Ross Sea Bottom Water Research Articles

Changes in Antarctic bottom water off the Wilkes Land coast in the Australian-Antarctic Basin

Long-term changes in Antarctic Bottom Water (AABW) off the Wilkes Land coast were examined using historical hydrographic surveys and data from Deep Argo floats. Since the 1980s at the latest, AABW has contracted by approximately 11–14 m yr−1 and undergone freshening by approximately −0.6 × 10−3 yr−1. The contraction and freshening of AABW has accelerated slightly over the past few decades, with the former being led by the latter. The contraction of AABW was mainly derived from thinning of the denser layers of AABW, which was compensated for by thickening of the upper AABW. This contrast between thinning and compensative thickening has strengthened gradually over time, suggesting that the meridional overturning circulation driven by AABW formation has weakened in the Australian-Antarctic Basin. Dissolved oxygen on isotherms in AABW appears to have remained constant since 1970, except for the possibility of some oxygenation after 2000, which was not statistically significant. After 2010, changes in AABW deviated from the long-term trends, with contraction and freshening of AABW accelerating significantly from 2011 to 2015 and then slowing down from 2015 to 2019. Meanwhile, the bottommost AABW, which is defined as the bottommost 300 m of the water column, increased in salinity significantly in the latter 2010s, mainly in response to contraction of AABW. These processes have resulted in the recent salinification of the bottommost AABW in the eastern Australian-Antarctic Basin, even in areas where the saltier Ross Sea Bottom Water has not arrived yet. In 2018, marked freshening and oxygenation of AABW was observed in the 110°E section, probably due to newly formed bottom water in the Vincennes Bay Polynya.

Spatial Variability of Antarctic Bottom Water in the Australian Antarctic Basin From 2018–2020 Captured by Deep Argo

AbstractThere are two varieties of Antarctic Bottom Water present in the Australian Antarctic Basin (AAB): locally produced Adélie Land Bottom Water (ALBW) and distantly produced Ross Sea Bottom Water (RSBW). Between 2014 and 2018, RSBW has rebounded from a multidecade freshening trend. The return of the salty RSBW to the AAB is revealed by six Deep Argo floats that have occupied the region from January of 2018 to March of 2020. The floats depict a zonal variation in temperature and salinity in the bottom waters of the AAB, driven by the inflow of RSBW. A simple Optimum Multiparameter Analysis based on potential temperature and salinity gives a sense of scale to the composition of the bottom waters, which are nearly 80% of the new, salty RSBW in the south‐east corner of the basin by 2019 and generally less than 40% to the west closer to the ALBW outflow region and the abyssal plain.

Reversal of freshening trend of Antarctic Bottom Water in the Australian-Antarctic Basin during 2010s

The Antarctic continental margin supplies the densest bottom water to the global abyss. From the late twentieth century, an acceleration in the long-term freshening of Antarctic Bottom Waters (AABW) has been detected in the Australian-Antarctic Basin. Our latest hydrographic observations reveal that, in the late 2010s, the freshening trend has reversed broadly over the continental slope. Near-bottom salinities in 2018–2019 were higher than during 2011–2015. Along 170° E, the salinity increase between 2011 and 2018 was greater than that observed in the west. The layer thickness of the densest AABW increased during the 2010s, suggesting that the Ross Sea Bottom Water intensification was a major source of the salinity increase. Freshwater content on the continental slope decreased at a rate of 58 ± 37 Gt/a in the near-bottom layer. The decadal change is very likely due to changes in Ross Sea shelf water attributable to a decrease in meltwater from West Antarctic ice shelves for the corresponding period.

Rebound of shelf water salinity in the Ross Sea

Antarctic Bottom Water (AABW) supplies the lower limb of the global overturning circulation and ventilates the abyssal ocean. In recent decades, AABW has warmed, freshened and reduced in volume. Ross Sea Bottom Water (RSBW), the second largest source of AABW, has experienced the largest freshening. Here we use 23 years of summer measurements to document temporal variability in the salinity of the Ross Sea High Salinity Shelf Water (HSSW), a precursor to RSBW. HSSW salinity decreased between 1995 and 2014, consistent with freshening observed between 1958 and 2008. However, HSSW salinity rebounded sharply after 2014, with values in 2018 similar to those observed in the mid-late 1990s. Near-synchronous interannual fluctuations in salinity observed at five locations on the continental shelf suggest that upstream preconditioning and large-scale forcing influence HSSW salinity. The rate, magnitude and duration of the recent salinity increase are unusual in the context of the (sparse) observational record.

