Brian A. King

Transport and variability of the Antarctic Circumpolar Current in Drake Passage

The baroclinic transport of the Antarctic Circumpolar Current (ACC) above 3000 m through Drake Passage is 107.3 ± 10.4 Sv and has been steady between 1975 and 2000. For six hydrographic sections (1993–2000) along the World Ocean Circulation Experiment (WOCE) line SR1b, the baroclinic transport relative to the deepest common level is 136.7 ± 7.8 Sv. The ACC transport is carried in two jets, the Subantarctic Front 53 ± 10 Sv and the Polar Front (PF) 57.5 ± 5.7 Sv. Southward of the ACC the Southern Antarctic Circumpolar Current transports 9.3 ± 2.4 Sv. We observe the PF at two latitudes separated by 90 km. This bimodal distribution is related to changes in the circulation and properties of Antarctic Bottom Water. Three realizations of the instantaneous velocity field were obtained with lowered ADCPs. From these observations we obtain near-bottom reference velocities for transport calculations. Net transport due to these reference velocities ranges from -28 to 43 Sv, consistent with previous estimates of variability. The transport in density layers shows systematic variations due to seasonal heating in near-surface layers. Volume transport-weighted mean temperatures vary by 0.40°C from spring to summer; a seasonal variation in heat flux of about 0.22 PW. Finally, we review a series of papers from the International Southern Ocean Studies Program. The average yearlong absolute transport is 134 Sv, and the standard deviation of the average is 11.2 Sv; the error of the average transport is 15 to 27 Sv. We emphasize that baroclinic variability is an important contribution to net variability in the ACC.

Argo profiling floats bring new era of in situ ocean observations

The Argo profiling float project will enable, for the first time, continuous global observations of the temperature, salinity, and velocity of the upper ocean in near-real time.This new capability will improve our understanding of the oceans role in climate, as well as spawn an enormous range of valuable ocean applications. Because over 90% of the observed increase in heat content of the air/land/sea climate system over the past 50 years occurred in the ocean [Leuitus et al., 2001], Argo will effectively monitor the pulse of the global heat balance.The end of 2003 was marked by two significant events for Argo. In mid-November 2003, over 200 scientists from 22 countries met at Argos first science workshop to discuss early results from the floats. Two weeks later, Argo had 1000 profiling floats—one-third of the target total—delivering data. As of 7 May that total was 1171.

Oceanic Fluxes on the WOCE A11 Section

Abstract The most southerly WOCE one-time section in the South Atlantic, designated A11, was occupied in January 1993. The cruise track lay across the cool subantarctic zone of the Circumpolar Current in the west and the warm subtropical gyre in the east. In this paper estimates of the flux of heat, salt, oxygen, and other tracers across the section are presented. A brief description of the distribution of physical and chemical properties is followed by a determination of the flux in the surface Ekman layer. Direct measurements of current shear made from a shipborne ADCP and estimates from cruise wind data and climatological data all yield a flux of 5 ± 1 Sv (Sv = 106 m3 s−1) equatorward. When combined with geostrophic estimates relative to a level near 3500 dbar, or the bottom where shallower, an initial guess for the flow field is derived. This initial guess is combined with absolute currents derived from the ADCP when the ship is underway. Unrealistic aspects of the circulation are found, and we conclu...

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 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.

Bottom Currents Derived from a Shipborne ADCP on WOCE Cruise A11 in the South Atlantic

Abstract This paper is divided into two parts: the first describing an enhancement of the shipborne ADCP system and the second describing information about bottom currents that have been obtained therefrom on WOCE cruise A11 in the South Atlantic, Punta Arenas to Cape Town. RRS Discovery has been fitted with a GPS3DF receiver and an array of antennas about its bridge, which determine the instantaneous attitude of the ship. The heading record from this instrument has been compared with the ships gyrocompass throughout the 40-day cruise. Differences have been calculated and examined according to heading, speed, and movements. Underway differences are found insensitive to heading except in a southerly sector, but on station, differences exhibit a sinusoidal dependence on heading with range 3°. Steaming from a station led to transient behavior with recognizable effects for 1 hour. The results are generally consistent with earlier reports and are attributed to gyro error. An improved heading measurement has a...

