Karen J. Heywood

Modification and pathways of Southern Ocean deep waters in the Scotia Sea

An unprecedented high-quality, quasi-synoptic hydrographic data set collected during the ALBATROSS cruise along the rim of the Scotia Sea is examined to describe the pathways of the deep water masses flowing through the region, and to quantify changes in their properties as they cross the sea. Owing to sparse sampling of the northern and southern boundaries of the basin, the modification and pathways of deep water masses in the Scotia Sea had remained poorly documented despite their global significance. Weddell Sea Deep Water (WSDW) of two distinct types is observed spilling over the South Scotia Ridge to the west and east of the western edge of the Orkney Passage. The colder and fresher type in the west, recently ventilated in the northern Antarctic Peninsula, flows westward to Drake Passage along the southern margin of the Scotia Sea while mixing intensely with eastward-flowing Circumpolar Deep Water (CDW) of the antarctic circumpolar current (ACC). Although a small fraction of the other WSDW type also spreads westward to Drake Passage, the greater part escapes the Scotia Sea eastward through the Georgia Passage and flows into the Malvinas Chasm via a deep gap northeast of South Georgia. A more saline WSDW variety from the South Sandwich Trench may leak into the eastern Scotia Sea through Georgia Passage, but mainly flows around the Northeast Georgia Rise to the northern Georgia Basin. In Drake Passage, the inflowing CDW displays a previously unreported bimodal property distribution, with CDW at the Subantarctic Front receiving a contribution of deep water from the subtropical Pacific. This bimodality is eroded away in the Scotia Sea by vigorous mixing with WSDW and CDW from the Weddell Gyre. The extent of ventilation follows a zonation that can be related to the CDW pathways and the frontal anatomy of the ACC. Between the Southern Boundary of the ACC and the Southern ACC Front, CDW cools by 0.15°C and freshens by 0.015 along isopycnals. The body of CDW in the region of the Polar Front splits after overflowing the North Scotia Ridge, with a fraction following the front south of the Falkland Plateau and another spilling over the plateau near 49.5°W. Its cooling (by 0.07°C) and freshening (by 0.008) in crossing the Scotia Sea is counteracted locally by NADW entraining southward near the Maurice Ewing Bank. CDW also overflows the North Scotia Ridge by following the Subantarctic Front through a passage just east of Burdwood Bank, and spills over the Falkland Plateau near 53°W with decreased potential temperature (by 0.03°C) and salinity (by 0.004). As a result of ventilation by Weddell Sea waters, the signature of the Southeast Pacific Deep Water (SPDW) fraction of CDW is largely erased in the Scotia Sea. A modified form of SPDW is detected escaping the sea via two distinct routes only: following the Southern ACC Front through Georgia Passage; and skirting the eastern end of the Falkland Plateau after flowing through Shag Rocks Passage.

Multidecadal warming of Antarctic waters

Decadal trends in the properties of seawater adjacent to Antarctica are poorly known, and the mechanisms responsible for such changes are uncertain. Antarctic ice sheet mass loss is largely driven by ice shelf basal melt, which is influenced by ocean-ice interactions and has been correlated with Antarctic Continental Shelf Bottom Water (ASBW) temperature. We document the spatial distribution of long-term large-scale trends in temperature, salinity, and core depth over the Antarctic continental shelf and slope. Warming at the seabed in the Bellingshausen and Amundsen seas is linked to increased heat content and to a shoaling of the mid-depth temperature maximum over the continental slope, allowing warmer, saltier water greater access to the shelf in recent years. Regions of ASBW warming are those exhibiting increased ice shelf melt. Seawater temperature around Antarctica is rising as more warmer, deeper water moves toward the surface. [Also see Perspective by Gille] Bringing up the problem of ice shelf melting Warm water intruding from below is heating up the ocean that covers the continental shelf of Antarctica. Schmidtko et al. report that Circumpolar Deep Water has been warming and moving further up onto the shelf around Antarctica for the past 40 years, causing higher rates of ice sheet melting (see the Perspective by Gille). These observations need to be taken into account when considering the potential for irreversible retreat of parts of the West Antarctic Ice Sheet. Science, this issue p. 1227; see also p. 1180

