Gd Williams

Estimates of the Southern Ocean general circulation improved by animal‐borne instruments

Over the last decade, several hundred seals have been equipped with conductivity-temperature-depth sensors in the Southern Ocean for both biological and physical oceanographic studies. A calibrated collection of seal-derived hydrographic data is now available, consisting of more than 165,000 profiles. The value of these hydrographic data within the existing Southern Ocean observing system is demonstrated herein by conducting two state estimation experiments, differing only in the use or not of seal data to constrain the system. Including seal-derived data substantially modifies the estimated surface mixed-layer properties and circulation patterns within and south of the Antarctic Circumpolar Current. Agreement with independent satellite observations of sea ice concentration is improved, especially along the East Antarctic shelf. Instrumented animals efficiently reduce a critical observational gap, and their contribution to monitoring polar climate variability will continue to grow as data accuracy and spatial coverage increase.

Sea-ice growth and water-mass modification in the Mertz Glacier polynya, East Antarctica, during winter

Abstract In July-September 1999, an extensive oceanographic survey (87 conductivity-, temperature-and depth-measuring stations) was conducted in the Mertz Glacier polynya over the Adélie Depression off the Antarctic coast between 145° and 150° E. We identify and describe four key water masses in this polynya: highly modified circumpolar deep water (HMCDW), winter water (WW), ice-shelf water (ISW) and high-salinity shelf water (HSSW). Combining surface velocity data (from an acoustic Doppler current-profiler) with three hydrographic sections, we found the HMCDW to be flowing westward along the shelf break (0.7 Sv), the WW and HSSW flowing eastwards underneath Mertz Glacier (2.0 Sv) and that there was a westward return flow of ISW against the continent (1.2 Sv). Using a simple box model for the exchanges of heat and fresh water between the principal water masses, we find that the polynya was primarily a latent-heat polynya with 95% of the total heat flux caused by sea-ice formation. This heat flux results from a fresh-water-equivalent sea-ice growth rate of 4.9−7.7 cm d−1 and a mass exchange between HMCDW and WW of 1.45 Sv The inferred ocean heat flux is 8−14 W m−2 and compares well with other indirect estimates.

Antarctic coastal polynya response to climate change

Sensitivity of sea ice formation and dense shelf water production to perturbations of air temperature, precipitation, and wind stress in an important Antarctic coastal polynya system is investigated. Shelf water formation in the Mertz Glacier Polynya is a major source of Adelie Land Bottom Water. Coupled ocean and sea ice model simulations for 1996–1999 span a transitional period of the system: The 1996–1997 strong polynya state is characterized by high sea ice growth and export, ocean to atmosphere heat flux, shelf water density, and rate of dense water export; in the 1998–1999 weak polynya state all these quantities are greatly reduced. The 1990s interannual variability in air temperature and precipitation is of similar magnitude to future increases as projected for the Southern Ocean by the IPCC assessment. We model the polynya with perturbed climate change forcing and find that the system shows a reduction in shelf water export in both the strong/weak modes. Overall, the dense water export is reduced by 40% for a 2°C surface warming, and by 33% for a 20 cm a−1 precipitation increase. In the weak polynya state that is more likely in future climate, shelf water export is reduced by 81% for the warming and by 65% for the freshening. The reduction in dense shelf water export implies a corresponding reduction in Antarctic Bottom Water formation.

A Southern Indian Ocean database of hydrographic profiles obtained with instrumented elephant seals

The instrumentation of southern elephant seals with satellite-linked CTD tags has offered unique temporal and spatial coverage of the Southern Indian Ocean since 2004. This includes extensive data from the Antarctic continental slope and shelf regions during the winter months, which is outside the conventional areas of Argo autonomous floats and ship-based studies. This landmark dataset of around 75,000 temperature and salinity profiles from 20–140 °E, concentrated on the sector between the Kerguelen Islands and Prydz Bay, continues to grow through the coordinated efforts of French and Australian marine research teams. The seal data are quality controlled and calibrated using delayed-mode techniques involving comparisons with other existing profiles as well as cross-comparisons similar to established protocols within the Argo community, with a resulting accuracy of ±0.03 °C in temperature and ±0.05 in salinity or better. The data offer invaluable new insights into the water masses, oceanographic processes and provides a vital tool for oceanographers seeking to advance our understanding of this key component of the global ocean climate.

