OS1.10

Giant icebergs can greatly impact the mass, freshwater and nutrient budgets of the ocean. They can deposit large amounts of freshwater at great distances from their origins, impacting upper-ocean stratification and mixing, and they can be important vectors for micronutrient delivery with impacts on primary production and carbon drawdown. Their impacts on advection, productivity and blocking of flows can be critical for zooplankton and regional ecosystem functioning, with consequences for higher trophic levels and local fisheries. Their breakouts from ice shelves create new opportunities for biological colonisation and carbon sinks and their collisions with the seabed (iceberg scour) can shape local and regional benthic biodiversity patterns and influence carbon sequestration.

In 2017, the A68 iceberg (around 6000 km²) calved from the Larsen C Ice Shelf on the Antarctic Peninsula. It subsequently moved eastward and northward, crossing the Scotia Sea to move, virtually intact, to within 300 km of the island of South Georgia (SG) in late 2020. This caused concern, following the impact of a previous iceberg, A38, on the SG ecosystem in 2003-2004. Further, given the advances in observing technology since the time of the previous iceberg, it afforded an unparalleled opportunity to study in detail the impacts of giant bergs on the ocean physical, biogeochemical and biological systems.

Diverse datasets were collected in response to this event. A research cruise on RRS James Cook was mobilised, to study the iceberg as it approached SG and fragmented into multiple smaller pieces. These measurements included physical parameters (including oxygen isotopes to inform on freshwater sources), dissolved inorganic nutrients, biosilica concentration, and composition of the phytoplankton community to inform bloom dynamics and primary production by the input of terrigenous material. Ocean gliders, deployed from the ship, surveyed the largest iceberg fragment in extremely close proximity and followed this for the remainder of its life, deconvolving the iceberg influence from frontal dynamics and assisting in understanding meltwater influence. Concurrently, Earth Observation (EO) techniques were employed including Sentinel-1 SAR imagery, Planet Labs very high-resolution optical imagery, MODIS Aqua and Terra imagery and satellite radar and laser altimetry. A sediment trap deployed on a mooring downstream of SG will be utilised to investigate the carbon export from the cruise period to that of the previous 10 years while enhanced observations on higher predator colonies will compare their foraging paths and breeding performance to those of previous years.

This presentation will discuss preliminary findings from the study of A68, including EO-derived quantifications of changing iceberg morphology, ice loss from fragmentation and basal melting, and the significance of fractures in dictating collapse fissures. Physical oceanographic data from the ship and gliders are used to determine the impact on water column stability, mixing and circulation on a range of scales. Biogeochemical and biological data reveal the impact of interacting processes on phytoplankton community biomass and species composition. Ecosystem implications and future directions of investigation will be outlined.

How to cite: Lucas, N., Brearley, A., Abrahamsen, P., Meredith, M., Hendry, K., Manno, C., Liszka, C., Gerrish, L., Shepherd, A., Braakmann-Folgmann, A., Fleming, A., Ratcliffe, N., Collins, M., Murphy, E., Barnes, D., and Tarling, G.: Physical, biogeochemical and ecological impacts of giant icebergs: a multidisciplinary study of iceberg A68 near South Georgia, Southern Ocean , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3914, https://doi.org/10.5194/egusphere-egu22-3914, 2022.

Maud Rise polynyas are rare events in the Weddell Sea (Atlantic sector of the Southern Ocean) that cause deep vertical mixing within the ocean column and large surface fluxes of heat with large impacts on  the local Weddell gyre circulation and on the Antarctic bottom water properties. Here we use a 1/12^o ocean reanalysis product to assess the dominant drivers of ocean stratification leading up to the polynya event of 2016 and 2017 in Maud Rise, Weddell Sea. We carry out a potential vorticity (PV) budget to identify the dynamical components of the regional circulation responsible for changes in ocean stratification that culminated in the formation of the 2017 polynya. During 2015, an exceptionally strong (about 2x that of the previous three years) buoyancy-driven destratification led to a shoaling of the pycnocline, and the restratification at the end of 2015 remained weak. During 2016 and 2017, the buoyancy-driven destratification decreased in strength, becoming weakest during the polynya of 2017. The destratification was once again strong in 2018, but this was balanced by a stratifying forcing from the surface stress and advective components, the latter of which was associated with a transport of denser (more saline and cooler) subsurface waters from the flanks of Maud Rise. These denser subsurface waters maintained a strong stratification through 2018. These results show how interannual anomalies in local sea ice production and regional circulation can promote or inhibit the formation of polynyas in the region. Furthermore, it suggests that the Maud Rise polynya opened in 2017 following a chain of perturbations that started at least back in 2015, contrary to the common view that the polynya was initiated solely  by a series of short-lived storms in 2017.

How to cite: Narayanan, A., Gülk, B., Roquet, F., and Garabato, A.: The oceanic drivers of the 2017 Maud Rise Polynya, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9100, https://doi.org/10.5194/egusphere-egu22-9100, 2022.

