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Locations of the various fronts in the Southern Ocean

Entry ID: southern_ocean_fronts

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Summary

Abstract: This information is gathered from the paper by Alejandro H. Orsi, Thomas Whitworth III, and Worth D. Nowlin Jr (1995) (see below for full reference).

This is a line dataset stored in the Geographical Information System (GIS). The fronts were derived as part of the analysis done in the lead up to the World Ocean Circulation Experiment (WOCE).

Hydrographic observations were used in this study of ... the Southern Ocean to improve our knowledge of large-scale aspects of the Antarctic Circumpolar Current (ACC). The flow associated with this current appears in the dynamic topography of the upper kilometre of water (50/1000 db), as a zonal band with continuous contours through the Drake Passage. This ACC band shows relatively large geostrophic shear, when compared with the adjacent cicrulations in the subtropical and subpolar regimes. However, the flow is similarly sheared in the southern limbs of the subtropical gyres, particularly near the western boundary of each ocean basin, making differentiation of the ACC and subtropical regimes difficult based on shear alone. The Subtropical Front (STF) is considered the northern limit of the Subantarctic Surface Waters (SSW).

Following Deacon's original definition of the Subtropical Convergence, the STF is delineated by means of property maps at a near-surface level (100m). There is a narrow transition between warm and saline surface waters of the subtropical regime and colder and fresher subantarctic waters. Although many stations have been added since Deacon prepared his maps, this band is found to closely follow this position of the Subtropical Convergence (now referred to as Front). Closely-spaced stations along the Greenwich Meridian reveal a sharp termination to the poleward extent of Upper Circumpolar Deep Water (UCDW) characteristics, which coincides with a frontal feature that separates the ACC from the Weddell Gyre. Property maps on a density surface characterising the UCDW core within the ACC show a clear transitional band around Antarctica, across which the selected isopycnal shoals to within 200 m of the surface. South of this band, the characteristic signal of the UCDW is no longer discernable; most of this water mass has been entrained into the surface mixed layer and its identifying isopycnal lies within Antarctic Surface Water. Examination of numerous transects through this transition allowed us to trace the southern limit of UCDW characteristics. Its location is in good agreement with the observed change in geostrophic shear between the ACC and the subpolar circulations. This water mass boundary is suggested as a useful definition of the southern boundary of the ACC. Recent high-qualty transects of the ACC at Drake Passage average 97 Sv (1 Sv = 10,000,000 cubic metres per second) of eastward baroclinic transport above 3000 m. In the open ocean far from Drake Passage, more consistent estimates of the current's baroclinic transport result naturally from the present definition of its southern boundary. A geostrophic transport function map for the Southern Ocean reveals that roughly 100 Sv are transported to the east above 3000 m at all longitudes within the proposed meridional boundaries of the circumpolar regime. The characteristic frontal structure between the meridional bounds of the ACC was examined. Previous observations have shown three deep-reaching fronts at both the Drake Passage and Greenwich Meridian. These ACC fronts appear as maxima in geostrophic transport and show distinct property characteristics at both locations. Examination of water mass properties (tracers) and volume transport estimates reveal three fronts on all synoptic sections across the ACC. The name southern ACC front is proposed here for the third, southernmost of these circumpolar current cores. Based on examination of property changes and zonal shear transports from all meridional sections, presented here, for the first time, are circumpolar paths for the Subantarctic front (SAF), Polar Front (PF) and the southern ACC front. As portrayed by zonally-averaged theta-S relations for waters above Lower Circumpolar Deep Water (LCDW), the fronts separate distinctly different water mass regimes. Zonally-averaged characteristic curves also show the northward spread and mixing along isopycnals of surface waters across the southern ACC front, which contribute to the subsurface temperature minimum layer of the Antarctic Zone. A similar influence is also implied across the Polar Front.

In the average, the characteristic temperature minimum of the Antarctic Surface Water becomes saltier and colder to the south of the Polar Front. It is the saltiest in the subpolar region, suggesting the progressive entrainment of UCDW from the ACC into the mixed layer of the subpolar regime. This identification of the major large-scale features of the ACC is expected to benefit future descriptive studies and numerical simulations. In particular, all three deep-reaching fronts of the ACC should be included in quantitative analyses and models of this current. The present delineation of its southern end may aid also in estimating and understanding property exchanges between the ACC and the subpolar regime.

