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EARTH SCIENCE > OCEANS > OCEAN CIRCULATION > WATER MASSES

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  • Metadata record for data from ASAC Project 2535 See the link below for public details on this project. Project 2535 'Variability and stability of Antarctic Bottom Water (AABW)' Metadata description (1) Model analysis of natural AABW variability:- We have assessed the interannual to multi-decadal variability of AABW in a global coupled climate model, focussing on variations in bottom water formation rates, T-S changes on AABW neutral surfaces, and the physical mechanisms controlling this variability. The global coupled climate model used is the CSIRO Mark 3 Coupled Climate Model, which incorporates sub-models of the ocean, atmosphere, sea-ice, and land-surface. The experiments were run over a global grid at approximate resolution of 1.9 degrees x 1.9 degrees x 18 levels in the atmosphere, and 1.875 degrees x 0.94 degrees x 31 levels in the ocean. Variables analysed include oceanic temperature, salinity and circulation on AABW density layers, sea-ice extent and thickness, atmospheric sealevel pressure, temperature, and winds. The model integration considered was run with steady CO2 levels for two hundred years in a quasi-steady state mode. Full details of the CSIRO Mark 3 Coupled Climate Model can be found in Gordon et al. (2002). Gordon, H.B., Rotstayn, L.D., McGregor J.L., Dix M.R., Kowalczyk E.A., O'Farrell S.P., 2002: The CSIRO Mk3 Climate System Model. CSIRO Division of Atmospheric Research Technical Paper, No. 60. 130pp. (2) Model simulations of CO2-induced change in AABW: We also ran simulations of climate change within the Canadian University of Victoria Earth System Climate Model of Intermediate Complexity at a global longitude x latitude resolution of 3.6 degrees x 1.8 degrees. The model includes a primitive equation three-dimensional, 19 level ocean model, a sea-ice model, a simple land and river model and a two dimensional energy-moisture balance atmospheric model. A number of sensitivity experiments on ocean mixing parameters and the sea-ice model were conducted to optimise the Southern Hemisphere climatology for the control experiment. The control case (CTRL) was integrated for 3100 years starting from idealised initial conditions. Three climate change experiments were conducted, in which atmospheric carbon dioxide concentrations are changed to 450 ppm, 750 ppm and 1000 ppm from a pre-industrial level of 280 ppm, over different temporal regimes. Full model experiment descriptions appear in Bates, Sijp, and England (2005).

