Weddell Sea Region Research Articles

Early Oligocene record of an “iceberg alley” in the Weddell Sea from quartz sand microtextural analysis at ODP Site 696

Abstract The greenhouse to icehouse transition at the Eocene-Oligocene boundary (34 Ma) marked the appearance of continental-scale glaciation in Antarctica. The material recovered from Ocean Drilling Program Site 696 is the only record spanning this major climatic shift in the Weddell Sea region. Using scanning electron microscopy, quartz microtextures in 13 samples across the Eocene-Oligocene transition were analyzed to understand the degree of glacial modification and the transportation history of the >150 µm material. The quartz grains were visually grouped, characterized, and interpreted by grain outline, relief, and surface microtextures. Glacial textures are present throughout the entire interval (34.4–33.2 Ma), with the proportion of iceberg-rafted grains in each sample decreasing into the early Oligocene (33.6 Ma), accompanied by an increase in the frequency of eolian and sea-ice-rafted grains. Mass accumulation rates reveal that the flux of iceberg-rafted debris increased coincident with the flux of eolian and sea-ice-rafted grains following 33.6 Ma, suggesting a strong coupling between land-ice development and high-latitude atmospheric processes. When compared with other Antarctic climate proxy data sets, the intensification of ice rafting at Site 696 occurred after ice-sheet inception in East Antarctica. The prominent influx of terrigenous material after 33.6 Ma points to strengthened glacial conditions accompanied by major changes in the environment of the Weddell Sea region, supporting the idea of a high-latitude role in climate perturbations, in agreement with interpretations of other global proxies.

60-YEAR REMOTE SENSING OBSERVATIONS REVEALED CROSS-CYCLE EVOLUTION OF THE BRUNT ICE SHELF IN THE CONTEXT OF GLOBAL WARMING

Abstract. The Antarctic Ice Sheet (AIS) holds the potential to raise global sea levels by a staggering 57 m. Ice shelves, extensions of these ice sheets into the ocean, play a crucial role in stabilizing the upstream ice sheet. The Brunt Ice Shelf (BIS), located in the Southern Weddell Sea region and proximal to the continental shelf break, stands as a vital indicator for the region's response to global climate warming. Hence, this study systematically employs a range of remote sensing and field data, including remote sensing data from 1963 to 2023 (ARGON images, Landsat1-9 images, Sentinel-1 images, ICESat-2 altimetry data, etc.) and in-situ ice flow velocity data since 1956, to conduct a comprehensive analysis of the BIS's calving cycle and evolution over nearly six decades. The results reveal a significant exacerbation of both dynamical and structural instabilities in the BIS compared to the calving cycle in the 1970s. Combining analyses of air temperature from meteorological station, water temperature from ocean models and in-situ measurements such as Conductivity-Temperature-Depth profiles, the study suggests that the increased instability of the BIS is likely linked to the intrusion of warmer currents over the continental shelf. This research provides valuable insights into the evolving behavior of the BIS in response to changing climate conditions, underscoring the critical role of remote sensing data in advancing our understanding of these processes on a global scale.

A contrast in sea ice drift and deformation between winter and spring of 2019 in the Antarctic marginal ice zone

Abstract. Two ensembles of buoys, deployed in the marginal ice zone (MIZ) of the north-eastern Weddell Sea region of the Southern Ocean, are analysed to characterise the dynamics driving sea ice drift and deformation during the winter-growth and the spring-retreat seasons of 2019. The results show that although the two buoy arrays were deployed within the same region of ice-covered ocean, their trajectory patterns were vastly different. This indicates a varied response of sea ice in each season to the local winds and currents. Analyses of the winter data showed that the Antarctic Circumpolar Current modulated the drift near the sea ice edge. This led to a highly energetic and mobile ice cover, characterised by free-drift conditions. The resulting drift and deformation were primarily driven by large-scale atmospheric forcing, with negligible contributions due to the wind-forced inertial response. For this highly advective coupled ice–ocean system, ice drift and deformation linearly depended on atmospheric forcing. We also highlight the limits of commercial floating ice velocity profilers in this regime since they may bias the estimates of sea ice drift and the ice type detection. On the other hand, the spring drift was governed by the inertial response as increased air temperatures caused the ice cover to melt and break up, promoting a counterintuitively less wind-driven ice–ocean system that was more dominated by inertial oscillations. In fact, the deformation spectra indicate a strong decoupling to large-scale atmospheric forcing. Further analyses, extended to include the deformation datasets from different regions around Antarctica, indicate that, for similar spatial scales, the magnitude of deformation varies between seasons, regions, and the proximity to the sea ice edge and the coastline. This implies the need to develop rheology descriptions that are aware of the ice types in the different regions and seasons to better represent sea ice dynamics in the MIZ.

