Alejandro H. Orsi

On the meridional extent and fronts of the Antarctic Circumpolar Current

Large-scale features of the Antarctic Circumpolar Current (ACC) are described using all historical hydrographic data available from the Southern Ocean. The geopotential anomaly of the sea surface relative to 1000 db reveals the highly-sheared eastward flow of the ACC and the strong steering of the current by the ridge system around Antarctica. The near-surface property distributions differentiate the ACC waters from the warmer and saltier waters of the subtropical regimes. The Subtropical Front (STF), interrupted only by South America, marks the northern most extent of subantarctic waters. Distributions of properties on isopycnal surfaces show an abrupt end to the characteristic signal of the Upper Circumpolar Deep Water (UCDW), as this water mass shoals southward and is entrained into the surface mixed layer. This sharp water mass boundary nearly coincides with the southernmost circumpolar streamline passing through Drake Passage. To its south are the weakly-sheared circulations of the subpolar regime. Inspection of many hydrographic crossings of this transition reveals that the poleward edge of the UCD W signal is a reasonable definition of the southern boundary of the ACC. At Drake Passage, three deep-reaching fronts account for most of the ACC transport. Well-established indicators of the Subantarctic Front and Polar Front are traced unbroken around Antarctica. The third deep-reaching front observed to the south of the Polar Front at Drake Passage also continues with similar characteristics as a circumpolar feature. It is called here the southern ACC front. Stations from multiple synoptic transects of these circumpolar fronts are used to describe the average property structure within each ACC zone. Between the STF and the southern boundary of the ACC, the shear transport of the circumpolar current above 3000 m is at all longitudes about 100 Sv (1 Sv = 106 m3 s−) eastward.

On the circulation and stratification of the Weddell Gyre

The availability of new, high-quality, hydrographic data has prompted a re-examination of the circulation in the Atlantic sector of the Southern Ocean. Dynamic topography maps and tracer distributions on selected isopycnal surfaces show that the Weddell Gyre is a large, elongated cyclone located south of the Antarctic Circumpolar Current (ACC), extending northeastward from the Antarctic Peninsula. Patterns of geostrophic shear and a southward turn of the ACC mark its northeastern end near 30°E.

On the total input of Antarctic waters to the deep ocean: A preliminary estimate from chlorofluorocarbon measurements

[1] Deep ocean inventories of dissolved chlorofluorocarbon-11 (CFC-11) along representative sections off Antarctica provide the first estimate of the overall strength of all dense water sources in the Southern Ocean. Their formation rates are reported for three density layers that span the main water masses involved in the lower limb of the Thermohaline Circulation (THC). The bottom layer is supplied via sinking of Antarctic Bottom Water (AABW) produced at a few continental shelves. The middle layer receives the offshore injection of ventilated Modified Circumpolar Deep Water (MCDW) produced along much of the lengthy Antarctic Slope Front. The top layer is ventilated by northward export of Antarctic Surface Water into the Upper Circumpolar Deep Water of the Antarctic Circumpolar Current. Average Southern Ocean inputs to the upper two layers of the deep ocean for the 1970-1990 period are derived on the basis of the CFC-11 distributions along meridional sections, the mean CFC-11 saturations of all water mass ingredients, and the inferred mixing leading to production of dense source waters. About 5.4 ± 1.7 Sv (1 Sv = 10 6 m 3 s -1 ) of near-freezing Shelf Water ventilate the bottom layer, and 4.7 ± 1.7 Sv and 3.6 ± 1.3 Sv of cold Antarctic Surface Water ventilate the middle and top layers. Therefore the total contribution of ventilated Southern Ocean waters to the lower limb of the global THC is about 14 Sv. This is close to the about 17 Sv estimated for North Atlantic near-surface sources from CFC-11 inventories. Their entrainment of 7.4 ± 2.4 Sv of CFC-poor subsurface Lower Circumpolar Deep Water during the formation and sinking of AABW and MCDW raises the total Southern Ocean input to the deep ocean to about 21 Sv.

Weddell Sea shelf water in the Bransfield Strait and Weddell-Scotia Confluence

Abstract The unusual stratification of the waters in the Weddell-Scotia Confluence between the Scotia and Weddell Seas and in the Bransfield Strait is traced to the influence of shelf waters from the northwest Weddell Sea. The shelf waters span the density range encompassed by the warm, salty Circumpolar Deep Water (CDW) of the Antarctic Circumpolar Current, and the colder and slightly fresher CDW in the Weddell Sea. An isopycnal mixture of these three source waters flows eastward from the tip of the Antaractic Peninsula in to the Weddell-Scotia Confluence region, and westward north of the Peninsula, where it flows downslope to renew the deep waters of the Bransfield Strait. This mixing scheme can occur year-round, in contrast to some previous explanations of the stratification in the region, which relied on the (unobserved) winter convective overturn of the water column.

Energetic plumes over the western Ross Sea continental slope

Rapid descent of dense Drygalski Trough (western Ross Sea, Antarctica) shelf water over the continental slope, within 100 to 250 m thick benthic plumes, is described. Speeds of up to 1.0 m/s are recorded flowing at an average angle of 35° to the isobaths, entraining ambient Lower Circumpolar Deep Water en route. This process is predominant in determining the concentration and placement of the shelf water injected into the deep sea as a precursor Antarctic Bottom Water. Nonetheless, a 4-hour duration pulse of undiluted shelf water was observed at depth (1407 m) directly north of the Drygalski Trough, moving at around 90 degrees to isobaths, and at a speed of 1.4 m/s. Thus the export of Ross Sea shelf water to the deep sea is accomplished within plumes descending at moderate angle to isobaths, punctuated by rapid downhill cascades.