  • Metadata record for data from ASAC Project 2960 See the link below for public details on this project Public The ocean's thermohaline circulation (THC) plays a fundamental role in global climate, transporting heat poleward and regulating the uptake of anthropogenic CO2. Multiple steady-states in the THC have been identified in the North Atlantic, including an "off" state where no deep water is formed, yet little is known regarding the possibility for multiple equilibria of the Southern Ocean THC. This study aims to (1) examine hysteresis behaviour and possible multiple equilibria of the Southern Ocean THC, and (2) quantify the role of the Southern Ocean THC by examining the difference between "on" and "off" states in various water-masses. Project objectives: The overarching goal of the proposed study is to explore the possibility of multiple steady-states of the Southern Ocean (SO) thermohaline circulation (THC) and to explore their role in the global climate system. Multiple steady-states in the ocean's THC have been identified in the Northern Hemisphere [e.g., Marotzke, 2000; Rahmstorf, 2002]. While substantial climate variability and change can be inferred from palaeoclimate data for the Southern Hemisphere, our understanding of the underlying physics of SO THC variability and the associated climate dynamics remains limited. It is also unclear how the Southern Ocean THC will change in the future. This study aims to: 1. Examine the hysteresis behaviour of the Southern Ocean thermohaline circulation in relation to surface freshwater forcing, both for AABW and AAIW, 2. Explore the possibility for multiple steady-states in the Southern Ocean THC, 3. Estimate how the present-day Southern Ocean THC may be changing in relation to this hysteresis diagram, and how this relates to global climate, and 4. Quantify the role of the present-day Southern Ocean THC by examining the difference between "on" and "off" states. Taken from the 2008-2009 Progress Report: Progress against objectives: Progress on this Antarctic Sciences project during 2008/2009 can be summarised as below. Each of the four main aims have been touched upon during the past 12 months, although the most significant progress has been against items 1, 3, and 4 as listed in Section 1.1 above. The existence of teleconnections of Southern Ocean freshwater anomalies to the North Atlantic THC was investigated, primarily in the context of past climates (Trevena, Sijp and England, 2008a). We found that a Southern Ocean freshwater pulse of comparable magnitude to meltwater pulse 1A, shuts down, instead of strengthens, NADW in a glacial climate simulation. Unlike a modern-day simulation, the glacial experiment is associated with a more fragile North Atlantic thermohaline circulation, whereby freshwater anomalies that propagate into the North Atlantic are able to dominate the bipolar density see-saw. The possibility for large-scale collapse and/or multiple steady-states in the Southern Ocean THC was also investigated using a coupled climate model of intermediate complexity. Also investigated was the impact of a slowdown of Antarctic Bottom Water (AABW) on regional Southern Hemisphere climate. This involved the gradual addition of meltwater anomalies to strategic locations of the Southern Ocean, then removal of these anomalies to explore whether the regional thermohaline circulation (THC) exhibits saddle-node instabilities (bifurcation points) as have been commonly found for the North Atlantic. We found that no stable AABW "off" state could persist, regardless of the freshwater anomaly imposed. We did, however, identify a significant impact on regional climate during the transient slow down of AABW (Trevena, Sijp and England, 2008b). In particular, during peak FW forcing, Antarctic surface sea and air temperatures decrease by a maximum of 2.5 degs C and 2.2 degs-C respectively. This is of a similar magnitude to the corresponding response in the North Atlantic. Taken from the 2009-2010 Progress Report: Progress against objectives: Progress on this Antarctic Sciences project during 2009/2010 can be summarised as below. Each of the four main aims have been touched upon during the past 12 months, although the most significant progress has been against items 2 and 4 as listed in Section 1.1 above. A large set of experiments were configured and analysed to examine Southern Ocean THC states in the global climate system. Specifically we conducted experiments using the Canadian University of Victoria Earth System Climate Model (the 'UVic' model) wherein the model is perturbed in some way to explore the possibility for multiple steady-states in the Southern Ocean THC. Where multiple steady states were obtained, the difference between "on" and "off" states was examined to quantify the role of the Southern Ocean THC in global climate. Three papers were published in the 2009/2010 period that were produced using support from this Antarctic Research project:- Sijp, W. P., M. H. England, and J.R. Toggweiler, 2009: Effect of ocean gateway changes under greenhouse warmth, J. Climate, 22, 6639-6652. In this study Southern Ocean gateway changes and the THC were examined under a suite of atmospheric CO2 levels, spanning pre-industrial (280 ppm) up to values relevant to the Eocene (1500 ppm). A markedly stronger gateway response is found under elevated CO2 levels, suggesting past work has underestimated the effects of gateway changes at the Oligocene-Eocene boundary. Sen Gupta, A., A. Santoso, A.S. Taschetto, C.C. Ummenhofer, J. Trevena and M.H. England, 2009: Projected changes to the Southern Hemisphere ocean and sea-ice in the IPCC AR4 climate models, J. Climate, 22, 3047-3078. In this study simulations of the Southern Ocean THC, water-masses, and mixed layer depth were examined and compared across a series of IPCC-class global climate models, under both present-day and climate change scenarios. Sijp, W. P. and M. H. England, 2009: The control of polar haloclines by along-isopycnal diffusion in climate models, J. Climate, 22, 486-498. In this study the ocean THC was shown to be sensitive to along-isopycnal diffusion rates in global climate models. This potentially impacts on past studies wherein multiple equilibria were obtained at unrealistic values of this mixing parameter.

