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Shadwick E.H.,University of Tasmania | Rintoul S.R.,University of Tasmania | Rintoul S.R.,CSIRO | Rintoul S.R.,Center for Australian Weather and Climate Research | And 11 more authors.
Geophysical Research Letters | Year: 2013

Dense shelf water formed in the Mertz Polynya supplies the lower limb of the global overturning circulation, ventilating the abyssal Indian and Pacific Oceans. Calving of the Mertz Glacier Tongue (MGT) in February 2010 altered the regional distribution of ice and reduced the size and activity of the polynya. The salinity and density of dense shelf water declined abruptly after calving, consistent with a reduction of sea ice formation in the polynya. Breakout and melt of thick multiyear sea ice released by the movement of iceberg B9B and the MGT freshened near-surface waters. The input of meltwater likely enhanced the availability of light and iron, supporting a diatom bloom that doubled carbon uptake relative to precalving conditions. The enhanced biological carbon drawdown increased the carbonate saturation state, outweighing dilution by meltwater input. These observations highlight the sensitivity of dense water formation, biological productivity, and carbon export to changes in the Antarctic icescape. © 2013. American Geophysical Union. All Rights Reserved.

Herraiz-Borreguero L.,Center for Australian Weather and Climate Research | Herraiz-Borreguero L.,University of Tasmania | Rintoul S.R.,Center for Australian Weather and Climate Research | Rintoul S.R.,University of Tasmania | Rintoul S.R.,Wealth from Oceans National Research Flagship
Journal of Geophysical Research: Oceans | Year: 2010

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. © 2010 by the American Geophysical Union.

Downes S.M.,University of Tasmania | Downes S.M.,Princeton University | Bindoff N.L.,Center for Australian Weather and Climate Research | Bindoff N.L.,Wealth from Oceans National Research Flagship | And 2 more authors.
Journal of Climate | Year: 2010

A multimodel comparison method is used to assess the sensitivity of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) formation to climate change. For the Intergovernmental Panel on Climate Change A2 emissions scenario (where atmospheric CO2 is 860 ppm at 2100), the models show cooling and freshening on density surfaces less than about 27.4 kg m-3, a pattern that has been observed in the late twentieth century. SAMW (defined by the low potential vorticity layer) and AAIW (defined by the salinity minimum layer) warm and freshen as they shift to lighter density classes. Heat and freshwater fluxes at the ocean surface dominate the projected buoyancy gain at outcrop regions of SAMW and AAIW, whereas the net increase in the Ekman flux of heat and freshwater contributes to a lesser extent. This buoyancy gain, combined with shoaling of the winter mixed layer, reduces the volume of SAMW subducted into the ocean interior by a mean of 8 Sv (12%), and the subduction of AAIW decreases by a mean of 14 Sv (23%; 1 Sv ≡ 106 m3 s-1). Decreases in the projected subduction of the key Southern Ocean upper-water masses imply a slow down in the Southern Ocean circulation in the future, driven by surface warming and freshening. A reduction in the subduction of intermediate waters implies a likely future decrease in the capacity of the Southern Ocean to sequester CO2. © 2010 American Meteorological Society.

Herraiz-Borreguero L.,CSIRO | Herraiz-Borreguero L.,Australian Antarctic Division | Rintoul S.R.,CSIRO | Rintoul S.R.,Australian Antarctic Division | Rintoul S.R.,Wealth from Oceans National Research Flagship
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2011

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. © 2011 Elsevier Ltd.

Meijers A.J.S.,University of Tasmania | Bindoff N.L.,University of Tasmania | Rintoul S.R.,Wealth from Oceans National Research Flagship
Journal of Atmospheric and Oceanic Technology | Year: 2011

A gravest empirical mode (GEM) projection of temperature and salinity fields over the circumpolar Southern Ocean is presented and is used in combination with satellite altimetry to produce gridded, fulldepth, time-evolving temperature, salinity, and velocity fields. Optimal interpolation of historical hydrography, including Argo floats, is used to produceGEMprojections of the circumpolar temperature and salinity fields. Parameterizing these fields by dynamic height allows the use of altimetric SSH values from 1992-2006 to create synoptic temperature and salinity fields at weekly intervals on a 1/ 3° grid at 36 depth levels. The satellite-derived temperature and salinity fields generally capture over 90% of the property variance below the thermocline, with RMS residuals of 1.16°C and 0.132 in salinity at the surface, decreasing to less than 0.45°C and 0.05 below 500 dbar. The combination of altimetry with theGEMfields allows the resolution of the subsurface structure of the filamentary fronts and eddy features. Velocity fields derived from the timeevolving temperature and salinity fields reproduce the Antarctic Circumpolar Current (ACC) velocity structure well, and are strongly correlated (r > 0.7) with in situ measurements from current meters and drifters, with RMS velocity residuals of 4.8-14.8 cm21 in the Subantarctic Front. © 2011 American Meteorological Society.

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