The large-scale mean horizontal circulation of the Weddell Gyre was determined solely from Argo floats drifting throughout the gyre since 2002. The circulation describes an elongated, double-celled gyre, where the eastern sub-gyre is stronger and subject to mesoscale variability in comparison to the considerably weaker western sub-gyre. Since positive long-term nutrient trends across the western sub-gyre have been associated with an increase in upwelling, this study aims to compare long-term nutrient trends in the western sub-gyre, from Kapp Norvegia to Joineville Island, to those along the Prime Meridian section, spanning the eastern sub-gyre. We find the strongest trends in surface Silicate in the central part of the western sub-gyre, where the horizontal circulation is weakest. Across the eastern sub-gyre, along the Prime Meridian, the strongest Silicate trends occur in the westward flowing southern limb, south of Maud Rise. This suggests that there are different dynamical causes of the nutrient trends in the east versus the west, since the strongest upwelling at the Prime Meridian occurs north of Maud Rise, where some of the lowest long term trends in nutrients were observed. We hypothesise that while increased upwelling may be the cause of positive long-term nutrient trends in the western Weddell Gyre, mesoscale variability and convection associated with Maud Rise in the eastern Weddell Gyre have a larger impact on nutrient concentrations, making long-term trends more challenging to extract.

How to cite: Reeve, K., Hoppema, M., Kanzow, T., Boebel, O., Geibert, W., and Strass, V.: How does the Weddell Gyre circulation influence long-term trends in nutrient concentrations?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9990, https://doi.org/10.5194/egusphere-egu22-9990, 2022.

Subantarctic Mode Water (SAMW) is a voluminous water mass that forms on the equatorward side of the Antarctic Circumpolar Current. The subduction and export of SAMW plays an important role in the global redistribution of heat, freshwater, nutrients, and dissolved gases such as oxygen and carbon dioxide. Recently the global Argo program of profiling floats provided for the first time near-global coverage of temperature and salinity in the upper 2000 m, revealing basin-wide spatial patterns of strong interannual SAMW variability. The same observations also indicate variations on decadal time scales. Combined with the output from an ocean state estimate, we investigate the mechanisms that drive the regional distribution of decadal variability of the SAMW.  We show that the decadal variability of SAMW volume and formation rate is strongly correlated with the decadal variability in the atmospheric circulation, in particular the zonal sea-level pressure gradients, governing the meridional wind component and meridional heat and moisture redistribution. These findings imply that strong quasi-decadal variability of surface heat and freshwater fluxes also governs the regional uptake of anthropogenic heat and carbon dioxide by SAMW.

How to cite: Cerovecki, I. and Haumann, A.: Mechanisms for decadal Subantarctic Mode Water variability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13277, https://doi.org/10.5194/egusphere-egu22-13277, 2022.

Water-mass transformation in the Weddell Sea is responsible for the generation of 50-70% of Antarctic Bottom Water exported to the global deep ocean, with effects for the deep marine sequestration of atmospheric CO₂. Uncertainties in the dynamics of this system urgently need to be addressed to assist with modelling the carbon cycle in the southern high latitudes, and identifying whether the Weddell Sea and the Southern Ocean may act as either a carbon source or sink as the global climate shifts.

In this study, we used sequential acid-reductive leaching and total digestion to obtain neodymium (Nd), lead (Pb), and uranium (U) concentrations and isotopic compositions from both the authigenic and the detrital fractions of sediments from a suite of long-cores and surface sediments around the Weddell Sea. Paired isotope analyses were carried out to reconstruct bottom water conditions during deposition, and determine the sedimentary provenance of lithogenic detritus. The combination allows us to observe the relationship between lithogenic and authigenic phases. Authigenic Nd and Pb isotope signatures were interpreted to reflect pore-water compositions, affected by a combination of bottom-water composition, lithology, and element release from sediments into the pore-water and overlying water column. We further assess whether authigenic U may serve as a proxy for bottom-water oxygenation and ocean productivity at our Southern Scotia Sea sites, giving insight to deep-ocean ventilation and bottom-water export rates from the Weddell Sea.

Our results suggest that detrital isotopic records indicate an increase in sediment delivery from the East Antarctic to the northwestern Weddell Sea during the deglacial. We hypothesize that this was the result of a strengthening in the Weddell Gyre or Antarctic Circumpolar Current at this time. Here, we present an updated dataset of new authigenic and detrital measurements from the Weddell Sea investigating this hypothesis, and we unveil new details of the dynamic nature of Weddell Sea circulation and ice-ocean interactions over the last 30 kyr.

How to cite: Bollen, M., Jaccard, S., Gutjahr, M., Müller, J., and Blaser, P.: Reconstructing Weddell Sea current variability since the LGM: insights from authigenic and detrital radioisotope analyses of marine sediments., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6923, https://doi.org/10.5194/egusphere-egu22-6923, 2022.

The Southern Ocean is recognized as a potential cause of the lower atmospheric concentration of CO₂ during ice ages, but the mechanism is debated. In the ice age Antarctic Zone, biogeochemical paleoproxy data suggest a reduction in the exchange of nutrients (and thus water and carbon) between the surface and the deep ocean. We report simple calculations with those data indicating that the decline in the supply of nutrients during peak glacials was extreme, >50% of the interglacial rate. Weaker wind-driven upwelling is a prime candidate for such a large decline, and new, complementary aspects of this mechanism are identified here. First, reduced upwelling would have resulted in a “slumping” of the pycnocline into the AZ. Second, it would have allowed diapycnal mixing to “mine” nutrients out of the upper water column, possibly causing an even greater slumping of the vertical nutrient gradient (or “nutricline”). These mechanisms would have reduced shallow subsurface nutrient concentrations, decreasing wintertime resupply of nutrients to the surface mixed layer, beyond the reduction in upwelling alone. They would have complemented two changes previously proposed to accompany a decline in upwelling: (1) halocline strengthening and (2) reduced isopycnal mixing in the deep ocean. Together, the above changes would have encouraged declines in the nutrient content and/or the formation rate of new deep water in the AZ, enhancing CO₂ storage in the deep ocean.

