Modeling the physical drivers of the decadal variability of the Southern Ocean carbon uptake

2021 ◽  
Author(s):  
Lavinia Patara ◽  
Torge Martin ◽  
Ivy Frenger ◽  
Jan Klaus Rieck ◽  
Chia-Te Chien
Keyword(s):  
Southern Ocean ◽  
Decadal Variability ◽  
Open Ocean ◽  
Global Ocean ◽  
Carbon Uptake ◽  
Positive Trend ◽  
The Past ◽  
Rate Of Increase ◽  
Ocean Biogeochemistry ◽  
Ocean Carbon

<p>Observational estimates point to pronounced changes of the Southern Ocean carbon uptake in the past decades, but the mechanisms are still not fully understood. In this study we assess physical drivers of the Southern Ocean carbon uptake variability in a suite of global ocean biogeochemistry models with 0.5º, 0.25º and 0.1º horizontal resolution as well as in a 3-member ensemble performed with an Earth System Model (ESM) sharing the same ocean biogeochemistry model. The ocean models show a positive trend of the Southern Ocean CO<sub>2</sub> uptake in the past decades, with a weakening of its rate of increase in the 1990s. The 0.1º model exhibits the strongest trend in the Southern Ocean carbon uptake. <span>Different physical drivers of the carbon up</span>take variability and of its trends (such as changes in stratification, ventilation, overturning circulation, and SST) are analyzed. A particular focus of this study is to assess the role of open-ocean polynyas in driving Southern Ocean carbon uptake. Open-ocean polynyas in the Southern Ocean have pronounced climate fingerprints, such as reduced sea-ice coverage, heat loss by the ocean and enhanced bottom water formation, but their role for the Southern Ocean carbon uptake has been as yet little studied. To this end we analyze conjunctly ESM simulations and an ocean-only sensitivity experiment where open-ocean polynyas are artificially created by perturbing the Antarctic freshwater runoff. We find that enhanced CO<sub>2</sub> outgassing takes place during the polynya opening, because old carbon-rich waters come in contact with the atmosphere. The concomitant increased uptake of anthropogenic CO<sub>2</sub> partially compensates the CO<sub>2</sub> outgassing. When the polynya closes, the ocean CO<sub>2</sub> uptake increases significantly, possibly fueled by abundant nutrients and higher alkalinity brought to the surface during the previous convective phase. Our results suggest that open-ocean polynyas could have a significant impact on the Southern Ocean CO<sub>2</sub> uptake and could thus modulate its decadal variability.</p><p> </p>

scholarly journals Strong sensitivity of Southern Ocean carbon uptake and nutrient cycling to wind stirring

2013 ◽  
Vol 10 (9) ◽  
pp. 15033-15076 ◽  
Author(s):  
K. B. Rodgers ◽  
O. Aumont ◽  
S. E. Mikaloff Fletcher ◽  
Y. Plancherel ◽  
L. Bopp ◽  
...  
Keyword(s):  
Carbon Cycle ◽  
Southern Ocean ◽  
Ad Hoc ◽  
Global Ocean ◽  
Nutrient Concentrations ◽  
Carbon Uptake ◽  
Carbon Cycle Model ◽  
Ocean Biogeochemistry ◽  
Ocean Carbon ◽  
The Impact

Abstract. Here we test the hypothesis that winds have an important role in determining the rate of exchange of CO2 between the atmosphere and ocean through wind stirring over the Southern Ocean. This is tested with a sensitivity study using an ad hoc parameterization of wind stirring in an ocean carbon cycle model. The objective is to identify the way in which perturbations to the vertical density structure of the planetary boundary in the ocean impacts the carbon cycle and ocean biogeochemistry. Wind stirring leads to reduced uptake of CO2 by the Southern Ocean over the period 2000–2006, with differences of order 0.9 Pg C yr−1 over the region south of 45° S. Wind stirring impacts not only the mean carbon uptake, but also the phasing of the seasonal cycle of carbon and other species associated with ocean biogeochemistry. Enhanced wind stirring delays the seasonal onset of stratification, and this has large impacts on both entrainment and the biological pump. It is also found that there is a strong sensitivity of nutrient concentrations exported in Subantarctic Mode Water (SAMW) to wind stirring. This finds expression not only locally over the Southern Ocean, but also over larger scales through the impact on advected nutrients. In summary, the large sensitivity identified with the ad hoc wind stirring parameterization offers support for the importance of wind stirring for global ocean biogeochemistry, through its impact over the Southern Ocean.