The Neodymium Isotope Fingerprint of Adélie Coast Bottom Water

AbstractAdélie Land Bottom Water (ALBW), a variety of Antarctic Bottom Water formed off the Adélie Land coast of East Antarctica, ventilates the abyssal layers of the Australian sector of the Southern Ocean as well as the eastern Indian and Pacific Oceans. We present the first dissolved neodymium (Nd) isotope and concentration measurements for ALBW. The summertime signature of ALBW is characterized by εNd = −8.9, distinct from Ross Sea Bottom Water, and similar to Weddell Sea Bottom Water. Adélie Land Shelf Water, the precursor water mass for wintertime ALBW, features the least radiogenic Nd fingerprint observed around Antarctica to date (εNd = −9.9). Local geology around Antarctica is important in setting the chemical signature of individual varieties of Antarctic Bottom Water, evident from the shelf water signature, which should be considered in the absence of direct wintertime observations.

Neodymium isotopic characterization of Ross Sea Bottom Water and its advection through the southern South Pacific

Since the inception of the international GEOTRACES program, studies investigating the distribution of trace elements and their isotopes in the global ocean have significantly increased. In spite of this large-scale effort, the distribution of neodymium isotopes (143Nd/144Nd, εNd) and concentrations ([Nd]) in the high latitude South Pacific is still understudied, specifically north of the Antarctic Polar Front (APF). Here we report dissolved Nd isotopes and concentrations from 11 vertical water column profiles from the South Pacific between South America and New Zealand and across the Antarctic frontal system. Results confirm that Ross Sea Bottom Water (RSBW) is represented by an εNd value of ∼−7, and for the first time show that these Nd characteristics can be traced into the Southeast Pacific until progressive mixing with ambient Lower Circumpolar Deep Water (LCDW) dilutes this signal north of the APF. That is, εNd behaves conservatively in RSBW, opening a path for studies of past RSBW behavior.Neodymium concentrations show low surface concentrations and a linear increase with depth north of the APF. South of the APF, surface [Nd] is high and increases with depth but remains almost constant below ∼1000 m. This vertical and spatial [Nd] pattern follows the southward shoaling density surfaces of the Southern Ocean and hence suggests supply of Nd to the upper ocean through upwelling of Nd-rich deep water. Low particle abundance due to reduced opal production and seasonal sea ice cover likely contributes to the maintenance of the high upper ocean [Nd] south of the APF. This suggests a dominant lateral transport component on [Nd] and a reduced vertical control on Nd concentrations in the South Pacific south of the APF.

Dissolved oxygen change and freshening of Antarctic Bottom water along 62°S in the Australian-Antarctic Basin between 1995/1996 and 2012/2013

Significant warming and freshening signals observed in the world׳s deep oceans are generally stronger southward. In the Australian-Antarctic Basin along 62°S – near the southern edge of the western Pacific and Indian oceans – we compared water properties of Antarctic Bottom Water observed in 1995/1996 and 2012/2013. The basin collects newly ventilated dense water from the Western Pacific sector of the Antarctic coast and the Ross Sea. Compared on pressure surfaces, freshening of more than 0.003g/kg in Absolute Salinity (0.003 in Practical Salinity Scale 1978 or PSS-78) was found below 3000dbar. The freshening was stronger (0.02g/kg or 0.02 in PSS-78) eastward and towards the bottom, suggesting that it originated from the regions of new bottom water production. We also detected warming (>0.2°C) on pressure surfaces below the surface layer. Freshening of similar magnitudes were observed on density surfaces (neutral densities >1028.1kg/m3). Adiabatic southward migration of water masses, observed by satellite altimetry, can explain the warming but not the freshening. A notable increase in dissolved oxygen on density surfaces was found near the bottom. The increase appears due to density decrease of near-bottom water, which can be explained by a change in the mixing ratio of bottom water from near the Adélie Depression (Adélie Land Bottom Water (ALBW), higher oxygen) with that from the more distant Ross Sea (Ross Sea Bottom Water (RSBW), lower oxygen): an increased contribution of ALBW and a decreased contribution of RSBW. This change might be associated with a decrease in the RSBW supply following an earlier ice-calving event in the polynya region. Oxygen rich (>242μmol/kg) and dense (>1028.35kg/m3) bottom water observed in 1995/1996 in troughs near 130°E disappeared in 2012/2013, which also might reflect changes in bottom water production along the coast, possibly in Mertz and Dibble Polynyas.