How to cite: Fripiat, F., Sigman, D., Ai, X., Studer, A., Kemeny, P., Hain, M., Wang, X., Ren, H., Haug, G., and Martinez-Garcia, A.: The Southern Ocean during the ice ages: A slumped pycnocline from reduced wind-driven upwelling?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2047, https://doi.org/10.5194/egusphere-egu22-2047, 2022.

The Southwest Atlantic section of the Southern Ocean is a highly energetic confluence zone where Pacific and Antarctic waters flow via the Antarctic Circumpolar Current (ACC) to merge with waters from the Atlantic Ocean and contribute to the global overturning circulation. However, there is an insufficient understanding of sub-mesoscale variability in the region. Such processes are known to play an important role in the vertical and lateral exchange of water masses, along with tracers such as carbon, atmosphere-ocean exchange, ocean productivity, and the mixing budget necessary to complete the overturning circulation. In particular, observations of the subsurface structure of frontal systems on high spatial scales are currently lacking, with typical hydrographic transects being too coarse to resolve sub-mesoscale processes and ACC filaments.

Here we present the first multichannel seismic images of ocean finestructure to the north of the North Scotia Ridge in the Southern Ocean, which cross several frontal systems and bathymetric features. High-resolution (O(10m)) sections of sub-surface thermohaline structure are revealed and analysed by combining the acoustic information with hydrographic (CTD, XBT and ARGO floats), current velocity (VMADCP) and satellite altimetry data. In addition, diapycnal mixing and potential vorticity estimates are generated from acoustic data. The sections reveal an intricate and complex pattern of oceanic finestructure: very high thermohaline gradients are present in shallow and intermediate waters of up to 700 m depth associated with Subantarctic Surface Water, Subantarctic Mode water and a mix of Antarctic Intermediate Water and Antarctic Surface Water; curving features, lenses and filaments with length scales of 100m-10km are found at the Polar Front and Southern ACC front; and steep continuous gradients with separate filaments are typically present in deeper sections (up to 2000 m) associated with Circumpolar Deep Water. Furthermore, acoustic reflections provide evidence that bathymetric features like the Maurice Ewing Bank or the Northeast Georgia Rise disrupt the flow of intermediate and deep water in the region and enhance diapycnal mixing.

How to cite: Ehmen, T., Sheen, K., Watson, A., Brearley, A., Palmer, M., Roper, D., and Thompson, A.: High resolution acoustic imaging of frontal dynamics and thermohaline finestructure in the Southwest Atlantic sector of the Southern Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-411, https://doi.org/10.5194/egusphere-egu22-411, 2022.

The southern boundary of the Antarctic Circumpolar Current (ACC) is often associated with the southern limit of Upper Circumpolar Deep Water, and so forms the boundary between warm ACC waters and colder waters within the marginal seas of Antarctica. Strong density gradients across the southern boundary constitute the frontal jet and are thought to modulate the heat transport across the southern boundary. It is well known that eddies cross the fronts of the ACC and are advected downstream, but how does an eddy interact with the southern boundary of the ACC? Does it change its frontal structure? Does it impact the intensity of the frontal jet? Can changes of the southern boundary’s frontal structure impact mixing? These are questions that we aim to discuss.

As part of the Robotic Observations And Modelling in the Marginal Ice Zone (ROAM-MIZ) project, profiling ocean glider observations at the Greenwich Meridian between 54-57 °S from the 20^th of October 2019 to the 18^th of February 2020 provide a unique data set of 5 highly resolved hydrographic transects that cross the southern boundary repeatedly. Θ/S diagrams from the hydrographic transects, maps of absolute dynamic topography and dive average currents are used to identify the location, properties and rotational direction of eddies crossing the meridional transects in close proximity to the southern boundary and the frontal jet. We demonstrate that a cyclonic eddy crossing the meridional transect significantly impacts the southern boundary's frontal structure. While the eddy is crossing the meridional transect, density gradients are strengthened and geostrophic velocities show a narrow and strong frontal jet (~50 km wide with velocities of ~80 cm/s). Shortly after the eddy has crossed the meridional transect, density gradients are weakened and geostrophic velocities show a broadened and weakened frontal jet (~75 km wide with velocities of ~60 cm/s). Mixing length scales (the length at which a water parcel can move before mixing laterally) are calculated for all transects with L_mix=Θ_rms/(∇_n Θ_m)  (Θ_rms a measure of temperature fluctuations , ∇_n the gradient along density surfaces  and Θ_m mean temperature field). Values of L_mix are near zero across the frontal jet while the eddy is crossing and near 40 km after the eddy has crossed the meridional transect. The increased mixing length scales indicate that the exchange of water parcels between ACC waters and waters further south is increased after the eddy has crossed the meridional transect.

How to cite: Oelerich, R., Heywood, K. J., Damerell, G. M., Swart, S., and du Plessis, M.: The Antarctic Circumpolar Current’s Southern Boundary at the Greenwich Meridian, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3614, https://doi.org/10.5194/egusphere-egu22-3614, 2022.

The Southern Ocean is a critical component of Earth’s climate system, but its remoteness makes it challenging to develop a holistic understanding of its processes from the small scale to the large scale. The Antarctic Circumnavigation Expedition (ACE, austral summer 2016/2017) surveyed a large number of variables describing the state of the ocean and the atmosphere, the freshwater cycle, atmospheric chemistry, and ocean biogeochemistry and microbiology. This circumpolar cruise included visits to 12 remote islands, the marginal ice zone, and the Antarctic coast.

