On the temporal variability of the Weddell Sea Deep Water masses (original) (raw)
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A seasonal cycle in the export of bottom water from the Weddell Sea
Nature Geoscience, 2010
Dense water formed over the Antarctic continental shelf rapidly descends into the deep ocean to spread as Antarctic Bottom Water, ventilating the global ocean 1,2. The coldest and most voluminous component is Weddell Sea Bottom Water 1,3,4,5,6,7. Here we present an 8-year observational record (3 April 1999 to 29 January 2007) of the benthic layer stratification within the Weddell Sea Bottom Water export path southeast of the South Orkney Islands. A pronounced bottom temperature seasonal cycle is apparent, with the coldest pulse in May/June, warmest in October/November, though the timing of these phases vary with year. The cold pulse thermohaline characteristics and mean bottom current of 0.1 m/s indicate an origin in the southwest Weddell Sea, with shelf water discharge during the prior austral summer. The coldest bottom occurred in 1999 and 2002; in 2000 the cold phase was absent. We propose that the Weddell Sea Bottom Water seasonal fluctuation is governed by the wind seasonal cycle over the Weddell Sea western margin, with the interannual fluctuations linked to variability of the wind-driven Weddell Gyre. The observations suggest a link of Weddell Sea Bottom Water generation to the Southern Annular Mode and El Niño Southern Oscillation. The ratio of Antarctic Bottom Water (AABW) to North Atlantic Deep Water (NADW) volume within the deep ocean is estimated as 1.7, with AABW covering more than twice the sea floor area than NADW 8. Observations within
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.
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.
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...
Representation of the Weddell Sea deep water masses in the ocean component of the NCAR-CCSM model
Antarctic Science, 2009
We examine Weddell Sea deep water mass distributions with respect to the results from three different model runs using the oceanic component of the National Center for Atmospheric Research Community Climate System Model (NCAR-CCSM). One run is inter-annually forced by corrected NCAR/ NCEP fluxes, while the other two are forced with the annual cycle obtained from the same climatology. One of the latter runs includes an interactive sea-ice model. Optimum Multiparameter analysis is applied to separate the deep water masses in the Greenwich Meridian section (into the Weddell Sea only) to measure the degree of realism obtained in the simulations. First, we describe the distribution of the simulated deep water masses using observed water type indices. Since the observed indices do not provide an acceptable representation of the Weddell Sea deep water masses as expected, they are specifically adjusted for each simulation. Differences among the water masses' representations in the three simulations are quantified through their root-mean-square differences. Results point out the need for better representation (and inclusion) of ice-related processes in order to improve the oceanic characteristics and variability of dense Southern Ocean water masses in the outputs of the NCAR-CCSM model, and probably in other ocean and climate models.