Energy Exchange between Jets of the Antarctic Circumpolar Current and Synoptic Eddies in the Drake Passage and Scotia Sea

We study the energy exchange between jets of the Antarctic Circumpolar Current (ACC) and synoptic eddies generated by them in the surface layer of the ocean in the Drake Passage and Scotia Sea based on 22-year-long satellite altimetry time series from the French CLS Agency (DT Global–MADT–Upd product, http://www.aviso.altimetry.fr) under the assumption, based on observations, that each of the jets is confined between two fixed contour lines of the absolute dynamic topography of the ocean. We calculate and analyze the 22-year evolution of the kinetic energy of each ACC jet and cyclonic and anticyclonic eddies generated by it. We demonstrate the fundamental dependence of fluctuations in jet energy on the phase of their meander and eddy formation, as well as their back absorption by jets. We calculate the mean and extreme energetic characteristics of jets and eddies and compare the jets in terms of the intensity of the generated eddies.     

Circumpolar bottom water in the Scotia Sea and the Drake Passage

The quantitative properties and circulation of the lower layer of circumpolar water in the Scotia Sea with density 28.16 < γ ⁿ < 28.26 (potential temperature 0.9° > θ > 0.2°C) are investigated using the original procedure for determination of boundaries between water masses. The primary objective of this work is data analyses of four Russian sections, which were occupied in the vicinity of the Shackleton Fracture Zone in 2003, 2005, and 2007. It is shown that the ridges in the Hero and Shackleton fracture zones essentially constrain overflow of the lower layer of circumpolar water, and thereby, they produce the conditions to the east of the Shackleton Ridge for transformation (freshening and warming) of this layer reaching the northern side of the Antarctic Circumpolar Current. These ridges also promote formation of several quasi-permanent and semi-enclosed abyssal and deep-water eddies adjacent to these ridges. The estimation of overflow of the lower part of the investigated layer with density 28.23 < γ ⁿ < 28.26 (0.9° > θ > 0.2°C) through the Shackleton Ridge based on LADCP measurements in 2007 is 0.5 Sv (0.1 Sv) to the east (west). The upper part of the overflow is estimated as 8.0 (7.9) Sv. Thus, the total transport of the lower layer of circumpolar water through the ridge is practically zero. It is confirmed by LADCP measurements carried out on the section across the Drake Passage in 2003.     

Antarctic bottom water in the Scotia Sea and the Drake Passage

The quantitative features and circulation of the Antarctic bottom water (AABW) in the Scotia Sea are investigated using an original procedure for the determination of the boundaries between the water masses. It is shown that the AABW is effectively transferred across the Antarctic Circumpolar Current (ACC) from the regions on the south flank of this current where the AABW penetrates into the Scotia Sea. This transfer results in the abyssal water cooling and freshening in the Yaghan Basin of the north Scotia Sea. Some rises and depressions in the bottom relief of the western and northern Scotia Sea are important features that impact the AABW transfer. It is shown that there is an additional path of the AABW transit transport to the North Atlantic passing through the western Scotia Sea. The existence of the semienclosed cyclonic abyssal water circulation in the South Shetland Trench and the westward transport of the Atlantic AABW along the Antarctic slope foot into the Pacific are proved.     

Seasonal cycle of circulation in the Antarctic Peninsula and the off-shelf transport of shelf waters into southern Drake Passage and Scotia Sea