The fields in this dataset are:
Subtropical front
Subantarctic front
Polar front
Southern Antarctic circumpolar current front (loop 1 and 2)
Antarctic circumpolar current
Boundary (southern boundary of the ACC)
longitude
latitude

Related URL

Link: GET DATA

Description: Download point for the data - excel

Link: GET DATA

Description: Download point for the data - GIS shapefile

Link: GET DATA

Description: The coordinates (traces) for these frontal distributions are available at the
investigators official web site under the DATABASE link.

Link: VIEW RELATED INFORMATION

Description: Citation reference for this metadata record and dataset

Geographic Coverage

(Click for Interactive Map)

Spatial coordinates

N: -30.0

S: -71.0

E: 180.0

W: -180.0

Temporal Coverage

Start Date: 1994-11-10

Location Keywords

OCEAN > SOUTHERN OCEAN

GEOGRAPHIC REGION > POLAR

Science Keywords

OCEANS > OCEAN CIRCULATION > FRONTS

OCEANS > OCEAN CIRCULATION > OCEAN CURRENTS

ISO Topic Category

OCEANS

Quality This dataset includes the following features of the Antarctic Circumpolar Current, a critical component of the global ocean circulation:
Subtropical Front (STF);
Subantarctic Front (SAF);
Southern Antarctic Circumpolar Current Front (sACCf);
Polar Front (PF);
Southern Boundary of the Antarctic Circumpolar Current.

The data resulted from analysis of historical hydrographic data of the Southern Ocean available up to 1990. This work was part of the lead up to the ongoing World Ocean Circulation Experiment (WOCE). The estimated position of the fronts and boundary are likely to change to some extent as more WOCE data are analysed.

Access Constraints The data are freely available for download from the AADC at the url given below.
Two versions of the dataset are available for download:
1) an excel spreadsheet of the data.
2) ArcView shapefiles - GIS version of the data.

Furthermore, the coordinates (traces) for these frontal distributions are available at the investigators official web site under the DATABASE link.

Use Constraints This data set conforms to the PICCCBY Attribution License
(http://creativecommons.org/licenses/by/3.0/).

Must be cited properly.
Abbreviations used for the fronts should be those given in the Quality field.

Ancillary Keywords

ACC

ANTARCTIC CIRCUMPOLAR CURRENT

Data Set Progress

COMPLETE

Originating Center

Data Center

Australian Antarctic Data Centre, Australia

Data Center URL: http://data.aad.gov.au

Data Center Personnel
Name: DATA OFFICER AADC Phone: +61 3 6232 3244 Fax: +61 3 6232 3351 Email: metadata at aad.gov.au Contact Address: Australian Antarctic Division 203 Channel Highway City: Kingston Province or State: Tasmania Postal Code: 7050 Country: Australia

Distribution

Distribution Media: HTTP

Distribution Size: 95 kb

Distribution Format: Excel

Fees: free

Distribution Media: HTTP

Distribution Size: 26 kb

Distribution Format: shapefile

Fees: free

Personnel

ALEJANDRO H ORSI

Role: INVESTIGATOR

Role: TECHNICAL CONTACT

Phone: +1 979 845 4014

Fax: +1 979 847 8879

Email: aorsi at tamu.edu

Contact Address:

Texas A and M University

Oceanography Department

College Station,

Province or State: Texas

Country: USA

URSULA HARRIS

Role: TECHNICAL CONTACT

Phone: +61 3 6232 3519

Fax: +61 3 6232 3351

Email: metadata at aad.gov.au

Contact Address:

Australian Antarctic Division

203 Channel Highway

City: Kingston

Province or State: Tasmania

Postal Code: 7050

Country: Australia

DAVE CONNELL

Role: DIF AUTHOR

Phone: +61 3 6232 3244

Fax: +61 3 6232 3351

Email: dave.connell at aad.gov.au

Contact Address:

Australian Antarctic Division

203 Channel Highway

City: Kingston

Province or State: Tasmania

Postal Code: 7050

Country: Australia

Publications/References

Alejandro H. Orsi, Thomas Whitworth III, and Worth D. Nowlin Jr (1995) On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep-Sea Research 42 (5), 641-673

Creation and Review Dates

DIF Creation Date: 2001-07-18

Last DIF Revision Date: 2008-04-10

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