Here, we use 111 of the observed variables to study the latitudinal gradients, seasonality, shorter-term variations, geographic setting of environmental processes, and interactions between them over the duration of 90 days. To reduce the dimensionality and complexity of the dataset and make the relations between variables interpretable we applied an unsupervised machine learning method, the sparse principal component analysis (sPCA), which describes environmental processes through 14 latent variables.

Our results provide a proof of concept that sPCA with uncertainty analysis is able to identify temporal patterns from diurnal to seasonal cycles, as well as geographical gradients and “hotspots” of interaction between environmental compartments. Our analysis provides novel insights into the Southern Ocean water cycle, atmospheric trace gases, and microbial communities. More specifically, we

• identify the important role of the oceanic circulations, frontal zones, and islands in shaping the nutrient availability that controls biological community composition and productivity;
• find that sea ice controls sea water salinity, dampens the wave field, and is associated with increased phytoplankton growth and net community productivity possibly due to iron fertilisation and reduced light limitation;
• elucidate the clear regional patterns of aerosol characteristics, stressing the role of the sea state, atmospheric chemical processing, and source processes near hotspots for the availability of cloud condensation nuclei and hence cloud formation.

A set of key variables and their combinations, such as the difference between the air and sea surface temperature, atmospheric pressure, sea surface height, geostrophic currents, upper-ocean layer light intensity, surface wind speed and relative humidity played an important role in our analysis, highlighting the necessity for Earth system models to represent them adequately.

In conclusion, our study highlights the use of sPCA to identify key ocean–atmosphere interactions across physical, chemical, and biological processes and their associated spatio-temporal scales. It thereby fills an important gap between simple correlation analyses and complex Earth system models.

The paper and links to data are available here: https://esd.copernicus.org/articles/12/1295/2021/

How to cite: Schmale, J. and the ACE-DATA Team: Exploring the coupled ocean and atmosphere system with a data science approach applied to observations from the Antarctic Circumnavigation Expedition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1611, https://doi.org/10.5194/egusphere-egu22-1611, 2022.

The Southern Ocean (SO) dynamics, and the various fronts of the Antarctic Circumpolar Current in particular, are well known to display a very energetic variability covering a wide range of spatial and temporal scales. Since a substantial fraction of such variability is known to be intrinsic, and therefore basically chaotic, predictability in this part of the world ocean is particularly poor.

In this context, the YOPP-endorsed IPSODES project of the Italian “Programma Nazionale di Ricerche in Antartide” (PNRA) is aimed at improving process understanding concerning the predictability of the SO dynamics through ensemble simulation (ES) hindcasts analyzed by means of various statistical techniques supported by dynamical interpretations, with special focus on multiscale interactions linking high-frequency (up to seasonal) and low-frequency (interannual and larger) variability. IPSODES uses existing state-of-the-art eddy-permitting global ocean-sea-ice model ESs and coupled global atmosphere-ocean-sea-ice model ESs developed for decadal climate predictions. Moreover, new ESs performed with an ocean model specifically developed for IPSODES are carried out: sensitivity numerical experiments to assess model uncertainty are performed with these new simulations. The study of transport of marine debris provides an application of such modelling effort, and contributes also to model validation through the use of an available valuable data set.

This contribution illustrates advances achieved so far in IPSODES towards improving our understanding of the predictability properties of oceanic variability of the SO dynamics.

How to cite: Zanchettin, D., Pierini, S., Aliani, S., Rubino, A., Zambianchi, E., Viana Barreto, R., de Ruggiero, P., and Colella, A.: Predictability of the Southern Ocean dynamics through ensemble simulation hindcasts, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2229, https://doi.org/10.5194/egusphere-egu22-2229, 2022.

A technique has been developed for assessing the linear long-term change in the structure of the gradient field of the Absolute Dynamic Topography (ADT) based on satellite altimetry data distributed on the website https://marine.copernicus.eu. This structure is understood as the alternation in the meridional direction of the zones of increased values ​​of the absolute values of the ADT gradient (jets) and the zones of their reduced values ​​(interjet spaces). The technique uses linear regression analyzes and makes it possible to calculate the meridional shift in the structure of the gradient field of the ADT and the change in the absolute values of the gradient of the ADT, as well as to estimate the calculation error.