The Role of Bottom Friction in Mediating the Response of the Weddell Gyre Circulation to Changes in Surface Stress and Buoyancy Fluxes

Abstract The Weddell Gyre is one of the dominant features of the Southern Ocean circulation and its dynamics have been linked to processes of climatic relevance. Variability in the strength of the gyre’s horizontal transport has been linked to heat transport toward the Antarctic margins and changes in the properties and rates of export of bottom waters from the Weddell Sea region to the abyssal global ocean. However, the precise physical mechanisms that force variability in the Weddell’s lateral circulation across different time scales remain unknown. In this study, we use a barotropic vorticity budget from a mesoscale eddy active model simulation to attribute changes in gyre strength to variability in possible driving processes. We find that the Weddell Gyre’s circulation is sensitive to bottom friction associated with the overflowing dense waters at its western boundary. In particular, an increase in the production of dense waters at the southwestern continental shelf strengthens the bottom flow at the gyre’s western boundary, yet this drives a weakening of the depth-integrated barotropic circulation via increased bottom friction. Strengthening surface winds initially accelerate the gyre, but within a few years the response reverses once dense water production and export increases. These results reveal that the gyre can weaken in response to stronger surface winds, putting into question the traditional assumption of a direct relationship between surface stress and gyre strength in regions where overflowing dense water forms part of the depth-integrated flow.

Differences in iodine chemistry over the Antarctic continent

High concentrations of iodine oxide (IO) have been reported over west Antarctica, with areas around the Weddell Sea showing a peak in spring. However, stations in east Antarctica show much lower values during summer, although observations over spring are still missing. Here, we present the first year-long observations of IO outside the Weddell Sea region using a multi-axis differential optical absorption spectrometer (MAX-DOAS) over the Bharati station (69.41°S, 76.19°E). Observations show that iodine chemistry is less active than over the Weddell Sea, even during springtime, with IO mixing ratios below 2 pptv throughout the sunlit period. A slight increase in IO is observed in spring, although it is a factor of 10 lower than the Weddell Sea region. We identify the variations in drivers in the different regions using sea ice concentrations, sea ice thickness and chlorophyll concentrations. We use a global model which uses a parameterization for iodine emissions based on a combination of these factors. The model reproduces the high concentrations over the Weddell Sea and the low concentrations over Bharati throughout the year, shedding light on the environmental factors, sources and chemistry of iodine in Antarctica. Even at small concentrations, iodine can enhance ozone loss caused by bromine chemistry over east Antarctica, although this impact is lower than in the west Antarctic.

West-warming East-cooling trend over Antarctica reversed since early 21st century driven by large-scale circulation variation

Antarctic climate changes prior to 2000 were characterized by a strong zonally asymmetric pattern. Over 90% of the land ice mass loss occurred around a limited area in West Antarctica, accompanied by a rapid surface warming rate about three times the global mean. In contrast, surface warming and glacier mass loss around East Antarctica are not significant, until the decades since 2000 when several individual stations show that the temperature trends might have reversed. The asymmetric climate changes between East- and West-Antarctica are largely attributable to the inter-decadal variabilities over the Pacific and Atlantic Oceans through tropical–polar teleconnections, leaving open the question of whether the post-2000 phase shift of the lower-latitude decadal variability causes a flip of the asymmetric Antarctic changes. Here, by synthesizing 26 in-situ observations and 6 reanalysis datasets using a statistical method and integrating the results with a series of climate model experiments, we find that the West-warming, East-cooling trend over Antarctica has systematically reversed in austral spring since the early 21st century, largely due to the atmospheric circulation anomaly over the Antarctic Peninsula–Weddell Sea region, which is associated to the teleconnection with Pacific and atmospheric internal variability. This reversal of the temperature seesaw suggests that substantial decadal-scale fluctuations of the Antarctic climate system exist, including for sea-ice and land-ice systems, superimposed on and modifying longer term changes.

