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OS1.12

The Southern Ocean around the latitudes of the Antarctic Circumpolar Current is a key region for the vertical and lateral exchanges of heat, carbon and nutrients, with significant impacts on the climate system as a whole. The role of the Southern Ocean as a sink of anthropogenic carbon and heat, and as a source of natural carbon in present and future climate conditions remains uncertain. To reduce this uncertainty, understanding the physical and biogeochemical processes underlying the Southern Ocean internal variability and its response to external forcing is critical. Recent advances in observational capabilities, theoretical frameworks, and numerical models (e.g. CMIP6 simulations) are providing a deeper insight into the three-dimensional patterns of Southern Ocean change. This session will discuss the current state of knowledge and novel findings concerning the role of the Southern Ocean in past, present, and future climates. In particular, it will address physical, biological, and biogeochemical processes, including interior ocean mixing and transport pathways, the cycling of carbon and nutrients, as well as ocean-ice-atmosphere interactions, and their wider implications for lower latitudes and the global climate.

Highlight: Solicited speaker Michael Meredith will report on the outcomes of the Polar Regions chapter of the recent "IPCC Special Report on the Ocean and Cryosphere in a Changing Climate" during this session.

Public information:
Please join our live text chat on the display items. The displays will be discussed in the order outlined in our program: https://tinyurl.com/y88p7g5o

There will be a joined virtual (video) coffee break (15:45-16:15 CEST) between sessions OS1.12 and OS1.13 as well as a follow-up online open bar (18:00- CEST). Please join us. You can find a registration link in the session program.

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Co-organized by BG4/CL4
Convener: Alexander HaumannECSECS | Co-conveners: Ivy FrengerECSECS, Lavinia Patara, Christian Turney
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| Attendance Thu, 07 May, 14:00–15:45 (CEST)

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Session materials Session summary Download all presentations (106MB)

Chat time: Thursday, 7 May 2020, 14:00–15:45

Chairperson: Alexander Haumann, Ivy Frenger, Lavinia Patara
D2737 |
EGU2020-164
| solicited
| Highlight
Michael Meredith, Martin Sommerkorn, Sandra Cassotta, Chris Derksen, Alexey Ekaykin, Anne Hollowed, Gary Kofinas, Andrew Mackintosh, Jess Melbourne-Thomas, Monica Muelbert, Geir Ottersen, Hamish Pritchard, Ted Schuur, Andrew Meijers, Andrew Hogg, Robert Hallberg, Alessandro Tagliabue, Shengping He, and Victoria Peck

Climate change in the polar regions exerts a profound influence both locally and over all of our planet.  Physical and ecosystem changes influence societies and economies, via factors that include food provision, transport and access to non-renewable resources.  Sea level, global climate and potentially mid-latitude weather are influenced by the changing polar regions, through coupled feedback processes, sea ice changes and the melting of snow and land-based ice sheets and glaciers.

Reflecting this importance, the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) features a chapter highlighting past, ongoing and future change in the polar regions, the impacts of these changes, and the possible options for response.  The role of the polar oceans, both in determining the changes and impacts in the polar regions and in structuring the global influence, is an important component of this chapter.

With emphasis on the Southern Ocean and through comparison with the Arctic, this talk will outline key findings from the polar regions chapter of SROCC. It will synthesise the latest information on the rates, patterns and causes of changes in sea ice, ocean circulation and properties. It will assess cryospheric driving of ocean change from ice sheets, ice shelves and glaciers, and the role of the oceans in determining the past and future evolutions of polar land-based ice. The implications of these changes for climate, ecosystems, sea level and the global system will be outlined.

How to cite: Meredith, M., Sommerkorn, M., Cassotta, S., Derksen, C., Ekaykin, A., Hollowed, A., Kofinas, G., Mackintosh, A., Melbourne-Thomas, J., Muelbert, M., Ottersen, G., Pritchard, H., Schuur, T., Meijers, A., Hogg, A., Hallberg, R., Tagliabue, A., He, S., and Peck, V.: Causes and consequences of Southern Ocean change: the IPCC SROCC assessment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-164, https://doi.org/10.5194/egusphere-egu2020-164, 2019

D2738 |
EGU2020-20837
Thomas Rackow, Sergey Danilov, Helge F. Goessling, Hartmut H. Hellmer, Dmitry V. Sein, Tido Semmler, and Thomas Jung

Despite ongoing global warming and strong sea ice decline in the Arctic, the sea ice extent around the Antarctic continent has not declined during the satellite era since 1979. This is in stark contrast to existing climate models that tend to show a strong negative sea ice trend for the same period; hence the confidence in projected Antarctic sea-ice changes is considered to be low. In the years since 2016, there has been significantly lower Antarctic sea ice extent, which some consider a sign of imminent change; however, others have argued that sea ice extent is expected to regress to the weak decadal trend in the near future.

In this presentation, we show results from climate change projections with a new climate model that allows the simulation of mesoscale eddies in dynamically active ocean regions in a computationally efficient way. We find that the high-resolution configuration (HR) favours periods of stable Antarctic sea ice extent in September as observed over the satellite era. Sea ice is not projected to decline well into the 21st century in the HR simulations, which is similar to the delaying effect of, e.g., added glacial melt water in recent studies. The HR ocean configurations simulate an ocean heat transport that responds differently to global warming and is more efficient at moderating the anthropogenic warming of the Southern Ocean. As a consequence, decrease of Antarctic sea ice extent is significantly delayed, in contrast to what existing coarser-resolution climate models predict.

Other explanations why current models simulate a non-observed decline of Antarctic sea-ice have been put forward, including the choice of included sea ice physics and underestimated simulated trends in westerly winds. Our results provide an alternative mechanism that might be strong enough to explain the gap between modeled and observed trends alone.