The seasonal cycle of circulation and transport in the Antarctic Peninsula shelf region is investigated using a high-resolution (∼2 km) regional model based on the Regional Oceanic Modeling System (ROMS). The model also includes a naturally occurring tracer with a strong source over the shelf (radium isotope ²²⁸Ra, t_1/2=5.8 years) to investigate the sediment Fe input and its transport. The model is spun-up for three years using climatological boundary and surface forcing and then run for the 2004–2006 period using realistic forcing. Model results suggest a persistent and coherent circulation system throughout the year consisting of several major components that converge water masses from various sources toward Elephant Island. These currents are largely in geostrophic balance, driven by surface winds, topographic steering, and large-scale forcing. Strong off-shelf transport of the Fe-rich shelf waters takes place over the northeastern shelf/slope of Elephant Island, driven by a combination of topographic steering, extension of shelf currents, and strong horizontal mixing between the ACC and shelf waters. These results are generally consistent with recent and historical observational studies. Both the shelf circulation and off-shelf transport show a significant seasonality, mainly due to the seasonal changes of surface winds and large-scale circulation. Modeled and observed distributions of ²²⁸Ra suggest that a majority of Fe-rich upper layer waters exported off-shelf around Elephant Island are carried by the shelfbreak current and the Bransfield Strait Current from the shallow sills between Gerlache Strait and Livingston Island, and northern shelf of the South Shetland Islands, where strong winter mixing supplies much of the sediment derived nutrients (including Fe) input to the surface layer.     

Structure and Variability of Mesoscale Perturbations of Ocean Currents in the Drake Passage and Scotia Sea

Oceanology - Satellite altimeter observation data collected over 22 years are used to analyze the structure and variability of mesoscale fluctuations of ocean currents in the Drake Passage and...     

Seismic structure and tectonics of the Shackleton Fracture Zone (Drake Passage,Scotia Sea)

The structural framework of the southern part of the Shackleton Fracture Zone has been investigated through the analysis of a 130-km-long multichannel seismic reflection profile acquired orthogonally to the fracture zone near 60° S. The Shackleton Fracture Zone is a 800-km-long, mostly rectilinear and pronounced bathymetric lineation joining the westernmost South Scotia Ridge to southern South America south of Cape Horn, separating the western Scotia Sea plate from the Antarctic plate. Conventional processing applied to the seismic data outlines the main structures of the Shackleton Fracture Zone, but only the use of enhanced techniques, such as accurate velocity analyses and pre-stack depth migration, provides a good definition of the acoustic basement and the architecture of the sedimentary sequences. In particular, a strong and mostly continuous reflector found at about 8.0 s two-way traveltime is very clear across the entire section and is interpreted as the Moho discontinuity. Data show a complex system of troughs developed along the eastern flank of the crustal ridge, containing tilted and rotated blocks, and the presence of a prominent listric normal fault developed within the oceanic crust. Positive flower structures developed within the oceanic basement indicate strike-slip tectonism and partial reactivation of pre-existing faults. Present-day tectonic activity is found mostly in correspondence to the relief, whereas fault-induced deformation is negligible across the entire trough system. This indicates that the E–W-directed stress regime present in the Drake Passage region is mainly dissipated along a narrow zone within the Shackleton Ridge axis. A reappraisal of all available magnetic anomaly identifications in the western Scotia Sea and in the former Phoenix plate, in conjunction with new magnetic profiles acquired to the east of the Shackleton Fracture Zone off the Tierra del Fuego continental margin, has allowed us to propose a simple reconstruction of Shackleton Fracture Zone development in the general context of the Drake Passage opening.     

Scleractinian Corals as an Indicator of Vertical Density of the Antarctic Circumpolar Current

Oceanology - The article presents data indicating the relationship of the bathymetric distribution of solitary endemic scleractinian corals inhabiting the Antarctic continental shelf and upper...     

Chaotic and regular trajectories in the Antarctic Circumpolar Current

Methods from chaos theory are applied to the analysis of the circulation in the Southern Ocean, using velocity fields produced by a realistic global ocean model. We plot the intersections of individual trajectories encircling Antarctica with a vertical plane in the Drake passage. This so-called Poincaré section shows a drastic difference between regular trajectories in a core region of the Antarctic Circumpolar Current (ACC), and chaotic, mixing trajectories in the surrounding region. It also shows that there is a region with overturning circulation of approximately 3.5 Sv in the ACC, with downwelling on the northern side and upwelling on the southern side, which may be related to the Deacon cell.     

