Abruptly attenuated carbon sequestration with Weddell Sea dense waters towards the end of the 21st century

Antarctic Bottom Water formation, such as in the Weddell Sea, is an efficient vector for carbon sequestration on time scales of centuries. Possible changes in carbon sequestration under changing environmental conditions are unquantified to date, mainly due to difficulties in simulating the relevant processes on high-latitude continental shelves. Using a model setup including both ice-shelf cavities and oceanic carbon cycling, we demonstrate that by 2100, deep-ocean carbon accumulation in the southern Weddell Sea is abruptly attenuated to only 40% of the rate in the 1990s in a high-emission scenario, while still being 4-fold higher in the 2080s. Assessing deep-ocean carbon budgets and water mass transformations, we attribute this decline to an increased presence of Warm Deep Water on the southern Weddell Sea continental shelf, a 16% reduction in sea-ice formation, and a 79% increase in ice-shelf basal melt. Altogether, these changes lower the density and volume of newly formed bottom waters and reduce the associated carbon transport to the abyss.

Thermohaline structure and circulation beneath the Langhovde Glacier ice shelf in East Antarctica

Basal melting of ice shelves is considered to be the principal driver of recent ice mass loss in Antarctica. Nevertheless, in-situ oceanic data covering the extensive areas of a subshelf cavity are sparse. Here we show comprehensive structures of temperature, salinity and current measured in January 2018 through four boreholes drilled at a ~3-km-long ice shelf of Langhovde Glacier in East Antarctica. The measurements were performed in 302–12 m-thick ocean cavity beneath 234–412 m-thick ice shelf. The data indicate that Modified Warm Deep Water is transported into the grounding zone beneath a stratified buoyant plume. Water at the ice-ocean interface was warmer than the in-situ freezing point by 0.65–0.95°C, leading to a mean basal melt rate estimate of 1.42 m a−1. Our measurements indicate the existence of a density-driven water circulation in the cavity beneath the ice shelf of Langhovde Glacier, similar to that proposed for warm-ocean cavities of larger Antarctic ice shelves.

Weddell seal behaviour during an exceptional oceanographic event in the Filchner-Ronne Ice Shelf in 2017

Rapid and regionally contrasting climate changes have been observed around Antarctica. However, our understanding of the impact of these changes on ecosystems remains limited, and there is an urgent need to better identify habitats of Antarctic species. The Weddell seal (Leptonychotes weddellii) is a circumpolar mesopredator and an indicative species of Antarctic marine communities. It has been extensively studied in the western Ross Sea and East Antarctica, and an understanding of its ecology in the Weddell Sea in the wintertime is emerging. We documented the behavioural response(s) of four Weddell seals from February to June in 2017 in the Filchner-Ronne Ice Shelf region and related these to unusual oceanographic conditions in 2017. Unexpectedly, we found that Weddell seals had the longest foraging effort within the outflow of Ice Shelf Water or at its turbulent boundary. They also foraged on the eastern side of the trough from April to June within the Modified Warm Deep Water and seem to take advantage of the unusual conditions of persistent inflow of warm waters through the winter. Linking animal behavioural responses to oceanographic conditions is informative for quantifying rarely recorded events and provides great insight into how predators may respond to changing conditions.

Evaluation of four coupled climate models in the Amundsen Sea, Antarctica

<p>The Amundsen Sea glaciers, in West Antarctica, are among the world’s fastest discharges of ice into the ocean. The rapid thinning of these ice shelves can be largely explained by basal melting driven by the ocean. Relatively warm water reaches the continental shelf in the Amundsen Sea and deep bathymetric troughs facilitate warm deep water flow to the base of the ice shelves. However, time sparse observational data, and even poorly known bathymetry, contribute to the difficulty of quantifying the key ocean mechanisms, and their variability, that transport warm water onto the Amundsen Sea continental shelf and guide it southward into the ice shelf cavities. Nonetheless these processes should be represented in the coupled climate models, such as those used for CMIP6, which are being used to project future sea level rise.</p><p>Here we leverage recent observational campaigns and gains in process understanding to assess how well four of these models, UKESM1 and the HadGEM-GC3.1 family of models, represent the ocean processes forcing warm water onto the Amundsen Sea continental shelf. The three HadGEM models have the same external forcing but different horizontal resolutions, 1/12, ¼ and 1 degree. The 1 degree resolution UKESM1 is based on HadGEM3.1 but includes atmospheric chemistry, aerosols and marine biogeochemistry. A key finding is the medium resolution (1/4°) HadGEM-GC3.1 model’s inability to allow warm deep water intrusion onto the continental shelf, associated with a strong westward slope current that is not present in the other models. The medium resolution model represents well the annual cycle of sea ice in the Amundsen Sea, but overall has significantly less sea ice around Antarctica, compared with the other models and satellite observations. Despite its low resolution, UKESM1 represents well all the main ocean features, including the shelf-break undercurrent, warm deep water and realistic sea ice. It captures more significant interannual variability, in contrast to the low resolution HadGEM, for which the interannual variability is more suppressed. Of the four models considered here, the best performing models are the 1/12° HadGEM and UKESM1, followed by the low resolution HadGEM model, which reasonably represents warmer deep water on the continental shelf and a shallower mixed layer. The medium resolution HadGEM, despite its better resolution is less realistic than the two low resolution models.</p>

