Figure 6 - uploaded by Alan D. Chave
Content may be subject to copyright.
Stream coordinates mean absolute velocities at the five pressure levels indicated in the legend. The location of the SAF was chosen based on the time mean location of the core (50.6°S) along the HEFR line; the direction of the core flow was based on the time mean downstream direction from the OI mapped Z 6 fields.
Source publication
1] The mean synoptic structure of the northern, strongest branch of the Antarctic Circumpolar Current southwest of Tasmania, at the Subantarctic Front (SAF), is estimated by a stream coordinates analysis of data from overlapping arrays of Inverted Echo Sounders (IESs) and Horizontal Electric Field Recorders deployed during the 1995–1997 Sub-Antarct...
Contexts in source publication
Context 1
... Figure 6 presents the mean stream coordinates ve- locities in plan view, where the core is placed at the time- mean location where it crosses the HEFR line; the core direction is given by the time-mean downstream direction from the OI mapped Z 6 fields, and the vectors are plotted along a line perpendicular to the time-mean core direction. The diffluence of the cross-stream flow is visually obvious, as is a small amount of backing and veering with height. ...
Context 2
... flow observed by PR02 is stronger than the flow observed during SAFDE, both near the core and at the flanks (Figure 8). These differences generally exceed the combined error bars of the SAFDE section (see Appendix A) and PR02 (see their Figure 6). There are a number of possible reasons for the differences. ...
Context 3
... By applying both the frozen field-vertical shear methods and alternately the OI method to the same data set, ML03 demonstrate the resulting mean current strength depends on the stream coordinates method chosen. They found that the vertical shear method leads to stronger along- stream flows, particularly along the flanks, because any diffluent baroclinic cross-stream flow (Figures 5 and 6) is rotated into the along-stream component by this technique. Bower and Hogg [1996] document further concerns about using the vertical shear method in the presence of a curved current path. ...
Similar publications
To examine the mesoscale distribution of Antarctic krill Euphausia superba Dana, length frequency and maturity stage data were subjected to multivariate analysis. The results showed that the population was heterogeneous with respect to size and maturity, with small immature krill found primarily inside the Bransfield Strait and large mature krill m...
Citations
... The working mode is mainly to measure the sound propagation time from the sea surface to the sea bottom. Since its inception in the 1970s, IES has been widely used in marine research with the help of technological developments and the progress of data processing methods (Meinen et al., 2003;Donohue et al., 2008;. ...
Mesoscale eddies are widespread in the oceans; however, tracking and observing them remains a challenge. Additionally, there are few studies of eddy–Kuroshio interactions based on large-scale in situ data. Data from a large array of 39 CPIESs deployed around the Luzon Strait in 2018–2019 (12 deployed east of Taiwan within a 3 × grid) were used to reveal the three-dimensional structure of mesoscale eddies and their interaction with Kuroshio. The results show a strengthening (weakening) of the tilt of the pycnocline, which has also caused a deeper (shallower) pycnocline in eastern Taiwan following an anticyclonic (cyclonic) interaction with Kuroshio, which resulted in a strengthening (weakening) of the Kuroshio. During the anticyclonic eddy impinging Kuroshio, the downward (upward) movement at the center of the anticyclonic eddy (Kuroshio) results in a positive (negative) anomaly structure for temperature, with corresponding positive (negative) anomalies for salinity above 600 m in the intermediate layer and negative (positive) anomalies below 600 m. In contrast, when the cyclonic eddies interact with the Kuroshio, the temperature and salinity anomalies change simultaneously, with the temperature showing an overall negative anomaly and the salinity showing a negative and positive anomaly above and below 600 m, respectively. The research in this study provides the basis for further research on energy exchange during eddy–Kuroshio interactions.
... The Subtropical Front marks the northern boundary of the ACC, separating Subantarctic and Subtropical surface waters, and is located too far north (~40 o S) to influence the core sites used in this study. The Subantarctic Front is the most northerly of the remaining two fronts (white line in Fig. 1) and is marked by SSTs ≥8 • C to the north and SSTs ≤7 • C to the south (Meinen et al., 2003). The final ACC front is the Polar Front (black line in Fig. 1) which is marked by the subsurface (200 m) 2 • C isotherm (Orsi et al., 1995) and generally corresponds to SSTs of ~2-3 • C (Dong et al., 2006). ...