How to cite: Rackow, T., Danilov, S., Goessling, H. F., Hellmer, H. H., Sein, D. V., Semmler, T., and Jung, T.: Antarctic sea ice decline delayed well into the 21st century in a high-resolution climate projection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20837, https://doi.org/10.5194/egusphere-egu2020-20837, 2020

D2739 |
EGU2020-12247
Marzieh H. Derkani, Katrin Hessner, Stefan Zieger, Filippo Nelli, Alberto Alberello, and Alessandro Toffoli

The Southern Ocean is the birthplace of the fiercest waves on the Earth, which play a fundamental role in global climate by regulating momentum, heat and gas exchanges between the atmosphere and ocean. At high latitudes, waves interact with Antarctic sea ice, another crucial player of the Earth's climate system, modulating its expansion in the winter and its retreat in summer and hence affecting the global albedo. Despite the impact of waves on climate, global wave models are considerably biased in the Southern Hemisphere, due to the scarcity of observations in these remote waters. This is exacerbated in the marginal ice zone, the region of ice-covered water between the compact ice or land and the open ocean, where surface waves, upper ocean and atmosphere interact with sea ice but the dominant physics are still largely unknown. To improve our understanding of physical processes in Southern Ocean and model capabilities, the Antarctic Circumnavigation Expedition (ACE) sailed these waters from December 2016 to March 2017 to acquire wave data (among other climate variables) both in the open ocean and Antarctic marginal ice zone. Observations were gathered using a radar-based wave and surface current monitoring system (WaMoS-II) built on board of the research icebreaker Akademik Tryoshnikov. Here, we discuss how these observations underpin the set up, calibration and validation of the WaveWatch III wave model over a domain covering the entire Southern Hemisphere, therefore spanning from tropical waters to the edge of sea ice (open waters only). The calibrated model will then be used to carry out a thorough assessment of different sea ice modules, to evaluate accuracy of predictions in the marginal ice zone. Test cases of waves-in-ice recorded during the Antarctic Circumnavigation Expeditions will be discussed in details.

How to cite: H. Derkani, M., Hessner, K., Zieger, S., Nelli, F., Alberello, A., and Toffoli, A.: Evaluation of the numerical wave model (WaveWatch III) for wave simulation in the Antarctic marginal ice zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12247, https://doi.org/10.5194/egusphere-egu2020-12247, 2020

D2740 |
EGU2020-7580
Simon Josey, Veronica Tamsitt, Ivana Cerovecki, Sarah Gille, and Eric Schulz

Wintertime surface ocean heat loss is the key driver of Subantarctic Mode Water (SAMW) formation. However, until now there have been very few direct observations of fluxes, particularly during winter. Here, we present results from the first concurrent (2015-17 with gaps), air-sea flux mooring deployments in two key SAMW formation regions: the Southern Ocean Flux Site (SOFS) in the Southeast Indian sector and the Ocean Observatories Initiative (OOI) mooring in the Southeast Pacific sector. Gridded Argo and ERA5 reanalysis provide temporal and spatial context for the mooring observations. Turbulent ocean heat loss is found to be on average 1.5 times larger at the Southeast Indian than Southeast Pacific sites with stronger extreme heat flux events in the Southeast Indian leading to larger cumulative winter heat loss. For the first time, we show that turbulent heat loss events in the Southeast Indian sector occur in two atmospheric regimes (a direct cold air pathway from the south and an indirect pathway circulating dry Antarctic air via the north). In contrast, heat loss events in the Southeast Pacific sector occur in a single atmospheric regime (cold air from the south). On interannual timescales, wintertime anomalies in net heat flux and mixed layer depth (MLD) are often correlated at the two sites, particularly when wintertime MLDs are anomalously deep. Using ERA5, we show that this is part of a larger zonal dipole in heat flux and MLD anomalies present in both the Indian and Pacific SAMW formation regions, associated with anomalous meridional atmospheric circulation. These recent results will be placed in the context of multidecadal variability in the SAMW formation region dominant heat flux patterns over the past 40 years over all 3 sectors of the Southern Ocean (Pacific, Indian and Atlantic).

How to cite: Josey, S., Tamsitt, V., Cerovecki, I., Gille, S., and Schulz, E.: New insights into concurrent air-sea heat flux forcing of Subantarctic Mode Water formation from mooring observations in the Southeast Indian and Southeast Pacific sectors of the Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7580, https://doi.org/10.5194/egusphere-egu2020-7580, 2020

D2741 |
EGU2020-19451
Pearse Buchanan, Robyn Tuerena, Alessandro Tagliabue, Claire Mahaffey, and Raja Ganeshram

Southern Ocean mode and intermediate waters supply nitrate-rich but silicate-poor waters to the lower latitudes, impeding diatom growth throughout the extra-polar ocean and weakening the ocean’s ability to absorb carbon dioxide from the atmosphere. This silicate deficiency is widely attributed to high silicate to nitrate uptake by iron-limited diatoms. Here, we show that nitrification, by rapidly regenerating nitrate in shallow waters, drives the silicate deficiency. Measurements of nitrate dual isotopes and complementary modelling independently suggest that 15-35% of the nitrate within mode waters is generated by nitrification. Our results reveal that without nitrification, the silicate deficiency would disappear, which would allow the diatomaceous niche to expand. Nitrification therefore provides a key buffering service that mitigates against change in the silicate deficit and subsequently restricts diatom dominance to the polar ocean. This insight highlights the critical importance for understanding Southern Ocean processes, such that the large-scale effects of ongoing environmental change may be realised.