Seasonal variability of ocean circulation near the Dotson Ice Shelf, Antarctica

Recent rapid thinning of West Antarctic ice shelves are believed to be caused by intrusions of warm deep water that induce basal melting and seaward meltwater export. Dotson Ice Shelf has a high basal melt rate due to southward ocean heat transport in the Dotson-Getz Trough. We deployed three bottom-moored instrument arrays along the ice shelf calving front, obtaining continuous records of temperature, salinity, and current velocity throughout 2014 and 2015. Southward deep water velocities were highest along the eastern channel slope, while northward outflows of freshened ice shelf meltwater spread at intermediate depth above the western slope. Inflow warm water along the eastern slope into the sub-Dotson cavity reached a maximum of 182 MW m− 1 in Summer, 3.5 times larger than the autumn/winter values of 51 MW m− 1. The inflow correlated with the local ocean surface stress curl. At the western slope meltwater outflows were strongest during autumn and weakest in spring, following the warm influx along the eastern slope with a ~ 2–3 months delay. Ocean circulation near Dotson Ice Shelf, affected by sea ice distribution and wind, appears to be a significant control on the inflow of warm water and subsequent ice shelf melting on seasonal time-scales.

Warm water flow and mixing beneath Thwaites Glacier ice shelf, West Antarctica

<p>The fate of the West Antarctic Ice Sheet is the largest remaining uncertainty in predicting sea-level rise through the next century, and its most vulnerable and rapidly changing outlet is Thwaites Glacier . Because the seabed slope under the glacier is retrograde (downhill inland), ice discharge from Thwaites Glacier is potentially unstable to melting of the underside of its floating ice shelf and grounding line retreat, both of which are enhanced by warm ocean water circulating underneath the ice shelf. Recent observations show surprising spatial variations in melt rates, indicating significant knowledge gaps in our understanding of the processes at the base of the ice shelf. Here we present the first direct observations of ocean temperature, salinity, and oxygen underneath Thwaites ice shelf collected by an autonomous underwater vehicle, a Kongsberg Hugin AUV. These observations show that while the western part of Thwaites has outflow of meltwater-enriched circumpolar deep water found in the main trough leading to Thwaites, the deep water (> 1000 m) underneath the central part of the ice shelf is in connection with Pine Island Bay - a previously unknown westward branch of warm deep water flow. Mid-depth water (700 - 1000 m) enters the cavity from both sides of a buttressing point and large spatial gradients of salinity and temperature indicate that this is a region of active mixing processes. The observations challenge conceptual models of ice-ocean interactions at glacier grounding zones and identify a main buttressing point as a vulnerable region of change currently under attack by warm water inflow from all sides: a scenario that may lead to ungrounding and retreat more quickly than previously expected.</p>

The horizontal circulation, upwelling and heat budget of the Weddell Gyre: an observation perspective

<p>The Weddell Gyre is an important region in that it feeds source water masses (and thus heat) toward the ice-shelves, and exports locally and remotely formed dense water masses to the global abyssal ocean. Argo float profiles and trajectories were implemented to capture the large-scale, long-term mean circulation of the entire Weddell Gyre, from which the heat budget has been diagnosed for a layer within Warm Deep Water (WDW), the main heat source to the Weddell Gyre. We show that heat is horizontally advected into the southern limb of the Weddell Gyre, and then removed from the southern limb by horizontal turbulent diffusion (1) northwards towards the gyre interior, and (2) southwards towards the ice shelves. Since the gyre is cyclonic, the heat that is turbulently diffused into the gyre interior is subsequently brought closer to the surface by upwelling. Upwelling is thus an important yet poorly understood feature of the dynamics of the Weddell Gyre. This study marks the beginnings of a project focused on improved understanding of the role of upwelling within the Weddell Gyre, and investigating the role of turbulent diffusion in redistributing heat towards the central gyre interior, as well as towards the ice shelves of Antarctica.</p>