  • Fieldwork was undertaken on Voyage 7 of 1997/98 season (April/May 1998) aboard Aurora Australis. The plan was to repeat the transect sampled in September 1996 to investigate seasonal differences between Spring and Autumn. We anticipated being able to sample further south on this voyage because the ice would be at close to its minimum extent. Sixteen stations were successfully sampled with the MIDOC multiple opening/closing net at 1000-750m, 750-500m, 500-250m and 250-0m depth ranges. The stations were at the following approximate positions: 47o14S:145o43E, 47o57S:145o14E, 49o54S:144o28E, 50o28S:144o15E, 51o15S:144o45E, 52o04S:144o24E, 52o59S:144o52E, 54o00S:145o01E, 53o59S:145o56E, 53o37S:144o32E, 57o00S:145o08E, 59o00S:144o56E, 58o12S:145o44E, 63o41S:139o03E, 64o17S:140o41E, 61o03S:139o40E. It was originally intended to sample 18 stations, some of which during both day and night, for a total of 26 samples, but consistently bad weather and the necessity to divert to Davis to pick up helicopters prevented the full programme being completed. However, data for the transect sampled in September of 1996 are available as a collection in the biodiversity database at the provided URL - MIDOC Fish catch from 1996/97 Voyage 1 WASTE (WOCE Antarctic Southern Transect Expedition).

  • The circumpolar Subantarctic Zone (SAZ) is a globally significant region of water mass formation and carbon dioxide uptake from the atmosphere. Here we synthesise the results of nine voyages over 8 years to describe the seasonal variation in mixed layer properties in the SAZ south of Australia for comparison with biogeochemical process studies carried out in late summer (March 1998) as part of the SAZ Project. Winter mixing extends to depths greater than 400 m, resulting in the formation of Subantarctic Mode Water. In summer the mixed layer shoals to 75-100 m, depths which are still sufficiently deep that phytoplankton growth may be light limited. Nitrate and phosphate concentrations are reduced in summer( e.g., nitrate decreases from greater than 15 to less than 5/ micro mol kg- 1) but remain well above limiting levels. Silicate in contrast, is low throughout the year (4/micro mol kg- 1 in winter and less than 2 / micro mol kg- 1 in summer). Water mass properties along a north-south hydrographic section in March 1998 suggest that near-surface waters spread from south to north across the Subantarctic Front (SAF), supplying cool, fresh, nutrient-rich water to the SAZ. As a consequence, the properties of the southern SAZ differ from those farther north: the mixed layer in the south is cooler, fresher, deeper, higher in nutrients, and bounded below by a halocline (rather than by a seasonal thermocline, as in the northern SAZ). The contrast between the northern and southern SAZ persists throughout the year, suggesting the cross-front exchange occurs year-round and likely contributes to the differences in seasonal thermal amplitude and algal biomass accumulation seen in satellite images. Density-compensated horizontal gradients of temperature and salinity are common in the mixed layer of the SAZ and the northern SAF, consistent with the hypothesis that the vigour of lateral mixing in the mixed layer is a strong function of the magnitude of the lateral density gradient. CTD Data are available for access via the provided URL. Data from the following voyages of the Aurora Australis were used: 1991-1992 Voyage 1 1992-1993 Voyage 9 1994-1995 Voyage 4 1994-1995 Voyage 7 1995-1996 Voyage 1 1996-1997 Voyage 1 1997-1998 Voyage 6

