Amundsen Sea

The Amundsen Sea is a marginal sea of the Southern Ocean lying off Marie Byrd Land in West Antarctica, between Cape Flying Fish to the east and Cape Dart on Siple Island to the west. Covering approximately 98,000 km² and reaching depths of up to 3,600 metres, it is one of the most remote and scientifically significant bodies of water on Earth. Although relatively small compared to many polar seas, the Amundsen Sea has become the focus of intense international scientific attention because it drains the most rapidly changing and climatically vulnerable sector of the West Antarctic Ice Sheet (WAIS) — a body of ice containing enough frozen water to raise global sea levels by three to five metres if it were to fully melt.

Unlike the Ross Sea and Weddell Sea — the two great Antarctic seas with long histories of exploration and established scientific infrastructure — the Amundsen Sea was barely visited before the late twentieth century. Its treacherous sea ice, severe weather, remoteness from any port, and poorly charted waters made it forbidding even by Antarctic standards. Yet the glaciers that calve into the Amundsen Sea — Thwaites, Pine Island, Smith, Kohler, Dotson, and Crosson among the most significant — are now understood to be the single largest contributors to observed sea level rise from the Antarctic continent, collectively losing ice at an accelerating rate that has doubled since the 1990s.

The sea takes its name from the Norwegian explorer Roald Amundsen, the first person to reach the South Pole (December 1911) and to navigate the Northwest Passage. It was formally named in his honour, though Amundsen himself never sailed these waters. The first dedicated scientific exploration of the sea came decades after his polar achievements, during the International Geophysical Year (1957–58) and in subsequent USAP (United States Antarctic Program) voyages. The modern era of Amundsen Sea science, however, began in earnest with the recognition in the 1990s and 2000s that its glaciers were retreating at alarming speed — a process since confirmed by satellite radar, airborne surveys, and direct oceanographic measurement.

For mariners and maritime professionals, the Amundsen Sea represents one of the most demanding operational environments on the planet. There are no ports, no permanent rescue facilities, and only limited hydrographic survey coverage. Voyage planning requires polar expertise, ice-rated vessels, and careful coordination with national Antarctic programs. Access is seasonal, practical only during January and February, and conditions can deteriorate rapidly at any time. Yet the scientific imperative of understanding what is happening to the glaciers of this remote sea has never been more urgent.

1. Geography & Physical Characteristics

The Amundsen Sea occupies the Pacific sector of the Southern Ocean, lying between approximately 100°W and 135°W longitude and extending poleward from around 70°S to the Antarctic coastline. It is bounded to the northeast by the Bellingshausen Sea (separated notionally at the Hobbs Coast) and to the west by the Ross Sea approaches. The coastline fronting the Amundsen Sea is part of Marie Byrd Land, a vast and largely featureless expanse of the West Antarctic Ice Sheet that rises from the coast toward the interior of the continent at elevations of up to 2,000 metres above sea level.

Pine Island Bay is the most prominent geographic feature of the Amundsen Sea's inner coast. It is a deeply indented embayment receiving ice from several of the most active glaciers on Earth. The bay is flanked by the Pine Island Glacier to the north and the Thwaites Glacier to the south — together these two glaciers drain approximately 180,000 km² of the West Antarctic Ice Sheet. The Pine Island Glacier has one of the fastest-moving ice fronts on the continent, flowing at velocities of up to 4 km per year in its main trunk, and has experienced repeated calving events that have produced large tabular icebergs in recent decades.

Other significant glaciers draining into the Amundsen Sea include the Smith Glacier, the Kohler Glacier, the Dotson Glacier (fronted by the Dotson Ice Shelf), and the Crosson Glacier (fronted by the Crosson Ice Shelf). All of these glaciers are grounded on bedrock that lies below sea level — in some cases more than 2,000 metres below sea level in the deep interior — a configuration that makes them inherently susceptible to a runaway retreat mechanism known as Marine Ice Sheet Instability (MISI). Under MISI, once a glacier retreats past a threshold point, the deepening of the bed inland means that the grounding line continues to retreat indefinitely without any additional external forcing.

