As insolation increases during the Antarctic summer, sea ice melts, and the iron in the sea ice is released into the ocean Iron leads to phytoplankton blooms at sea ice margins and polynya in Antarctica, a high nutrient and low chlorophyll region. However, it is difficult to investigate the direct effect of sea ice on chlorophyll a changes because the observed regions are physically difficult to access. Therefore, this study intends to investigate the sea ice effect on chlorophyll a variation by using the decreasing salinity when sea ice melts in the Ross Sea, Antarctica. Using Random Forest, an ensemble bagging tree method of machine learning, the relationship was analyzed by 11 variables, including physical variables (sea surface temperature, photosynthetically available radiation, atmospheric temperature, wind, and salinity) as input data and chlorophyll a data observed from satellites as output data. As a result, the square of the correlation coefficient (R²) of the test data set was 0.97, and the root mean square error (RMSE) was 0.41 mg m^-3, showing high accuracy. In addition, the importance of salinity was identified by calculating the variable importance in the model. These results provide the importance of salinity in predicted future chlorophyll a changes in the Antarctic Ocean due to climate change.

How to cite: Yang, H.-J., Baek, J.-Y., and Jo, Y.-H.: Interrelationship of sea ice-salinity-chlorophyll a changes using multi-satellite based on machine learning analysis in Ross Sea, Antarctica, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10869, https://doi.org/10.5194/egusphere-egu23-10869, 2023.

The uptake of carbon by the Southern Ocean plays a critical role in mitigating atmospheric CO₂ increases, but its magnitude and temporal and spatial variability are still subject to large uncertainties due to the scarcity of observations, and model disagreements. The Southeast Pacific is one such region where deep, nutrient and carbon-rich circumpolar deep waters upwell, but also where Subantarctic Mode Water and Antarctic Intermediate Water are formed and subducted, carrying with them high loadings of anthropogenic carbon dioxide into the ocean interior. The processes driving the upper ocean carbon levels are a balance of biological activity and heat-flux driven solubility effects in response to changing physical dynamics. While the former is thought to drive the region being a net annual carbon sink, the depth at which exported organic matter is remineralised will have a large effect on whether it remains there on climatically-important timescales. Here we present a multi-year biogeochemical timeseries from the OOI mooring located in the region, combined with observations from profiling BGC-Argo floats, a 6-week process cruise in austral summer 2019-2020 (as part of the UK CUSTARD programme), and outputs from data assimilation models to investigate the effects of interannual variability in mixed layer dynamics on primary production, carbon export and long-term carbon sequestration. We find a strong relationship between winter mixed layer depths and densities, biological activity the following summer, and impacts on the magnitude and distribution of subsurface remineralisation, providing insight into the controls on carbon uptake in a region of global significance for climate regulation

How to cite: Brown, P., Trucco Pignata, P., Oliver, S., García-Ibañez, M., Conde Pardo, P., Bakker, D., and Martin, A.: Interannual variability in the physical and biological drivers of carbon sequestration in the southeast Pacific Subantarctic Mode Water Formation region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16070, https://doi.org/10.5194/egusphere-egu23-16070, 2023.

The Bransfield Strait (BS) is a relatively narrow region located between the South Shetland Islands (SSI) and the Antarctic Peninsula (AP), where the dominant cyclonic circulation is composed by two major inflows, which appear to influence the development of the seasonal chlorophyll bloom. On the one hand, the Bransfield Current transports Transitional Zonal Water with Bellingshausen influence (TBW) northeastwards along the SSI slope. TBW is characterised by well-stratified, relatively warm (Ɵ > -0.4ºC) and fresh (<34.45) waters. On the other hand, the Antarctic Coastal Current (CC) transports Transitional Zonal Water with Weddell influence (TWW) southwestwards along the AP coastline, being distinguished by homogeneous, colder (Ɵ < -0.4ºC) and saltier (>34.45) waters (Sangrà et al., 2017). These two water masses confront each other forming the Peninsula Front (PF; García et al., 1994; López et al., 1999). Interestingly, the chlorophyll-a (chl-a) spatial distribution in the BS has already been linked in the past to the spatial distribution of both water masses and their water column vertical stability, among other factors (Lipski and Rakusa-Suszczewski, 1990; Basterretxea and Arístegui, 1999). Thus, higher chl-a concentrations have been reported around the SSI and Gerlache Strait where TBW flows, while lower concentrations have been traditionally found north off the SSI and closer to the AP coastline, where more homogeneous surface waters prevail (AASW and TWW, respectively) (Corzo et al., 2005).

