Subantarctic Mode Water variability influenced by mesoscale eddies south of Tasmania

[1] Subantarctic Mode Water (SAMW) is formed by deep mixing on the equatorward side of the Antarctic Circumpolar Current. The subduction and export of SAMW from the Southern Ocean play an important role in global heat, freshwater, carbon, and nutrient budgets. However, the formation process and variability of SAMW remain poorly understood, largely because of a lack of observations. To determine the temporal variability of SAMW in the Australian sector of the Southern Ocean, we used a 15 year time series of repeat expendable bathythermograph sections from 1993 to 2007, seven repeat conductivity-temperature-depth sections from 1991 to 2001, and sea surface height maps. The mean temperature of the SAMW lies between 8.5°C and 9.5°C (mean of 8.8°C, standard deviation of 0.3°C), and there is no evidence of a trend over the 18 year record. However, the temperature, salinity, and pycnostad strength of the SAMW can change abruptly from section to section. In addition, the SAMW pool on a single section often consists of two or more modes with distinct temperature, salinity, and vertical homogeneity characteristics but similar density. We show that the multiple types of mode water can be explained by the advection of anomalous water from eddies and meanders of the fronts bounding the Subantarctic Zone and by recirculation of SAMW of different ages. Our results suggest that infrequently repeated sections can potentially produce misleading results because of aliasing of high interannual variability

Circumpolar habitat use in the southern elephant seal : implications for foraging success and population trajectories

In the Southern Ocean, wide-ranging predators offer the opportunity to quantify how animals respond to differences in the environment because their behavior and population trends are an integrated signal of prevailing conditions within multiple marine habitats. Southern elephant seals in particular, can provide useful insights due to their circumpolar distribution, their long and distant migrations and their performance of extended bouts of deep diving. Furthermore, across their range, elephant seal populations have very different population trends. In this study, we present a data set from the International Polar Year project; Marine Mammals Exploring the Oceans Pole to Pole for southern elephant seals, in which a large number of instruments (N = 287) deployed on animals, encompassing a broad circum-Antarctic geographic extent, collected in situ ocean data and at-sea foraging metrics that explicitly link foraging behavior and habitat structure in time and space. Broadly speaking, the seals foraged in two habitats, the relatively shallow waters of the Antarctic continental shelf and the Kerguelen Plateau and deep open water regions. Animals of both sexes were more likely to exhibit area-restricted search (ARS) behavior rather than transit in shelf habitats. While Antarctic shelf waters can be regarded as prime habitat for both sexes, female seals tend to move northwards with the advance of sea ice in the late autumn or early winter. The water masses used by the seals also influenced their behavioral mode, with female ARS behavior being most likely in modified Circumpolar Deepwater or northerly Modified Shelf Water, both of which tend to be associated with the outer reaches of the Antarctic Continental Shelf. The combined effects of (1) the differing habitat quality, (2) differing responses to encroaching ice as the winter progresses among colonies, (3) differing distances between breeding and haul-out sites and high quality habitats, and (4) differing long-term regional trends in sea ice extent can explain the differing population trends observed among elephant seal colonies.Publisher PDFPeer reviewe

​​Observing Antarctic Bottom Water in the Southern Ocean​

Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system

Subantarctic mode water: distribution and circulation

The subduction and export of subantarctic mode water (SAMW) as part of the overturning circulation play an important role in global heat, freshwater, carbon and nutrient budgets. Here, the spatial distribution and export of SAMW is investigated using Argo profiles and a climatology. SAMW is identified by a dynamical tracer: a minimum in potential vorticity. We have found that SAMW consists of several modes with distinct properties in each oceanic basin. This conflicts with the previous view of SAMW as a continuous water mass that gradually cools and freshens to the east. The circulation paths of SAMW were determined using (modified) Montgomery streamlines on the density surfaces corresponding with potential vorticity minima. The distribution of the potential vorticity minima revealed “hotspots” where the different SAMW modes subduct north of the Subantarctic Front. The subducted SAMWs follow narrow export pathways into the subtropical gyres influenced by topography. The export of warmer, saltier modes in these “hotspots” contributes to the circumpolar evolution of mode water properties toward cooler, fresher and denser modes in the east

Poleward shift of Circumpolar Deep Water threatens the East Antarctic Ice Sheet

Future sea-level rise projections carry large uncertainties, mainly driven by the unknown response of the Antarctic Ice Sheet to climate change. During the past four decades, the contribution of the East Antarctic Ice Sheet to sea-level rise has increased. However, unlike for West Antarctica, the causes of East Antarctic ice-mass loss are largely unexplored. Here, using oceanographic observations off East Antarctica (80–160° E) we show that mid-depth Circumpolar Deep Water has warmed by 0.8–2.0 °C along the continental slope between 1930–1990 and 2010–2018. Our results indicate that this warming may be implicated in East Antarctic ice-mass loss and coastal water-mass reorganization. Further, it is associated with an interdecadal, summer-focused poleward shift of the westerlies over the Southern Ocean. Since this shift is predicted to persist into the twenty-first century, the oceanic heat supply to East Antarctica may continue to intensify, threatening the ice sheet’s future stability.</p