  • Papers arising from phytoplankton experiments associated with the SAZ (Subantarctic Zone) project. This work was complete as part of ASAC (AAS) project 1156. Taken from the abstracts of the referenced papers: Subantarctic Southern Ocean surface waters in the austral summer and autumn are characterised by high concentrations of nitrate and phosphate but low concentrations of dissolved iron (Fe, ~0.05 nM) and silicic acid (Si, less than 1 micro M). During the Subantarctic Zone AU9706 cruise in March 1998 we investigated the relative importance of Fe and Si in controlling phytoplankton growth and species composition at a station within the subantarctic water mass (46.8 degrees S, 142 degrees E) using shipboard bottle incubation experiments. Treatments included unamended controls; 1.9 nM added iron (+Fe); 9 micro M added silicic acid (+Si); and 1.9 nM added iron plus 9 micro M added silicic acid (+Fe+Si). We followed a detailed set of biological and biogeochemical parameters over 8 days. Fe added alone clearly increased community growth rates and nitrate drawdown and altered algal community composition relative to control treatments. Surprisingly, small, lightly silicified pennate diatoms grew when Fe was added either with or without Si, despite the extremely low ambient silicic acid concentrations. Pigment analyses suggest that lightly silicified chrysophytes (type 4 haptophytes) may have preferentially responded to Si added either with or without Fe. However, for many of the parameters measured the +Fe+Si treatments showed large increases relative to both the +Fe and +Si treatments. Our results suggest that iron is the proximate limiting nutrient for chlorophyll production, photosynthetic efficiency, nitrate drawdown, and diatom growth, but that Si also exerts considerable control over algal growth response, suggesting that both Fe and Si play important roles in structuring the subantarctic phytoplankton community. The influence of irradiance and iron (Fe) supply on phytoplankton processes was investigated, north (47 degrees S, 142 degrees E) and south (54 degrees S, 142 degrees E) of the subantarctic Front in austral autumn (March 1998). At both sites, resident cells exhibited nutrient stress. Shipboard perturbation experiments examined two light (mean in situ and elevated) and two Fe (nominally 0.5 and 3 nM) treatments under silicic acid-replete conditions. Mean in situ light levels (derived from incident irradiances, mixed layer depths (MLDs), wind stress, and a published vertical mixing model) differed at the two sites, 25% of incident irradiance I0 at 47 degrees S and 9% I0 at 54 degrees S because of MLDs of 40 (47S) and 90 m (54S), when these stations were occupied. The greater MLD at 54S is reflected by tenfold higher cellular chlorophyll a levels in the resident phytoplankton. In the 47S experiment, chlorophyll a levels increased to greater than 1 micro gram per litre only in the high-Fe treatments, regardless of irradiance levels, suggesting Fe limitation. This trend was also noted for cell abundances, silica production, and carbon fixation rates. In contrast, in the 54S experiment there were increases in chlorophyll a (to greater than 2 micro grams per litre), cell abundances, silica production, and carbon fixation only in the high-light treatments to which Fe had been added, suggesting that Fe and irradiance limit algal growth rates. Irradiance by altering algal Fe quotas is a key determinant of algal growth rate at 54S (when silicic acid levels are nonlimiting); however, because of the integral nature of Fe/light colimitation and the restricted nature of the current data set, it was not possible to ascertain the relative contributions of Fe and irradiance to the control of phytoplankton growth. On the basis of a climatology of summer mean MLD for subantarctic (SA) waters south of Australia the 47 and 54S sites appear to represent minimum and maximum MLDs, where Fe and Fe/ irradiance, respectively, may limit/colimit algal growth. The implications for changes in the factors limiting algal growth with season in SA waters are discussed.

  • Oceanographic measurements were conducted in the South Indian Ocean sector during the southern summer of 2002/2003 on Aurora Australis voyage au0304, V4 2002/2003. A total of 64 vertical CTD stations were taken, in a krill survey area in the vicinity of Mawson, and approximately following WOCE I08 meridional transect passing up the western flank of the Kerguelen Plateau and then continuing south across the Princess Elizabeth Trough to the Antarctic continental shelf. Over 1050 Niskin bottle samples were collected using a SeaBird 24 bottle rosette sampler, with samples collected for the analysis of salinity, dissolved oxygen, nutrients, and biological parameters. Full-depth current profile data were collected by either 1 or 2 lowered acoustic Doppler profilers (LADCP) attached to the CTD rosette package. Near surface current data were also collected using a ship mounted ADCP. An array of 8 moorings comprising current meters and thermosalinographs were deployed along the western flank of the Kerguelen Plateau, for the Deep Western Boundary Current Experiment. Ship's underway data, (including bathymetry, met. sensors and sea surface salinity/temperature/fluorescence) are included in the cruise data set; an offset correction was applied to the underway sea surface salinity and temperature data, derived from comparison with near surface CTD data. A summary of all data and important data quality information is presented in the data report. Note that LADCP data are not included here. This work was completed as part of ASAC projects 1250 and 2312. Models of climate change project a decrease in the global ocean overturning circulation, significantly impacting climate and ocean ecosystems. The Deep Western Boundary Current experiment commenced on this voyage aims to measure the northward transport of Antarctic Bottom Water east of the Kerguelen Plateau so that future change in this component of the global thermohaline circulation can be detected.