How to cite: Buchanan, P., Tuerena, R., Tagliabue, A., Mahaffey, C., and Ganeshram, R.: Nutrient budgets in Southern Ocean mode waters controlled by nitrification, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19451, https://doi.org/10.5194/egusphere-egu2020-19451, 2020

D2742 |
EGU2020-18205
Lydia Keppler, Peter Landschützer, Nicolas Gruber, Siv Lauvset, and Irene Stemmler

We present a monthly climatology of dissolved inorganic carbon (DIC) in the upper 2000 m of the Southern Ocean north of 65°S, based on a 2-step neural network method that establishes and applies statistical relationships between global fields of physical and biogeochemical properties and direct DIC measurements from the GLODAPv2.2019 database from 2004 trough 2017. We test our method using synthetic data from a global hindcast simulation of the HAMOCC ocean biogeochemistry model, and independent observational datasets. At the month and location of biogeochemical floats from the SOCCOM array, our estimate is on average ~10 μmol kg-1 lower than the calculated DIC based on the float measurements. This difference can be partially explained by the difference in time period (SOCCOM floats used: 2014 through 2017; our estimate: 2004 through 2017). We find that the surface seasonal cycle of DIC has a mean amplitude of ~20 μmol kg-1 in the Southern Ocean, and the months of the highest surface DIC concentrations tend to be in austral spring when vertical mixing dominates the seasonal maximum. We also find that the nodal depth of DIC, the depth where the phase of the seasonal cycle of DIC shifts due to photosynthesis near the surface and remineralisation below, partially extends to several hundred meters. Using the nodal depth allows us for the first time to estimate the basin-wide seasonal net community production (NCP) based on direct DIC measurements. We find a mean NCP of ~2 mol C m-2, which is considerably lower than in the temperate northern hemisphere.

How to cite: Keppler, L., Landschützer, P., Gruber, N., Lauvset, S., and Stemmler, I.: Seasonal carbon dynamics in the Southern Ocean based on a neural network mapping of ship measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18205, https://doi.org/10.5194/egusphere-egu2020-18205, 2020

D2743 |
EGU2020-10433
Margaux Brandon, Stéphanie Duchamp-Alphonse, Amaëlle Landais, Elisabeth Michel, Gulay Isguder, and Thomas Extier

During the last 800,000 years, atmospheric CO2 concentrations have varied with an amplitude of more than 100 ppm, with the fastest increases registered during deglaciations. The mechanisms behind the increases of CO2 are still discussed since several parameters are involved. Biological productivity on land and in the ocean played a major role in the variations of atmospheric CO2. Particularly, productivity variations in the Southern Ocean along deglaciations are key because changes in the efficiency of the Soft Tissue Pump (STP) and the Carbonate Counter Pump (CCP) in the Subantarctic Zone significantly impact the exchanges between ocean and atmospheric reservoirs. As calcifying organisms, coccolithophores and planktonic foraminifera represent the major producers of CaCO3 and are therefore good tools to reconstruct past variations of CCP.

Among the last 9 deglaciations, Termination V registers the strongest global productivity (20% higher) compared to the other 8 interglacial periods. Associated with the Mid-Brunhes event, it is followed by the warm MIS 11, the longest interglacial (~ 30 ka). MIS 11 also registers a strong carbonate production in the ocean, most probably favoured by the low eccentricity during this period. Studying the variations of the CCP during this specific period of time is therefore important to better understand its relation with biospheric productivity changes and its impact on atmospheric CO2.

Here we present micropaleontological (coccoliths and foraminifera) and geochemical (CaCO3) data from marine core MD04-2718, located in the Indian sector of the Southern Ocean (48°53 S; 65°57 E) throughout Termination V and MIS 11, that we compared with other productivity data from the Southern Ocean as well as reconstruction of global biospheric productivity data (Δ17O of O2).

 Results show that coccolith and foraminifera abundances and masses increase during Termination V and MIS 11. The good correlation between variations of CaCO3 in the sediment and calcite mass from coccoliths and foraminifera shells proves that exported CaCO3 is essentially of planktonic origin and reveals that CCP significantly increases over this period.

We suggest that the strengthening of CCP through the increase in production and export of calcite associated to coccolith and foraminifera in the Southern Ocean may have contributed to increase the atmospheric CO2 during Termination V and MIS 11, while the strong biological productivity registered during this period would have permitted to maintain the CO2 level relatively low

How to cite: Brandon, M., Duchamp-Alphonse, S., Landais, A., Michel, E., Isguder, G., and Extier, T.: Variations of the Carbonate Counter Pump in the Southern Ocean during the Mid-Brunhes event and their contribution to the global biospheric productivity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10433, https://doi.org/10.5194/egusphere-egu2020-10433, 2020

D2744 |
EGU2020-18595
Luke Roberts, Rhiannon Jones, Matthew Donnelly, and Katharine Hendry

Argo is an array of automated profiling floats, which have allowed the rapid development of high-resolution and high-quality oceanographic data acquisition. The international program has been in operation since the 1990s providing continuous hydrographic data globally. There are now over a million individual float profiles, contributing to our understanding of global ocean physical properties, such as circulation processes at both a local and regional scale. With these innovations come the challenges of data processing, and compilation of user-friendly data products. For example, the Southern Ocean is a critical region that modulates our climate, via heat exchange, carbon storage, biogeochemistry, and primary productivity. An improved quantified understanding of Southern Ocean currents, informed by Argo, must be implemented in policy-relevant high-resolution climate models to advance our understanding of future change.

 

In May 2019, a new collaboration was formed between the Southern Ocean Argo Resource Centre (British Oceanographic Data Centre) and the University of Bristol. The aims were two-fold: to produce a method for characterising Southern Ocean frontal zones using Argo floats, and to train early career researchers in the University sector in data processing and management. We have created a publicly available code that characterises physical features of the Antarctic Circumpolar Current using Argo float profiles, using minimal software, and without the need to access high-performance computers. The code categorises each profile based on the temperature and salinity ‘fingerprints’ of zones between each Southern Ocean front. This allows the user to produce output surface plots from user-specified time-slices and geographic areas, and so compare frontal movement in time and space.