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.

Decadal changes in the South Indian Ocean thermocline

A significant change in properties of the thermocline is observed across the whole Indian Ocean 32°S section between 1987 and 2002. This change represents a reversal of the pre-1987 freshening and decreasing oxygen concentrations of the upper thermocline that had been interpreted as a fingerprint of anthropogenic climate change. The thermocline at the western end of the section (40°–70°E) is occupied by a single variety of mode water with a potential temperature of around 13°C. The thermocline at the eastern end of the 32°S section is occupied by mode waters with a range of properties cooling from 11°C at 80°E to 9°C near the Australian coast. The change in –S properties between 1987 and 2002 is zonally coherent east of 80°E, with a maximum change on isopycnals at 11.6°C. Ages derived from helium–tritium data imply that the mode waters at all longitudes take about the same time to reach 32°S from their respective ventilation sites. Dissolved oxygen concentration changes imply that all of the mode water reached the section 20% faster in 2002 than in 1987.

An Advanced Method to Estimate Deep Currents from Profiling Floats

Abstract Subsurface ocean currents can be estimated from the positions of drifting profiling floats that are being widely deployed for the international Argo program. The calculation of subsurface velocity depends on how the trajectory of the float while on the surface is treated. The following three aspects of the calculation of drift velocities are addressed: the accurate determination of surfacing and dive times, a new method for extrapolating surface and dive positions from the set of discrete Argos position fixes, and a discussion of the errors in the method. In the new method described herein, the mean drift velocity and the phase and amplitude of inertial motions are derived explicitly from a least squares fit to the set of Argos position fixes for each surface cycle separately. The new method differs from previous methods that include prior assumptions about the statistics of inertial motions. It is concluded that the endpoints of the subsurface trajectory can be estimated with accuracy better tha...

The deep waters from the Southern Ocean at the entry to the Argentine Basin

Hydrographic data from the World Ocean Circulation Experiment (WOCE) and South Atlantic Ventilation Experiment (SAVE) in the region of transition between the Scotia Sea and the Argentine Basin are examined to determine the composition of the deep water from the Southern Ocean that enters the Atlantic, and to describe the pathways of its constituents. The deep current that flows westward against the Falkland Escarpment is formed of several superposed velocity cores that convey waters of different origins: Lower Circumpolar Deep Water (LCDW), Southeast Pacific Deep Water (SPDW), and Weddell Sea Deep Water (WSDW). Different routes followed by the WSDW upstream of, and through, the Georgia Basin, lead to distinctions between the Lower-WSDW (σ4>46.09) and the Upper-WSDW (46.04<σ4 <46.09). The Lower-WSDW flows along the South Sandwich Trench, then cyclonically in the main trough of the Georgia Basin. Although a fraction escapes northward to the Argentine Basin, a comparison of the WOCE data with those from previous programmes shows that this component had disappeared from the southwestern Argentine Basin in 1993/1994. This corroborates previous results using SAVE and pre-SAVE data. A part of the Upper-WSDW, recognizable from different θ–S characteristics, flows through the Scotia Sea, then in the Georgia Basin along the southern front of the Antarctic Circumpolar Current. Northward leakage at this front is expected to feed the Argentine Basin through the northern Georgia Basin. The SPDW is originally found to the south of the Polar Front (PF) in Drake Passage. The northward veering of this front allows this water to cross the North Scotia Ridge at Shag Rocks Passage. It proceeds northward to the Argentine Basin around the Maurice Ewing Bank. The LCDW at the Falkland Escarpment is itself subdivided in two cores, of which only the denser one eventually underrides the North Atlantic Deep Water (NADW) in the Atlantic Ocean. This fraction is from the poleward side of the PF in Drake Passage. It also crosses the North Scotia Ridge at Shag Rocks Passage, then flows over the Falkland Plateau into the Atlantic. The lighter variety, from the northern side of the PF, is thought to cross the North Scotia Ridge at a passage around 55°W. It enters the Argentine Basin in the density range of the NADW.