Adélie penguins north and east of the ‘Adélie gap’ continue to thrive in the face of dramatic declines elsewhere in the Antarctic Peninsula region

While population declines among Adélie penguins and population increases among gentoo penguins on the Western Antarctic Peninsula are well established, the logistical challenges of operating in the sea ice-heavy northern tip of the Antarctic Peninsula have prohibited reliable monitoring of seabirds in this region. Here we describe the findings of an expedition to the northern and eastern sides of the Antarctic Peninsula—a region at the nexus of two proposed Marine Protected Areas—to investigate the distribution and abundance of penguins in this region. We discovered several previously undocumented penguin colonies, completed direct surveys of three colonies initially discovered in satellite imagery, and re-surveyed several colonies last surveyed more than a decade ago. Whereas our expectation had been that the Peninsula itself would divide the areas undergoing ecological transition and the apparently more stable Weddell Sea region, our findings suggest that the actual transition zone lies in the so-called "Adélie gap," a 400-km stretch of coastline in which Adélies are notably absent. Our findings suggest that the region north and east of this gap represents a distinct ecoregion whose dynamics stand in sharp contrast to surrounding areas and is likely to be impacted by future conservation measures.

Can Ammonium Records in Antarctic Ice Cores be a Proxy for Sea Ice Fluctuations?

Abstract Ammonium (NH4+) accumulation and variability in Antarctic ice cores possess signals of various biological, anthropogenic, and atmospheric processes. However, the use of NH4+ variability in Antarctic ice cores for paleoclimate reconstructions is not well established. Addressing this, we have utilized, published NH4+ ice core records from the Dronning Maud Land (DML), Antarctica to assess the sources of NH4+, its transport, major forcing factors, and their mechanism in controlling the variability of ammonium signals in coastal ice cores. The first principle component (PC1) derived from PC analysis of NH4+ data comprises ~43% of total variability and also depicts a significant spatial correlation with the NSIDC Sea ice concentration (SIC) variability in the Weddell Sea region of the Southern Ocean. In addition, the PC1 shows strong 2-9 year periodicities related to El Niño Southern Oscillation along with two major decadal to centennial periodicities of ~40 and 100 years. The control of sea ice fluctuations and related oceanic processes (ENSO) provide evidence of the marine sources and influence of ocean-related processes as a major controlling factor for the NH4+ variability in DML ice cores. The cumulative accumulation of the NH4+ produced from enhanced biological decomposition, and unutilized ammonium during nitrification under high SIC conditions are possible mechanisms for the increased supply. The correlation of ERA5 surface air temperatures with Nino3.4 suggests that during El Niño conditions, the air temperatures significantly dropped in the Weddell Sea sector, fostering the high SIC conditions. The findings of this study regarding the role of the SIC in the ammonium variability in Antarctic ice cores would help us better interpret the paleo NH4+ records and their applications to deduce the paleoclimate conditions in coastal Antarctic regions.

Analysis of extreme wind events in the Weddell Sea region (Antarctica) at Belgrano II Station

The objective of the present work is to analyze the wind pattern (direction and speed) at Belgrano II Station, evaluating the influence of large-scale atmospheric systems on extreme wind speed events. The study area is the Weddell Sea, located at the southern end of the Atlantic Ocean, between latitudes of 83° S and 55° S and longitudes of 84° W and 20° E. Daily meteorological data were used in the period from 1979 to 2015, whose variables are: Sea Level Pres-sure (SLP), Wind Speed (WS), Wind Direction (WD) and Air Temperature at 2 Meters (T2M), collected at Belgrano II Station, under the care of the Ar- gentine Antarctic Institute (AAI). To analyze the anomalies in the fields of air temperature, atmospheric circulation and surface pressure on a large scale, we used the reanalysis data obtained from the ERA-Interim database, which are: air temperature at 2 m (oC), geopotential height (m) at the 500 hPa level, and zonal and meridional components of wind (ms−1) on the surface. The ex-treme event was defined as values above the 95% percentile of each month of the year. The results indicate that the wind direction at Belgrano II Station was predominantly southeast. However, when the wind speed was predominant from the northeast, approximately 20% of the cases presented wind speeds above 16 ms−1. Therefore, when there is a change in synoptic conditions near the Belgrano II station, the predominance of the winds becomes northeasterly, with an intensification in its speed. An extra-tropical cyclone approaching the Belgrano II region caused the wind to become of the northeastern on the days before the extreme event, so that two days beforehand there was an increase in air temperature in surface. When we analyzed the wind speed in Belgrano II, we identified that there was not a marked change and over the years, however, we identified that the seasonality presented changes, with more intense winds between the months of May and September.