How to cite: Roberts, L., Jones, R., Donnelly, M., and Hendry, K.: A new method for characterising the Antarctic Circumpolar Currents using Argo float temperature and salinity profiles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18595, https://doi.org/10.5194/egusphere-egu2020-18595, 2020

D2745 |
EGU2020-1957
Roman Tarakanov and Alexander Gritsenko

We have analyzed the fine structure of Antarctic Circumpolar Current jets in the Drake Passage based on CTD and SADCP measurements over two hydrographic sections in January 2010 and October–November 2011. Eleven jets with a local horizontal velocity maximum were revealed in 2010, and nine jets were in 2011. These individual jets were various combinations of 12 jets of the Antarctic Circumpolar Current, which we revealed earlier in the region south of Africa on the basis of the section data in December 2009. Daily satellite altimetry data available at http://www.aviso.altimetry.fr were also used to interpret the synoptic patterns of currents over the sections. These results allow us to suggest that the multi-jet structure with a number of jets exceeding nine reported by Sokolov&Rintoul, 2009 is common for the entire circumpolar circle and even for regions with significant contraction of the ACC, such as the Drake Passage. However, the question about the number of jets and its temporal and spatial permanency remains open. Investigation was supported by Russian Foundation of Basic Research grant No 18-05-00283.

How to cite: Tarakanov, R. and Gritsenko, A.: Jets of the Antarctic Circumpolar Current in the Drake Passage Based on Hydrographic Section Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1957, https://doi.org/10.5194/egusphere-egu2020-1957, 2020

D2746 |
EGU2020-13460
Yuri Cotroneo, Lavinia Patara, Milena Menna, Pierpaolo Falco, Jan Klaus Rieck, Giulio Notarstefano, Giannetta Fusco, Giorgio Budillon, and Pierre-marie Poulain

Mesoscale variability and associated eddy fluxes play crucial roles in the ocean dynamics, transport of water mass properties and ecology of the upper ocean. In the Southern Hemisphere, where the nearly zonal flow of the Antarctic Circumpolar Current (ACC) acts as a barrier to the direct poleward transport toward the Antarctica, the eddy flux across the ACC is the main mechanism that guarantees the heat budget and distributes physical and biogeochemical properties between subtropical and polar regions. We focused on a high dynamical region located between the South-West Indian Ridge and the South Pacific Ridge. In this area, the interaction between the ACC and the major bathymetric features produces relatively large values of eddy kinetic energy and eddy heat fluxes as well as a relevant forcing for the ACC path.

The aim of this study is to evaluate the actual efficiency of mesoscale eddies to exchange heat and other properties across the different ACC fronts and to describe the vertical properties of the eddies, their tracks and evolution. To this end, we used in-situ and satellite data in conjunction with a hindcast simulation from 1958 to 2018 performed with a 1/10° ocean biogeochemistry model.

Eddies are identified and tracked in both the model output and altimetry data while their thermohaline properties and vertical extension are described using model outputs and in situ data, which include available repeated XBT sections (i.e. New Zealand – Antarctica and Hobart – Antarctica) and Argo float profiles located inside these structures.

Thanks to the joint analysis of model and observational data, we are able to 1) assess the ability of the 1/10° ocean model of simulating the eddy field properties, and to 2) better interpret the spatial and temporal variability of the observed eddy characteristics in the larger and longer framework of the ocean simulation.

How to cite: Cotroneo, Y., Patara, L., Menna, M., Falco, P., Rieck, J. K., Notarstefano, G., Fusco, G., Budillon, G., and Poulain, P.: Characterization of the ocean mesoscale eddies in the Antarctic Circumpolar Current from in situ, model and remotely sensed data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13460, https://doi.org/10.5194/egusphere-egu2020-13460, 2020

D2747 |
EGU2020-5944
Daan Boot, Rene van Westen, and Henk Dijkstra

The Maud Rise Polynya, a large hole in the Antarctic sea-ice, was first observed in the 1970s and reappeared again in 2017. The general paradigm is that the polynya formed due to deep convection caused by static instability of the water column. There is, however, no consensus on the processes responsible for the initialisation of deep convection. Both atmospheric and oceanic processes have been suggested by observational and model studies. Deep convection is viewed as an irregular event caused by densification of the surface layer. Heat accumulation in the subsurface layer is also considered to be vital for the formation of the polynya. This study investigates the initiation of deep convection using a simple 1D convective model introduced by Martinson et al. (1981) which is further extended with a dynamical subsurface layer. This extended version of the model allows us to study the contribution of both surface- and subsurface forcing on the initiation of deep convection. Two model set-ups with different subsurface characteristics have been used: (1) with a constant subsurface layer; (2) with periodic subsurface accumulation of heat and/or salt. Model set-up 1 results in either one or no polynya events. This does not agree with observations, since multiple polynya events have been observed. Model set-up 2 results in in periodically returning polynyas with the same period as the subsurface accumulation. Therefore, model set-up 2 is able to again multiple events as observed. Adding noise to the simulations does not change the conclusions for both model set-ups. The results suggest that subsurface forcing is a dominant process in Maud Rise Polynya formation. Our results indicate that densification of the surface layer plays a much smaller role than previously assumed by various literature. Based on these results and previous studies, we suggest that subsurface processes govern both the initial formation and reccurrence of the Maud Rise Polynya.

How to cite: Boot, D., van Westen, R., and Dijkstra, H.: A new convective model for the Maud Rise Polynya, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5944, https://doi.org/10.5194/egusphere-egu2020-5944, 2020