Wind-Driven Transport Fluctuations through Drake Passage: A Southern Mode

Abstract It is proposed that, for periods between about 10 and 220 days, the variability in Antarctic circumpolar transport is dominated by a barotropic mode that follows f/H contours almost everywhere. Theoretical arguments are given that suggest the possible importance of this mode and show that bottom pressure to the south of the current should be a good monitor of its transport. The relevance of these arguments to eddy-resolving models is confirmed by data from the Fine Resolution Antarctic Model and the Parallel Ocean Climate Model. The models also show that it may be impossible to distinguish the large-scale barotropic variability from local baroclinic processes, given only local measurements, although this is not generally a problem to the south of the Antarctic Circumpolar Current. Comparison of bottom pressures measured in Drake Passage and subsurface pressure on the Antarctic coast, with wind stresses derived from meteorological analyses, gives results consistent with the models, showing that wi...

High mixing rates in the abyssal Southern Ocean

Mixing of water masses from the deep ocean to the layers above can be estimated from considerations of continuity in the global ocean overturning circulation. But averaged over ocean basins, diffusivity has been observed to be too small to account for the global upward flux of water, and high mixing intensities have only been found in the restricted areas close to sills and narrow gaps. Here we present observations from the Scotia Sea, a deep ocean basin between the Antarctic peninsula and the tip of South America, showing a high intensity of mixing that is unprecedented over such a large area. Using a budget calculation over the whole basin, we find a diffusivity of (39 ± 10) × 104 m2 s-1, averaged over an area of 7 × 105 km2. The Scotia Sea is a basin with a rough topography, situated just east of the Drake passage where the strong flow of the Antarctic Circumpolar Current is constricted in width. The high basin-wide mixing intensity in this area of the Southern Ocean may help resolve the question of where the abyssal water masses are mixed towards the surface.

Sustained monitoring of the southern ocean at Drake Passage: Past achievements and future priorities

Drake Passage is the narrowest constriction of the Antarctic Circumpolar Current (ACC) in the Southern Ocean, with implications for global ocean circulation and climate. We review the long-term sustained monitoring programs that have been conducted at Drake Passage, dating back to the early part of the twentieth century. Attention is drawn to numerous breakthroughs that have been made from these programs, including (1) the first determinations of the complex ACC structure and early quantifications of its transport; (2) realization that the ACC transport is remarkably steady over interannual and longer periods, and a growing understanding of the processes responsible for this; (3) recognition of the role of coupled climate modes in dictating the horizontal transport and the role of anthropogenic processes in this; and (4) understanding of mechanisms driving changes in both the upper and lower limbs of the Southern Ocean overturning circulation and their impacts. It is argued that monitoring of this passage remains a high priority for oceanographic and climate research but that strategic improvements could be made concerning how this is conducted. In particular, long-term programs should concentrate on delivering quantifications of key variables of direct relevance to large-scale environmental issues: In this context, the time-varying overturning circulation is, if anything, even more compelling a target than the ACC flow. Further, there is a need for better international resource sharing and improved spatiotemporal coordination of the measurements. If achieved, the improvements in understanding of important climatic issues deriving from Drake Passage monitoring can be sustained into the future.