Ocean‐Sea Ice Processes and Their Role in Multi‐Month Predictability of Antarctic Sea Ice

AbstractAntarctic sea ice is a critical component of the climate system and a vital habitat for Southern Ocean ecosystems. Understanding the underlying physical processes and improving Antarctic sea ice prediction is of broad interest. Using the model data, we investigate sea ice and upper ocean predictability at interannual timescales in the Weddell Sea region. We find that oceanic predictability is largely confined to the Winter Water layer and responds to seasonal modifications of the water column, mainly driven by sea ice processes. Predictability depends not only on the depth of the Winter Water layer, but also on how strongly stratified its base is. Predictability is lost when warm Circumpolar Deep Water with no sea ice‐related memory entrains into the mixed layer. We show the strong dependence of sea ice predictability on the local upper ocean vertical structure, which suggests that both are likely to change in a warming climate.

Modelling evidence for late Eocene Antarctic glaciations

It is generally believed that a large scale Antarctic ice sheet formed at the Eocene-Oligocene transition (34.44-33.65 Ma). However, oxygen isotope excursions during the late Eocene (38-34 Ma) and geomorphic evidence of glacial erosion suggest that there were ephemeral continental scale glaciations before the Oi-1 event. Here, we investigate the Antarctic ice sheet evolution over a multi-million year timescale during the late Eocene up to the early Oligocene with the most recent estimates of carbon dioxide evolution over this time period and different bedrock elevation reconstructions. A novel ice sheet-climate modelling approach is applied where the Antarctic ice sheet model VUB-AISMPALEO is coupled to the emulated climate from HadSM3 using the coupler CLISEMv1.0. Our modelling results show that short-lived continental scale Antarctic glaciation might have occurred during the late Eocene when austral summer insolation reached a minimum in a narrow range of carbon dioxide concentrations. The Antarctic ice sheet first reached the coast in Prydz Bay and later in the Weddell Sea region, supporting the glaciomarine sediments dated prior to the EOT.

Weddell Sea polynya analysis using SMOS–SMAP apparent sea ice thickness retrieval

Abstract. The Weddell Sea is known to feature large openings in its winter sea ice field, otherwise known as open-ocean polynyas. An area within the Weddell Sea region that has repeatedly featured open-ocean polynyas in the past is that which encompasses the Maud Rise seamount. Within this area, after 40 years of intermittent, smaller openings, a larger, more persistent polynya appeared in early September 2017​​​​​​​ and remained open for approximately 80 d until spring ice melt. In this study we present proof that polynya-favorable activity in the Maud Rise area is taking place more frequently and on a larger scale than previously assumed. By investigating thin (< 50 cm) apparent sea ice thickness (ASIT) retrieved from the satellite microwave sensors Soil Moisture and Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP), we find an anomaly of thin sea ice spanning an area comparable to the polynya of 2017 over Maud Rise which occurred in September 2018. In this paper, we look at sea ice above Maud Rise in August and September of 2017 and 2018 as well as all years from 2010 until 2020 in an 11-year time series. Using fifth-generation ECMWF Reanalysis (ERA5) surface wind reanalysis data, we corroborate previous findings (e.g., Campbell et al., 2019; Francis et al., 2019; Wilson et al., 2019) on the strong impact that storm activity can have on sea ice above Maud Rise and help consolidate the theory that the evolution of the Weddell Sea polynya is controlled by local atmospheric as well as oceanographic variability. Based on the results presented, we propose that the Weddell Sea polynya, rather than being a binary phenomenon with one principal cause, is a dynamic process caused by various different preconditioning factors that must occur simultaneously for it to appear and persist. Moreover, we show that rather than an abrupt stop to anomalous activity above Maud Rise in 2017, the very next year shows signs of polynya-favorable activity that, although insufficient to open the polynya, were present in the region. This phenomenon, as we have shown in the 11-year SMOS record, was not unique to 2018 and was also identified in 2010, 2013 and 2014. It is demonstrated that L-band microwave radiometry from the SMOS and SMAP satellites can provide additional useful information, which helps to better understand dynamic sea ice processes like polynya events when compared to the use of satellite sea ice concentration products alone.