D2748 |
EGU2020-5883
Gaston Manta, Sabrina Speich, and Johannes Karstensen

The variability of the Atlantic Meridional Overturning Circulation (AMOC) is known to have considerable impacts on the global climate system. In particular, the South Atlantic is thought to control the stability of the AMOC. That’s why significant resources have been invested in measuring the AMOC in the South Atlantic since 2009 mainly through mooring deployments accompanied by yearly cruise campaigns in the eastern and western boundaries. In January 2017, the RV Maria S. Merian conducted the first hydrographic (GO-SHIP) transect along the SAMOC-SAMBA line located at 34.5°S in the South Atlantic. In this study, we present analyses of the data obtained during the cruise. Volume and heat transports were calculated using the slow varying geostrophic density field and Ekman transport from daily satellite wind stress data. The general structure of these transports is in close agreement with other estimations at near latitudes. Two defined overturning cells are distinguished with differences in the abyssal circulation between the west and east. While the west shows Antarctic Bottom Waters flowing northward (ɣ > 28.27 kg.m-3) and Lower Circumpolar Deep Waters (LCDW, 28.10 kg.m-3 <  ɣ < 28.27 kg.m-3) above flowing southward, only LCDW are found in the east, flowing northward and southward from 6°E, respectively. The section presented a positive mass imbalance. By assuming the latter to be the barotropic transport component we equally distributed it along the vertical in order to satisfy a net-zero transport. The overturning maximum is located at 1250 meters deep and is 18 Sv with net northward heat transport. Moreover, by using the Laxenaire et al., (2018) eddy detection method, we identified the various mesoscale eddies crossed during the cruise. This analysis provided evidence of the presence of 13 cyclonic and 12 anticyclonic eddies. Among the anticyclonic eddies, three were identified as Agulhas rings. One of these eddies was located close to the Brazilian slope and was more than 5 years old since its first detection. During the cruise, a particularly intense cyclonic eddy was crossed near the African coast. The upper layer velocity within the eddies exceeded 1 m.s-1, with an asymmetric stronger northward flow. Although mesoscale eddies dominate the upper 1000 meters circulation, no significant differences were found in the overturning circulation when replacing the upper 1200 meters layer of 34.5°S observed section with a climatology generated with the ARGO profiles located outside eddies.

How to cite: Manta, G., Speich, S., and Karstensen, J.: Overturning Circulation and mesoscale eddies in the first GO-SHIP section at 34.5ºS across the South Atlantic during January 2017, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5883, https://doi.org/10.5194/egusphere-egu2020-5883, 2020

D2749 |
EGU2020-12128
Ivana Cerovecki and Andrew Meijers

The dominant Subantarctic Mode Water (SAMW) formation regions are located in the Indian, and in the Pacific sector of the Southern Ocean. Strong wintertime (Jul-Sep) surface air pressure anomalies with variance maxima at approximately 100°E and 150°W drive a zonal dipole structure in the SAMW formation and thickness, in both the Indian and Pacific sector of the Southern Ocean. This has been documented within gridded Argo data for years 2005-2019. A much weaker surface air pressure anomaly variance maxima is located in the Atlantic Ocean centered at approximately 25°W.

Anomalously strong positive pressure anomalies result in deepening of the wintertime mixed layers and an increase in the SAMW formation in the eastern part of the Pacific and Indian sector; these effects are due to cold southerly winds, strengthened zonal winds and increased surface ocean heat loss. 
Anomalously strong negative pressure anomalies result in shoaling of the wintertime mixed layers and a decrease in SAMW formation in these regions, while at the same time deepening the wintertime mixed layers and increasing SAMW formation in the western Indian Ocean and in the central Pacific.

In years with strong El Nino, the interannual variability of the strength of two surface air pressure anomalies does not co-vary in phase with each other. Strong isopycnal heave in SAMW density range emanates from locations where winter surface air pressure anomalies and mixed layers are most strongly coupled.  

How to cite: Cerovecki, I. and Meijers, A.: Strong atmospheric surface pressure anomalies drive a see-saw in Subantarctic Mode Water formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12128, https://doi.org/10.5194/egusphere-egu2020-12128, 2020

D2750 |
EGU2020-4670
Yu Hong, Yan Du, Tangdong Qu, and Wenju Cai

Analysis of the Argo data reveals that the Subantarctic Mode Water (SAMW) in the South Indian Ocean, characterized by a vertical potential vorticity minimum, decreases by 10% in volume from 2004 to 2015. Most of this volume decrease occurs in the density range 26.8-26.9 kg m-3 which forms southwest of Australia, while a slight volume increase occurs in 26.6-26.8 kg m-3. Further analysis of the data indicates that a reduction of subtropical high and westerly winds in the South Indian Ocean weakens (intensifies) the E-P, heat loss, Ekman pumping and shoals (deepens) the mixed layer southwest of Australia (west of 90°E), which leads the decrease in 26.8-26.9 kg m-3 (increase in 26.6-26.8 kg m-3) by 3 years (see the figure below). This result suggests that the subtropical wind system variation plays an important role in the volume variation of SAMW in the South Indian Ocean in the Argo period.

How to cite: Hong, Y., Du, Y., Qu, T., and Cai, W.: Variability of the Subantarctic Mode Water volume in the South Indian Ocean during 2004-2018, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4670, https://doi.org/10.5194/egusphere-egu2020-4670, 2020

D2751 |
EGU2020-8015
Ivy Frenger, Ivana Cerovecki, and Matthew Mazloff

Deep waters upwell in the Southern Ocean, replete with nutrients. Some of these nutrients enter lighter mode and intermediate waters (MIW), fueling upper ocean productivity in the otherwise nutrient depleted (sub)tropical waters. However some of the upwelled nutrients are retained in the Southern Ocean or leak into denser bottom waters (AABW), making them unavailable for upper ocean productivity. Despite its fundamental importance for the global ocean productivity, this “reshuffling” of nutrients between Southern Ocean water masses, and its driving forces and temporal variability, have not been quantified to date.

We analyze the globally major limiting macronutrient, nitrate (NO3), using the results of a data-assimilating coupled ocean-sea-ice and biogeochemistry model, the Biogeochemical Southern Ocean State Estimate (B-SOSE), for the years 2008 – 2017. Using a water mass framework, applied to five day averaged SOSE output south of 30oS, we quantify the processes controlling NO3 inventories and fluxes. The water mass framework enables us to assess the relative importance of physical processes (such as surface buoyancy fluxes and diapycnal mixing) and biogeochemical processes (such as productivity and remineralization) in driving the transfer of NO3 from upwelling deep waters (CDW) to MIW and AABW, and its interannual variability.