The continental shelf of the Amundsen Sea is relatively deep compared to other Antarctic shelves, averaging 400 to 500 metres depth, with submarine troughs and channels incised by former glacier flow that reach depths exceeding 1,000 metres in places. These deep troughs are significant because they provide pathways for warm Circumpolar Deep Water to flow from the open Southern Ocean directly to the grounding lines of the major glaciers. Siple Island and Bear Peninsulaare notable coastal features that partially shelter inner reaches of the sea from the open Southern Ocean. The Thwaites Eastern Ice Shelf and the Thwaites Glacier Tongue are the major floating ice features of the sea, both of which have undergone dramatic structural changes in recent years as warming ocean water undermines them from below.

2. Oceanography & Ice Shelf Dynamics

The oceanography of the Amundsen Sea is dominated by the intrusion of Circumpolar Deep Water (CDW) — a warm, relatively salty water mass that circulates within the Antarctic Circumpolar Current (ACC) at intermediate depths. CDW in the Southern Ocean has a temperature of approximately 0.5°C to 1.2°C above the freezing point of seawater at depth, which is warm enough to drive intense basal melting when it comes into contact with the undersides of floating ice shelves. In the Amundsen Sea, CDW does not remain offshore: it crosses the relatively deep continental shelf and flows along submarine glacial troughs directly to the grounding lines of the major outlet glaciers.

The mechanism of basal melting — the melting of ice from below by ocean heat rather than from above by atmospheric warming — is fundamentally different from the surface melting processes that affect Arctic glaciers and temperate ice sheets. Because the Amundsen Sea glaciers are not losing ice primarily from their surfaces (which remain cold), they are instead losing it from the underside of their floating ice shelves. The ice shelves act as buttresses, holding back the flow of inland ice; when basal melting thins or removes the ice shelf, the upstream glacier accelerates and discharges more ice into the ocean. Basal melt rates beneath the Thwaites and Pine Island ice shelves have been measured at 50 to 100 metres per year in the most active zones — extraordinarily high by any comparison.

The surface waters of the Amundsen Sea are strongly influenced by seasonal sea ice. In winter, sea ice extends far northward, insulating the ocean surface and limiting exchange with the atmosphere. In summer, ice retreats, permitting sunlight to penetrate the surface layer and drive phytoplankton blooms. Meltwater from the sea ice creates a fresh, cold surface layer (Antarctic Surface Water) that sits above the warmer CDW beneath. Wind patterns — driven by the position of the Amundsen Sea Low, a persistent atmospheric low-pressure system centred over the sea — influence how strongly CDW is upwelled onto the shelf and how much melting occurs. Changes in wind patterns associated with climate variability (including the El Niño–Southern Oscillation) modulate CDW intrusion on interannual timescales, but the long-term trend is one of increasing ocean heat delivery and accelerating ice loss.

Oceanographic measurements in the Amundsen Sea have been conducted primarily by research vessels equipped with CTD (Conductivity, Temperature, Depth) instruments and by autonomous underwater vehicles (AUVs) capable of penetrating beneath floating ice shelves. The ITGC deployed the Boaty McBoatfish and other AUVs beneath the Thwaites Ice Shelf to map the cavity geometry and measure the temperature and salinity of water melting the glacier from below — among the most technically demanding oceanographic measurements ever attempted. These data have confirmed that warm CDW is directly accessing the glacier grounding zone, driving basal melt at rates that are challenging even the most pessimistic model projections.

3. Marine Ecology & Biodiversity

Despite its extreme conditions, the Amundsen Sea supports a productive and characteristic Antarctic marine ecosystem. The biological community is structured around the seasonal pulse of primary production that occurs as sea ice retreats in spring and summer, exposing surface waters to Antarctic sunlight and triggering intense phytoplankton blooms. These blooms — dominated by diatoms and other microalgae — form the energetic base of a food web that supports some of the most abundant wildlife populations in the Southern Ocean.