Two 26-year series of dependences of the mean annual absolute values ​​of the ADT gradient on latitude and on the ADT have been analyzed. Analysis of the dependence on latitude showed that in the ACC band (42°–57°S), there are three zones of increased gradients conventionally corresponding to the cores of the Subantarctic (SAC), South Polar (SPC) and South Antarctic (SthAC) currents. Analysis of the dependence on ADT showed that in the ACC band (-130–20 cm in ADT units) there are four such zones; an additional zone is observed in the SPC. In general, in the ACC band for 26 years of observations, a shift in the structure of the gradient field of the ADT in latitude by 0.05±0.10° to the north is noted. At the same time, in the zones of SAC, SPC, and SthAC the shifts on average are 0.16°±0.15 ° to the south, 0.30°±0.14° to the north and 0.03°±0.26° to the north, respectively. The extreme values ​​of the shift in the SAC and SPC zones reach 0.4° to the south and 1.4° to the north, respectively. In the ACC band relative to the ADT, a positive shift in the structure of the gradient field of the ADT is observed amounting to 8.3±1.0 cm. This shift is mainly due to the corresponding increase in the ocean level at geographic points. However, for separate zones within the ACC, the shift can differ significantly from the mentioned value due to the meridional shift of the structure of the ADT gradient in geographic coordinates. In particular, in the boundary zone between SAC and SPC it reaches 17 cm. In the ACC band, an increase in the absolute value of the ADT gradient is also observed, 1.9±2.7×10^-3 cm/km, which is equivalent to an increase in the ADT difference across the ACC by about 3 cm, which corresponds to the difference in the increase in ADT at geographical points on the southern and northern periphery of the current. At the same time, in the SAC zone, a decrease in the absolute value of the ADT gradient by 8.0±4.2×10^‑3 cm/km is observed, and in the SPC and SthAC, on the contrary, an increase by 10.6±4.3×10^-3 cm/km and 8.2±5.4×10^-3 cm/km, respectively.

How to cite: Tarakanov, R.: The long-term linear meridional shift of the jet structure of the Antarctic Circumpolar Current south of Africa, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1965, https://doi.org/10.5194/egusphere-egu22-1965, 2022.

We examine the current surface biases in sea-surface temperature (SST), sea-ice fraction, and winds over the Southern Ocean in the FOCI climate model and demonstrate various methods to reduce them. We examine and tune biases in both atmosphere-only simulations with ECHAM6 and OpenIFS 43r3 and coupled models FOCI (ECHAM6+NEMO) and FOCI-OpenIFS (OpenIFS + NEMO). Over the Southern Ocean both coupled climate models suffer from a warm SST bias, low sea-ice fraction, and surface westerlies with a maximum too far north. We explore how modifying ocean mixing parameters, air-sea coupling frequency and ice model parameters impacts surface biases. 

Shortening coupling frequency in the FOCI model from 3-hourly to hourly reduces both the warm SST bias and the low sea-ice fraction bias, while the northward bias of the westerly wind maximum is largely unchanged. This suggests that the SST and sea-ice fraction biases are related to a lack of wind gustiness and not the biases in mean the winds. Similarly, reducing the horizontal tracer diffusion in the ocean from 600 m2/s to 300 m2/s also reduces the warm SST bias and the low sea-ice fraction bias. The cooling of the Southern Ocean surface is likely due to a reduced vertical heat transport by the tracer diffusion, which is along iso-neutral surfaces. Combined, both reducing the coupling frequency and re-tuning the horizontal mixing parameters acts to reduce the Southern Ocean surface biases more than either one alone. 

The two coupled models, FOCI and FOCI-OpenIFS, share identical ocean model configurations, NEMO ORCA05, but produces warm SST biases in different ways. OpenIFS suffers from a strong cloud radiative forcing bias which is not existent in ECHAM. Hence, reducing the SST and sea-ice fraction biases in FOCI-OpenIFS requires improvements in the cloud scheme rather than tuning oceanic mixing parameters. 

How to cite: Kjellsson, J., Wahl, S., Martin, T., and Park, W.: Understanding and reducing surface biases over the Southern Ocean in the FOCI climate model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10363, https://doi.org/10.5194/egusphere-egu22-10363, 2022.

Mesoscale eddies play a key role in Southern Ocean dynamics, upwelling and transformation of water masses, the surface heat flux and therefore in storage of heat at deeper layers. To better understand local processes but also basin-scale implications, we apply regional ocean grid refinement to the entire region south of 28˚S in the fully coupled climate model FOCI. This two-way nesting configuration (FOCI-ORION10X) enables us to resolve the entire Southern Ocean at 0.1˚ yielding an eddy-rich simulation in this region whereas eddies are parameterized in the remainder of the global ocean running on a 0.5˚ grid. We contrast our high-resolution simulation with the non-eddying pre-industrial control run of FOCI. Heat uptake and redistribution in mean states of three pre-industrial simulations with FOCI-ORION10X are investigated: one 100 years after starting the model from rest, and two branching off from two different FOCI reference runs without and with regular open ocean deep convection in the Weddell Sea.

Net surface heat fluxes are significantly enhanced by up to 50% in the eddy-rich nested simulations compared to the non-eddying reference simulations. In our simulations, eddy kinetic energy (EKE) is largest in the Brazil–Malvinas Confluence Zone and the Agulhas Current system, regions of large upward surface heat flux, i.e. ocean heat loss. Heat uptake occurs farther south in the region of the Antarctic Circumpolar Current associated with a broad band of enhanced EKE between 45˚S and 55˚S. Explicitly simulating instead of parameterizing eddies also impacts Southern Ocean upwelling and heat convergence at mid-latitudes. We explore and quantify the associated impact on heat storage in three mean states representing different states of Southern Ocean mean temperature and bottom water volume, which affects the meridional overturning circulation strength.

How to cite: Zeller, M. and Martin, T.: Role of mesoscale dynamics in Southern Ocean heat uptake and storage, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4852, https://doi.org/10.5194/egusphere-egu22-4852, 2022.

How to cite: Hague, M., Münnich, M., and Gruber, N.: Coupling between Southern Ocean Heat and Carbon: The role of atmospheric boundary conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13431, https://doi.org/10.5194/egusphere-egu22-13431, 2022.