An analytical diagnostic model of the Antarctic Circumpolar Current

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南极绕极流线性理论的一个摄动解

为了模拟南极绕极流区的流动，首先将环形条带区从Drake海峡处分开，并展成以经纬度为坐标的矩形区域。将模拟区划分为边界区和内部区。与Munk的大洋环流理论不同．在我们所考虑的线性化的涡度方程中保留了经向摩擦作用。这是基于绕极流流轴处的强剪切产生的经向摩擦项与β效应同等重要的判断。另外．内部区的北边界取为流线，而2个侧边界区的北边界视为南北水体交换的通道。基于巴西暖流穿过北边界流入和秘鲁寒流穿过北边界流出，以及常年有大于100Sv(平均约134Sv)的水体自西向东穿过Drake海峡的观测事实，引入了净通量条件。计算出了不同于Sverdrup流的更集中的内部区带状流的流线分布，这与观测大体一致。在Drake海峡处引入匹配条件，从而得到了完整的边界解。计算结果与理论分析指出，通过Drake海峡的流量远小于南极绕极流所带动水体的纬向通量是因为Drake海峡偏离绕极流流轴(也是南大洋西风带的主轴)。受南美大陆的阻挡，另一部分水体通过秘鲁寒流流出该海区。     

The Southern Antarctic Circumpolar Current Front: physical and biological coupling at South Georgia

The coupling of physics and biology was examined along a 160 km long transect running out from the north coast of South Georgia Island and crossing the Southern Antarctic Circumpolar Current Front (SACCF) during late December 2000. Surface and near surface potential TS properties indicated the presence of three water types: a near-shore group of stations characterised by water which became progressively warmer and fresher closer to South Georgia, an offshore grouping in which sea surface temperatures and those at the winter water level were relatively warm (1.8°C and 0.5°C, respectively), and a third in which surface and winter water temperatures were cooler and reflected the presence of the SACCF. The transect bisected the SACCF twice, revealing that it was flowing in opposite directions, north-westward closest to South Georgia and south-eastwards at its furthest point from the island. The innermost limb was a narrow intense feature located just off the shelf break in 2000–3500 m of water and in which rapid surface baroclinic velocities (up to 35 cm s⁻¹) were encountered. Offshore in the outermost limb, shown subsequently to be a mesoscale eddy that had meandered south from the retroflected limb of the SACCF, flow was broader and slower with peak velocities around 20 cm s⁻¹. Chlorophyll a biomass was generally low (<1 mg m⁻³) over much of the transect but increased dramatically in the region of the innermost limb of the SACCF, where a deepening of the surface mixed layer was coincident with a subsurface chlorophyll maximum (7.4 mg m⁻³) and elevated concentrations down to 100 m. The bloom was coincident with depleted nutrient concentrations, particularly silicate, nitrate and phosphate, and although ammonium concentrations were locally depleted the bloom lay within an elevated band (up to 1.5 mmol m⁻³) associated with the frontal jet. Increased zooplankton abundance, higher copepod body carbon mass and egg production rates all showed a strong spatial integrity with the front. The population structure of the copepods Calanoides acutus and Rhincalanus gigas at stations within the front suggested that rather than simply resulting from entrainment and concentration within the jet, increased copepod abundance was the result of development in situ. Estimates of bloom duration, based on silicate and carbon budget calculations, set the likely duration between 82 and 122 d, a figure supported by the development schedule of the two copepod species. Given this timescale, model outputs from FRAM and OCCAM indicated that particles that occurred on the north side of South Georgia in December would have been in the central-southern Scotia Sea 2–3 months earlier, probably in sea ice affected regions.     