A tool to evaluate accessibility due to sea-ice cover: a case study of the Weddell Sea, Antarctica

AbstractSea ice is the major constraint on human activities in the Southern Ocean. Depending on a vessel's ice class, human mobility may be restricted or even prevented altogether by sea-ice conditions. This may imply limited access to research or monitoring stations, preferred fishing grounds or attractive tourist sites. Here, we introduce a statistical model that evaluates the sea-ice cover with two measures: 1) accessibility (i.e. the probability that a given area is navigable by vessels at a given time) and 2) repeated accessibility (i.e. the probability that a given area is navigable by vessels at a given time and again at least once within a defined timespan). We use daily sea-ice concentration data from 2002 to 2020 to demonstrate this tool and its functioning regarding the spatiotemporal variability of sea-ice cover in the wider Weddell Sea region. These findings reflect known characteristics of sea-ice distribution and dynamics in the Weddell Sea, confirming the functionality of our simple tool for determining repeated accessibility of certain areas. Such a tool may facilitate the planning of research and monitoring activities in the Southern Ocean, as well as in Arctic seas.

A Model-Based Climatology of Low-Level Jets in the Weddell Sea Region of the Antarctic

Low-level jets (LLJs) are climatological features in polar regions. It is well known that katabatic winds over the slopes of the Antarctic ice sheet are associated with strong LLJs. Barrier winds occurring, e.g., along the Antarctic Peninsula may also show LLJ structures. A few observational studies show that LLJs occur over sea ice regions. We present a model-based climatology of the wind field, of low-level inversions and of LLJs in the Weddell Sea region of the Antarctic for the period 2002–2016. The sensitivity of the LLJ detection on the selection of the wind speed maximum is investigated. The common criterion of an anomaly of at least 2 m/s is extended to a relative criterion of wind speed decrease above and below the LLJ. The frequencies of LLJs are sensitive to the choice of the relative criterion, i.e., if the value for the relative decrease exceeds 15%. The LLJs are evaluated with respect to the frequency distributions of height, speed, directional shear and stability for different regions. LLJs are most frequent in the katabatic wind regime over the ice sheet and in barrier wind regions. During winter, katabatic LLJs occur with frequencies of more than 70% in many areas. Katabatic LLJs show a narrow range of heights (mostly below 200 m) and speeds (typically 10–20 m/s), while LLJs over the sea ice cover a broad range of speeds and heights. LLJs are associated with surface inversions or low-level lifted inversions. LLJs in the katabatic wind and barrier wind regions can last several days during winter. The duration of LLJs is sensitive to the LLJ definition criteria. We propose to use only the absolute criterion for model studies.

Water masses in the Atlantic Ocean: characteristics and distributions

Abstract. A large number of water masses are presented in the Atlantic Ocean, and knowledge of their distributions and properties is important for understanding and monitoring of a range of oceanographic phenomena. The characteristics and distributions of water masses in biogeochemical space are useful for, in particular, chemical and biological oceanography to understand the origin and mixing history of water samples. Here, we define the characteristics of the major water masses in the Atlantic Ocean as source water types (SWTs) from their formation areas, and map out their distributions. The SWTs are described by six properties taken from the biased-adjusted Global Ocean Data Analysis Project version 2 (GLODAPv2) data product, including both conservative (conservative temperature and absolute salinity) and non-conservative (oxygen, silicate, phosphate and nitrate) properties. The distributions of these water masses are investigated with the use of the optimum multi-parameter (OMP) method and mapped out. The Atlantic Ocean is divided into four vertical layers by distinct neutral densities and four zonal layers to guide the identification and characterization. The water masses in the upper layer originate from wintertime subduction and are defined as central waters. Below the upper layer, the intermediate layer consists of three main water masses: Antarctic Intermediate Water (AAIW), Subarctic Intermediate Water (SAIW) and Mediterranean Water (MW). The North Atlantic Deep Water (NADW, divided into its upper and lower components) is the dominating water mass in the deep and overflow layer. The origin of both the upper and lower NADW is the Labrador Sea Water (LSW), the Iceland–Scotland Overflow Water (ISOW) and the Denmark Strait Overflow Water (DSOW). The Antarctic Bottom Water (AABW) is the only natural water mass in the bottom layer, and this water mass is redefined as Northeast Atlantic Bottom Water (NEABW) in the north of the Equator due to the change of key properties, especially silicate. Similar with NADW, two additional water masses, Circumpolar Deep Water (CDW) and Weddell Sea Bottom Water (WSBW), are defined in the Weddell Sea region in order to understand the origin of AABW.

