southern Indian Ocean over the last two decades, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12750, https://doi.org/10.5194/egusphere-egu21-12750, 2021.

Peter Landschützer, Toste Tanhua, and Stefan Raimund and the Team Malizia and Team Newrest

The surface partial pressure of carbon dioxide (pCO2) is one of the main quantitates determining the ocean sink strength for CO2 and knowledge of surface ocean pCO2 plays a vital role in monitoring the global carbon budget. However, measuring pCO2 via infrared absorption requires repeated calibration and drift corrections, and therefore ships are still the major platform for these measurements. Given the limited number and availability of pCO2 observations, scientists have fostered collaborations with industrial partners, participating in the Ships of Opportunity (SOOP) program, to collect valuable pCO2 measurements. One fleet, however, has thus far been largely overlooked: sailing yachts. Modern sensor technology to-date allows for low weight and low energy consumption equilibrator systems that can be successfully mounted on recreational and high-performance sailing yachts with good quality data. Here we present the first results from 3 years of autonomous measurements aboard two IMOCA yachts, Seaexplorer -Yacht Club de Monaco (previously Malizia) and Newrest –Art & Fenêtres using a SubCtech flat membrane equilibrator system. First results indicate that sailing yachts provide crucial high frequency measurements to study open and coastal ocean systems, are well suited to study mesoscale variations in the ocean carbon sink and provide measurements beyond industrial shipping routes (e.g. the Southern Ocean). In summary, sail yachts are a promising way forward in order to complement the current observing system for the global ocean carbon cycle in a changing climate.

How to cite: Landschützer, P., Tanhua, T., and Raimund, S. and the Team Malizia and Team Newrest: Sailing meets Science, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1471, https://doi.org/10.5194/egusphere-egu21-1471, 2021.

Pradeebane Vaittinada Ayar and Jerry Tjiputra

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.

Global ocean interior carbon
Lydia Keppler, Peter Landschützer, Nicolas Gruber, and Siv K. Lauvset

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.


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.

Jens Daniel Müller, Donghe Zhu, Luke Gregor, Are Olsen, Nico Lange, Siv Lauvset, Toste Tanhua, Masao Ishii, Fiz Fernandez Perez, Brendan Carter, Richard Feely, Rik Wanninkhof, and Nicolas Gruber

Surface ocean pCO2-based estimates and models indicate that the ocean sink for anthropogenic CO2 (Cant) has continued to increase unabatedly over the past decade. However, the most recent global and observation-based estimate of the accumulation of Cant in the ocean interior by Gruber et al. (2019) does not extend beyond 2007, preventing an independent assessment of this increase in the magnitude of the sink.

Here, we exploit about 50,000 additional observations of dissolved inorganic carbon (DIC) and other relevant biogeochemical parameters, to extend the Gruber et al. analysis based on the eMLR(C*) method to the 2010s. These data were collected from all major ocean basins over the past decade by GO-SHIP and associated programs, and assembled through GLODAPv2.2020 into an internally consistent data product. We refine the eMLR(C*) method in three ways to achieve the updated storage estimates: (1) the uncertainty assessment is improved, based on a coupled analysis of observations and synthetic data generated from an ocean biogeochemical model, (2) the robustness of the multiple linear regression models is increased, using more stringent predictor and model selection procedures, and (3) the mapping of the Cant fields relies on a MLR ensemble approach that takes into account co-occurring temporal changes of the predictor variables salinity, temperature and oxygen.

Initial results show that the ocean has continued to act as a strong Cant sink with an average uptake rate of 2.8 ± 0.3 Pg C yr-1 between the reference years 2007 and 2015. This represents a small increase in rate compared to 2.6 ± 0.3 Pg C yr-1 determined for the 1994 through 2007 period. This increase is slightly smaller than expected on the basis of the growth of atmospheric CO2 over the period, but associated uncertainties are too large to make a conclusive statement about whether the ocean carbon sink is slowing down. Initial analyses of the synthetic data indicate that variable ocean circulation and limited sampling, especially the small number of cruises in the Indian Ocean, represent the biggest sources of uncertainty for the eMLR(C*)-based estimate. However, our preliminary sink estimate is in good agreement with recent air-sea CO2 flux-based uptake estimates, based on an ensemble of surface pCO2 interpolation techniques once these fluxes are adjusted for the river carbon input driven outgassing of natural CO2.

How to cite: Müller, J. D., Zhu, D., Gregor, L., Olsen, A., Lange, N., Lauvset, S., Tanhua, T., Ishii, M., Perez, F. F., Carter, B., Feely, R., Wanninkhof, R., and Gruber, N.: The continued accumulation of anthropogenic carbon in the global ocean during the 2010s, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10161, https://doi.org/10.5194/egusphere-egu21-10161, 2021.