scholarly journals Strong sensitivity of Southern Ocean carbon uptake and nutrient cycling to wind stirring

2014 ◽  
Vol 11 (15) ◽  
pp. 4077-4098 ◽  
Author(s):  
K. B. Rodgers ◽  
O. Aumont ◽  
S. E. Mikaloff Fletcher ◽  
Y. Plancherel ◽  
L. Bopp ◽  
...  
Keyword(s):  
Carbon Cycle ◽  
Southern Ocean ◽  
Ad Hoc ◽  
Global Ocean ◽  
Relative Reduction ◽  
Carbon Uptake ◽  
Carbon Cycle Model ◽  
Ocean Biogeochemistry ◽  
Ocean Carbon ◽  
The Impact

Abstract. Here we test the hypothesis that winds have an important role in determining the rate of exchange of CO2 between the atmosphere and ocean through wind stirring over the Southern Ocean. This is tested with a sensitivity study using an ad hoc parameterization of wind stirring in an ocean carbon cycle model, where the objective is to identify the way in which perturbations to the vertical density structure of the planetary boundary in the ocean impacts the carbon cycle and ocean biogeochemistry. Wind stirring leads to reduced uptake of CO2 by the Southern Ocean over the period 2000–2006, with a relative reduction with wind stirring on the order of 0.9 Pg C yr−1 over the region south of 45° S. This impacts not only the mean carbon uptake, but also the phasing of the seasonal cycle of carbon and other ocean biogeochemical tracers. Enhanced wind stirring delays the seasonal onset of stratification, and this has large impacts on both entrainment and the biological pump. It is also found that there is a strong reduction on the order of 25–30% in the concentrations of NO3 exported in Subantarctic Mode Water (SAMW) to wind stirring. This finds expression not only locally over the Southern Ocean, but also over larger scales through the impact on advected nutrients. In summary, the large sensitivity identified with the ad hoc wind stirring parameterization offers support for the importance of wind stirring for global ocean biogeochemistry through its impact over the Southern Ocean.

scholarly journals Evaluating CMIP5 ocean biogeochemistry and Southern Ocean carbon uptake using atmospheric potential oxygen: Present-day performance and future projection

2016 ◽  
Vol 43 (5) ◽  
pp. 2077-2085 ◽  
Author(s):  
C. D. Nevison ◽  
M. Manizza ◽  
R. F. Keeling ◽  
B. B. Stephens ◽  
J. D. Bent ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Carbon Uptake ◽  
Future Projection ◽  
Ocean Biogeochemistry ◽  
Ocean Carbon ◽  
Potential Oxygen

scholarly journals Utilizing the Drake Passage Time-series to understand variability and change in subpolar Southern Ocean pCO<sub>2</sub>

2017 ◽  
Author(s):  
Amanda R. Fay ◽  
Nicole S. Lovenduski ◽  
Galen A. McKinley ◽  
David R. Munro ◽  
Colm Sweeney ◽  
...  
Keyword(s):  
Time Series ◽  
Southern Ocean ◽  
Seasonal Cycle ◽  
Drake Passage ◽  
Passage Time ◽  
Global Ocean ◽  
Carbon Uptake ◽  
Surface Ocean ◽  
Ocean Carbon

Abstract. The Southern Ocean is highly under-sampled for the purpose of assessing total carbon uptake and its variability. Since this region dominates the mean global ocean sink for anthropogenic carbon, understanding temporal change is critical. Underway measurements of pCO2 collected as part of the Drake Passage Time-series (DPT) program that began in 2002 inform our understanding of seasonally changing air-sea gradients in pCO2, and by inference the carbon flux in this region. Here, we utilize all available pCO2 observations collected in the subpolar Southern Ocean to evaluate how the seasonal cycle, interannual variability, and long-term trends in surface ocean pCO2 in the Drake Passage region compare to that of the broader subpolar Southern Ocean. Our results indicate that the Drake Passage is representative of the broader region in both seasonality and long term pCO2 trends shown through the agreement of timing and amplitude of seasonal cycles as well as trend magnitudes. The high temporal density of sampling by the DPT is critical to constraining estimates of the seasonal cycle of surface pCO2 in this region, as winter data remain sparse in areas outside of the Drake Passage. From 2002–2015, data show that carbon uptake has strengthened with surface ocean pCO2 trends less than the global atmospheric trend in the Drake Passage and the broader subpolar Southern Ocean. Analysis of spatial correlation shows Drake Passage pCO2 to be representative of pCO2 and its variability up to several hundred kilometers upstream of the region. We also compare DPT data from 2016 and early 2017 to contemporaneous pCO2 estimates from autonomous biogeochemical floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) so as to highlight the opportunity for evaluating data collected on autonomous observational platforms. Though SOCCOM floats sparsely sample the Drake Passage region for 2016–2017, their pCO2 estimates typically fall within the range of underway observations. Going forward, continuation of the Drake Passage Time-series will reduce uncertainties in Southern Ocean carbon uptake seasonality, variability, and trends, and provide an invaluable independent dataset for post-deployment quality control of sensors on autonomous floats. Together, these datasets will vastly increase our ability to monitor change in the ocean carbon sink.