Antarctic krill (Euphausia superba) are the keystone species of the Amundsen Sea ecosystem, as throughout the Southern Ocean. Krill aggregate in enormous swarms, feeding on phytoplankton and ice algae, and in turn sustain virtually every larger animal in the ecosystem. The crabeater seal (Lobodon carcinophaga) — despite its misleading name, it feeds almost exclusively on krill — is the most abundant pinniped species on Earth, with populations numbering in the tens of millions around Antarctica, including substantial numbers in the Amundsen Sea pack ice. Weddell seals (Leptonychotes weddellii) are associated with fast ice and coastal areas, where they maintain breathing holes through the sea ice in winter.

Minke whales (Balaenoptera bonaerensis — the Antarctic minke) are abundant in the Amundsen Sea during summer, exploiting krill concentrations in ice-edge waters. They are among the most numerous baleen whales in Antarctic waters and represent an important link between the krill community and higher trophic levels. The remoteness of the Amundsen Sea has largely protected its cetacean populations from the historical whaling pressure that devastated populations in more accessible Antarctic waters during the twentieth century.

Adélie penguins (Pygoscelis adeliae) and emperor penguins(Aptenodytes forsteri) are the characteristic seabirds of the Amundsen Sea. Emperor penguins breed at colonies on the fast ice of the West Antarctic coast during winter, returning to sea to feed in summer. Adélie penguins nest on ice-free rocky outcrops around the coast and adjacent island groups during the summer breeding season. Antarctic silverfish (Pleuragramma antarcticum) are an ecologically critical fish species in the Amundsen Sea, serving as a key prey item for penguins, seals, and whales. Unlike most polar fish, silverfish spawn in sea ice cavities, making them particularly vulnerable to changes in sea ice extent and timing. There are no commercial fisheries in the Amundsen Sea; its extreme remoteness makes commercial exploitation impractical, and the area falls under the management framework of the CCAMLR (Commission for the Conservation of Antarctic Marine Living Resources).

4. Navigation & Vessel Access

The Amundsen Sea falls within NAVAREA XIV, the Antarctic navigational warning area coordinated by Maritime New Zealand on behalf of the IMO. NAVAREA XIV covers the Pacific sector of the Southern Ocean south of 55°S. Navigational warnings are issued via SafetyNET on Inmarsat-C and via Navtex where coverage exists, covering ice hazards, new obstructions, and research vessel operations. There are no VTS (Vessel Traffic Service) authorities, no port state control facilities, and no SAR (Search and Rescue) bases within reach of the Amundsen Sea. The nearest SAR coordination capability is operated by Chile (MRCC Punta Arenas) and the United States Coast Guard, but response times to the remote Amundsen Sea would be measured in days.

There are no ports or harbours anywhere in or adjacent to the Amundsen Sea. Research vessels operating in the area typically depart from Punta Arenas, Chile or Lyttelton, New Zealand — the two nearest supply ports with Antarctic logistics infrastructure — after passage through the Drake Passage or across the Southern Ocean respectively. Vessel access is practical only during the Antarctic summer window of approximately late December through late February. Outside this window, sea ice consolidates rapidly and even icebreakers face extreme difficulty. Within the summer window, ice conditions vary greatly from year to year and require voyage planning based on the most current satellite sea ice analysis (available from the US National Ice Center, the Australian Antarctic Division, and the UK's British Antarctic Survey).