Influence of Ross Sea Bottom Water changes on the warming and freshening of the Antarctic Bottom Water in the Australian-Antarctic Basin

Abstract. Changes to the properties of Antarctic Bottom Water in the Australian-Antarctic Basin (AA-AABW) between the 1990s and 2000s are documented using data from the WOCE Hydrographic Program (WHP) and repeated hydrographic surveys. Strong cooling and freshening are observed on isopycnal layers denser than γn = 28.30 kg m−3. Changes in the average salinity and potential temperature below this isopycnal correspond to a basin-wide warming of 1300 ± 200 GW and freshening of 24 ± 3 Gt year−1. Recent changes to dense shelf water in the source regions in the Ross Sea and George V Land can explain the freshening of AA-AABW but not its extensive warming. An alternative mechanism for this warming is a decrease in the supply of AABW from the Ross Sea (RSBW). Hydrographic profiles between the western Ross Sea and George V Land (171–158° E) were analyzed with a simple advective-diffusive model to assess the causes of the observed changes. The model suggests that the warming of RSBW observed between the 1970s and 2000s can be explained by a 21 ± 23% reduction in RSBW transport and the enhancement of the vertical diffusion of heat resulting from a 30 ± 7% weakening of the abyssal stratification. The documented freshening of Ross Sea dense shelf water leads to a reduction in both salinity and density stratification. Therefore the direct freshening of RSBW at its source also produces an indirect warming of the RSBW. A simple box model suggests that the changes in RSBW properties and volume transport (a decrease of 6.7% is assumed between the year 1995 and 2005) can explain 51 ± 6% of the warming and 84 ± 10% of the freshening observed in AA-AABW.

Climatology and decadal variability of the Ross Sea shelf waters

The World Ocean Database 2001 data located in the Ross Sea (named WOD01 and containing data in this region since 1928) are merged with recent data collected by the Italian expeditions (CLIMA dataset) in the period November 1994-February 2004 in the same area. From this extended dataset, austral summer climatologies of the main Ross Sea subsurface, intermediate and bottom water masses: High Salinity Shelf Water (HSSW), Low Salinity Shelf Water (LSSW), Ice Shelf Water (ISW) and Modified Circumpolar Deep Water (MCDW) have been drawn. The comparison between the WOD01_1994 climatologies (a subset of the WOD01 dataset until April 1994) and the CLIMA ones for the period 1994/95-2003/04 showed significant changes occurred during the decade. The freshening of the Ross Sea shelf waters which occurred during the period 1960-2000, was confirmed by our analysis in all the main water masses, even though with a spatially varying intensity. Relevant variations were found for the MCDW masses, which appeared to reduce their presence and to deepen; this can be ascribed to the very limited freshening of the MCDW core, which allowed an increased density with respect to the surrounding waters. Variations in the MCDW properties and extension could have relevant consequences, e.g. a decreased Ross Ice Shelf basal melting or a reduced supply of nutrients, and may also be indicative of a reduced thermohaline circulation within the Ross Sea. Shelf Waters (SW) having neutral density γn > 28.7 Kg m-3, which contribute to form the densest Antarctic Bottom Waters (AABW), showed a large volumetric decrease in the 1994/95-2003/04 decade, most likely as a consequence of the SW freshening.

Long-term bottom water warming in the north Ross Sea

We measured potential temperature, salinity, and dissolved oxygen profiles from the surface to the bottom at two locations in the north Ross Sea (65.2°S, 174.2°E and 67.2°S, 172.7°W) in December 2004. Comparison of our data with previous results from the same region reveals an increase in potential temperature and decreases in salinity and dissolved oxygen concentration in the bottom layer (deeper than 3000 m) over the past four decades. The changes were significantly different from the analytical precisions. Detailed investigation of the temperature, salinity, dissolved oxygen and σ 3 value distributions and the bottom water flow in the north Ross Sea suggests a long-term change in water mass mixing balance. That is to say, it is speculated that the influence of cool, saline, high-oxygen bottom water (high-salinity Ross Sea Bottom Water) formed in the southwestern Ross Sea has possibly been decreased, while the influences of relatively warmer and fresher bottom water (low-salinity Ross Sea Bottom Water) and the Adelie Land Bottom Water coming from the Australia-Antarctic Basin have increased. The possible impact of global warming on ocean circulation needs much more investigation.

Importance of the eastern Indian Ocean for the abyssal Pacific

The role of the eastern Indian Ocean in controlling the abyssal circulation in the Pacific is examined using a 1° × 1° world ocean model. Thanks to the improvement of bottom topography and the incorporation of a bottom boundary layer model, the bottom water formation off the Adélie Coast is produced. This bottom water is not only the source of the abyssal water in the eastern Indian but also has a great influence on the whole abyssal Pacific. The lower part of Circumpolar Deep Water (CDW), which enters the abyssal Pacific passing through south of New Zealand, changes its property through mixing with this bottom water flowing underneath CDW in the Australian‐Antarctic Basin and the South Australia Basin. The tracer experiments show that the water ventilated off the Adélie Coast occupies about 20% of the abyssal Pacific, whereas North Atlantic Deep Water (NADW) occupies about 50%, the Weddell Sea bottom water about 20%, and the Ross Sea bottom water about 10%. The flux of the ventilated water estimated by the tracer experiments is 17.1 Sv for NADW, 6.3 Sv from the Weddell Sea, 6.2 Sv from off the Adélie Coast, and 3.5 Sv from Ross Sea in this model. The total flux of 33.1 Sv is not far from the estimate from observations, though it may overestimate by 1–4 Sv. This does not directly indicate that the real world ocean should have the large amount of the bottom water sources in the Pacific‐Indian sector. However, the model offers a picture of the water properties consistent with the observation.