Anthropogenic activities during the past two centuries have caused an increase in atmospheric CO₂ which has driven a linear increase in oceanic CO₂ uptake. The Southern Ocean (SO, < 35ᵒS) is one of the major uptake areas for anthropogenic CO₂, responsible for ~40% of ocean CO₂ sink. Apart from the linear increase in the CO₂ sinking trend, in the SO pronounced variations have been observed in recent decades, driven by natural processes, but the exact mechanisms behind them are still debated. Aiming to fill this knowledge gap, we investigated the natural drivers of CO₂ flux variations in the SO using existing observation-based datasets between the years 1982-2019. We removed the long-term linear trend in the time series of CO₂ flux and other indexes to focus on decadal variations. We found that two mechanisms explain the interannual to decadal variations in the SO: Ekman upwelling and eddy kinetic energy, by their controls on different components of surface pCO₂ variations. The pattern of variability in Ekman upwelling during the time period studied was markedly circumpolar, and the time series of its 1^st principal component was strongly correlated with the detrended SAM Index (r=0.81, p<0.05). Similarly, leading EOF maps of CO₂ flux anomalies and the components of surface pCO₂ changes (i.e., nonthermal and thermal) show that their variations were dominantly symmetric. As previously shown, weakening of SO CO₂ sink in the 1990s coincides with intense positive SAM episodes. Following the late 1990s, the intensity of SAM decreased, which strengthened the CO₂ sink in the early 2000s. At the same time, the relative contribution of the thermal component grew south of the Polar Front, indicating positive temperature anomalies during this period. Such warming events, following intense and recursive SAM episodes were reported before and were attributed to the increased mesoscale eddy activity in the region. In agreement with these studies, our results show that eddy kinetic energy increased after intense SAM periods with a lagged response of ~2 years, and a positive temperature anomaly in low frequency was observed following these peaks. This warming prevented the CO₂ uptake rate from reaching immediately to its potential strength in the absence of strong westerlies, and explains the growing effect of the thermal pCO₂ component.

How to cite: Yilmaz, E., Bernardello, R., and Martin, A. P.: Natural processes behind the CO2 sink variability in the Southern Ocean during the last three decades, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7827, https://doi.org/10.5194/egusphere-egu22-7827, 2022.

The Southern Ocean plays a major role in both the global oceanic uptake of anthropogenic CO₂ and its interannual variations. The size and origin of the interannual variability in the Southern Ocean CO₂ fluxes is debated. Observation-based estimates suggest a large variability (+/- 0.11 PgC/yr) while Global Ocean Biogeochemistry Models (GOBMs) simulate almost no variability. Studying the air-sea fluxes of O₂ can provide independent information that help resolve this data-model inconsistency. Oceanic O₂ is influenced by the same physical and biogeochemical processes as CO₂, but unlike CO₂, its variability is not masked by a large anthropogenic flux. Here, we used 26 years (1994-2019) of monthly O₂ fluxes from 9 GOBMs. These model outputs were compared to air-sea O₂ fluxes inferred from an atmospheric inversion of precisely quantified changes in atmospheric O₂ and CO₂ levels. The 26-year time series of air-sea O₂ fluxes from all GOBMs and the atmospheric inversion exhibited similar temporal variations. This could be linked to the Southern Annular Mode and its influence on air-sea heat flux forcing that induced large-scale changes in observed wintertime Mixed Layer Depth (MLD). However, the amplitude of the interannual variability in air-sea O₂ fluxes was two times higher in the atmospheric inversion than in GOBMs. It possible that this was induced by the general overestimation of the mean wintertime MLD by the GOBM and subsurface vertical gradients in oxygen saturation lower than observed. Implications of these results for the variability in air-sea fluxes of CO₂ will be discussed.

How to cite: Mayot, N., Le Quéré, C., Manning, A., Willis, D., Gruber, N., Schwinger, J., Séférian, R., Ilyina, T., Hauck, J., Resplandy, L., Bopp, L., Keeling, R., and Rödenbeck, C.: Origin and magnitude of interannual variabilities in Southern Ocean air-sea O2 and CO2 fluxes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12773, https://doi.org/10.5194/egusphere-egu22-12773, 2022.

The Southern Ocean (SO) plays an important role in the uptake, transport and storage of carbon by the global oceans. These properties are dominated by the rise in anthropogenic CO₂ in the atmosphere, but they are modulated by climate variability and climate change. Here we explore the effect of climate variability and climate change in the SO using a combination of modelling and observations to identify climate fingerprints in dissolved inorganic carbon (DIC). We conduct an ensemble of hindcast model simulations using the NEMO-PlankTOM12 global ocean biogeochemical model. We use the model to isolate the changes in DIC due to anthropogenic CO₂ alone and the changes due to climatic drivers (both climate variability and climate change) and determine their relative roles in the emerging total DIC trends and patterns. We analyse the DIC climate fingerprint since 1995, across spatial scales in the SO, and check the extent to which they are detectable in the GLODAPv2.2020 observations. Model results were subsampled to the observations to directly compare the climate fingerprints. Results show that in the surface ocean, both anthropogenic CO₂ and climatic drivers act to increase DIC concentration, with the influence of anthropogenic CO₂ dominating at lower latitudes (<55°S) and the influence of climatic drivers dominating at higher latitudes (>55°S). This pattern is present in all basins. In the subsurface ocean, climatic drivers act to decrease DIC concentration, opposing the influence of anthropogenic CO₂, with a stronger decrease at lower latitudes (<50°S). These patterns resulted in a climate fingerprint specific to SO change and were detectable in the observations. However, the model underestimates the surface DIC increase and the spatial and depth variability of the subsurface DIC decrease. We use the model to directly attribute the climate fingerprint to various climate drivers and discuss timescales for unambiguous detectability of the fingerprint in observations.