All vessels operating in the Amundsen Sea must comply with the IMO Polar Code(entered into force 1 January 2017), which establishes mandatory requirements for vessel construction, equipment, operations, and environmental protection in polar waters. The sea's classification as Category A ice (the most demanding) means that vessels must be purpose-built ice-strengthened ships or icebreakers to operate safely. Research icebreakers that have operated in the Amundsen Sea include the RV Nathaniel B. Palmer (USA), the RRS Sir David Attenborough (UK), the RV Araon (South Korea), and the RV Polarstern (Germany). Navigation within the sea requires continuous watch for growlers (small, low-lying ice fragments invisible to radar), bergy bits, and large tabular icebergs calved from the major glaciers — some exceeding 100 km in length. Charting of the Amundsen Sea is incomplete; significant areas have never been systematically surveyed by modern multibeam echo sounder, and charted depths should be treated with caution.

Communication in the Amundsen Sea relies entirely on satellite systems, primarily Iridium satellite phones and Inmarsat-C for safety communications and GMDSS compliance. HF radio propagation is unreliable at high southern latitudes. EPIRB registration and functionality must be verified before departure, and personal locator beacons (PLBs) should be carried by all crew. Fuel is carried for the entire voyage without any possibility of bunkering en route, making fuel management a critical voyage planning consideration.

5. Exploration & Scientific History

The Amundsen Sea was one of the last major Antarctic seas to receive systematic exploration, largely because its sea ice conditions and remoteness from the principal routes of early Antarctic expeditions made access extremely difficult. Early cartographic knowledge of the area was fragmentary — vague coastlines derived from brief sightings by nineteenth-century seal hunters and explorers navigating the wider Southern Ocean.

The first recorded scientific foray into the Amundsen Sea was made by the Bear, the ship of the Byrd Antarctic Expedition, under the command of Lawrence Gouldin 1929. Gould's voyage mapped sections of the coast and provided the first systematic observations of the ice edge and coastal geography, laying the foundation for subsequent United States Antarctic Program activities in the region. The sea was named in honour of Roald Amundsen — who had died in 1928 on an Arctic search-and-rescue mission — during the mapping work of this era.

The International Geophysical Year (IGY, 1957–58) marked the beginning of coordinated scientific activity across Antarctica, including establishment of the first permanent US stations and overland traverses across the ice sheet. The ITASE (International Trans-Antarctic Scientific Expedition), a series of overland traverses across the WAIS conducted from the 1990s through the 2000s, collected ice cores and surface snow samples across the interior of the ice sheet, providing records of past climate and accumulation rates that were essential context for understanding the dynamic changes observed at the coast.

The WAIS Divide Ice Core project (drilling completed 2011) recovered a 3,405-metre ice core from a site 160 km from the flow divide of the West Antarctic Ice Sheet, providing a 68,000-year climate record of extraordinary resolution. The WAIS Divide record demonstrated that Antarctic climate changes during the last glacial period were closely coupled to climate changes in the Northern Hemisphere on short (decadal) timescales — a finding with important implications for understanding the pace at which future warming may drive Antarctic ice loss. The recognition during the 2000s and 2010s that Pine Island and Thwaites glaciers were retreating at dramatically accelerating rates transformed the Amundsen Sea from a scientific backwater into one of the most intensively studied ocean-ice systems on the planet, ultimately leading to the establishment of the ITGC.

6. Environmental Crisis & Sea Level Rise

The Amundsen Sea sector of Antarctica is at the centre of one of the most consequential environmental crises of the twenty-first century. The glaciers draining into this sea are losing ice at a rate that has approximately doubled since the 1990s and now constitutes the largest single source of sea level rise from the Antarctic Ice Sheet. Current estimates suggest that the Amundsen Sea glaciers are collectively contributing approximately 0.4–0.6 mm to global sea level rise per year — a small number that nonetheless represents an accelerating trend with potentially catastrophic long-term consequences.