Variability of the southern Antarctic Circumpolar Current front north of South Georgia

South Georgia (f54jS, 37jW) is an island in the eastern Scotia Sea, South Atlantic that lies in the path of the Antarctic Circumpolar Current (ACC). The southern ACC front (SACCF), one of three major fronts associated with the ACC, wraps anticyclonically around South Georgia and then retroflects north of the island. This paper investigates temporal variability in the position of the SACCF north of South Georgia that is likely to have an effect on the South Georgia ecosystem by contributing to the variability in local krill abundance. A meridional hydrographic section that crossed the SACCF three times demonstrates that the SACCF is associated with a geopotential anomaly of 4.5 J kg 1 in the eastern Scotia Sea. A high resolution (1/4j1/4j) map of historical geopotential anomaly shows the mean position of the SACCF retroflection north of South Georgia to be at 36jW, 400 km further east than in previous work. It also reveals temporal variability associated with the SACCF in the South Georgia region. A near-surface drifter provides evidence for variability in the western extent of the SACCF north of South Georgia and for the presence of eddies in the region. Output from a 3-year (1993–1995) high frequency wind forced run of the eddy-permitting Ocean Circulation and Climate Advanced Modelling project (OCCAM) model, used to investigate the frontal variability, shows two periods of anomalous westward extent of the SACCF north of South Georgia and associated eddy-shedding. The SACCF variability affects the near-surface transport of passive drifters into the region with implications for the South Georgia ecosystem.

Variability of Subantarctic mode water and Antarctic intermediate water in the Drake Passage during the late-twentieth and early-twenty-first centuries

A time series of the physical and biogeochemical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage between 1969 and 2005 is constructed using 24 transects of measurements across the passage. Both water masses have experienced substantial variability on interannual to interdecadal time scales. SAMW is formed by winter overturning on the equatorward flank of the Antarctic Circumpolar Current (ACC) in and to the west of the Drake Passage. Its interannual variability is primarily driven by variations in wintertime air–sea turbulent heat fluxes and net evaporation modulated by the El Nino–Southern Oscillation (ENSO). Despite their spatial proximity, the AAIW in the Drake Passage has a very different source than that of the SAMW because it is ventilated by the northward subduction of Winter Water originating in the Bellingshausen Sea. Changes in AAIW are mainly forced by variability in Winter Water properties resulting from fluctuations in wintertime air–sea turbulent heat fluxes and spring sea ice melting, both of which are linked to predominantly ENSO-driven variations in the intensity of meridional winds to the west of the Antarctic Peninsula. A prominent exception to the prevalent modes of SAMW and AAIW formation occurred in 1998, when strong wind forcing associated with constructive interference between ENSO and the southern annular mode (SAM) triggered a transitory shift to an Ekman-dominated mode of SAMW ventilation and a 1–2-yr shutdown of AAIW production. The interdecadal evolutions of SAMW and AAIW in the Drake Passage are distinct and driven by different processes. SAMW warmed (by 0.3°C) and salinified (by 0.04) during the 1970s and experienced the reverse trends between 1990 and 2005, when the coldest and freshest SAMW on record was observed. In contrast, AAIW underwent a net freshening (by 0.05) between the 1970s and the twenty-first century. Although the reversing changes in SAMW were chiefly forced by a 30-yr oscillation in regional air–sea turbulent heat fluxes and precipitation associated with the interdecadal Pacific oscillation, with a SAM-driven intensification of the Ekman supply of Antarctic surface waters from the south contributing significantly too, the freshening of AAIW was linked to the extreme climate change that occurred to the west of the Antarctic Peninsula in recent decades. There, a freshening of the Winter Water ventilating AAIW was brought about by increased precipitation and a retreat of the winter sea ice edge, which were seemingly forced by an interdecadal trend in the SAM and regional positive feedbacks in the air–sea ice coupled climate system. All in all, these findings highlight the role of the major modes of Southern Hemisphere climate variability in driving the evolution of SAMW and AAIW in the Drake Passage region and the wider South Atlantic and suggest that these modes may have contributed significantly to the hemispheric-scale changes undergone by those waters in recent decades.