  • Oceanographic measurements were conducted in the vicinity of the Mertz Polynya, encompassing 2 consecutive seasonal cycles from 1998 to 2000. In the southern winter of 1999, a total of 92 CTD/LADCP vertical profile stations were taken, most to within 20 m of the bottom, with 3 laps completed around the boundary of a box adjacent to the Mertz Glacier. Over 700 Niskin bottle water samples were collected for the measurement of salinity, dissolved oxygen, nutrients, oxygen 18, dimethyl sulphide, and biological parameters, using a 12 bottle rosette sampler mounted on a 24 bottle frame. Additional CTD vertical profiles were taken in April 1998, July 1998 and February 2000. Near surface current data were collected on all cruises using ship mounted ADCP. Two mooring arrays comprising thermosalinographs, current meters and upward looking sonars were deployed in the region of the Polynya. The first array of 7 moorings was deployed in April 1998. The second array of 4 moorings was deployed in the winter of 1999. All 11 Polynya moorings were recovered in February 2000. A summary of all data and data quality is presented in the data report. This work was completed as part of ASAC projects 2223 and 189.

  • Oceanographic measurements were conducted in the vicinity of the Amery Ice Shelf on two cruises, during the southern summers of 2000/2001 and 2001/2002. A CTD transect parallel to the front of the Amery Ice Shelf was occupied on both cruises, including repeat occupations on each cruise. A total of 100 CTD vertical profile stations were taken near the ice shelf, most to within 20 m of the bottom, and over 1150 Niskin bottle water samples were collected for the measurement of salinity, dissolved oxygen, nutrients, helium, tritium, oxygen 18 and biological parameters, using a 12 bottle rosette sampler mounted on either a 24 or 12 bottle frame. On the first cruise, an additional 39 CTD stations were occupied around an experimental krill survey area in the vicinity of Mawson. Additional CTD stations were taken at the end of each cruise for calibration of CTD instrumentation from borehole sites on the Amery Ice Shelf. Near surface current data were collected on both cruises using a ship mounted ADCP. An array of 9 moorings comprising current meters, thermosalinographs and upward looking sonars were deployed along the ice shelf front in February 2001 during the first cruise, and retrieved on the second cruise in February 2002. A summary of all data and data quality is presented in the data report.

  • Oceanographic measurements were conducted in the Southern Ocean Indian sector during the southern summer of 2004/2005 on Aurora Australis voyage au0403, V3 2004/2005. Data were collected during a complete occupation of CLIVAR meridional section I9S; and then along a transect up the northeastern flank of the Kerguelen Plateau, south across the Princess Elizabeth Trough and onward to the Antarctic continental shelf. A total of 115 CTD vertical profile stations were taken, most to within 30 m of the bottom. Over 2450 Niskin bottle water samples were collected for the measurement of salinity, dissolved oxygen, nutrients, CFCs, dissolved inorganic carbon, alkalinity, oxygen-18, methane, selenium and biological parameters, using a 24 bottle rosette sampler. Full depth current profiles were collected by a lowered acoustic Doppler profiler (LADCP) attached to the rosette package, while near surface current data were collected by a ship mounted ADCP. An array of 8 current meter and thermosalinograph moorings, deployed 2 years earlier on cruise au0304, were recovered from the vicinity of the Kerguelen Plateau. Ship's underway data (including bathymetry, met. sensors and sea surface parameters) are included in the cruise data set; an offset correction was applied to the underway sea surface salinity and temperature data, derived from comparison with near surface CTD data. A summary of all data and important data quality information is presented in the data report. LADCP data are not included in this data set. This work was completed as part of ASAC projects 2312 and 2572.