Surface geostrophic currents across the Antarctic circumpolar current in Drake Passage from 1992 to 2004

The Southern Ocean plays an important role in the global overturning circulation as a significant proportion of deep water is converted into intermediate and deeper water masses in this region. Recently, a secular trend has been reported in wind stress around the Southern Ocean and it is thought theoretically that the strength of the ACC is closely related to wind stress, so one consequence should be a corresponding increase in ACC transport and hence changes in the rate of the global overturning. There are no long-term data sets of ACC transport and so we must examine other data that may also respond to changing wind stress. Here we calculate surface currents in Drake Passage every seven days over 11.25 years from 1992 to 2004. We combine surface velocity anomalies calculated from satellite altimeter sea surface heights with measured surface currents. Since 1992, the UK has regularly occupied WOCE hydrographic section SR1b across the ACC in Drake Passage. From seven hydrographic sections surface currents are estimated by referencing relative geostrophic velocities from CTD sections with current measurements made by shipboard and lowered acoustic Doppler current profilers. Combining the seven estimates of surface currents with the altimeter data reduces bias in the estimates of average currents over time through Drake Passage and we show that surface current anomalies estimated by satellite and in situ observations are in good agreement. The strongest surface currents are found in the Subantarctic and Polar Fronts with average speeds of 50 cm/s and 35 cm/s, respectively and are inversely correlated, so that maximum westward flow in one corresponds to minimum westward flow in the other. The average cross-sectional weighted surface velocity from 1992 to 2004 is 16.7 ± 0.2 cm/s. A spectral analysis of the average surface current has only weakly increasing energy at higher frequencies and there is no dominant mode of variability. The standard deviation of the seven day currents is 0.68 cm/s and a running 12 month average has only a slightly smaller standard deviation of 0.52 ± 0.16 cm/s. The southern annular mode (SAM) measures the circumpolar average of wind stress and like the surface currents its spectrum has slightly increased energy at frequencies greater than 1 cpy. A cospectral analysis of these, averaging cospectra of five slightly overlapping 36 month segments improve statistical reliability, suggests that there is coherence between them at 1 cpy with the currents leading changes in the Southern annular mode. We conclude that the SAM and average Drake Passage surface currents are weakly correlated with no dominant co-varying modes, and hence predicting Southern Ocean transport variability from the SAM is not likely to give significant results and that secular trends in surface currents are likely to be masked by weekly and interannual variability.     

The low-frequency variance of the ocean surface wave field in the area of the Antarctic Circumpolar Current

The low-frequency variance of the surface wave in the area of the Antarctic Circumpolar Current (ACC) and its correlation with the antarctic circumpolar wave (ACW) are focused on. The analysis of the series of 44 a significant wave height (SWH) interannual anomalies reveals that the SWH anomalies have a strong periodicity of about 4-5 a and this signal propagates eastward obviously from 1985 to 1995, which needs about 8 a to complete a mimacircle around the earth. The method of empirical orthogonal function (EOF) is used to analyze the filtered monthly SWH anomalies to study the spatio-temporal distributions and the propagation characteristics of the low-frequency signals in the wave field. Both the dominant wavenumber-2 pattern in space and the propagation feature in the south Pacific, the south Atlantic and the south Indian ocean show strong consistency with the ACW. So it is reasonable to conclude that the ACW signal also exists in the wave field. The ACW is important for the climate in the Southern Ocean, so it is worth to pay more attention to the large-scale effect of the surface wave, which may also be important for climate studies.     

Interaction between Rossby Waves and a Jet Flow: Basic Equations and Verification for the Antarctic Circumpolar Current

Izvestiya, Atmospheric and Oceanic Physics - Abstract—The article focuses on the interaction of Rossby waves in the ocean with zonal jet flows. A new approach is proposed to show that...     

