Quantifying Antarctic deep waters in SODA reanalysis product (original) (raw)
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Antarctic Bottom Water variability in a coupled climate model
2008
The natural variability of the Weddell Sea variety of Antarctic Bottom Water (AABW) is examined in a long-term integration of a coupled climate model. Examination of passive tracer concentrations suggests that the model AABW is predominantly sourced in the Weddell Sea. The maximum rate of the Atlantic sector Antarctic overturning (atl) is shown to effectively represent the outflow of Weddell Sea deep and bottom waters and the compensating inflow of Warm Deep Water (WDW). The variability of atl is found to be driven by surface density variability, which is in turn controlled by sea surface salinity (SSS). This suggests that SSS is a better proxy than SST for post-Holocene paleoclimate reconstructions of the AABW overturning rate. Heat-salt budget and composite analyses reveal that during years of high Weddell Sea salinity, there is an increased removal of summertime sea ice by enhanced wind-driven ice drift, resulting in increased solar radiation absorbed into the ocean. The larger ice-free region in summer then leads to enhanced air-sea heat loss, more rapid ice growth, and therefore greater brine rejection during winter. Together with a negative feedback mechanism involving anomalous WDW inflow and sea ice melting, this results in positively correlated-S anomalies that in turn drive anomalous convection, impacting AABW variability. Analysis of the propagation of-S anomalies is conducted along an isopycnal surface marking the separation boundary between AABW and the overlying Circumpolar Deep Water. Empirical orthogonal function analyses reveal propagation of-S anomalies from the Weddell Sea into the Atlantic interior with the dominant modes characterized by fluctuations on interannual to centennial time scales. Although salinity variability is dominated by along-isopycnal propagation, variability is dominated by isopycnal heaving, which implies propagation of density anomalies with the speed of baroclinic waves.
Journal of Marine Research, 1999
We present measurements of the stable isotopes of oxygen ( 18 O and 16 O) from seawater samples collected during the Antarctic Deep Out ow Experiment (ADOX) cruises in the Southern Ocean and southern Indian Ocean, February to . The data are used in conjunction with hydrographic data to infer characteristics of the formation and circulation of water masses found in the region. The waters on the continental shelf of Antarctica (adjacent to the Princess Elizabeth Trough; PET) are isotopically the lightest found due to the injection of about 1% of glacial meltwater, and are probably advected to the region from farther east by the current associated with the Antarctic Slope Front. They appear to be locally disassociated from the Antarctic Surface Water and Winter Water (WW) farther north in the PET. The WW of the Enderby Basin is isotopically lighter than the PET WW and also fresher, indicating the presence of an additional component of glacial meltwater or high-latitude precipitation. North of the Antarctic Circumpolar Current, the surface d 18 O values correlate strongly with salinity, but extrapolate to an apparent freshwater endmember which is isotopically too light to be reasonable; advection and mixing of the water masses dominate over the local water balance at this location.
Journal of Geophysical Research, 2003
1] Interhemispheric signal transmission in the Atlantic Ocean connects the deep water production regions of both hemispheres. The nature of these interactions and large-scale responses to perturbations on timescales of years to millennia have been investigated using a global three-dimensional ocean general circulation model coupled to a dynamicthermodynamic sea ice model. The coupled model reproduces many aspects of today's oceanic circulation. A set of experiments shows the sensitivity to changes in different surface boundary conditions. Buoyancy changes in the Weddell and Labrador Seas exert a direct effect on the overturning cells of the respective hemisphere. They influence the density structure of the deep ocean and thereby lead to alterations in the strength of the ACC. Changing the wind stress south of 30°S influences the magnitude of the deep water production of both hemispheres. The interhemispheric effect in these experiments cannot be explained solely by advective mechanisms. Switching off the wind stress over the latitude band of the Drake Passage leads to a slow gradual decrease of the water mass transport in the ACC, resulting in an almost complete cessation. The model results prove the necessity to continue integrations over thousands of years until new equilibria are established.
Antarctic Intermediate Water Mass Formation in Ocean General Circulation Models
Journal of Physical Oceanography, 2001
Antarctic Intermediate Water is formed at the high mid-latitudes of the Southern Ocean. In many ocean general circulation model simulations with coarse resolution and a z-coordinate, a mid-depth salinity minimum characteristic of this intermediate water is reproduced. However, for the real ocean it remains unclear which are the dominant processes in the formation of this water mass and which are the source regions for contributing surface waters. To elucidate such processes and quantify intermediate water formation rates, two experiments with an ocean general circulation model were conducted. In one experiment, the traditional parameterization of horizontal and vertical mixing was applied, while the second model included the Gent-McWilliams parameterization for an eddy-induced transport velocity. In the latter application, the production and meridional export of intermediate water was found to be larger than in the first experiment. Furthermore, mid-latitude convective mixing, which had been argued to be the main intermediate water mass formation mechanism was found to be not as important as in previous model results. Passive tracer experiments indicated that diapycnal mixing and circumpolar subduction might also play important roles as water mass formation processes in this particular ocean circulation model.