Cooling and ventilating the Abyssal Ocean

The abyssal ocean is filled with cold, dense waters that sink along the Antarctic continental slope and overflow sills that lie south of the Nordic Seas. Recent inte- grations of chlorofluorocarbon-11 (CFC) measurements are similar in Antarctic Bottom Water (AABW) and in lower North Atlantic Deep Water (NADW), but Antarctic inputs are ≈ 2 ◦ C colder than their northern counterparts. This indicates comparable ventilation rates from both polar re- gions, and accounts for the Southern Ocean dominance over abyssal cooling. The decadal CFC-based estimates of recent ventilation are consistent with other hydrographic observa- tions and with longer-term radiocarbon data, but not with hypotheses of a 20 th -centuryslowdown in the rate of AABW formation. Significant variabilityis not precluded bythe available ocean measurements, however, and interannual to decadal changes are increasinglyevident at high latitudes.

Southwest Pacific Ocean Water-Mass Changes between 1968/69 and 1990/91*†

Abstract Water-mass changes are estimated in the southwest Pacific Ocean by comparing a meridional hydrographic section along 170°W between 60°S and the equator occupied in 1968/69 during the Southern Cross cruise and again in 1990 during a NOAA Climate and Global Change cruise. Another comparison is made using a hydrographic section along 35°S between the date line and 169°W occupied in 1969 during USNS Eltanin cruise 40 and again in 1991 during a Mapkiwi cruise. The most robust change consists of cooling (and freshening) on isopycnals, with peak differences exceeding −1.0°C (−0.25 pss) at the base of the subtropical thermocline. The cooling and freshening starts above the stratification minimum of the Subantarctic Mode Water and persists to below the salinity minimum of the Antarctic Intermediate Water. Amplitudes are largest at 48°S, near where these water masses subduct, and decay toward 20°S, near the axis of the Subtropical Gyre. This change is likely the result of surface warming and/or freshening ...

Antarctic Bottom Water production and export by tides in the Ross Sea

[1] Current measurements in the western Ross Sea show that Antarctic Bottom Water is produced at the outer edge of the Continental Shelf by tidal stirring of Antarctic Surface Water, Circumpolar Deep Water and Shelf Water. Current amplitudes during spring tides are greater than 1 m/s, sufficient to carry Circumpolar Deep Water onto the shelf from offshore and stir at least the lower portion of the water column to a nearly homogeneous mixture. The Antarctic Bottom Water produced every day during spring tides does not appear to have a seasonal signal. Its export rate is estimated to be 1.95 ± 1.85 × 10 6 m 3 /s.

On the Variability of Antarctic Circumpolar Current Fronts Inferred from 1992–2011 Altimetry*

AbstractAntarctic Circumpolar Current (ACC) fronts, defined as water mass boundaries, have been known to respond to large-scale atmospheric variabilities, especially the Southern Hemisphere annular mode (SAM) and El Nino–Southern Oscillation (ENSO). Distinct patterns of localized variability in meridional front displacements during 1992–2011 are derived from the analysis of satellite sea surface height data. Major basin-scale differences are found between the southeast Pacific (150°–90°W) and the southeast Indian (75°–150°E) sectors of the ACC. Frontal positions in the southeast Pacific show large year-to-year meridional fluctuations, attributed mostly to ENSO and in part SAM, and no apparent seasonal cycles or long-term trends. In contrast, summer (winter) frontal locations in the southeast Indian extend farther to the south (north) of their long-term mean distribution. A southward drift of ACC fronts is indicated over the Indian sector during the past two decades. This long-term shift is not directly re...

Role of tides on the formation of the Antarctic Slope Front at the Weddell‐Scotia Confluence

The structure of the Antarctic Slope Front (ASF) and the associated Antarctic Slope Current (ASC) on the Scotia Sea side of the Weddell-Scotia Confluence (WSC) is described using data from a hydrographic survey and three 1 year long moorings across the continental slope. The ASC in this region flows westward along isobaths with an annual mean speed of ∼0.2 m s−1, with time variability dominated by the K1 and O1 tidal diurnal constituents, a narrowband oscillation with ∼2-week period attributable to the spring/neap tidal cycle, and seasonal variability. Realistic and idealized high-resolution numerical simulations are used to determine the contribution of tides to the structure of the ASF and the speed of the ASC. Two simulations forced by realistic atmospheric forcing and boundary conditions integrated with and without tidal forcing show that tidal forcing is essential to reproduce the measured ASF/ASC cross-slope structure, the time variability at our moorings, and the reduced stratification within the WSC. Two idealized simulations run with tide-only forcing, one with a homogeneous ocean and the other with initial vertical stratification that is laterally homogeneous, show that tides can generate the ASC and ASF through volume flux convergence along the slope initiated by effects including the Lagrangian component of tidal rectification and mixing at the seabed and in the stratified ocean interior. Climate models that exclude the effects of tides will not correctly represent the ASF and ASC or their influence on the injection of intermediate and dense waters from the WSC to the deep ocean.