scholarly journals Utilizing the Drake Passage Time-series to understand variability and change in subpolar Southern Ocean <i>p</i>CO<sub>2</sub>

2018 ◽  
Vol 15 (12) ◽  
pp. 3841-3855 ◽  
Author(s):  
Amanda R. Fay ◽  
Nicole S. Lovenduski ◽  
Galen A. McKinley ◽  
David R. Munro ◽  
Colm Sweeney ◽  
...  
Keyword(s):  
Time Series ◽  
Southern Ocean ◽  
Seasonal Cycle ◽  
Drake Passage ◽  
Passage Time ◽  
Global Ocean ◽  
Carbon Uptake ◽  
Surface Ocean ◽  
Ocean Carbon

Abstract. The Southern Ocean is highly under-sampled for the purpose of assessing total carbon uptake and its variability. Since this region dominates the mean global ocean sink for anthropogenic carbon, understanding temporal change is critical. Underway measurements of pCO2 collected as part of the Drake Passage Time-series (DPT) program that began in 2002 inform our understanding of seasonally changing air–sea gradients in pCO2, and by inference the carbon flux in this region. Here, we utilize available pCO2 observations to evaluate how the seasonal cycle, interannual variability, and long-term trends in surface ocean pCO2 in the Drake Passage region compare to that of the broader subpolar Southern Ocean. Our results indicate that the Drake Passage is representative of the broader region in both seasonality and long-term pCO2 trends, as evident through the agreement of timing and amplitude of seasonal cycles as well as trend magnitudes both seasonally and annually. The high temporal density of sampling by the DPT is critical to constraining estimates of the seasonal cycle of surface pCO2 in this region, as winter data remain sparse in areas outside of the Drake Passage. An increase in winter data would aid in reduction of uncertainty levels. On average over the period 2002–2016, data show that carbon uptake has strengthened with annual surface ocean pCO2 trends in the Drake Passage and the broader subpolar Southern Ocean less than the global atmospheric trend. Analysis of spatial correlation shows Drake Passage pCO2 to be representative of pCO2 and its variability up to several hundred kilometers away from the region. We also compare DPT data from 2016 and 2017 to contemporaneous pCO2 estimates from autonomous biogeochemical floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) so as to highlight the opportunity for evaluating data collected on autonomous observational platforms. Though SOCCOM floats sparsely sample the Drake Passage region for 2016–2017 compared to the Drake Passage Time-series, their pCO2 estimates fall within the range of underway observations given the uncertainty on the estimates. Going forward, continuation of the Drake Passage Time-series will reduce uncertainties in Southern Ocean carbon uptake seasonality, variability, and trends, and provide an invaluable independent dataset for post-deployment assessment of sensors on autonomous floats. Together, these datasets will vastly increase our ability to monitor change in the ocean carbon sink.

Detecting Climate Fingerprints in Southern Ocean Carbon using a Biogeochemical Ocean Model

2021 ◽  
Author(s):  
Rebecca Wright ◽  
Corinne Le Quéré ◽  
Erik Buitenhuis ◽  
Dorothee Bakker
Keyword(s):  
Climate Change ◽  
Climate Variability ◽  
Southern Ocean ◽  
Observational Data ◽  
Ecosystem Dynamics ◽  
Global Ocean ◽  
Subsurface Waters ◽  
Climatic Drivers ◽  
Ocean Carbon ◽  
And Storage