Measurements beneath an Antarctic ice shelf using an autonomous underwater vehicle

The cavities beneath Antarctic ice shelves are among the least studied regions of the World Ocean, yet they are sites of globally important water mass transformations. Here we report results from a mission beneath Fimbul Ice Shelf of an autonomous underwater vehicle. The data reveal a spatially complex oceanographic environment, an ice base with widely varying roughness, and a cavity periodically exposed to water with a temperature significantly above the surface freezing point. The results of this, the briefest of glimpses of conditions in this extraordinary environment, are already reforming our view of the topographic and oceanographic conditions beneath ice shelves, holding out great promises for future missions from similar platforms.

On the sources of Weddell Gyre Antarctic Bottom Water

In March–April 1995, as part of the World Ocean Circulation Experiment section A23, we completed 49 hydrographic stations across the Weddell Gyre and southern Antarctic Circumpolar Current, from the Antarctic continental shelf (72.5°S, 16.5°W) to South Georgia (55°S, 34.5°W). Chlorofluorocarbon (CFC-11, CFC-12, and CFC-113) data collected at these stations reveal that distinct sources renew the Antarctic Bottom Water (defined as waters with potential temperatures less than 0°C) of the Weddell Gyre. Weddell Sea Bottom Water (defined as waters with potential temperatures less than −0.7°C) formed in the western Weddell Sea has CFC concentrations about 5 to 6 times higher in the eastward flowing northern Weddell Gyre than in the westward flowing southern limb. Our CFC measurements suggest that distinct sources of Weddell Sea Bottom Water exist in the western Weddell Sea, in agreement with previous descriptions based on potential temperature and salinity signals. In the northern Weddell Gyre, high CFC concentrations in Weddell Sea Deep Water, potential temperatures between 0°C and −0.7°C, confirm the long-recognized sources for this water mass in the western and southwestern Weddell Sea. In the southern Weddell Gyre at about 20°W and along the Antarctic continental slope, Weddell Sea Deep Water with potential temperatures around −0.45°C shows a deep CFC maximum about 1000 m above the seafloor. CFC concentrations in this deep southern core are about 80% of those of new Weddell Sea Deep Water in the northern Weddell Gyre near 30°W. The A23 CFC and hydrographic data are not consistent with the hypothesis that Weddell Sea Deep Waters are derived from a single source in the western Weddell Sea. Instead, these tracers suggest that an important portion of the Weddell Sea Deep Water in the southern Weddell Gyre originates outside the western Weddell Sea, probably near the Amery Basin and environs, around 75°E. These features of the circulation and renewal of the deep Weddell Gyre should be carefully considered in simulations dealing with fluxes, pathways, and formation rates of Antarctic Bottom Water.

Seasonal and interannual changes in the North Atlantic subpolar gyre from Geosat and TOPEX/POSEIDON altimetry

Sea surface slopes from the altimetric satellites Geosat and TOPEX/POSEIDON are used to calculate eddy kinetic energy of the North Atlantic subpolar gyre (40–65°N, 60°–5°W). Using two years of data from each satellite (December 1986 to December 1988 and October 1992 to September 1994, respectively), interannual differences in the North Atlantic Current (NAC) are revealed. In regions of strong currents the eddies are driven by baroclinic instability of the mean flow, are not seasonally varying, and may therefore be used as a surrogate for the mean flow itself. Wind stress curl fields for the same periods show that the northward and southward shifts in the current branches across the Mid-Atlantic Ridge are related to interannual differences in the winter wind stress curl pattern when the zero in wind stress curl is well defined and wind stress is at a maximum. Outside the NAC, the eddies are driven primarily by wind stress, indicated by a significant seasonality. Time series of eddy kinetic energy and wind stress are generated for the 4 years, and the magnitude, phase, and significance of annual and semiannual signals are determined. Interannual changes in the timing of the maximum eddy kinetic energy are associated with that of the maximum wind stress, with each being shifted by a month or two later or earlier. Throughout much of the northern North Atlantic a significant annual signal in eddy kinetic energy is observed, lagging the wind stress by, on average, 6 weeks. A negligible lag is found in the region of the East Greenland Current and Irminger Sea, whereas the greatest lag, about 2 months, is found in the Labrador Sea.