Thwaites Glacier — nicknamed the “Doomsday Glacier” in popular media and scientific commentary — is the single glacier most closely watched by glaciologists worldwide. Thwaites drains an area the size of Florida (approximately 192,000 km²) and contains enough ice to raise global sea levels by approximately 65 centimetres if it were to fully melt. More critically, Thwaites is considered a potential “keystone” glacier: its collapse could destabilise neighbouring glaciers and trigger a cascading retreat of the wider West Antarctic Ice Sheet, which holds enough ice for 3–5 metres of sea level rise. The ice shelf that currently buttresses Thwaites is estimated to have weakened dramatically in recent decades; satellite radar interferometry has revealed a series of large cracks suggesting that partial ice shelf collapse may occur within years to decades.

Pine Island Glacier has undergone repeated large calving events, losing massive sections of its ice shelf in 2001, 2007, 2011, 2013, 2015, 2017, and 2020. Each calving event reduces the buttressing effect on the inland glacier and is followed by a period of acceleration. The glacier's grounding line — where the glacier transitions from grounded ice to floating ice shelf — has retreated by approximately 30 km since the 1990s. Both Pine Island and Thwaites sit in overdeepened basins where the bed deepens inland, meaning that Marine Ice Sheet Instability could drive irreversible retreat without any further increase in ocean warming — the warming already delivered may be sufficient.

The mechanism driving ice loss is the delivery of warm Circumpolar Deep Water to glacier grounding lines, a process modulated in part by the Amundsen Sea Low — a semi-permanent atmospheric low-pressure system whose position and intensity influence wind-driven upwelling of CDW onto the continental shelf. Evidence suggests that changes in the Amundsen Sea Low associated with ozone depletion and greenhouse gas forcing have increased CDW intrusion in recent decades. The interaction between atmospheric forcing, ocean circulation, and ice dynamics in the Amundsen Sea represents one of the most complex and consequential feedback systems in the Earth's climate. Sea level rise projections that do not account for potential Marine Ice Sheet Instability in the WAIS are almost certainly underestimates of the risk facing low-lying coastal communities worldwide.

7. Scientific Research Programmes

The International Thwaites Glacier Collaboration (ITGC) is the largest joint UK-US Antarctic science programme in more than 70 years, launched in 2018 with £25 million in combined funding from the Natural Environment Research Council (NERC) and the US National Science Foundation (NSF). The ITGC encompasses eight major scientific projects covering ocean-ice interaction, glacier dynamics, bed topography mapping, ice shelf structural integrity, glacial history, and risk assessment. Its findings are intended to provide the most accurate possible estimates of the rate and timing of Thwaites Glacier retreat and the associated sea level rise implications.

The ITGC's TARSAN (Thwaites-Amundsen Regional Survey and Network) project deployed oceanographic moorings, ice-tethered profilers, and autonomous underwater vehicles to measure ocean temperature, salinity, and circulation patterns beneath and adjacent to the Thwaites Ice Shelf. The MELT (Melting at Thwaites grounding zone and its control on sea level) project used a hot-water drill to penetrate 600 metres through the Thwaites Eastern Ice Shelf, deploying instruments through the drill hole to measure conditions in the sub-shelf ocean cavity — the first time humans had directly observed conditions beneath Thwaites. The instruments recorded water temperatures approximately 2°C above freezing, confirming vigorous basal melting at the grounding zone.

Supporting national programmes include NASA's Operation IceBridge (now succeeded by ICESat-2 satellite measurements), which conducted thousands of hours of airborne radar and laser altimetry surveys over the Amundsen Sea sector, providing high-resolution maps of ice thickness, surface elevation change, and bed topography. The BedMachine Antarcticadataset, compiled by NASA from diverse sources, has revealed the deep and complex topography of the WAIS bed in unprecedented detail, showing that several Amundsen Sea glaciers sit in overdeepened basins that extend hundreds of kilometres inland — dramatically expanding the volume of ice at risk from Marine Ice Sheet Instability. Satellite missions including ESA's CryoSat-2 and the Copernicus Sentinel series provide ongoing monitoring of ice surface elevation and velocity across the Amundsen Sea drainage basin, delivering near-real-time data on the continuing acceleration of the major outlet glaciers.