Effects of partial meridional barriers on the Antarctic Circumpolar Current—Buoyancy-driven three-layer model

The transport and vertical structure of the Antarctic Circumpolar Current (ACC) are examined, especially the component of the current driven by buoyancy, by using a three-layer model. We investigate the effects of the South American peninsula, the island arc to the east, and the Macquarie ridge, which are modeled as partial meridional barriers overlapping meridionally each other. We found that the buoyancy-driven component is given as a function of the transport out of the Weddell Sea (S _W ) and the sum of the transports into the North Atlantic (S _A ) and the North Pacific (S _P ) out of the Southern Ocean. The buoyancy-driven current flows westward, ifS _W andS _A +S _P are positive. The transport depends on the value ofS _W more thanS _A +S _P by one order of magnitude within a realistic range of parameters. The most predominant term in the transport equation is inversely proportional to the difference between the Coriolis parameters at the tips of the partial meridional barriers. Thus, the magnitude of the transport strongly depends on the overlapping length of the meridional barriers. The eastward current of the ACC is driven by the predominant eastward wind stress in the Southern Ocean, although a part of the wind-driven component is canceled by the westward buoyancy-driven component. The vertical structure of the ACC is found to be attributed to the surface wind-driven circulation and the deep and bottom buoyancy-driven circulation.     

Improving the estimate of wind energy input into the Ekman layer within the Antarctic Circumpolar Current

Based on the data and method offered by Liu et al. (2009), the direct wind and Stokes drift-induced energy inputs into the Ekman layer within the Antarctic Circumpolar Current (ACC) area are reestimated since the results of the former have been proved to be underestimated. And the result shows that the total rate of energy input into the Ekman-Stokes layer within the ACC area is 852.41 GW, including 649.75 GW of direct wind energy input (76%) and 202.66 GW of Stoke drift-induced energy input (24%). Total increased energy input, due to wave-induced Coriolis-Stokes forcing added to the classical Ekman model, is 52.05 GW, accounting for 6.5% of the wind energy input into the classical Ekman layer. The long-term variability of direct wind and Stokes drift-induced energy inputs into the Ekman layer within the ACC is also investigated, and the result shows that the Stokes drift hinders the decadal increasing trend of direct wind energy input. Meanwhile, there is a period of 4-5 a in the energy spectrums, as same as the Antarctic circumpolar wave.     

Late Quaternary stable isotope record and meltwater discharge anomaly events to the south of the Antarctic Polar Front,Drake Passage

Marine isotope stages (MISs) 1 to 5 were identified in the planktonic ¹⁸O record in sediment core DP00-02 just south of the Antarctic Polar Front in the Drake Passage, Antarctica. The oxygen isotope record, based on Neogloboquadrina pachyderma sinistral, is correlated with the contemporaneous global ¹⁸O stratigraphy. Marked deviations from the global climate curve suggest a local/regional overprint, particularly during MIS 3 which is considered a colder time period in the ocean record than MIS 1 and MIS 5 during the last interglacial. The comparison shows that negative ¹⁸O shifts in core DP00-02 during MIS 3 are larger than mean global changes which show a shift equal to or smaller than 0.5. The isotope shift, exceeding the glacial-interglacial ice volume effect, probably resulted from changes in the isotope composition of seawater, which is linearly related to decreases in salinity rather than to increases in sea-surface temperature. Increased ice-rafted debris (IRD) content during this interval indicates a strong influx of IRD from melting ice shelves and icebergs, which may be related to upwelling of warmer circumpolar deep water.     

Particulate carbohydrate fluxes in the Bransfield Strait and the Drake Passage

Carbohydrate fluxes were determined for five drifting sediment traps (max. 30 h) in the Bransfield Strait and one moored trap array (52 days) in the Drake Passage in December/January 1980. In a Phaeocystis-dominated zone in the Bransfield Strait the carbohydrate composition was comparable to that of water column suspended material with absolute fluxes of 9.0 and 56.7 mg carbohydrate carbon m⁻²d⁻¹. In Bransfield Strait waters dominated by bacillariophyceae, fluxes were 75.4, 92.6 and 276.8 mg carbohydrate carbon m⁻²d⁻¹ with a pronounced dominance of glucose. This is attributed to the presence of the reserve polysaccharide chrysolaminarin in diatom resting spores. Drake Passage fluxes were 1–2 orders of magnitude lower with a distinct degradation of glucose with depth.