Ocean Dynamics, 2017
The evolution of the deep salinity-maximum associated with the Lower Circumpolar Deep Water (LCDW) is 15 assessed using a set of 37 hydrographic sections collected over a 20 year period in the Southern Ocean as part of 16 the WOCE/CLIVAR programme. A circumpolar decrease in the value of the salinity maximum is observed 17 eastwards from the North Atlantic Deep Water (NADW) in the Atlantic sector of the Southern Ocean through 18 the Indian and Pacific sectors to Drake Passage. Isopycnal mixing processes are limited by circumpolar fronts, 19 and in the Atlantic sector this acts to limit the direct poleward propagation of the salinity signal. Limited 20 entrainment occurs into the Weddell Gyre, with LCDW entering primarily through the eddy-dominated eastern 21 limb. A vertical mixing coefficient, κV of (2.86 ± 1.06) x 10-4 m 2 s-1 and an isopycnal mixing coefficient, κI of 22 (8.97 ± 1.67) x 10 2 m 2 s-1 are calculated for the eastern Indian and Pacific sectors of the Antarctic Circumpolar 23 Current (ACC). A κV of (2.39 ± 2.83) x 10-5 m 2 s-1 , an order of magnitude smaller, and a κI of (2.47 ± 0.63) x 24 10 2 m 2 s-1 , three times smaller, are calculated for the southern and eastern Weddell Gyre reflecting a more 25 turbulent regime in the ACC and a less turbulent regime in the Weddell Gyre. In agreement with other studies, 26 we conclude that the ACC acts as a barrier to direct meridional transport and mixing in the Atlantic sector 27 evidenced by the eastward propagation of the deep salinity-maximum signal, insulating the Weddell Gyre from 28 short-term changes in NADW characteristics. 29 Manuscript text Click here to download Manuscript LCDW-sub-v4.4-text.docx Click here to view linked References
Nature Communications, 2020
Weddell Sea-derived Antarctic Bottom Water (AABW) is one of the most important deep water masses in the Southern Hemisphere occupying large portions of the deep Southern Ocean (SO) today. While substantial changes in SO-overturning circulation were previously suggested, the state of Weddell Sea AABW export during glacial climates remains poorly understood. Here we report seawater-derived Nd and Pb isotope records that provide evidence for the absence of Weddell Sea-derived AABW in the Atlantic sector of the SO during the last two glacial maxima. Increasing delivery of Antarctic Pb to regions outside the Weddell Sea traced SO frontal displacements during both glacial terminations. The export of Weddell Sea-derived AABW resumed late during glacial terminations, coinciding with the last major atmospheric CO2 rise in the transition to the Holocene and the Eemian. Our new records lend strong support for a previously inferred AABW overturning stagnation event during the peak Eemian interg...
On the export of Antarctic Bottom Water from the Weddell Sea
Deep-sea Research Part Ii-topical Studies in Oceanography, 2002
A survey of the current field over the South Scotia Ridge, obtained with a lowered Acoustic Doppler Current Profiler (LADCP), is presented. There is a pattern of northward (southward) flow on the western (eastern) side of each of four deep passages in the ridge, which is supported by tracer measurements. The net full-depth LADCP-referenced geostrophic transport over the ridge is 22±7 Sv (1 Sv=106 m3 s−1) northward, with the jets on either side of the passages transporting 5–10 Sv in alternating directions. The corresponding Weddell Sea Deep Water (WSDW) transport over the ridge is 6.7±1.7 Sv. This is a factor of 4 larger than the only previous estimate in the literature, and suggests that a significant proportion of the Antarctic Bottom Water (AABW) invading the world ocean abyss escapes the Weddell Sea via the Scotia Sea.The net full-depth and WSDW transports over the ridge are modified to 7±6 and 4.7±0.7 Sv, respectively, by a box inverse model of the western Weddell Gyre. The model incorporates the WOCE A23 crossing of the central part of the gyre and a set of five constraints synthesizing our previous oceanographic knowledge of the region. It diagnoses that 9.7±3.7 Sv of AABW are formed in the Weddell Sea, and that comparable amounts are exported over the South Scotia Ridge (∼48%) and further east (∼52%) assuming that no AABW enters the Weddell Gyre from the Indian Ocean. The WSDW fraction with neutral density γn>28.31 kg m−3 transported over the ridge upwells in the Scotia Sea at a rate of 6×10−6 m s−1, an order of magnitude larger than many basin-scale estimates of deep upwelling in the literature. In contrast, the Weddell Sea Bottom Water exported to the eastern Weddell Gyre entrains upward at a rate of 8×10−7 m s−1, more typical of other open-ocean regions. When their different ventilation histories are considered, the comparable transports and disparate upwelling rates of the AABW exported over the South Scotia Ridge and farther east may be crucial to our understanding of teleconnections between the Weddell Sea and the global ocean.
Journal of Physical Oceanography, 2002
The ocean distributions of chlorofluorocarbons (CFCs) have been measured extensively in order to determine the mechanisms, rates, and pathways associated with thermohaline deep water formation. Model temperature, salinity and CFC-11 fields from the National Center for Atmospheric Research (NCAR) global ocean climate model are compared against observations with emphasis on the patterns of Antarctic Bottom Water (AABW) production, properties, and circulation in the Southern Ocean. The model control simulation forms deep water as observed in both the Weddell and Ross Seas, though not along other sectors of the Antarctic coast. Examination of the deep water CFC-11 distribution, total inventory, and profiles along individual observational sections demonstrate that the decadal-scale deep water ventilation in the model Southern Ocean is both too weak and too restricted to the Ross and Weddell Sea source regions. A series of sensitivity experiments is conducted to determine the factors contributing to these deficiencies. The incorporation of a simple bottom boundary layer (BBL) scheme leads to only minor reductions in overall model-data error. The limited impact of the BBL may reflect in part other model large-scale circulation problems, for example the lack of saline Circumpolar Deep Water along the Antarctic slope, and the coarse vertical resolution of the model. The surface boundary conditions in the permanent sea ice covered regions are a more major factor, leading to inadequate formation of dense, cold and relatively saline shelf waters, the precursors of AABW. Improved model-data agreement is found by combining the BBL parameterization with reasonably small adjustments in the surface restoring salinities on the Weddell and Ross Shelfs, justified by under sampling of winter conditions in standard climatologies. The modified salinities result in increased AABW production and enhanced signature of Shelf Water properties in the deep Southern Ocean similar in character to the effect of coupling with an active sea ice model.