A look beneath the ice shelves of western Dronning Maud Land, East Antarctica: subglacial topography linked to ice shelf stability

<p>Antarctica’s ice shelves play a key role in stabilizing their related ice sheets. The ice shelves of western Dronning Maud Land – including the Ekström, Atka, Jelbart, Fimbul and Vigrid ice shelves – currently buttress a catchment that comprises an ice volume equivalent to 0.95 meters of sea level. Any future increase in ice shelf mass loss, with basal melting likely being the main cause, will inevitably accelerate ice sheet drainage and contribute to global sea level rise. Since basal melting largely depends on ice-ocean interactions, it is crucial to attain reliable and consistent bathymetry models to estimate water and heat exchange beneath these ice shelves. We have constructed bathymetry models for an area of about 63,000 km<sup>2</sup> beneath the ice shelves of western Dronning Maud Land by inverting airborne gravity data, tied to radar, seismic, and offshore depth reference points. New high-resolution airborne magnetic data across the ice shelves point to Jurassic intrusions and seaward-dipping reflectors originating from Gondwana breakup; enabling us to consider geological density variations as part of the bathymetry modelling process. Our bathymetric models reveal deep glacial troughs beneath the ice shelves, and sills close to the continental shelf breaks which currently limit the possible entry of Warm Deep Water from the Southern Ocean. The present-day average thermocline depth is comparable to the average depths of saddles along the sills, which present gateways into the sub-ice cavities. This leads us to suggest a high sensitivity for these ice shelves to changes in ocean temperature and especially thermocline depth in the future. Once a significant amount of warm water overtops the sills, the deep troughs will allow for fast access to the grounding line, after which it seems there may be little to stop basal melting from rapidly eroding the ice shelves of western Dronning Maud Land.</p>

Recent Changes in the Larsen-Weddell System

<p>A warming planet, and particularly the warming Pacific Ocean, has led to major changes in the Larsen-Weddell System. While somewhat less significant than those in the adjacent Amundsen-Bellingshausen Sea and its coastal ice, the changes are nonetheless dramatic indicators of a closely interconnected system, driven by increased westerly winds and their impact on surface melting and ice drift. The system very likely will see further major changes if warming continues through the 22<sup>nd</sup> Century.</p><p>A warming trend in the central Pacific over the past ~80 years has induced air circulation changes over the Southern Ocean and Antarctic Peninsula. A rise in the mean speed of westerly-northwesterly winds across the northern Peninsula led to more frequent foehn events, which in turn increased surface melting on the eastern Peninsula ice shelves, and were responsible for reduced sea ice cover and more frequent shore leads on the eastern edges of the ice shelves. This likely led to greater sub-ice-shelf circulation, possibly including solar-warmed surface water (in summer) and modified Weddell Deep Water (mWDW). Around 1986, structural evidence in the form of more disrupted shear zones and increased rifting suggests that the Larsen A and B ice shelves began to thin and weaken. At this progressed, a combination of increased surface ponding and reduced backstress on the iceshelves led to a series of catastrophic break-ups due to hydrofracture, in 1995 (Larsen A shelf) and 2002 (Larsen B).  More recently, thinning detected by altimetry on the northern Larsen C may have contributed to new fracturing and calving of a large iceberg there in 2016 (iceberg A-68), setting the ice shelf front significantly farther to the west than has previously been observed (since 1898).</p><p>Looking forward, if the trend in increased westerly winds and Southern Annular Mode index continues, it is anticipated (modelled) that the large clockwise Weddell Gyre will increase in mean flow speed, and that warm deep water entrained from the Antarctic Circumpolar Current will more frequently mix with the mid- to deep ocean layers in the Weddell Gyre. One outcome of this is likely to be advection of warm deep water into the Ronne Ice Shelf cavity, dramatically increasing the heat available for sub-ice-shelf melting there and potentially changing ice sheet flux from the outlet glaciers significantly.</p>