Climate projections
Enhui Liao, Laure Resplandy, Junjie Liu, and Kevin Bowman

El Niño events weaken the strong natural oceanic source of CO2 in the Tropical Pacific Ocean, partly offsetting the simultaneous release of CO2 from the terrestrial biosphere during these events. Yet, uncertainties in the magnitude of this ocean response and how it will respond to the projected increase in extreme El Niño in the future (Cai et al., 2014) limit our understanding of the global carbon cycle and its sensitivity to climate. Here, we examine the mechanisms controlling the air-sea CO2 flux response to El Niño events and how it will evolve in the future, using multidecadal ocean pCO2 observations in conjunction with CMIP6 Earth system models (ESMs) and a state‐of‐the‐art ocean biogeochemical model. We show that the magnitude, spatial extent, and duration of the anomalous ocean CO2 drawdown increased with El Niño intensity in the historical period. However, this relationship reverses in the CMIP6 projections under the high emission scenario. ESMs project more intense El Niño events, but weaker CO2 flux anomalies in the future. This unexpected response is controlled by two factors: a stronger compensation between thermally-driven outgassing and non-thermal drawdown (56% of the signal); and less pronounced wind anomalies limiting the impact of El Niño on air-sea CO2 exchanges (26% of the signal). El Niños should no longer reinforce the net global oceanic sink in the future, but have a near-neutral effect or even release CO2 to the atmosphere, reinforcing the concurrent release of CO2 from the terrestrial biosphere.

How to cite: Liao, E., Resplandy, L., Liu, J., and Bowman, K.: More extreme El Niño events reduce ocean carbon uptake in the future, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1756, https://doi.org/10.5194/egusphere-egu21-1756, 2021.

Timothée Bourgeois, Nadine Goris, Jörg Schwinger, and Jerry Tjiputra

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.

James Orr and Lester Kwiatkowski

Ocean acidification implies long-term changes in ocean CO2 system variables modulated by changes in seasonal amplitudes. Further modulation, yet unexplored, may come from changes in timing of the annual cycle. For the CO2 partial pressure (pCO2), a winter high and summer low are observed in Arctic Ocean surface waters because thermal effects are outweighed by those from biology. Here the same timing was found with 9 Earth system models under historical forcing. Yet under a high-end CO2 emission scenario, those models project that the summer low (relative to the annual mean) eventually reverses sign across most of the Arctic Ocean. In most models, that sign reversal inverses the chronological order of the annual high and low. The high moves from spring to summer and the low moves from summer to spring. The cause is the projected dramatic warming in summer sea surface temperature provoked by earlier retreat of seasonal sea ice. The increase in the summer pCO2 extreme over this century is 29±9% greater than if there had been no change in seasonal timing, only the enhanced sensitivity of pCO2 to its driving variables. Thus the projected change in extreme summer pCO2 is 150±50 μatm higher. Outside of the Arctic Ocean, projected changes in seasonal timing of pCO2 are small.

How to cite: Orr, J. and Kwiatkowski, L.: Projected disruption in seasonal timing of Arctic Ocean pCO2, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14935, https://doi.org/10.5194/egusphere-egu21-14935, 2021.

Jens Terhaar, Olivier Torres, Timothée Bourgeois, and Lester Kwiatkowski

The uptake of anthropogenic carbon (Cant) by the ocean leads to ocean acidification, causing the reduction of pH and the calcium carbonate saturation states of aragonite (Ωarag) and calcite (Ωcalc). 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  Cant and ocean acidification in the Arctic Ocean over the 21st 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 Ωarag/calc is reduced by 44–64 %. The strong reduction in projection uncertainties of Ωarag/calc can be attributed to compensation between Cant uptake and alkalinity reduction in the latest models. Specifically, ESMs with a large increase in Arctic Ocean Cant over the 21st century tend to simulate a relatively weak concurrent freshening and alkalinity reduction, while ESMs with a small increase in Cant simulate a relatively strong freshening and concurrent alkalinity reduction. Although both mechanisms contribute to Arctic Ocean acidification over the 21st century, the increase in Cant 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 Cant inventory, is found to generally hold in CMIP6. However, a coincident constraint on Arctic declines in Ωarag/calc is not apparent in the new generation of models. This is due to both the reduction in Ωarag/calc projection uncertainty and the weaker direct relationship between projected changes in Arctic Ocean Cant and Ωarag/calc. In CMIP6, models generally better simulate maximum sea surface densities in the Arctic Ocean and consequently the transport of Cant into the Arctic Ocean interior, with simulated historical increases in Cant 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.