Marine Isotope Stage (MIS) 5e (130–116 ka) represents an important ‘process analogue’ for understanding the climatic feedbacks and responses likely active under future anthropogenic warming. Reconstructing the Southern Ocean (SO) palaeoenvironment during MIS 5e and comparing it to the present day provides insights into the different responses of the SO sectors to a warmer climate. This study presents new records from seven marine sediment cores for MIS 5e together with their surface sediment records; all cores are located south of 55 oS. We investigate changes in diatom species assemblage and the accompanying variations in sea surface temperatures, winter sea-ice extent (WSIE) and glacial meltwater flux. All records show warmer conditions and a reduced WSIE during MIS 5e relative to the surface sediments. While the Pacific and Indian Sector records present very stable conditions throughout MIS 5e, the Atlantic Sector records display much more changeable conditions, particularly with respect to the WSIE. These variable conditions are attributed to higher iceberg and glacial meltwater flux in the Weddell Sea. This evidence for increased iceberg and glacial meltwater flux in the Weddell Sea during MIS 5e may have significant implications for understanding the stability of the West Antarctic Ice Sheet, both during MIS 5e and under future warming.
... The modern STF is located at around 41 o S in the Atlantic and Indian sectors of the SO and on average at 39 o S in the Pacific sector (Fig. 1). To the south of the STF is the Subantarctic Front (SAF) which is marked by SSTs greater than~6e8 C (Meinen et al., 2003). The SAF is currently located at an average latitude of 45 o S, 48 o S and 57 o S in the Atlantic, Indian and Pacific sectors, respectively (Fig. 1). ...
... The MIS 5e frontal positions (Table 3 & Fig. 6b) are inferred from the reconstructed SSTs (cf. Bianchi and Gersonde, 2002), assuming the same temperature relations across the fronts as in the modern ocean (Orsi et al., 1995;Sikes et al., 2002;Meinen et al., 2003;Dong et al., 2006). The frontal positions inferred for MIS 5e are shown alongside the reconstructed MIS 5e SST maxima and anomalies in Fig. 6. ...
The peak of the Last Interglacial, Marine Isotope Stage (MIS) 5e (130–116 ka), provides a valuable ‘process analogue’ for validating the climatic feedbacks and forcings likely active under future anthropogenic warming. Reconstructing exact timings of MIS 5e peak warming and minimum winter sea-ice extent (WSIE) throughout the Southern Ocean (SO) will help to identify the interactions and feedbacks within the ice-ocean system. Here we present a new MIS 5e marine sediment record from the SW Atlantic sector together with 28 published core records (chronologies standardised to the LR04 δ¹⁸O benthic stack; Lisiecki and Raymo, 2005) to investigate the timing and sequence of minimum WSIE and peak warming across the SO. Sea-surface temperatures (SSTs) peaked earliest in the Indian (20oE–150oE) and Atlantic (70oW–20oE) sectors, at 128.7 ± 0.8 ka and 127.4 ± 1.1 ka respectively, followed by the Pacific sector (150oE–70oW) at 124.9 ± 3.6 ka. The interval of minimum WSIE for all three sectors occurred within the period from 129–125 ka, consistent with the ∼128 ka sea salt flux minimum in Antarctic ice cores. Minimum WSIE appears to have coincided with peak July insolation at 55 oS, suggesting it could be linked with the mildest winters. The reduced WSIE during MIS 5e would have likely reduced the production of deep- and bottom water masses, inhibiting storage of CO2 in the abyssal ocean and lowering nutrient availability in SO surface waters. Examining a wide spatial range of proxy records for MIS 5e is a critical step forward in understanding climatic interactions and processes that will be active under warmer global temperatures.