&lt;p&gt;The Southern Ocean plays an important role in the uptake, transport and storage of carbon by the global oceans. These properties are dominated by the response to the rise in anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; in the atmosphere, but they are modulated by climate variability and climate change. Here we explore the effect of climate variability and climate change on ocean carbon uptake and storage in the Southern Ocean. We assess the extent to which climate change may be distinguishable from the anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; signal and from the natural background variability. We use a combination of biogeochemical ocean modelling and observations from the GLODAPv2020 database to detect climate fingerprints in dissolved inorganic carbon (DIC).&lt;/p&gt;&lt;p&gt;We conduct an ensemble of hindcast model simulations of the period 1920-2019, using a global ocean biogeochemical model which incorporates plankton ecosystem dynamics based on twelve plankton functional types. We use the model ensemble to isolate the changes in DIC due to rising anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; alone and the changes due to climatic drivers (both climate variability and climate change), to determine their relative roles in the emerging total DIC trends and patterns. We analyse these DIC trends for a climate fingerprint over the past four decades, across spatial scales from the Southern Ocean, to basin level and down to regional ship transects. Highly sampled ship transects were extracted from GLODAPv2020 to obtain locations with the maximum spatiotemporal coverage, to reduce the inherent biases in patchy observational data. Model results were sampled to the ship transects to compare the climate fingerprints directly to the observational data.&lt;/p&gt;&lt;p&gt;Model results show a substantial change in DIC over a 35-year period, with a range of more than +/- 30 &amp;#181;mol/L. In the surface ocean, both anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; and climatic drivers act to increase DIC concentration, with the influence of anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; dominating at lower latitudes and the influence of climatic drivers dominating at higher latitudes. In the deep ocean, the anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; generally acts to increase DIC except in the subsurface waters at lower latitudes, while climatic drivers act to decrease DIC concentration. The combined fingerprint of anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; and climatic drivers on DIC concentration is for an increasing trend at the surface and decreasing trends in low latitude subsurface waters. Preliminary comparison of the model fingerprints to observational ship transects will also be presented.&lt;/p&gt;

The Southern Ocean carbon sink 1985-2018: first results of the RECCAP2 project

2021 ◽  
Author(s):  
Judith Hauck ◽  
Luke Gregor ◽  
Cara Nissen ◽  
Eric Mortenson ◽  
Seth Bushinsky ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Seasonal Cycle ◽  
Carbon Sink ◽  
Global Ocean ◽  
Biogeochemical Models ◽  
Wide Range ◽  
Data Products ◽  
First Results ◽  
Ocean Carbon ◽  
Indian Sector

&lt;p&gt;The Southern Ocean is the main gateway for anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; into the ocean owing to the upwelling of old water masses with low anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; concentration, and the transport of the newly equilibrated surface waters into the ocean interior through intermediate, deep and bottom water formation. Here we present first results of the Southern Ocean chapter of RECCAP2, which is the Global Carbon Project&amp;#8217;s second systematic study on Regional Carbon Cycle Assessment and Processes. In the Southern Ocean chapter, we aim to assess the Southern Ocean carbon sink 1985-2018 from a wide range of available models and data sets, and to identify patterns of regional and temporal variability, model limitations and future challenges.&lt;/p&gt;&lt;p&gt;We gathered global and regional estimates of the air-sea CO&lt;sub&gt;2&lt;/sub&gt; flux over the period 1985-2018 from global ocean biogeochemical models, surface pCO&lt;sub&gt;2&lt;/sub&gt;-based data products, and data-assimilated models. The analysis on the Southern Ocean quantified geographical patterns in the annual mean and seasonal amplitude of air-sea CO&lt;sub&gt;2&lt;/sub&gt; flux, with results presented here aggregated to the level of large-scale ocean biomes.&lt;/p&gt;&lt;p&gt;Considering the suite of observed and modelled estimates, we found that the subtropical seasonally stratified (STSS) biome stands out with the largest air-sea CO&lt;sub&gt;2&lt;/sub&gt; flux per area and a seasonal cycle with largest ocean uptake of CO&lt;sub&gt;2&lt;/sub&gt; in winter, whereas the ice (ICE) biome is characterized by a large ensemble spread and a pronounced seasonal cycle with the largest ocean uptake of CO&lt;sub&gt;2&lt;/sub&gt; in summer. Connecting these two, the subpolar seasonally stratified (SPSS) biome has intermediate flux densities (flux per area), and most models have difficulties simulating the seasonal cycle with strongest uptake during the summer months.&lt;/p&gt;&lt;p&gt;Our analysis also reveals distinct differences between the Atlantic, Pacific and Indian sectors of the aforementioned biomes. In the STSS, the Indian sector contributes most to the ocean carbon sink, followed by the Atlantic and then Pacific sectors. This hierarchy is less pronounced in the models than in the data-products. In the SPSS, only the Atlantic sector exhibits net CO&lt;sub&gt;2&lt;/sub&gt; uptake in all years, likely linked to strong biological production. In the ICE biome, the Atlantic and Pacific sectors take up more CO&lt;sub&gt;2&lt;/sub&gt; than the Indian sector, suggesting a potential role of the Weddell and Ross Gyres.&lt;/p&gt;&lt;p&gt;These first results confirm the global relevance of the Southern Ocean carbon sink and highlight the strong regional and interannual variability of the Southern Ocean carbon uptake in connection to physical and biogeochemical processes.&lt;/p&gt;