How to cite: Wright, R., Le Quéré, C., Willis, D., Bakker, D., and Mayot, N.: Detecting Climate Fingerprints in Southern Ocean Carbon Using a Global Ocean Biogeochemical Model and Observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2239, https://doi.org/10.5194/egusphere-egu22-2239, 2022.

Recent advances in the development of hardware have pushed the explicit resolution of mesoscale (and, using nesting approaches, even sub-mesoscale) processes in global coupled ocean-circulation biogeochemical models within reach. This adds realism to the models in that previously-parameterized processes can now be explicitly resolved. Showcasing examples of our modeling work in the Baltic Sea, the subtropical North Atlantic and the Southern Ocean we will put the relevance of this paradigm to the test. We report on surprisingly small effects of explicitly-resolved mesoscale and even submesoscale features on a variety of domain-averaged entities such as air-sea and carbon cluxes in Boussinesq-approximated general ocean circulation models.

How to cite: Dietze, H. and Löptien, U.: Eddies, Winds, and Carbon in coupled ocean-circulation biogeochemical models: from the Baltic Sea to the Southern Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2980, https://doi.org/10.5194/egusphere-egu22-2980, 2022.

The Southern Ocean accounts for 40% of the uptake of anthropogenic CO₂ by the global ocean, which in turn absorbs a quarter of all anthropogenic CO₂ emissions, mitigating climate change. Despite its importance, sampling of the Southern Ocean is sparse and biased towards the summer months, and consequentially uncertainties in the carbon sink and its variability are largest there. Recently, autonomous platforms have begun to provide year-round coverage of the parameters needed to estimate the Southern Ocean carbon sink; however, these new observations cannot address the historical sparsity. We present a new estimate of the sink to address historically sparse wintertime sampling through interpretation of subsurface summertime observations to produce new ‘pseudo’ wintertime observations of surface fCO₂, boosting the wintertime spatiotemporal coverage by 22% and improving the spatial distribution. We show through a commonly used machine learning technique mapping method, that enhanced wintertime coverage does not significantly alter estimates of the flux or its variability at the sub-basin scale. After adjusting for surface boundary layer temperature effects, we find a strong mean sink south of 35°S of 1.29 ± 0.29 PgC yr^-1 for 2004-2018, consistent with recent independent estimates from atmospheric data.

How to cite: Mackay, N.: New wintertime observations allow re-examination of Southern Ocean carbon sink variability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9764, https://doi.org/10.5194/egusphere-egu22-9764, 2022.

The Southern Ocean plays a fundamental role in in the global carbon cycle and is estimated to absorb  ~40% of anthropogenic carbon-dioxide (CO₂) emissions. Recent studies have highlighted  the potentially large  decadal variability of this uptake, and the uncertainties associated with estimates derived from different  ocean carbon measurement technologies. The majority of these estimates of Southern Ocean CO₂ uptake are commonly derived from ‘bottom-up’ analyses of oceanic carbon measurements. An independent means of estimating air-sea CO₂ fluxes is provided by ‘top-down’ analyses, which employ inverse methods or data assimilation techniques  combining atmospheric CO₂ measurements with numerical transport model analyses. Robust regional flux estimates from such top-down methods require an atmospheric observational network with sufficient spatial coverage. At present, however, long-term measurements of atmospheric CO₂ are only available at a limited number of sites in the Southern Ocean region. Given this sparse atmospheric sampling there is an urgent need for expansion of the current Southern Ocean atmospheric CO₂ measurement network.

The British Antarctic Survey has identified a number of locations (including the sub-Antarctic and South Atlantic Islands of Tristan da Cunha, South Georgia and the Falklands) where new systems for long-term observations of CO₂ could be established. In this analysis we present results from a set of Observing System Sampling Experiments (OSSEs) using the GEOS-Chem atmospheric transport model, in combination with the Local Ensemble Transform Kalman Filter method (Chen et al. 2021) to identify the effectiveness of these locations towards providing improved constraints on Southern Ocean air-sea fluxes.  Our assessment of potential sampling sites is derived from metrics quantifying the uncertainty reduction of regional oceanic CO₂ flux estimates.

References

Chen et al.  (2021) Variability of North Atlantic CO₂ fluxes for the 2000–2017 period estimated from atmospheric inverse analyses. Biogeosciences, 18 (15). pp. 4549-4570. ISSN 1726-4189.

How to cite: Suntharalingam, P., Chen, Z., Jones, A., and Buchanan, D.: Assessing Potential Atmospheric CO2 Monitoring Sites for Improved  Estimation of Southern Ocean CO2 Uptake, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12483, https://doi.org/10.5194/egusphere-egu22-12483, 2022.