Our results show that two thirds of the NO3 supplied to MIW occurs through lightening, or transforming, of CDW waters during the course of the overturning circulation. The other third of the NO3 supplied to MIW occurs through upward mixing of NO3 from NO3-enriched CDW. This means that physical processes determine the mean MIW NO3 content. Biology does not have a net effect on MIW NO3: while biological uptake draws down the MIW concentration of  NO3 near the surface, remineralization of organic matter compensates for this MIW loss below the surface. Also, we find that the productivity in the subtropical waters south of 30oS is fed through both, the canonical upward mixing of NO3 through the thermocline, and through the near surface supply from MIW. Thus, again, water mass transformation is playing a large role in nutrient distributions. 

In ongoing work, we assess the drivers of variability of the reshuffling of NO3 between water masses and their potential sensitivity to climate change.

How to cite: Frenger, I., Cerovecki, I., and Mazloff, M.: Reshuffling of Nutrients in the Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8015, https://doi.org/10.5194/egusphere-egu2020-8015, 2020

D2752 |
EGU2020-3957
Francois Fripiat, Aflredo Martinez-Garcia, Dario Marconi, Sarah E. Fawcett, Daniel M. Sigman, and Gerald H. Haug

The circulation of the ocean plays a fundamental role in restoring the surface nutrients necessary to maintain global ocean biological production. However, our quantitative understanding of the physical mechanisms that return deep-ocean water and nutrients to the upper ocean is currently limited. The nitrate isotopes are investigated here as a new data constraint on the percentage of gross water transport into the global pycnocline that derives from the Southern Ocean as opposed to the deep ocean (which we term the “pycnocline recipe”). Based on a comparison between large-scale observations of nitrate isotopes and the output of a box model, we estimate that the pycnocline recipe is 75 ± 10%; this result implies that ~ 64% of the nutrients supplied to the low latitude pycnocline pass through the Southern Ocean. Our simulations also highlight the shortcomings of a purely advective view of the ocean’s transport of water and nutrients, confirming that mixing with both the deep ocean and the Southern Ocean ventilating area are key to the exchange of water and nutrients between the pycnocline and higher-density deep and polar surface waters. Our calculations support a pure advective-diffusive balance in the deep ocean. In contrast, in the Southern Ocean, our findings provide independent evidence for the importance of air-sea fluxes of momentum and buoyancy in driving the circulation.

How to cite: Fripiat, F., Martinez-Garcia, A., Marconi, D., Fawcett, S. E., Sigman, D. M., and Haug, G. H.: Nitrate isotopic constraints on routes of nutrient supply to global ocean pycnocline, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3957, https://doi.org/10.5194/egusphere-egu2020-3957, 2020

D2753 |
EGU2020-3950
Jenna Balaguer, Florian Koch, and Scarlett Trimborn

The Southern Ocean (SO) accounts for over 40% of anthropogenically derived CO2 uptake. It is the world’s largest High-Nutrient Low-Chlorophyll (HNLC) region and the scarcity of trace metals such as iron (Fe) drives phytoplankton composition and biomass build up. Besides Fe, manganese (Mn) is the second most abundant trace metal since it is present in the thylakoids. As dissolved manganese (dMn) concentrations in the Atlantic sector of the SO are very low (0.04 nM), phytoplankton growth may not only be limited by Fe but also by Mn availability, a theory previously described by Martin et al. (1990). However, mechanistic studies investigating the effects of multiple trace metals limiting or co-limiting on growth and photosynthesis are lacking. This study focuses on the identification of the Fe-Mn co-limitation of natural phytoplankton assemblages to elucidate the impact of different Fe and Mn additions on species composition. To this end, two shipboard Fe-Mn addition bottle incubation experiments were conducted during the ‘RV Polarstern’ expedition PS97 in the Western and Eastern Drake Passage (DP) in 2016. This study highlights the importance of Mn in the otherwise Fe-limited Drake Passage. From microscopy samples, the addition of Fe and Mn together triggered the highest abundance of the genus Fragilariopsis sp. in the Western DP. In the Eastern DP, the nanophytoplankton fraction, detected by flow cytometry, reached the highest abundance only when both trace elements were provided, confirmed by highest chlorophyll-a build up. Moreover, the distinct response of Mn depletion relative to the Fe depletion support the findings that Fe and Mn do not substitute to each other. This experimental study highlights that both trace elements act as drivers of the ecology across the Drake Passage.

How to cite: Balaguer, J., Koch, F., and Trimborn, S.: Limitation by iron and manganese on phytoplankton communities in the Drake Passage.
, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3950, https://doi.org/10.5194/egusphere-egu2020-3950, 2020

D2754 |
EGU2020-5855
J. Scott P. McCain, Eric P. Achterberg, Alessandro Tagliabue, and Erin M. Bertrand

Iron (Fe) limits primary productivity in a large part of the ocean; but recent geochemical profiles and bottle incubation assays indicate that manganese (Mn) can also limit primary productivity in the Southern Ocean. Fe and Mn can interact to influence primary productivity, but the extent to which these elements colimit phytoplankton in the Southern Ocean is uncertain. In addition, current models are insufficient to assess colimitation as they assume a single, most scarce, resource. In order to examine Fe and Mn colimitation in phytoplankton, we developed a modeling framework to predict proteomic profiles under varying Fe, Mn, and light conditions. In our model, proteins are optimally allocated to various coarse-grained cellular pools, governed by environmental conditions. We predict that Fe controls cellular Mn quotas, largely because of paired stoichiometry within photosynthetic machinery. Our model suggests that the diffusion-limitation paradigm of Fe should be revisited, as diffusive flux of Fe does not appear to be limiting. We then use our model to explore various cellular mechanisms leading to phytoplankton Fe limitation. Lastly, using Fe and Mn biogeochemical model output, we predict regions in the Southern Ocean where Fe/Mn colimitation is most likely to occur.