In this work we aim to provide a further understanding on the bio-physical coupling occurring in the Bransfield Strait, focused on the physical drivers controlling the surface distribution of the chl-a bloom and the location of the PF at seasonal and interannual scales. To do this we use various remotely-sensed observations over the period 1998-2018: Sea Surface Temperature (SST), Sea-Ice Coverage (SIC), chlorophyll-a, wind stress and Photosynthetically Active Radiation (PAR). Preliminary results confirm that the spatial distribution of the surface chl-a bloom in the Bransfield Strait is strongly influenced by the location of the PF, both seasonally and interannually. Also, a shift in the strength of the chl-a bloom has been identified, where significantly stronger events are found from 2005 onwards; when mean chl-a bloom values are slightly greater, and about 0.61 mg m^-3, than in previous years, when they averaged about 0.49 mg m^-3. We hypothesize this shift might be linked to observed changes in the seasonal evolution of the SIC and SST over the same period. Ongoing analyses attempt to elucidate the major mechanisms accounting for this apparent variability of the bio-physical coupling controlling the chl-a blooms in the Bransfield Strait.

How to cite: Veny, M., Aguiar-González, B., Marrero-Díaz, Á., Pereira-Vázquez, T., and Rodríguez-Santana, Á.: Spatio-temporal variability of the Peninsula Front and the surface chlorophyll-a bloom in the Bransfield Strait, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13417, https://doi.org/10.5194/egusphere-egu23-13417, 2023.

The Weddell Gyre plays a role in connecting the deep ocean to the surface through upwelling, and also in feeding heat towards the Antarctic ice shelves, regulating the density of water masses that feed the deepest limb of the global overturning circulation. Using Argo floats freely drifting throughout the Weddell Gyre, we describe its horizontal circulation as an elongated double-gyre system, with stronger transports in the east than in west, impacting water property distribution. The eastern sub-gyre region is also associated with stronger upwelling rates than in the west, as shown by radionuclide concentrations. To gain insight to long-term changes in the Weddell Gyre, nutrient concentrations can also be investigated as oceanic tracers. We determine long-term trends in surface silicates, a necessary nutrient for silicifying phytoplankton, from ship-based measurements since 1996, and find that the strongest increase is found in the central western sub-gyre region. In association with the eastern sub-gyre, long-term trends along the Prime Meridian are strongest (albeit weaker than in the central western sub-gyre) in the westward flowing southern limb of the gyre, downstream of Maud Rise. We hypothesize that there are different dynamical drivers, such as wind-driven upwelling (west) and turbulent mixing (east), which cause the positive silicate trends in the east versus the west, which are investigated accordingly.

How to cite: Reeve, K. A., Hoppema, M., Kanzow, T., Boebel, O., Geibert, W., and Strass, V.: The Weddell Gyre: what drives a long-term increase in surface nutrients?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8176, https://doi.org/10.5194/egusphere-egu23-8176, 2023.

Since it was recognised that the Southern Ocean plays a crucial role in global climate, the region has generally been understood in a zonally symmetric framework. While this simplification has provided valuable insight, recent work suggests that significant zonal asymmetries exist in several key parameters such as SST, mixed layer depth and air-sea CO2 flux. This is true both for the mean state and for changes at multiple time scales. 

Of particular interest here are changes in ocean heat content (ΔOHC) over the past three decades. Using an eddy-permitting ocean model forced by ERA5 reanalysis, we find significant asymmetries in ΔOHC both within and north of the ACC, which is robustly reflected in a suite of hindcast and reanalysis models, as well as observation based temperature reconstructions. In our model, asymmetry stems largely from a southward displacement of ΔOHC in the Indian basin, where warming occurs primarily within ACC, as opposed to north of it in the Atlantic and Pacific. However, significant asymmetries are also found within the sea ice zone south of 60^o S, where the Ross Sea warms to a much greater degree than other basins. 

In order to better understand the sources of this asymmetric warming, we run several model experiments which decompose the total OHC into components originating from wind, heat and freshwater flux changes. We find roughly equal contributions from wind and surface heat flux north of the ACC, with asymmetric changes in the westerlies driving anomalous convergence of heat. Within and south of the ACC all three forcings play an important role, although this depends strongly on the basin. Overall, we conclude that much of the asymmetries in ΔOHC originate from asymmetries in the surface flux changes, with an important secondary role played by variability in the mean state. These findings have two important implications. First, studies which only consider zonally averaged quantities will likely mask significant variability, and therefore miss important regional and local processes. Second, the impact of multi-decadal climate variability on the Southern Ocean is not manifested in a zonally symmetric fashion, which may have important implications for future changes. 

