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A Deep Cyclonic Gyre in the Australian–Antarctic Basin Michael S

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A Deep Cyclonic Gyre in the Australian–Antarctic Basin Michael S 1 A deep cyclonic gyre in the Australian–Antarctic Basin Michael S. McCartney Physical Oceanography Department, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543 USA Kathleen A. Donohue Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA Abstract The traditional image of ocean circulation between Australia and Antarctica is of a dominant belt of eastward flow, the Antarctic Circumpolar Current, with comparatively weak adjacent westward flows that provide anticyclonic circulation north and cyclonic circulation south of the Antarctic Cir- cumpolar Current. This image mostly follows from geostrophic estimates from hydrography using a bottom level of no motion for the eastward flow regime and typically yield transports near 170 Sv. Net eastward transport for this region of about 145 Sv results from subtracting those westward flows. This estimate is compatible with the canonical 134 Sv through Drake Passage with augmenta- tion from Indonesian Throughflow (around 10 Sv). A new image is developed from World Ocean Circulation Hydrographic Program sections I8S and I9S. These provide two quasi-meridional crossings of the South Australian Basin and the Australian– Antarctic Basin, with full hydrography and two independent direct-velocity measurements (shipboard and lowered acoustic Doppler current profilers). These velocity measurements indicate that the belt of eastward flow is much stronger, 271 ± 49 Sv, than previously estimated because of the presence of eastward barotropic flow. Substantial recirculations exist adjacent to the Antarctic Circumpolar Current: to the north a 38 ± 30 Sv anticyclonic gyre and to the south a 76 ± 26 Sv cyclonic gyre. The net flow between Australia and Antarctica is estimated as 157 ± 58 Sv, which falls within the expected net transport of 145 Sv. The 38 Sv anticyclonic gyre in the South Australian Basin involves the westward Flinders Current along southern Australia and a substantial 33 Sv Subantarctic Zone recirculation to its south. The cyclonic gyre in the Australian–Antarctic Basin has a substantial 76 Sv westward flow over the conti- nental slope of Antarctica, and 48 ±6 Sv northward-flowing western boundary current along the Kerguelen Plateau near 57ºS. The cyclonic gyre only partially closes within the Australian–Antarctic Basin. It is estimated that 45 Sv bridges westward to the Weddell Gyre through the southern Prin- cess Elizabeth Trough and returns through the northern Princess Elizabeth Trough and the Fawn Trough – where a substantial eastward 38 Sv current is hypothesized. There is evidence that the cyclonic gyre also projects eastward past the Balleny Islands to the Ross Gyre in the South Pacific. 2 The western boundary current along Kerguelen Plateau collides with the Antarctic Circumpolar Cur- rent that enters the Australian–Antarctic Basin through the Kerguelen–St. Paul Island Passage, forming an energetic Crozet–Kerguelen Confluence. Strongest filaments in the meandering Crozet-Kerguelen Confluence reach 100 Sv. Dense water in the western boundary current intrudes beneath the densest water of the Antarctic Circumpolar Current; they intensely mix diapycnally to produce a high poten- tial vorticity signal that extends eastward along the southern flank of the Southeast Indian Ridge. Dense water penetrates through the Ridge into the South Australian Basin. Two escape pathways are indicated, the Australian–Antarctic Discordance Zone near 125°E and the Geelvinck Fracture Zone near 85°E. Ultimately, the bottom water delivered to the South Australian Basin passes north to the Perth Basin west of Australia and east to the Tasman Basin. 3 Abstract .......................................................................................................................................... 1 1. Introduction................................................................................................................................ 4 2. Data .......................................................................................................................................... 7 3. Qualitative description of regional circulation elements and water masses ...................................... 10 3.1. The Western Boundary Current............................................................................................ 12 3.2. Sources for the Western Boundary Current ........................................................................... 13 3.3. Antarctic Circumpolar Current west of the Australian–Antarctic Basin ...................................... 15 3.4. Conditions west of and in the Fawn Trough .......................................................................... 17 3.5. Conditions west of and in the Kerguelen–St. Paul Island Passage............................................ 18 3.6. Antarctic Circumpolar Current at I8S .................................................................................... 20 3.7. The Crozet–Kerguelen Confluence........................................................................................ 21 3.7.1. Waters colder than 0.7ºC ............................................................................................. 22 3.7.2. Waters warmer than 0.7ºC ........................................................................................... 23 3.7.3. The front associated with the Crozet–Kerguelen Confluence ............................................ 24 3.7.4. Potential vorticity structure of the Crozet–Kerguelen Confluence ...................................... 25 3.8. Antarctic Circumpolar Current at I9S .................................................................................... 26 3.9. Leakage of cold water through the Australian–Antarctic Discordance....................................... 27 4. Combining the geostrophic shear with a velocity reference. .......................................................... 29 4.1. ADCP referencing of geostrophic shear................................................................................. 29 4.2. SADCP-referenced transport adjustment............................................................................... 31 4.3. Final transport adjustments................................................................................................. 32 4.3.1. Transport between I9S and Australia............................................................................. 33 4.3.2. Transport south of I8S in the southern Princess Elizabeth Trough .................................... 35 4.3.3. Transport south of I9S over the continental slope off the Adélie Coast. ............................ 35 4.3.4. The final SADCP bias adjustment. ................................................................................. 36 4.4. Discussion of the quality of the transport estimates ............................................................... 37 4.4.1. The remaining errors of SADCP referencing and its effect on transport estimates .............. 37 4.4.2. How strong is the Antarctic Slope Current? .................................................................... 38 4.4.3. Summary of estimated section transports, implied convergence, and error. ...................... 40 5. Quantification the strength and structure of the regional circulation............................................... 40 5.1 The large-scale circulation in the southeastern Indian Ocean................................................... 41 5.2 The dominant current elements ............................................................................................ 45 5.2.1. Flow through Kerguelen–St. Paul Island Passage ............................................................ 45 5.2.2. Flow along and across the Southeast Indian Ridge ......................................................... 48 5.2.3. Westward recirculation south of Australia....................................................................... 50 5.2.4. The collision of the western boundary and Fawn Trough currents east of Kerguelen Plateau ............................................................................................................................................ 51 5.2.5. The Crozet–Kerguelen Confluence meander field ............................................................ 53 5.2.6. The connection between the cyclonic gyres of the Weddell–Enderby and Australian–Antarctic Basins ................................................................................................................................... 55 5.2.7. The zonal overturning and the eastward closure of the cyclonic gyre................................ 57 5.2.8. The connection between the cyclonic gyre of the Australian–Antarctic Basin and the Ross Gyre ..................................................................................................................................... 60 5.3. The flow of bottom water from the Australian–Antarctic Basin to the South Australian Basin .... 61 5.3.1. Hypothesized northward dense water flow through the Geelvinck Fracture Zone ............... 61 5.3.2. Escape of dense water out of the cyclonic gyre in the Australian–Antarctic Basin............... 63 6. Conclusions .............................................................................................................................. 65 Acknowledgments.......................................................................................................................... 67 References ...................................................................................................................................
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