Two modes of bottom water in the Australian‐Antarctic Basin

Recent data from the deep Australian‐Antarctic Basin show that waters colder than about −0.4°C are fresher by as much as 0.02 than data collected prior to 1994. It is possible that the freshening represents a fundamental change in the sources of bottom water for the basin, but it is more likely that the fresh mode recurs periodically and is underrepresented in the historical database. The freshening may be caused by a change in fresh bottom water input from the Antarctic coast between 120°E – 160°E or by a reduction in the outflow of saline Ross Sea Bottom Water.

On the circulation and water masses over the Antarctic continental slope and rise between 80 and 150°E

The circulation and water masses in the region between 80 and 150°E and from the Antarctic continental shelf to the Southern Boundary of the Antarctic Circumpolar Current (ACC) (∼62°S) are described from hydrographic and surface drifter data taken as part of the multi-disciplinary experiment, Baseline Research Oceanography Krill and the Environment (BROKE). Two types of bottom water are identified, Adelie Land Bottom Water, formed locally between 140 and 150°E, and Ross Sea Bottom Water. Ross Sea Bottom Water is found only at 150°E, whereas Adelie Land Bottom Water is found throughout the survey region. The bottom water mass properties become progressively warmer and saltier to the west, suggesting a westward flow. All of the eight meridional CTD sections show an Antarctic Slope Front of varying strength and position with respect to the shelf break. In the water formation areas (between 140 and 150°E) and 104°E, the Antarctic Slope Front is more “V” shaped, while elsewhere it is one-sided. The shape of the slope front, and the presence or absence of water formation there, are consistent with other meridional sections in the Weddel Sea and simple theories of bottom-water formation (Gill, 1973. Deep-Sea Research 20, 111–140; Whitworth et al., 1998. In: Jacobs and Weiss (Eds.), Ocean, Ice and Atmosphere: Interactions at the Antartic Continental Margin, Antarctic Research Series. American Geophysical Union, Washington, pp. (1−27). ADCP surface velocities and buoy drift tracks show a strong westward flow over the shelf and slope regions. In the region 90–100°E there is a strong eastward flow of the waters just south of the Southern Boundary of the ACC, suggesting a recirculation of the westward slope current and the presence of a weak cyclonic gyre. Using the ADCP velocities as a reference for the CTD data, the average westward transport in this region is 29.4±14.7 Sv.

Radium-226 and Ra-Ba relationships in Antarctic and Pacific waters

Three 226Ra concentration profiles have been measured from the Antarctic south of New Zealand. These profiles show that the Ra concentration in the deep- and bottom-water region is fairly uniform at about 9.2 × 10 −14g/kg. However, south of the Antarctic Convergence, a slightly lower Ra concentration associated with higher salinity is found near the bottom where the Ross Sea Bottom Water spreads. In the Antarctic surface water, a gradual increase of radium concentration is observed from north to south across the Convergence. These features are closely correlated with the hydrographic data. The Ra concentration in the deep water increases from the North Atlantic through the Antarctic to the Pacific by a factor of three. In the Pacific, it increases from the southwest to the northeast. The trend of increase is similar to the abyssal water temperature, and shows that Ra, like heat, is injected continually into the deep water column as the water circulates. Ra and Ba are not linearly correlated over the entire ocean. The Ra/Ba ratio is not constant, and tends to increase from the surface water to the bottom water due to different input mechanisms for these two elements in the ocean.

Barium at Geosecs III in the southwest Pacific

A detailed, highly precise barium profile has been measured at Geosecs III, off Tonga, in the southwest Pacific. There is a smooth increase from the surface values of 4.7 μg/kg to a deep maximum of 18.4 μg/kg at 2.974 km. Below this maximum there is an abrupt decrease to 14.3 μg/kg in the deep Western Boundary Current. Comparison with published data shows recent Pacific and Antarctic barium profiles to be consistent within 2%. Model calculations do not require any in situ injection of barium (or of silicate or alkalinity) in the water column; the form of the profile can be explained by vertical diffusion and advection between horizontally advecting water masses. From the θ- S curve it appears that a minor core of higher salinity water, possibly derived from Ross Sea bottom water, or the North Atlantic deep water core in the Circumpolar Current, is present immediately above the deep Boundary Current.