Variability of Antarctic Bottom Water formation and export from the Weddell Sea    

The Weddell Sea is the main supplier for Antarctic Bottom Water (AABW) to the World Ocean. At its margins, by interaction with various shelf water types and the ice shelves, its precursors, Weddell See Deep and Bottom Water (WSDW, WSBW) are formed. Changes in their formation rates – caused by environmental changes – could modify the strength and variability of the Meridional Overturning Circulation (MOC) and, thus, affect climate and climate change. Changes in the bottom water formation process and in the amount of bottom water formed might also influence the anthropogenic carbon uptake and storage of the deep ocean.

Our project contributes to the International Polar Year (IPY) in the framework of CASO (Climate in Antarctica and the Southern Ocean). CASO aims to understand, interpret and predict climate variability and change in the southern polar region and its impact on Antarctic and global processes.

The aims of our proposal are:

• To estimate WSDW and WSBW formation in the Weddell Sea, especially to determine variability of their formation and to consider links to changing environmental conditions (i.e. degradation of ice shelves, enhanced melting, warming, freshening) that might lead to changing water mass distribution and/or composition.

• To determine the export of WSDW and WSBW through the South Scotia Ridge and across Greenwich Meridian and the import of deep water from easterly sources and their variability.

• To assess the contribution of the Southeast Pacific Deep Slope Water (SPDSW) to the transport of the Antarctic Circumpolar Current.

Major tools to achieve these aims are measurement and analysis of trace gasses. Chlorofluorocarbons (CFCs) as anthropogenic transient tracers provide information about internal transit times and the renewal of water masses (CFC inventories, transit time distributions). Noble gases (helium isotopes, neon) as steady state tracers mark (also qualitative) contributions of glacial melt water or entrained water from the Antarctic Circumpolar Current into the Weddell Gyre (multiparameter analysis).


Fig. 1. WSBW formation and basal melt rates in the western and southern Weddell Sea: Schematic of water mass spreading in the southern and western Weddell Sea. Green arrows = Weddell Sea Bottom Water (WSBW), thin black arrows = High Salinity Shelf Water (HSSW), and dotted black arrows = Ice Shelf Water (ISW). Numbers in circles denote derived WSBW formation rates (solid) and ice shelf basal melt rates (blue dotted). From Huhn et al., 2008.


Fig. 2. Composition of recently formed WSBW in the western Weddell Sea: Vertical sections of A: ⁴He [nmol/kg], B: CFC-12 [ppt]. Used are bottle data from stations along the ISPOL drift track on the slope of the Antarctic Peninsula in the northwestern Weddell Sea from south (left) to north (right). Isotherms of 0°C and -0.7°C mark the transitions between WDW/WSDW and WSDW/WSBW, respectively. The deep -1.7°C isotherm is included to mark the cold bottom layer. In A the enhancement of Helium within the WSBW (θ < -0.7°C) due to contributions of glacial melt water originating from the western Larsen Ice Shelf is clearly visible; in B the cold bottom layer is high in CFC-12, indicating its recent ventilation from surface near water masses. From Huhn et al., 2008.


Fig. 3. Import of anthropogenic CO ₂ in a deep boundary current into the Weddell Sea inferred using CFC based transit time distributions: (Left) From a time series of observed CFC-11 (green dots) and CFC-12 (blue dots) on the Greenwich Meridian section (core of recently ventilated deep water on the slope of the Antarctic Continent, approx. 2000-4000m) we derived transit time distributions (TTDs; TTD based CFCs are displayed as solid lines). (Right) Applying this TTD (black curve) to anthropogenic carbon (pCant, red dotted curve) we inferred the time evolution of pCant in this deep water (thick red curve). In preparation.

Scientists

Prof. Dr. Monika Rhein
Institute of Environmental Physics
University of Bremen

Dr. Oliver Huhn
Institute of Environmental Physics
University of Bremen

Research areas

Weddell Sea (including Drake Passage and Greenwich Meridian), 80°S-50°S / 70°W-20°E

Publications

Bluhm K, Croot P, Huhn O, Rohardt G, Lochte K, 2011. Distribution of Iodide and Iodate in the Atlantic sector of the Southern Ocean during austral summer. Deep Sea Research Part II: Topical Studies in Oceanography (03 March 2011) doi:10.1016/j.dsr2.2011.02.002

Hellmer HH, Huhn O, Gomis D, Timmermann R, 2011. On the freshening of the northwestern Weddell Sea continental shelf. Ocean Science Discussions 7, 2013-2042.

Sudre J, Garcon V, Provost C, Sennechael N, Huhn O, Lacombe M, 2011. Short-term variations of deep water masses in Drake Passage revealed by a multiparametric analysis of the ANT-XXIII/3 bottle data. Deep Sea Research Part II: Topical Studies in Oceanography 58, 2592-2612.

Huhn O, Hellmer HH, Rhein M, Roether W, Rodehacke C, Schodlok M, Schröder M, 2008. Evidence of deep and bottom water formation in the western Weddell Sea. Deep-Sea Res. II 55, 1098-1116.

Huhn O, Roether W, Steinfeldt R, 2008. Age spectra in North Atlantic Deep Water along the South American continental slope, 10°N - 30°S, based on tracer observations, Deep-Sea Res. I, 55(10), 1252-1276.

Rodehacke CB, Hellmer HH, Huhn O, Beckmann A, 2007. Ocean/ice shelf interaction in the southern Weddell Sea: Results of a regional numerical helium/neon simulation. Ocean Dynamcs, 57, 1-11.

Huhn O, Hellmer HH, Rhein M, Rodehacke C, Roether W, Schodlok MP, Schrödder M, 2008. Evidence of deep- and bottom-water formation in the western Weddell Sea. Deep-Sea Research II, doi: 10.1016/j.dsr2.2007.12.015

Huhn O, et al., 2008. Chlorofluorocarbons, helium, and neon measured on water bottle samples during POLARSTERN cruise ANT-XXII/2 (ISPOL). http://doi.pangaea.de/10.1594/PANGAEA.729117

Homepage

Find more about Antarctic Bottom Water formation and export from the Weddell Sea at http://www.ocean.uni-bremen.de/

Research funding organisation

German Research Foundation

Project number: RH 25/27
Funding period: July 2007 - June 2009