The Southern Ocean plays an important role in the climate system by regulating excess CO₂ in the atmosphere. A large part of the anthropogenic CO₂ (C_ant) absorbed in surface waters of the Southern Ocean is isolated from the atmosphere at mid-latitudes through the sinking of mode waters down to 1500m. Since the Southern Ocean CO₂ sink and mode waters formation vary from inter-annual to decadal scales, one would expect the C_ant content in these waters to be also variable. This has been suggested through modeling studies but it is challenging to detect these changes based on observations.  This study attempts to estimate the evolution of C_ant accumulation in mode waters of the Southern Indian Ocean over the period 1985-2019 based on observations from the Global Ocean Data Analysis Project (GLODAPv2_2020) and the long-term monitoring program OISO (Océan Indien Service d’Observations). The comparison of three data-based diagnostic approaches showed the strength of the eMLR(C*) method (Clement and Gruber, 2018) for estimating temporal variations in the accumulation of C_ant. The increase in C_ant estimated between 1985 and 2019 show a relatively good agreement for the three methods in the different types of mode waters identified in the Indian Ocean:  the mean trend is between +1.02 and +1.09 μmol kg^-1 yr^-1 in the Subtropical Mode Water (STMW), between +0.73 and +1.02 μmol kg^-1 yr^-1 in the Subantarctic Mode Water (SAMW) and between +0.25 and +0.51 μmol kg^-1 yr^-1 in the Antarctic Intermediate Water (AAIW). However, on shorter periods we found larger discrepancies between the eMLR(C*) method and the two other techniques (back-calculation and TrOCA), the latter showing larger uncertainties. The mean increase in C_ant between 1994 and 2007 estimated using the eMLR(C*) is +1.34 (± 0.18) μmol kg^-1 yr^-1 in the STMW, +1.05 (± 0.05) μmol kg^-1 yr^-1 in the SAMW and +0.60 (± 0.11) μmol kg^-1 yr^-1 in the AAIW, which is consistent with previous results obtained over the same time period using the same method (Gruber et al, 2019). Interestingly, the trends estimated with this method in recent years (between 2007 and 2017) weakened by about half in all mode waters, in STMW (+0.73 (± 0.20) μmol kg^-1 yr^-1), the SAMW (+0.49 (± 0.20) μmol kg^-1 yr^-1) and AAIW (+0.26 (± 0.42) μmol kg^-1 yr^-1). Due to the important contribution of mode waters in the storage of C_ant, these results could significantly reduce the oceanic inventories of C_ant in recent years at both the regional and global scales. The reduction in C_ant trends in mode waters of the Southern Indian Ocean raises questions on the external/internal processes that control mode waters formation and air-sea CO₂ exchanges in the Southern Ocean at a decadal scale.

How to cite: Barut, G., Clerc, C., Leseurre, C., LoMonaco, C., and Metzl, N.: Recent changes in the accumulation of anthropogenic carbon in mode waters of the Southern Indian Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3858, https://doi.org/10.5194/egusphere-egu22-3858, 2022.

Primary productivity in the Southern Ocean plays a key role in global biogeochemical cycles. While much focus has been placed on phytoplankton seasonality, interannual fluctuations exceed the amplitude of the seasonal cycle across large swaths of the Antarctic Circumpolar Current. Interannual variability of surface chlorophyll, a proxy for phytoplankton biomass, is typically linked to changes in the ocean circulation associated with the Southern Annular Mode (SAM). However, it is important to note that variations in annual mean chlorophyll may reflect processes occurring across a broad range of timescales from sub-seasonal to multi-annual. Here, we apply a timeseries decomposition method to satellite-derived surface chlorophyll in order to separate the low-frequency and high-frequency contributions to the interannual variability. Throughout most of the Southern Ocean, interannual variations are dominated by the sub-seasonal component, which is not strongly correlated with the SAM. The multi-annual component, while correlated with the SAM, only accounts for about 10% of the total chlorophyll variance. This suggests that year-to-year variations in annual mean chlorophyll are related to high-frequency events driven by intermittent forcing at small scales, such as storms and eddies, rather than low-frequency climate variability. Consequently, interannual variations of primary productivity are highly localized and do not remain correlated over large regions.

How to cite: Prend, C., Keerthi, M., Lévy, M., Aumont, O., Gille, S., and Talley, L.: Year-to-year variations of Southern Ocean primary productivity driven by sub-seasonal forcing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6327, https://doi.org/10.5194/egusphere-egu22-6327, 2022.

In the Subantarctic Zone (SAZ) of the southeast Pacific, the densest, coolest, and freshest Subantarctic Mode Water (SAMW) is formed. There, water masses reset their physicochemical characteristics interchanging properties with the atmosphere, and play a critical role in global climate through their impact on the overturning circulation and oceanic heat and carbon uptake. We estimate the magnitude, variability and uncertainty of the air-sea flux of oxygen from five years of hourly observations around the Observatories Initiative (OOI) Southern Ocean mooring.

The magnitude of oxygen fluxes depends greatly on the parameterization used, particularly for high wind events. Hence, there is a need for validation of the high wind speed regime at high latitudes. Surface waters remain undersaturated from autumn to mid-spring, when most of the annual oxygen uptake occurs. We calculate a total annual flux into the ocean of -12.6 ± 3.4 mol m^-2 yr^-1, with a thermal component of -10.3 ± 2.6 mol m^-2 yr^-1 and a non-thermal component of -1.0 ± 0.3 mol m^-2 yr^-1. These results provide the first estimate of oxygen fluxes for the region from high-frequency observations, surpassing previous estimates for the entire SAZ by one order of magnitude.

How to cite: Trucco-Pignata, P., Brown, P., Bakker, D., Venables, H., Naveira-Garabato, A., Carvalho, F., Pabortsava, K., García-Ibáñez, M., White, S., Henson, S., and Martin, A.: Surface oxygen balance in the Subantarctic Mode Water Formation region., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12662, https://doi.org/10.5194/egusphere-egu22-12662, 2022.