On the circulation and stratification of the Weddell Gyre (original) (raw)
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The contribution of the Weddell Gyre to the lower limb of the Global Overturning Circulation
Journal of Geophysical Research: Oceans, 2014
The horizontal and vertical circulation of the Weddell Gyre is diagnosed using a box inverse model constructed with recent hydrographic sections and including mobile sea ice and eddy transports. The gyre is found to convey 42 6 8 Sv (1 Sv 5 106 m3 s-1) across the central Weddell Sea and to intensify to 54 6 15 Sv further offshore. This circulation injects 36 6 13 TW of heat from the Antarctic Circumpolar Current to the gyre, and exports 51 6 23 mSv of freshwater, including 13 6 1 mSv as sea ice to the midlatitude Southern Ocean. The gyre's overturning circulation has an asymmetric double-cell structure, in which 13 6 4 Sv of Circumpolar Deep Water (CDW) and relatively light Antarctic Bottom Water (AABW) are transformed into upper-ocean water masses by midgyre upwelling (at a rate of 2 6 2 Sv) and into denser AABW by downwelling focussed at the western boundary (8 6 2 Sv). The gyre circulation exhibits a substantial throughflow component, by which CDW and AABW enter the gyre from the Indian sector, undergo ventilation and densification within the gyre, and are exported to the South Atlantic across the gyre's northern rim. The relatively modest net production of AABW in the Weddell Gyre (6 6 2 Sv) suggests that the gyre's prominence in the closure of the lower limb of global oceanic overturning stems largely from the recycling and equatorward export of Indian-sourced AABW.
Weddell Gyre: structure of the eastern boundary
Deep Sea Research Part I: Oceanographic Research Papers, 1993
Recetwd 12 August It~l: m relt.wd form 14 Aprd lt)t)2, a¢cepted 13 :~htv 19t)2) Ab,.tract-ln Januar)-Februar.~ lt)88 a survey m the eastern part of the Weddell Gwe b,, three U S S R research ~.ev, els shov.ed a comphcated h~drographlc structure that results from the mten,,r,e mteractmn between the ~,,aters ot the Wcddell Gyre and Antarctic Ctrcumpolar Current Three types of me'~o,.cale features ~ere found, warm-core eddies ol circumpolar origin, ~tth maximum temperature value,, m the range 1 7-2 I°C. warm-core eddies [ormed at the WeddeU Front ~,lth maxtmttm temperatures of (! S-I 4~C. and cold-core lcature,, lrom the Cold Regime of the V~eddell G)re ~,~th a maxm~um deep ~ater temperature ot II 7"C The mflo~ ol the C=reumpolar Deep Water into the g)re ,s taohtated b~, the .,,harp ,,outh~ard exeur,,=on of the Ant,,rctle Ctrcumpol,,r Current tore at abot, t 26°E due to the topographical corr, tramt o! the mtdotcan rtdge [ll|en,.e ~,~,drll,-torc eddie'., [ornllng ,tt the Polar }'FOnt alld drilling ,,outll~,~,ard arc ~,ls,blc m GEOSAT ,thmleter data .t,, relaw, el) high sea-level ~artabdm, The pure Cold Rcgtmc dccp ,.~,,ttcr t.,, lib ,I nl,tMilltnn tempcr,,ture-_tl 5"C occur', olll) In ~sol,ttcd p,ttthe,, v, c,,t of 21 "E INTRODUC'i ION "l'Jtl: Weddell Gyre consists of a large elongated cyclonic c|rculation cell, bounded in tile south and west by Antarctica. The northern alld e;.tsterli bott|ldaries are free Ik~r eXCllailge with neighbouring regions of the Southern Ocean. Surface circulation within tile Weddell Gyrc area reflects wind influence at tile sea surface. Trajectories of drifting buoys and icebergs do not indicate the southward eastern limb of the gyre, moving approximately in a zonal direction, whereas in tile west they mark the northward Ilow very well. Deep water circulation is different, and the distributions of water properties are indicative of a clockwise rotation. Therefore the Weddell Gyre in thought to provide a mechat|ism for the advection of the Circumpolar Deep Water (CDW) poleward from the Antarctic Circumpolar Current (ACC). This inIlu~ of relatively warm and salty deep water feeds the oceanatmosphere exchanges, resulting in the incorporation of large amounts of tile deep water into the st|three layer (GoRno,~ and HuBrR, 1990). It has long been thought that the CDW mostly enters the Weddcll Gyrc somewhere east of 20°E (DEacON, 1979) and is transported to the west-southwest by the southern limb of the Weddell Gyre. Eastward flow predominates at the northern limb of the Weddcll Gyrc, transporting relatively cold and less sahne deep water away from the western boundary of the gyrc. Temperature
A numerical study of the circulation in the northwestern Weddell Sea
Deep Sea Research Part II: Topical Studies in Oceanography, 2002