Sea ice production variability in Antarctic coastal polynyas

Enhanced sea ice production (SIP) in Antarctic coastal polynyas forms dense shelf water (DSW), leading to Antarctic Bottom Water (AABW) formation that ultimately drives the lower limb of the meridional overturning circulation. Some studies suggest that the variability of SIP in Antarctic coastal polynyas is driven by the influence of atmospheric forcing, i.e., surface winds and air temperature. Our previous mapping of SIP in 13 major Antarctic coastal polynyas from 1992 to 2007, using a heat flux calculation with ice thickness data derived from satellite data, is extended here to examine the interannual and seasonal variability of SIP from 1992 to 2013. The interannual variability of total ice production correlates more strongly with polynya extent than with atmospheric forcing, with the exception of the Shackleton Polynya, which correlates well with wind. There is no coherent signal in the interannual variability between the major Antarctic coastal polynyas. We find that stochastic changes to the coastal “icescape,” i.e., ice shelves, floating glaciers, fast ice, together with offshore first-year ice, are also important factors driving SIP variability on multiyear time scales. Both the Ross Ice Shelf Polynya and Mertz Glacier Polynya experienced a significant reduction in SIP due to calving events and the repositioning of icebergs and fast ice. Our results also show opposing trends between polynya-based SIP and sea ice extent in key regions of Antarctic sea ice change. Close monitoring of coastal icescape dynamics and change is essential to better understand the long-term impact of coastal polynya variability and its influence on regional AABW production.

The suppression of Antarctic bottom water formation by melting ice shelves in Prydz Bay

A fourth production region for the globally important Antarctic bottom water has been attributed to dense shelf water formation in the Cape Darnley Polynya, adjoining Prydz Bay in East Antarctica. Here we show new observations from CTD-instrumented elephant seals in 2011–2013 that provide the first complete assessment of dense shelf water formation in Prydz Bay. After a complex evolution involving opposing contributions from three polynyas (positive) and two ice shelves (negative), dense shelf water (salinity 34.65–34.7) is exported through Prydz Channel. This provides a distinct, relatively fresh contribution to Cape Darnley bottom water. Elsewhere, dense water formation is hindered by the freshwater input from the Amery and West Ice Shelves into the Prydz Bay Gyre. This study highlights the susceptibility of Antarctic bottom water to increased freshwater input from the enhanced melting of ice shelves, and ultimately the potential collapse of Antarctic bottom water formation in a warming climate.

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 Amerys 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.

Beyond point measurements: sea ice floes characterized in 3-D

A new methodology for coincident floe-scale measurements of the surface elevation, snow depth, and ice draft (the thickness below the water line) of Antarctic sea ice has been demonstrated during two recent research voyages: the Australian-led Sea Ice Physics and Ecosystem Experiment II (SIPEX II) to East Antarctica in September–November 2012 and the United Kingdom–led Ice Mass Balance in the Bellingshausen Sea (ICEBell) voyage to the Weddell and Bellingshausen Seas in November 2010

Antarctic Bottom Water production from the Vincennes Bay Polynya, East Antarctica

One year moorings at depths greater than 3000m on the continental slope off Vincennes Bay, East Antarctica, reveal the cold (<-0.5 degrees C) and fresh (<34.64) signals of newly formed Antarc ...

Cumulative solar irradiance and potential large-scale sea ice algae distribution off East Antarctica (30°E-150°E)

We present a computational model of the large-scale cumulative light exposure of sea ice in the Southern Ocean off East Antarctica (30°E–150°E). The model uses remotely sensed or modelled sea ice concentration, snow depth over sea ice, and solar irradiance data, and tracks sea ice motion over the season of interest in order to calculate the cumulative exposure of the ice field to photosynthetically active radiation (PAR). Light is the limiting factor to sea ice algal growth over winter and early spring, and so the results have implications for the estimation of algal biomass in East Antarctica. The model results indicate that highly light-exposed ice is restricted to within a few degrees of the coast in the eastern part of the study region, but extends much further north in the 30°E–100°E sector. The relative influences of sea ice motion, solar flux, and snow depth variations on interannual variations in model predictions were evaluated. The model estimates of cumulative PAR were found to correlate with satellite estimates of subsequent open-water chlorophyll-a concentration, consistent with the notion that sea ice algae can provide inocula for phytoplankton blooms.