... To eliminate the influences of these mesoscale features, hydrographic transects are often projected into stream-coordinates in which the location is not defined by distance, but by some essential characteristics of the currents, such as the geopotential height or an isothermal contour. The coordinate transformation can effectively filter out the temporal variability associated with meandering fronts and mesoscale eddies Sun and Watts, 2001;Meinen et al., 2003). Sun and Watts (2002) chose geopotential height as the streamfunction parameter and examined the water masses in stream-coordinates with six CTD transects south of Australia. ...
A streamfunction EOF method is applied to a time series of hydrographic sections in the Southern Ocean south of Australia to study water mass variations. Results show that there are large thermohaline variations north of the Subantarctic Front (SAF) at 300–1500 dbar level, indicating upwelling and downwelling of the Antarctic Intermediate Water (AAIW) along isopycnal surfaces. Based on the latest altimeter product, Absolute Dynamic Topography, a mechanism due to frontal wave propagation is proposed to explain this phenomenon, and an index for frontal waves is defined. When the frontal wave is in positive (negative) phase, the SAF flows northeastward (southeastward) with the fresh AAIW downwelling (upwelling). Such mesoscale processes greatly enhance cross-frontal exchanges of water masses. Spectral analysis shows that frontal waves in the Southern Ocean south of Australia are dominated by a period of about 130 days with a phase speed of 4 cm/s and a wavelength of 450 km.
... The width of the PF (165 km) is comparable to other studies of Southern Ocean jets in stream coordinates. Meinen et al. [2003] use data from south of Tasmania to study the SAF and cite its width as 220 km. Sokolov and Rintoul [2007] estimate the width of the PF from satellite data as 40-90 km (converted here from degrees latitude in their Figure 3). ...
The Polar Front (PF) is studied using 4 years of data collected by a line of current- and pressure-recording inverted echo sounders in Drake Passage complemented with satellite altimetry. The location of the PF is bimodal in latitude. A northern and southern PF exist at separate times, separated geographically by a seafloor ridge - the Shackleton Fracture Zone - and hydrographically by 17 cm of geopotential height. Expressed in stream coordinates, vertical structures of buoyancy are determined with a gravest empirical mode analysis. Baroclinic velocity referenced to zero at 3500 dbar, width, and full transport (about 70 Sv) of the jets are statistically indistinguishable; the two jets alternate carrying the baroclinic transport rather than coexisting. Influences of local bathymetry and deep cyclogenesis manifest as differences in deep reference velocity structures. Downstream reference velocities of the PF-N and PF-S reach maximum speeds of 0.09 and 0.06 m s-1, respectively. Buoyancy fields are indicative of upwelling and poleward residual circulation at the PF. Based on potential vorticity and mixing lengths, the northern and southern PF both act as a barrier to cross-frontal exchange while remaining susceptible to baroclinic instability. This article is protected by copyright. All rights reserved.
... In this paper, the 1247 profiles are used to examine the vertical and lateral structure of 3-D velocity along float trajectories moving with the ACC. The semi-Lagrangian float sampling provides an unprecedented view of the evolution of velocity and tracer structure within the ACC, contrasting the more common cross-stream view from hydrographic transects and moorings [Phillips and Rintoul, 2002;Meinen et al., 2003]. We use these data to test the order one assumptions of the equivalent barotropic framework: that the deep flow is parallel and proportional to the surface flow, and that vertical motion is small. ...
... They also infer strong vertical motion from the horizontal velocity and temperature records: typical values of 1 mm s 21 and peaks up to 5 mm s 21 . This location was reoccupied in 1995 by the SAFDE array [Meinen et al., 2003;Tracey et al., 2006] with a large array of current meters, inverted echo sounders and horizontal electric field recorders, spanning a much larger part of the ACC than the AUSSAF moorings. Tracey et al. [2006] examine daily maps of the flow field measured by the SAFDE array. ...