Accelerated Climate Changes in Weddell Sea Region of Antarctica Detected by Extreme Values Theory

On 13 February 2020, The Guardian, followed by many other newspapers and websites, published the news that on 9 February 2020, Antarctic air temperatures rose to about 20.75 ∘C in a base logged at Seymour Island. This value has not yet been validated by the WMO (World Meteorological Organization), but it is not the first time that an extreme temperature was registered in these locations. The recorded temperatures have often been described as “abnormal and anomalous”, according to a statement made by scientists working at the Antarctic bases. Since polar regions have shown the most rapid rates of climate change in recent years, this abnormality is of primary interest in the context of vulnerability of the Antarctic to climate changes. Using data detected at different Antarctic bases, we investigate yearly maxima and minima of recorded temperatures, in order to establish whether they can be considered as usual extreme events or abnormal. We found evidence for disagreement with the extreme values theory, indicating accelerated climate changes in the Antarctic, that is, a local warming rate that is much faster than global averages.

Evaluation of Three Numerical Weather Prediction Models for the Weddell Sea Region for the Austral Winter 2013

AbstractIt is widely recognized that numerical weather prediction (NWP) results for the Antarctic are relatively poor compared to the mid‐latitudes. In this study, we evaluate output from three operational NWP systems: the ECMWF, Global Forecast System (GFS) and Antarctic Mesoscale Prediction System (AMPS), for the Austral winter (June‐August) of 2013 for the Weddell Sea region, paying special attention to regional patterns of error statistics. This is the first evaluation of NWP systems over the Southern Ocean that also addresses the accuracy of forecasted vertical profiles. In the evaluation, we use data from land‐ and ship‐based automatic weather stations (AWS) and radiosoundings. While the ECMWF and AMPS forecasts are on average biased cold and dry near the surface, the GFS forecasts are on average biased warm and moist. The near‐surface wind speed is on average overestimated by the AMPS forecasts, whereas it is slightly underestimated by the forecasts of the other two NWP systems. Among the variables investigated, all three NWP systems forecast the near‐surface specific humidity most accurately, followed by the temperature and then the wind speed. The forecast quality for the near‐surface and upper‐air wind speed degrades the most rapidly with increasing lead time, compared to the other variables. ECMWF is the overall best NWP system when compared against both the near‐surface and upper‐air observations, followed by AMPS and then GFS. The generally poorest model performance is found in locations with complex terrain along the coast of the Antarctic continent, and the best over the ocean.

Automated iceberg tracking with a machine learning approach applied to SAR imagery: A Weddell sea case study

Drifting icebergs represent a significant hazard for polar navigation and are able to impact the ocean environment around them. Freshwater flux and the associated cooling from melting icebergs can locally decrease salinity and temperature and thus affect ocean circulation, biological activity, sea ice, and –on larger spatial scales– the whole climate system. However, despite their potential impact, the large-scale operational monitoring of drifting icebergs in sea ice-covered regions is as of today typically restricted to giant icebergs, larger than 18.5 km in length. This is due to difficulties in accurately identifying and following the motion of much smaller features in the polar ocean from space. So far, tracking of smaller icebergs from satellite imagery thus has been limited to open-ocean regions not covered by sea ice. In this study, a novel automated iceberg tracking method, based on a machine learning-approach for automatic iceberg detection, is presented. To demonstrate the applicability of the method, a case study was performed for the Weddell Sea region, Antarctica, using 1213 Advanced Synthetic Aperture Radar (ASAR) satellite images acquired between 2002 and 2011. Overall, a subset of 414 icebergs (3134 re-detections in total) with surface areas between 3.4 km2 and 3612 km2 were investigated with respect to their prevalent drift patterns, size variability, and average disintegration. The majority of the tracked icebergs drifted between 1.3 km and 2679.2 km westward around the Antarctic continent, following the Antarctic Coastal Current (ACoC) and the Weddell Gyre, at an average drift speed of 3.6 ± 7.4 km day−1. The method also allowed us to estimate an average daily disintegration (i.e. iceberg area decrease) rate of ~0.13% (~37% year−1) for all icebergs. Using the sum of all detected individual surface area reductions, we estimate a total iceberg mass decrease of ~683 Gt year−1, which can be freshwater input and/or new ‘child’ icebergs calved from larger icebergs. The extension to an automated long-term tracking method for icebergs is challenging as the iceberg shape can vary significantly due to abrupt disintegration or calving of bergy bits. However, our machine learning approach extended by automatic shape-based tracking capabilities proved to be a reliable alternative for automatic detection and tracking of icebergs, even under the ambiguous SAR background signatures often found in the Southern Ocean. In particular, the method works in the challenging near-coastal environment where the presence of sea ice and coastal ocean dynamics such as surface waves usually pose major obstacles for other approaches.