Regional circulation and its impact on upper ocean variability south of Tasmania

Ocean colour images of the Subantarctic Zone (SAZ) south of Tasmania show a higher biomass in the east than in the west. To identify the main features of the regional circulation and the physical drivers of the east/west contrast, we used World Ocean Circulation Experiment hydrographic sections SR3 and P11S (west and east of Tasmania, respectively), Argo float profiles and trajectories, and high resolution climatology. The East Australian Current and the Tasman Outflow are the mechanisms driving the variability in the eastern Subantarctic Zone. This region has a weak flow and an enhanced input of subtropical waters through eddies, interleaving and a subsurface salinity maximum intruding from the north to south. In the western Subantarctic Zone, the regional circulation is dominated by a northwestward circulation and a deep reaching anticyclonic recirculation. The South Tasman Rise acts as a barrier, inhibiting exchange between waters southeast and southwest of Tasmania. The regional circulation and mixing processes result in the strong contrast in water properties between the eastern and western Subantarctic Zone: cooler and fresher in the west and warmer and saltier in the east. The Subantarctic Mode Water (SAMW) pycnostad is more prominent in the west, with a local variety of SAMW associated with the anticyclonic recirculation west of the South Tasman Rise. Antarctic Intermediate Water (AAIW) formed in the southeastern Pacific and southwestern Atlantic Oceans meet in the SAZ south of Tasmania. Cool, fresh, and well-ventilated AAIW is found in the west and southeast SAZ. Relatively warm, salty and low oxygen AAIW enters the SAZ from the Tasman Sea, after having traversed the Pacific Ocean subtropical gyre. Enhanced input of subtropical water high in micronutrients (such as iron) in the east likely supports the higher surface biomass observed there. The physical processes responsible for maintaining the east/west contrast south of Tasmania (e.g. regional circulation, eddies and intrusions) are likely to drive variability in physical and biogeochemical properties of SAMW, AAIW, and the Subantarctic Zone elsewhere in the Southern Ocean

Ocean-ice shelf interaction in East Antarctica

Assessments of the Antarctic contribution to future sea level rise have generally focused on ice loss in West Antarctica. This focus was motivated by glaciological and oceanographic observations that showed ocean warming was driving loss of ice mass from the West Antarctic Ice Sheet (WAIS). Paleoclimate studies confirmed that ice discharge from West Antarctica contributed several meters to sea level during past warm periods. On the other hand, the much larger East Antarctic Ice Sheet (EAIS) was generally considered to be relatively stable because of being largely grounded above sea level and therefore protected from ocean heat flux. However, recent studies suggest that a large part of the EAIS is grounded well below sea level and that the EAIS also retreated and contributed several meters to sea level rise during past warm periods. We use ocean observations from three ice shelf systems to illustrate the variety of ocean-ice shelf interactions taking place in East Antarctica and to discuss the potential vulnerability of East Antarctic ice shelves to ocean heat flux. The Amery and the Mertz are “cold cavity” ice shelves that exhibit relatively low area-averaged basal melt rates, although substantial melting and refreezing occurs beneath the large and deep Amery Ice Shelf. In contrast, new oceanographic measurements near the Totten Ice Shelf show that warm water enters the sub-ice-shelf cavity and drives rapid basal melting, as is seen in West Antarctica. Totten Glacier is of particular interest because it holds a marine-based ice volume equivalent to at least 3.5 m of global sea level rise, an amount comparable to the entire marine-based WAIS, and recent glaciological measurements show the grounded portion of Totten Glacier is thinning and the grounding line is retreating. Multiple lines of evidence support the hypothesis that parts of the EAIS are more dynamic than once thought. Given that the EAIS contains a volume of marine-based ice equivalent to 19 m of global sea level rise, the potential for ocean-driven melt to destabilize the marine-based ice sheet needs to be accounted for in assessments of future sea level rise

Circulation of modified Circumpolar Deep Water and basal melt beneath the Amery Ice Shelf, East Antarctica

Antarctic ice sheet mass loss has been linked to an increase in oceanic heat supply, which enhances basal melt and thinning of ice shelves. Here we detail the interaction of modified Circumpolar Deep Water (mCDW) with the Amery Ice Shelf, the largest ice shelf in East Antarctica, and provide the first estimates of basal melting due to mCDW. We use subice shelf ocean observations from a borehole site (AM02) situated ?70 km inshore of the ice shelf front, together with open ocean observations in Prydz Bay. We find that mCDW transport into the cavity is about 0.22?±?0.06 Sv (1 Sv?=?106 m3 s?1). The inflow of mCDW drives a net basal melt rate of up to 2?±?0.5 m yr?1 during 2001 (23.9?±?6.52 Gt yr?1 from under about 12,800 km2 of the north-eastern flank of the ice shelf). The heat content flux by mCDW at AM02 shows high intra-annual variability (up to 40%). Our results suggest two main modes of subice shelf circulation and basal melt regimes: (1) the “ice pump”/high salinity shelf water circulation, on the western flank and (2) the mCDW meltwater-driven circulation in conjunction with the “ice pump,” on the eastern flank. These results highlight the sensitivity of the Amery's basal melting to changes in mCDW inflow. Improved understanding of such ice shelf-ocean interaction is crucial to refining projections of mass loss and associated sea level rise