An eddy-permitting simulation is used to study the circulation of the northwestern Weddell Sea and its interaction with the Scotia Sea. The analysis focuses on the circulation pathways, associated stratification, and volume transports. Comparison between model results and observations show reasonable agreements with respect to the modeled thermohaline stratification and circulation such as export of Weddell Sea Deep Water through the Weddell-Scotia Confluence region. Using the model results, we estimated the relative contributions of the two main routes of escape of the Weddell Sea deep waters into the Scotia Sea and the South Sandwich Trench. The main route for inter-basin exchange is found to be through the Scotia Sea (via the South Orkney Passage and the Bransfield Strait). Our simulation does not show advective transport of deep or bottom waters through the South Sandwich Trench, and Lagrangian analysis of float trajectories indicates that the fluxes in this region are more likely related to eddy-driven mixing than to mean flow advection. The model shows, in addition, some sub-basin scale features that have not been reported in the observations. The Weddell western boundary current is seen as a diffuse, filamentous feature. A southflowing jet was present over the outer shelf off the Antarctic Peninsula. Retroflection and return southward flow is seen for from the South Sandwich Trench, and there was a small inflow of Scotia Sea waters into the Powell Basin. r
Earth System Science Data, 2016
The Weddell Gyre plays a crucial role in the modification of climate by advecting heat poleward to the Antarctic ice shelves and by regulating the density of water masses that feed the lowest limb of the global ocean overturning circulation. However, our understanding of Weddell Gyre water mass properties is limited to regions of data availability, primarily along the Prime Meridian. The aim of this paper is to provide a data set of the upper water column properties of the entire Weddell Gyre. Objective mapping was applied to Argo float data in order to produce spatially gridded, time-composite maps of temperature and salinity for fixed pressure levels ranging from 50 to 2000 dbar, as well as temperature, salinity and pressure at the level of the sub-surface temperature maximum. While the data are currently too limited to incorporate time into the gridded structure, the data are extensive enough to produce maps of the entire region across three time-composite periods (2001-2005, 2006-2009 and 2010-2013), which can be used to determine how representative conclusions drawn from data collected along general RV transect lines are on a gyre scale perspective. The work presented here represents the technical prerequisite for addressing climatological research questions in forthcoming studies. The data sets are available in netCDF format at
Journal of Geophysical Research, 1995
The upward flux of heat from the subsurface core of Warm Deep Water (WDW) to the perennially ice-covered sea surface over the continental slope in the western Weddell Sea is estimated using data obtained during February-June 1992 from a drifting ice station. Through the permanent pycnocline the diapycnal heat flux is estimated to be about 3 W m-2, predominantly because of double-diffusive convection. There is no evidence that shear-driven mixing is important in the pycnocline. The estimated mean rate of heat transfer from the mixed layer to the ice is 1.7 W m-2, although peak heat fluxes of up to 15 W m-2 are found during storms. It is hypothesized that isopycnal mixing along sloping intrusions also contributes to the loss of heat from the WDW in this region; however, we are unable to quantify the fluxes associated with this process. Intrusions occur intermittently throughout this experiment but are most commonly found near the boundary of the warm-core current and the shelf-modified water to the east. These heat fluxes are significantly lower than the basin-averaged value of 19 W m-2 (Fahrbach et al., 1994) that is required to balance the heat budget of the Weddell Gyre. Other studies suggest that shelf processes to the west of the ice station drift track and more energetic double-diffusive convection in the midgyre to the east could account for the difference between our flux estimates for this region and those based on the basin-scale heat budget. 106 m 3 s-1) with most (-o90%) of the transport being contained in a boundary current located within 500 km of the shelf break [Fahrbach et al., 1994]. Water in the Gyre loses a significant amount of heat as it travels from the eastern Weddell to the northern tip of the Antarctic Peninsula. Fahrbach et al. [1994] estimate that the oceanic heat loss is equivalent to a flux to the atmosphere of 19 W m-2 when averaged over the entire Weddell Sea. Several different mechanisms are responsible for the observed cooling. For example, Muench et al. [1990] found that double-diffusive fluxes in the central Gyre, away from boundaries, were comparable to the basinscale average flux of 19 W m-2. High fluxes might also