How to cite: McCain, J. S. P., Achterberg, E. P., Tagliabue, A., and Bertrand, E. M.: Iron and manganese colimitation in the Southern Ocean examined with a proteomic allocation model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5855, https://doi.org/10.5194/egusphere-egu2020-5855, 2020

D2755 |
EGU2020-12282
Paul Spence, Laurie Menviel, and Darryn Waugh

The Southern Ocean is one of today's largest sink of carbon, having absorbed about 10\% of the anthropogenic carbon emissions. Southern Ocean's dynamics are principally modulated by the strength of the Southern Hemispheric westerlies,  which are projected to increase over the coming century. Here, using a high-resolution ocean-sea-ice-carbon cycle model, we explore the impact of idealized changes in Southern Hemispheric westerlies on the ocean carbon storage . We find that a 20\% strengthening of the Southern Hemispheric westerlies leads to a $\sim$25 Gt loss of natural carbon, while an additional 13 Gt of anthropogenic carbon is absorbed compared to the control run, thus resulting in a net loss of $\sim$12 GtC from the ocean over a period of 42 years. This tendency is enhanced if the westerlies are also shifted polewards, with a total natural carbon loss of almost 37 GtC, and an additional anthropogenic carbon uptake of 18 GtC. While both experiments display a large natural carbon loss south of 10$^\circ$S, the amplitude is three times greater in the poleward strengthening case, which is  not fully compensated by the increase in anthropogenic carbon content. However, the poleward wind shift leads to significant differences in the pattern of DIC change due to a weakening of the upper overturning cell,  which leads to an increase in natural and total carbon north of 35$^\circ$S in the upper 2000 m.

How to cite: Spence, P., Menviel, L., and Waugh, D.: Natural carbon release over-rides anthropogenic carbon uptake when Southern Hemispheric westerlies strengthen, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12282, https://doi.org/10.5194/egusphere-egu2020-12282, 2020

D2756 |
EGU2020-10767
Lavinia Patara, Jan Klaus Rieck, Toste Tanhua, Iris Kriest, and Claus Boening

Recent studies highlighted an increase in the Southern Ocean ventilation since several decades and, at the same time, pronounced decadal fluctuations in its carbon sink. The role of changing ventilation for the anthropogenic CO2 uptake (and thus for the overall Southern Ocean carbon sink) and for oxygen concentrations of mode and intermediate waters (with possible impacts on low-oxygen regions downstream) are still poorly understood. The aim of this study is to assess the role of changing Southern Ocean ventilation for the uptake and storage of atmospheric gases such as CFC-12, CO2 and oxygen. A related question is whether CFC-12 can be used as a proxy of anthropogenic CO2 in the Southern Ocean, since CO2 equilibrates significantly more slowly in seawater than CFC-12 and, while the solubility of CFC-12 increases with decreasing temperature and salinity, the solubility of anthropogenic CO2 decreases. We developed a suite of global configurations based on the NEMO-LIM2 ocean sea ice model including the passive tracer CFC-12 and the biogeochemical model MOPS. The suite includes ORCA05 (1/2° resolution), ORCA025 (1/4° resolution) and ORION10 (featuring a 1/10° nest between 68°S and 30°S). Hindcast and sensitivity experiments performed with ORCA025 under the JRA-55-do atmospheric forcing are used to unravel the role of changing wind and buoyancy forcing on the gas uptake and storage. First results highlight that anthropogenic CO2 is taken up in lighter density classes than CFC-12, meaning that increased ventilation of lighter mode waters would be particularly effective in taking up anthropogenic CO2. This effect is more pronounced in the higher-resolution model ORION10, indicating that mesoscale eddies inject anthropogenic CO2 in lighter waters than lower-resolution models.

How to cite: Patara, L., Rieck, J. K., Tanhua, T., Kriest, I., and Boening, C.: The role of changing Southern Ocean circulation for the uptake and storage of CFC-12, anthropogenic CO2 and oxygen, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10767, https://doi.org/10.5194/egusphere-egu2020-10767, 2020

D2757 |
EGU2020-1755
Karl Stein, Axel Timmermann, Eun Young Kwon, and Tobias Friedrich

The Southern Ocean (SO) played a prominent role in the exchange of carbon between ocean and atmosphere on glacial timescales through its regulation of deep ocean ventilation. Previous studies indicated that SO sea ice could dynamically link several processes of carbon sequestration, but these studies relied on models with simplified ocean and sea ice dynamics or snapshot simulations with general circulation models. Here we use a transient run of the LOVECLIM intermediate complexity climate model, covering the past eight glacial cycles, to investigate the orbital-scale dynamics of deep ocean ventilation changes due to SO sea ice. Cold climates increase sea ice cover, sea-ice export, and Antarctic Bottom Water formation, which are accompanied by increased SO upwelling, stronger poleward export of Circumpolar Deep Water, and a reduction of the atmospheric exposure time of surface waters by a factor of ten. Moreover, increased brine formation around Antarctica enhances deep ocean stratification, which could act to decrease vertical mixing by a factor of four compared to the current climate. The impact of the two mechanisms on carbon sequestration was then tested within a steady-state carbon cycle. The two mechanisms combined can reduce atmospheric carbon by 40 ppm, of which approximately 30 ppm is due to ocean stratification. Moreover, ocean stratification from increased SO sea ice production acts early within glacial cycles to amplify the carbon cycle response.

How to cite: Stein, K., Timmermann, A., Kwon, E. Y., and Friedrich, T.: Timing and magnitude of Southern Ocean sea ice/carbon cycle feedbacks over the last eight glacial cycles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1755, https://doi.org/10.5194/egusphere-egu2020-1755, 2019

D2758 |
EGU2020-22442
Patrizia Giordano, Federico Giglio, Mariangela Ravaioli, Marco Capello, Laura Cutroneo, Robert B. Dunbar, David A. Mucciarone, Walker O. Smith, Clara Manno, and Leonardo Langone

The export of particulate organic carbon (POC) from the sea surface is an essential part of the biological pump. Export fluxes are the result of what is produced in surface water and how much is consumed during particle sinking in the water column. In the Ross Sea, fluxes of POC and total mass are well correlated implying that particle fluxes are dominated by biogenic debris.