How to cite: Hague, M., Gruber, N., and Münnich, M.: Zonally Asymmetric Response of Southern Ocean Heat Content to Wind, Heat and Freshwater Forcing at Multi-decadal Time Scales, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3865, https://doi.org/10.5194/egusphere-egu23-3865, 2023.

Previous studies have identified intense climatic change in the Southern Ocean. However, the response of ACC transport to climate change is not fully understood. In this study, by using in-situ ocean bottom pressure (OBP) records and five GRACE products, long-term variations of ACC transport are studied. Our results confirm the reliability of GRACE CSR mascon product in ACC transport estimation at the Drake Passage. Superimposed on interannual variability, ACC transport exhibits an obvious increasing trend (1.32±0.07Sv year^-1) during the GRACE era. Based on results of a mass-conservation ocean model simulation, we suggest that the acceleration of ACC is associated with intensified westerly winds and loss of land ice in Antarctica.

How to cite: Yang, C., Cheng, X., Huang, D., Qin, J., Zhou, G., and Chen, J.: Acceleration of Antarctic Circumpolar Current at the Drake Passage during the GRACE era, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5783, https://doi.org/10.5194/egusphere-egu23-5783, 2023.

The Weddell Sea is located in the Southern Ocean, bounded to the south and west by the Antarctic continent and the Antarctic Peninsula, respectively, and to the north by the Antarctic Circumpolar Current. The cyclonic Weddell Gyre stands out as the dominant feature of the basin circulation, driven by wind and thermohaline forcing as well as topographic steering. Importantly, it is the primary source region of Antarctic Bottom Water (AABW), thus becoming one of the key regions for the global thermohaline circulation. Furthermore, the geographical location of the Western Boundary Current System (WBCS) developed in the gyre allows the leakage of near-freezing subsurface waters into the Bransfield Strait. This cold-water pathway has been recently suggested to maintain regionally low rates of glacier retreat.  In this work, we perform the inter-comparison between NEMO-based and HYCOM-based global ocean circulation models at different resolutions over the WBCS domain. To this aim, we analyse the horizontal and vertical structure of the WBCS and its volume transport along the historical ADELIE transect (SOS-Climate II campaign; https://doi.pangaea.de/10.1594/PANGAEA.864578), which extends oceanward from the northernmost tip of the Antarctic Peninsula and across the WBCS. The choice of this transect is not trivial as it captures the hydrodynamic of the WBCS before water masses either leave the basin or recirculate within the gyre.  

Preliminary results support that both eddy-resolving models are in agreement about the major features of the hydrography and dynamic structure of the WBCS as compared to previous modelling studies. Both reproduce the spatial distribution of the Antarctic Coastal Current (CC), the Antarctic Slope Front (ASF) and the Weddell Front (WF), as reported in Thompson and Heywood (2008). Talking about the NEMO-based model at a lower resolution (¼^o), the multi-jet structure of the WBCS is absent, appearing only one major branch. We attribute this mismatch mostly due to a resolution issue. Regarding the volume transport, we find the modelled WBCS displays a seasonal cycle in all cases of study, where minimum values are found in September-December while maximum are in March-July, as also reported Wang et al. [2012]. A major difference occurs towards the interior of the gyre, where the HYCOM-based model exhibits a significantly stronger and wider current branch (~150 km) east of the WF, and whose description is absent in the literature. In previous studies this domain is traditionally excluded and, when volume transport estimates from the NEMO-based model and the HYCOM based model were computed, they both yielded an average transport about 30 Sv, which agrees well with a transport about 24 Sv reported by Wang et al. (2012) and Jullion et al. (2014), also based on modelling estimates across a similar but shorter transect.

These results encourage us to further explore these models in ongoing analyses about the natural variability of the WBCS of the Weddell Gyre and major forcing controlling its variability. We expect that a better understanding of the governing processes will allow us to assess the potential downstream impact of local water masses after their exit from the Gyre. 

How to cite: Pereira-Vázquez, T., Aguiar-González, B., Marrero-Díaz, Á., Machín, F., Veny, M., and Rodríguez-Santana, Á.: On the Western Boundary Current System of the Weddell Gyre: a model intercomparison, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10045, https://doi.org/10.5194/egusphere-egu23-10045, 2023.