Mixing on the continental slope of the southern Weddell Sea
Journal of Geophysical Research, 2009
1] Shipboard hydrography and current profiles collected in 2003 and time series from moored current meters deployed in late 1990s are analyzed to study the variability of mixing in the southeastern Weddell Sea. Profiles of eddy diffusivity K r are inferred from fine-scale shear (vertical derivative of horizontal velocity) and strain (vertical derivative of isopycnal displacement) variance using parameterizations which relate the internal wave energy to the dissipation rate at small scales. The highest mixing rates are seen near the bottom where the eddy diffusivities are elevated by 1 order of magnitude from those in the interior and exceed 10 À4 m 2 s À1 . The observations show latitudinal variability in K r , particularly near the bottom, where K r significantly increases near 74°28 0 S, the critical latitude for lunar semidiurnal (M 2 ) tides. In this region, the critical latitude coincides with near-critical topography on the upper continental slope, a situation which favors generation of M 2 internal waves. Consistent with the results from fine-scale shear and strain parameterizations, which indicate highest bottom diffusivities near the critical latitude, independent analysis of current time series from moored instruments shows a thickening of the frictional bottom boundary layer near the critical latitude. Semidiurnal tidal dynamics at the upper continental slope together with the critical latitude effects lead to mixing that might significantly affect the regional heat budget and the circulation in the study area.
The new bathymetric charts of the Weddell Sea: AWI BCWS
Antarctic Research Series, 1998
From data collected by RV Polarstern, and additional echosoundings provided by national hydrographic offices, research institutions and the International Hydrographic Organization (IHO) Digital Bathymetric Data Center, the 1:1,000,000 Bathymetric Chart of the Weddell Sea (AWI BCWS) series has been developed. The heterogeneity of bathymetric data and the lack of observations for ice-covered areas required the incorporation of supplementary geophysical and geographical information. A new semi-automatic procedure was developed for terrain modeling and contouring. In coastal regions, adjacent sub-glacial information was included in order to model the bathymetry of the transition zone along the Antarctic ice edge. Six sheets of the AWI BCWS series in the scale of 1:1,000,000 covering the southern Weddell Sea from 66°S to 78°S and from 68°W to 0°E were recently completed and included in the 1997 GEneral Bathymetric Chart of the Oceans (GEBCO) Digital Atlas CD-ROM. On the basis of these six 1:1,000,000 AWI BCWS sheets, a generalized l:3,000,000-scale bathymetric chart was compiled for the entire southern Weddell Sea. That chart is included in this volume and is described with regard to its significance to other disciplines.
Montero-Serrano et al GRL 2011 Atlantic Gyres nf
1] The neodymium (Nd) isotopic composition (expressed in epsilon units, "Nd) of reef framework-forming coldwater corals provides unique measures of water mass provenance and mixing within the Northeast Atlantic today and in the past. A reconstruction of near thermocline water "Nd from cold-water corals of the Gulf of Cádiz and Porcupine Seabight spanning over the past 300,000 years, now revealed that climate cooling during Marine Isotope Stages (MIS) 7.2 and MIS 8/9 led to a retraction of the mid-depth Subpolar Gyre (mSPG) to the west. Conversely, Northern Hemisphere warming and increasing fresh water fluxes to the northwest (Labrador Sea) favor a stronger eastward extension of the mSPG blocking the northward flow of temperate Atlantic water as observed during the early MIS 1 and the early stage MIS 5.5. These changes are likely the result of large-scale south-north displacement of the westerlies similar to present-day observations that the North Atlantic Oscillation (NAO) is linked with mid-depth ocean circulation. Based on these observations, we hypothesize that further climate warming will also strengthen the mSPG leading to a salt and temperature decrease in the Northeast Atlantic whereas salinity and temperature will increase in the temperate Atlantic. However, the amplitude of such changes on North Atlantic overturning remains to be tested.
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