Here, we report new and reference data of vertical particle fluxes to below the productive layer obtained on decadal time scales (1990-2017) by automatic sediment traps tethered to moorings in the western Ross Sea (Antarctica). Compilation of all data available in the Ross Sea (23 sites, >1000 samples) shows that annual POC fluxes to below 200 m average 4.4±3.3 g C m-2  y-1. Particle fluxes are relatively low when primary production is high (spring-summer) followed by enhanced sedimentation in late summer-fall. The high degree of decoupling between production and sedimentation is unusual compared to records of Antarctic Peninsula and may represent low grazing rates. Furthermore, data exhibit a large interannual variability and a decreasing trend over time, with a clear shift after 2000. Do the reduced export fluxes depend on lower biological production, enhanced OM consumption, or other processes (e.g., lateral transfer of biogenic particles outside the study area)?

Satellite observations allow us to reconstruct the seasonal and interannual change of chlorophyll biomass, and sea ice extent and duration. Water temperature recorded at mid-depth is used to monitor the different intrusion over time of CDW, the main driver of temporal variability of Fe supply for the Ross Sea. Time series of particle fluxes, chlorophyll, sea ice cover and mid-depth temperature will be compared in order to test if the recent reduction of downward particle fluxes depend on primary production changes.

How to cite: Giordano, P., Giglio, F., Ravaioli, M., Capello, M., Cutroneo, L., Dunbar, R. B., Mucciarone, D. A., Smith, W. O., Manno, C., and Langone, L.: Long time-series of export fluxes in the western Ross Sea (Antarctica), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22442, https://doi.org/10.5194/egusphere-egu2020-22442, 2020

D2759 |
EGU2020-19394
Andres Rigual-Hernandez, Thomas W. Trull, Scott D. Nodder, José A. Flores, Helen Bostock, Fátima Abrantes, Ruth S. Eriksen, Francisco J. Sierro, Diana M. Davies, Anne-Marie Ballegeer, Miguel A. Fuertes, Alba González Lanchas, and Lisa C. Northcote

Coccolithophores are ubiquitous marine unicellular algae belonging to the Class Prymnesiophyceae, division Haptophyta. They are distinct from other phytoplankton groups in their capacity to produce minute calcite platelets (termed coccoliths) with which they cover their cells. During the Cretaceous and throughout the Cenozoic era, pelagic sedimentation of carbonate was largely controlled by coccolithophores as evidenced by their major contribution to deep-sea oozes and chalks. In the modern Southern Ocean, coccolithophores represent an important component of the phytoplankton communities and carbon cycle. However, their contribution to total Particulate Inorganic Carbon (PIC) for large regions of the Southern Ocean remains undocumented.

Here we report the Particulate Inorganic Carbon (PIC) and coccolithophore fluxes collected over a year by sediment traps placed at two sites of the subantarctic Southern Ocean. We present coccolith mass estimates of the most abundant coccolithophore species and quantitatively partition annual PIC fluxes amongst heterotrophic calcifying plankton and coccolithophores. Our results reveal that coccolithophores account for approximately half of the annual PIC export in the subantarctic Southern Ocean. Moreover, in contrast to satellite estimations, that mainly reflect coccospheres and detached coccoliths of Emiliania huxleyi, less abundant but larger species make the largest contribution to CaCO3. Lastly, comparison of our data with previous studies suggest that projected environmental change in the Southern Ocean may result in a decline of coccolithophore PIC production and export.

 

How to cite: Rigual-Hernandez, A., Trull, T. W., Nodder, S. D., Flores, J. A., Bostock, H., Abrantes, F., Eriksen, R. S., Sierro, F. J., Davies, D. M., Ballegeer, A.-M., Fuertes, M. A., González Lanchas, A., and Northcote, L. C.: Cocccolithophore contribution to carbonate export to the deep sea in the Australian-New Zealand sector of the subantarctic Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19394, https://doi.org/10.5194/egusphere-egu2020-19394, 2020

D2760 |
EGU2020-7505
Sophie M. Alexander, Philip F. Sexton, Pallavi Anand, and Steven M. Bohaty

The onset of the Cenozoic global cooling trend represents a major transition in Earth’s climate from the extremely warm early Eocene ‘greenhouse’ towards much colder unipolar glaciation by the end of the Eocene. Data across this interval of profound climatic change have long suggested contemporaneous changes to marine biological productivity and the carbon cycle. Modern observations, conceptual models from the last glacial cycle and models documenting change at Cenozoic timescales, all predict heterogeneous biological productivity responses between high and low latitudes. We test for this heterogeneity between latitudes across the onset of the Cenozoic global cooling trend. Here we utilise bulk sediment elemental compositions, and concentration of benthic foraminifera and ichthyoliths to reconstruct changes in palaeoproductivity, export and preservation of CaCO3 across the early-to-middle Eocene interval (~42 to 50 Ma) at multiple deep-sea sites in the Atlantic basin. We also present evidence of changes in bottom-water oxygenation across the Southern Atlantic sites. We find opposing trends in Biogenic barium (BioBa) versus benthic foraminifera accumulation rates (BFAR) and Ichythyolith Accumulation Rates (IAR) for many of our sites across the high southern latitudes. These trends could be explained by increased organic carbon flux to the sediment, which would have increased the BFAR and IAR while at the same time potentially causing oxygen depletion and reductive barite dissolution in the sediments driving our observed synchronous decreases in BioBa and Mn/Al. This trend of synchronous decreases in BioBa and Mn/Al is evident within the Atlantic Sector of the Sub-Antarctic Zone. Within the Antarctic zone, an opposing trend of decreasing BioBa coincides with increasing Mn/Al and BFAR, suggesting a differing mode of production, export and fate of marine CaCO3. Ongoing work seeks to determine the nature of biological productivity responses across the high and low latitude oceans during the Eocene ‘greenhouse’ to ‘icehouse’ transition.

How to cite: Alexander, S. M., Sexton, P. F., Anand, P., and Bohaty, S. M.: Ocean productivity and bottom water oxygenation across the onset of the Cenozoic cooling trend, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7505, https://doi.org/10.5194/egusphere-egu2020-7505, 2020