Effect of Antarctic sea ice on chlorophyll concentration in the Southern Ocean

Sea-ice extent is very sensitive to climate change and its minor variations can significantly affect the regional biota of the Southern Ocean (SO). Chlorophyll-a concentration (Chl-a) is a primary proxy for the understanding of phytoplankton distribution and primary productivity in the oceans. Therefore, analysing the relation between Chl-a and sea-ice extent could be significant in understanding the role of SO sea-ice extent on regional Chl-a variability. Local Chl-a variability in the SO was analysed in five major sectors, utilizing 39 years of remotely sensed sea ice and 15 years of Chl-a and Sea Surface Temperature (SST) datasets. Katabatic winds blowing offshore from the Antarctic coast were found to enhance the sea-ice extent during winter and diminish sea-ice melting in the austral summer in the SO. Thus, cold polar wind enhanced the sea-ice extent whereas warm subtropical winds reduced its formation. The Weddell Sea region showed the highest monthly averaged sea-ice extent of 6.2 × 106km2 during September (austral spring), whereas monthly averaged sea-ice extent was less than 2.9 × 106km2 for any other region. The largest sea-ice extent of 2 × 104km2 was observed in the Ross Sea during the austral summer (December, January, and February) owing to the katabatic winds. All other regions reported sea-ice extents below 1.15 × 104km2. Melting of sea ice during the austral summer was responsible for enhanced Chl-a in the SO through increased availability of nutrients in the near surface waters. Considering Chl-a as a proxy for productivity in the SO, the western sector (Weddell, Bellam and Ross Sea regions) was found to be more productive than the eastern sector (Indian and Pacific Ocean regions) during the study period. To understand the vertical variation of physical and biogeochemical variables and their influence on Chl-a variability, data from a few active Biogeochemical-Argo floats in the region were analysed. It was concluded that while the surface Chl-a was directly influenced by SST variations and sea-ice melting, the nutricline influenced the depth of Deep Chlorophyll Maximum (DCM) and Chl-a vertical distribution in the SO.

Enhanced Poleward Flux of Atmospheric Moisture to the Weddell Sea Region (ODP Site 690) During the Paleocene‐Eocene Thermal Maximum

AbstractEarth's hydrological cycle was profoundly perturbed by massive carbon emissions during an ancient (56 Ma) global warming event referred to as the Paleocene‐Eocene thermal maximum (PETM). One approach to gaining valuable insight into the response of the hydrological cycle is to construct sea‐surface salinity (SSS) records that can be used to gauge changes in the rates of evaporation and precipitation during the PETM in such climatically sensitive areas as the circum‐Antarctic region. Here, we pair oxygen isotope (δ18O) and magnesium‐calcium (Mg/Ca) measurements to reconstruct PETM sea‐surface temperatures (SSTs) and δ18O composition of seawater (δ18Osw) at austral Site 690 (Weddell Sea). Several discrepancies emerge between the δ18O‐ and Mg/Ca‐based SST records, with the latter indicating that the earliest PETM was punctuated by a short‐lived ~4°C increase in local SSTs. Conversion of the δ18Osw values to SSS reveals a ~4 ppt decrease ~50 ka after peak PETM warming at Site 690. This negative SSS (δ18Osw) anomaly coincides with a prominent minimum in the planktic foraminifer δ18O record published for the Site 690 PETM section. Thus, our revised interpretation posits that this δ18O minimum signals a decrease in surface‐ocean δ18Osw fostered by a transient increase in mean annual precipitation in the Weddell Sea region. The results of this study corroborate the view that the poleward flux of atmospheric moisture temporarily increased during a distinctive stage of the PETM.