We examine the vertical structure of the horizontal flow and diagnose vertical velocities in the Antarctic Circumpolar Current (ACC) near the Kerguelen Plateau using EM-APEX profiling floats. Eight floats measured horizontal velocity, temperature and salinity profiles to 1600 dbar, with a vertical spacing of 3-5 dbar four times per day over a period of approximately 3 months. Horizontal velocity profiles show a complex vertical structure with strong rotation of the velocity vector through the water column. The distribution of rotation angles from 1247 profiles is approximately Gaussian and rotations of either sign are equally likely. Fourty percent of profiles with speed greater than 5cms-1 have a depth-integrated rotation of less than 15 degrees over 1300 dbar, while the other 60% demonstrate significantly stronger rotation. Consequently, most profiles do not conform to the equivalent barotropic model (deep flow parallel and proportional to the surface flow) used in simplified dynamical models and in Gravest Empirical Mode climatologies of the ACC. Nevertheless, since we find the mean rotation to be zero, an equivalent barotropic assumption is valid to first order. Vertical velocities inferred using conservation of mass and a gradient wind balance in natural coordinates have magnitudes on the order of 100 m/day. We find robust patterns of upwelling and downwelling phase-locked to meanders in the flow, as found in earlier studies. With the advent of high-resolution observations such as those presented here, and high-resolution models, we can advance to a more complete understanding of the rich variability in ACC structure that is neglected in the equivalent barotropic model.
... The hydrographic analysis by Sun and Watts [7] reveals that the temperature and salinity fields in the Antarctic Circumpolar Current (ACC) exhibit a low-dimensional coherent structure if projected onto a baroclinic streamfunction coordinate such as sound travel time or dynamic height, which is called Geostrophic Empirical Mode (GEM). The GEM fields enable us to study the water mass formation, mean structure, and low-frequency variability of the ACC in stream coordinate [8][9][10]. ...
Recent discovery of low-dimensional coherent structure in oceanic currents along with a new merged altimeter product called Absolute Dynamic Topography (ADT) makes it possible to derive subsurface ocean information from satellite remote sensing data. An altimetric geostrophic empirical mode (η-GEM) is developed in this study by projecting hydrographic transects onto the ADT sea surface height coordinate, based on which the four-dimensional thermohaline and velocity structures of oceanic currents are reconstructed from satellite surface observations. In the WOCE/SR3 area, the η-GEM fields capture more than 95% of the total thermal variance. The GEM-derived flow has equivalent-barotropic structure and represents the velocity profile better than traditional dynamic modes. Comparison with mooring observations also demonstrates that the η-GEM provides good estimates of the deep thermohaline fields.
... In contrast, location in stream-coordinates is defined not by distance but by a streamfunction parameter such as SSH or geopotential. The meso-scale meander we see in geographical space does not contaminate a streamwise mean because streamfunction is tightly related to the velocity structure of the ACC [13,14]. The ACC is not a purely zonal current, and zonal averaging of its velocity is influenced strongly by the subtropical and subpolar gyres. ...
Interannual variability of the Antarctic Circumpolar Current (ACC) strength is studied in stream-coordinate with twenty-year Absolute Dynamic Topography data from satellite altimetry. The stream-coordinate projection method separates the ACC from adjacent subtropical and subpolar gyres, enabling consideration of the zonal asymmetry of the ACC rather than assuming that the ACC is a purely zonal flow. It is shown that the ACC strength has large interannual variations with two recent peaks around 2000 and 2009. The interannual variability appears mainly in the Indo-Pacific sector of the Southern Ocean and the strongest signal is located south of Australia. The intensification of the westerly wind in 1998 and 2008 appears to cause the strengthening of the ACC via baroclinic processes.
... ADT 数据包含平均动力地形和融合的海表高度异常 2 个部分, 其中平均动力地形是通过一种逆方法把最 新的大地水准面模型、 卫星高度计平均海表高度和现 场观测结合起来计算得到 [10] , 海表高度异常是利用 一种投影方法把 Topex/Poseidon, Envisat, Jason 和 ERS 系列卫星观测融合起来得到 [12] . 数据自 1992 年 [13,14] . 同 时 , [19,20] . ...
... Deep pressures are leveled by adjusting records to the same geopotential surface under the assumption that long-time averages of near bottom currents and bottom pressures are in geostrophic balance. These methods have been successful in many regions, including the ACC [Meinen et al., 2003;Watts et al., 2001]. ...
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.