scholarly journals Sources of Uncertainty in Future Projections of the Carbon Cycle

2016 ◽  
Vol 29 (20) ◽  
pp. 7203-7213 ◽  
Author(s):  
Alan J. Hewitt ◽  
Ben B. B. Booth ◽  
Chris D. Jones ◽  
Eddy S. Robertson ◽  
Andy J. Wiltshire ◽  
...  
Keyword(s):  
Carbon Cycle ◽  
Climate Models ◽  
Climate Model ◽  
Initial Conditions ◽  
Carbon Fluxes ◽  
Global Ocean ◽  
Lead Times ◽  
Carbon Uptake ◽  
The North ◽  
Ocean Carbon

Abstract The inclusion of carbon cycle processes within CMIP5 Earth system models provides the opportunity to explore the relative importance of differences in scenario and climate model representation to future land and ocean carbon fluxes. A two-way analysis of variance (ANOVA) approach was used to quantify the variability owing to differences between scenarios and between climate models at different lead times. For global ocean carbon fluxes, the variance attributed to differences between representative concentration pathway scenarios exceeds the variance attributed to differences between climate models by around 2025, completely dominating by 2100. This contrasts with global land carbon fluxes, where the variance attributed to differences between climate models continues to dominate beyond 2100. This suggests that modeled processes that determine ocean fluxes are currently better constrained than those of land fluxes; thus, one can be more confident in linking different future socioeconomic pathways to consequences of ocean carbon uptake than for land carbon uptake. The contribution of internal variance is negligible for ocean fluxes and small for land fluxes, indicating that there is little dependence on the initial conditions. The apparent agreement in atmosphere–ocean carbon fluxes, globally, masks strong climate model differences at a regional level. The North Atlantic and Southern Ocean are key regions, where differences in modeled processes represent an important source of variability in projected regional fluxes.

scholarly journals TREE RING RECONSTRUCTION OF MODERN RADIOCARBON DIOXIDE VARIABILITY OVER THE SOUTHERN OCEAN

2021 ◽  
Author(s):  
Rachel Corran
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
Tree Ring ◽  
Deep Water ◽  
Latitudinal Gradient ◽  
Carbon Sink ◽  
Carbon Uptake ◽  
Co2 Uptake ◽  
Anthropogenic Carbon ◽  
Ocean Carbon

<p><b>The Southern Ocean is the largest ocean carbon sink region. However, its trend of increasing carbon uptake has shown variability over recent decades. It is important to understand the underlying mechanisms of anthropogenic carbon uptake such that the future response of the Southern Ocean carbon sink under climate forcing can be predicted. </b></p><p>The carbon uptake of the Southern Ocean is characterised by the balance of outgassing of CO2 from carbon-rich deep water and sequestration of anthropogenic carbon into surface waters. Atmospheric radiocarbon dioxide (Del14CO2) in the Southern Hemisphere is sensitive to the release of CO2 from the upwelling of ‘old’ 14C-depleted carbon-rich deep water at high southern latitudes, but is insensitive to CO2 uptake into the ocean. Thus Del14CO2 has the potential to be used as a tracer of the upwelling observed, thereby isolating the outgassing carbon component. </p><p>The Southern Ocean Region has limited atmospheric Del14CO2 measurements, with sparse long-term sampling sites and few shipboard flask measurements. Therefore in this PhD project I exploit annual growth tree rings, which record the Del14C content of atmospheric CO2, to reconstruct Del14CO2 back in time. Within tree ring sample pretreatment for 14C measurement I automate the organic solvent wash method at the Rafter Radiocarbon Laboratory. I present new annual-resolution reconstructions of atmospheric Del14CO2 from tree rings, from coastal sites in New Zealand and Chile, spanning a latitudinal range of 44 S to 55 S, for the period of interest, 1985 – 2015. Data quality analysis using a range of replicate 14C measurements conducted within this project leads to assignment of apx 1.9 ‰ uncertainties for all results, in line with atmospheric measurements. </p><p>In this project I also develop a harmonised dataset of atmospheric Del14CO2 measurements in the Southern Hemisphere for this period from different research groups, including the new tree ring Del14CO2 records alongside existing data. The harmonised atmospheric Del14CO2 dataset has a wide range of applications, but specifically here allows investigation of temporal and spatial variability of atmospheric Del14CO2 over the Southern Ocean over recent decades, thereby also considering the role of upwelling in recent Southern Ocean carbon sink variability. Backward trajectories are produced for the tree ring sites from an atmospheric transport model, to help inform interpretation of results. </p><p>Over recent decades a latitudinal gradient of 3.7 ‰ is observed between 41 S and 53 S in the New Zealand sector, with a smaller gradient of 1.6 ‰ between 48 S and 55S in the Chile sector. This is consistent with other studies, with the spatial variability of atmospheric Del14CO2 attributed to air-sea 14C disequilibrium associated with carbon outgassing from 14C-depleted carbon-rich deep water upwelling at around 60 S, driving a latitudinal gradient of atmospheric Del14CO2 in the Southern Hemisphere, with longitudinal variability also observed. A stronger atmospheric Del14CO2 latitudinal gradient is observed in the 1980s/1990s relative to later 1990s/2000s. Stronger atmospheric Del14CO2 latitudinal gradients observed in 1980s/1990s suggest stronger deep water upwelling thereby greater associated outgassing of 14C-depleted CO2. These Del14CO2-based observations are consistent with modelling studies that predict changes in deep-water upwelling have controlled decadal variability in CO2 uptake, and are consistent with observation-based studies of decadal changes in rate of CO2 uptake of the Southern Ocean. The results presented in this thesis present the first observation-based confirmation that decadal changes in the strength of deep-water upwelling can explain decadal changes in the rate of CO2 uptake. </p>

scholarly journals Sea-ice Induced Southern Ocean Subsurface Warming and Surface Cooling in a Warming Climate

2020 ◽  
Author(s):  
F. Alexander Haumann ◽  
Nicolas Gruber ◽  
Matthias Münnich
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Heat Content ◽  
Ocean Surface ◽  
Open Ocean ◽  
Global Ocean ◽  
Content Increase ◽  
Surface Cooling ◽  
Cold Surface

&lt;p&gt;Much of the Southern Ocean surface south of 55&amp;#176; S cooled and freshened between at least the early 1980s and the early 2010s. Many processes have been proposed to explain the unexpected cooling, including increased winds or increased surface freshwater fluxes from either the atmosphere or glacial meltwater. However, these mechanisms so far failed to fully explain the surface trends and the concurrently observed warming of the subsurface (100 to 500 m). Here, we argue that these trends are predominantly caused by an increased wind-driven northward transport of sea ice, enhancing the extraction of freshwater near Antarctica and releasing it in the open ocean. This conclusion is based on factorial experiments with a regional ocean model. In all experiments with an enhanced northward transport of sea ice, the open-ocean surface between the Subantarctic Front and the sea-ice edge is cooled by strengthening the salinity dominated oceanic stratification. The strengthened stratification reduces the downward mixing of cold surface water and the upward heat loss of the warmer waters below, thus warming the subsurface. This sea-ice induced subsurface warming mostly occurs around West Antarctica, where it likely enhances ice-shelf melting. Moreover, it could account for about 8&amp;#177;2% of the global ocean heat content increase between 1982 and 2011. Antarctic sea-ice changes thereby may have contributed to the slowdown of global surface warming over this period. The important role of sea-ice in driving changes in the high-latitude Southern Ocean are robust across all considered sensitivity cases, although their magnitude is sensitive to the forcing and the role of salinity in controlling the vertical stratification in the mean state. It remains yet unclear whether these sea-ice induced changes are associated with natural variability or a response to anthropogenic forcing.&lt;/p&gt;

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