Interaction of the Antarctic Circumpolar Current with topography south ...
Neal, V. T. 1974. International Southern Ocean Studies, 1974-1975. Antarctic Journal oft/ic US., 9(6): 289-290. Nowlin, W. D., S. L. Patterson, R D. Pillsbury, and G. C. Anderson. 1975. Contributions of it/v Melville to FDRAKE 1975. Antarctic Jour. nalof the US., 10(4): 144-146.
Interaction of the Antarctic Circumpolar Current with topography south of New Zealand R. A. HEATH
New Zealand Oceanographic Institute Wellington, New Zealand H. L. BRYDEN
Department of Physical Oceanography Woods Hole Oceanographic Institution Woods Hole, Massachusetts 02543
Wearn, R. B., and P. K. Park. 1975. Contributions of ARA Islas Orcadas to FDRAKE 1975. AntarcticJournal of the US., 10(4): 141-142. Wust, C. 1936. Schictung und Zirkulation der Atlantischen Ozeans. Wissenschafihiche Ergebnisse der Deutschen Atlantischen Expedition "Meteor" 1925-27. Bd. VI, Tell 1,2, Die Stratosphare.
S. P. HAYES
Pacific Marine Environmental Laboratory National Oceanic and Atmospheric Administration Seattle, Washington 98105 The cruise southeast of New Zealand by the iiv Tangaroa from 4 to 24 April 1978 (figure 1) marked the start of concentrated International Southern Ocean Studies (iSos) designed to investigate the interaction of the Antarctic Circumpolar Current with the topography of the Macquarie Ridge/Campbell Plateau region south of New Zealand and the formation of subantarctic mode water on the Campbell Plateau. The purpose of this initial experiment was to study the spatial and temporal variability of low frequency motions
FIgure 1. Cruise track of niv Tan geroa 4-24 April 1978, shown on a bathymetric chart of the region southeast of New Zealand. Also shown are cm station and mooring positions and dynamic height (0-1,000 meters) contours drawn from the CTD data. 76
ANTARCTIC JOURNAL
and their effects on the mean circulation and to study the late winter formation of subantarctic mode water (McCartney, 1977), which is hypothesized to be similar to the formation of subtropical mode water south and east of the Gulf Stream and Kuroshio (Masuzawa, 1969; Worthington, 1959). Participating in the Tangaroa cruise were Harry Bryden from Woods Hole Oceanographic Institution, Ron Heath and Bruce Shakespeare from the New Zealand Oceanographic Institute, Dennis Root and Robert Still from Oregon State University, and David Spell from the Pacific Marine Environmental Laboratory. During the cruise, six moorings equipped with Aanderaa current meters measuring temperature, pressure, and current speed and direction were deployed: five in a cluster array near 49°30'S. 170°W. on the abyssal plain of the southwestern Pacific basin (figure 2), and one at 51°10'S.175°E. on the slope of the Campbell Plateau. In addition, one drifting buoy provided by George Cresswell (Commonwealth Scientific and Industrial Organization, Cronulla, Australia) was launched at 50°30'S.176°40'W. and will be tracked by satellite on its journey across the Pacific. Near the cluster array, 13 CTD (conductivity-temperature-depth) stations to 1,500 meters depth were made to determine the spatial scales of the density structure. A larger scale survey of 26 CTD stations (figure 1) was made to define the broad scale density structure and associated circulation southeast of New Zealand.
500
The mooring on the slope of Campbell Plateau was equipped with five thermistor chains as well as two current meters to study the winter overturning of subantarctic mode water and its relationship to the production of antarctic intermediate water (McCartney, 1977). The CTD station near this mooring (figure 3) revealed a 100-meter-deep mixed layer with temperature and salinity values similar to those of subantarctic mode water. That such a deep mixed layer was present in early autumn suggests that this is a region of active formation of subantarctic mode water and that the formation may take place throughout the year rather than only during late winter. 34.20 34.40
SALINITY 34.80 35.00 35.20
34.60
TEMPERATURE
2.00 4.00 6.00 8.00 10.00
N
1000
M 4900\_4 ^
W
1000
\92000
91000
2m
4000 4900
I: \
1100'I km
J
Depth Scale
10 km Horizontal le
Figure 2. Positions of current meters in the cluster array near 49030'S.1700W. From the cluster array measurements and the smaller scale CTD survey, the dynamics of low frequency motions or eddies will be studied and the effects of the eddies on the mean circulation will be estimated. We plan to investigate whether the eddies are freely propagating waves, atmospherically forced waves, or locally generated eddies growing in a baroclinic instability process. Following analysis of Drake Passage measurements (Bryden, in preparation), we will estimate the rate of energy exchange between eddies and mean circulation. October 1978
26.00
27.00 SIGMA-T
28100
Figure 3. Temperature and salinity v. pressure from the CTD station near the thermistor chain mooring at 51010'S.1750E. Preliminary analysis of the CTD data showed that the cluster array was set in a strong frontal region where the dynamic height between 1,000 meters depth and the surface changed by 10 dynamic centimeters over about 15 nautical miles. Because the CTD data in this region revealed significant amounts of subtropical water intermixed in subantarctic water, this front was probably the southeastward extension of the subtropical convergence located by Ridgway (1975). Within this front there was a pronounced tongue of water with a salinity maximum at a depth of about 150 meters. The presence of such a tongue extending southward from the Subtropical Convergence was first observed by Deacon (1937) and more recently has been analyzed by Heath (1976). The Subantarctic Front within which the thermistor chain mooring was located passes south of the cluster array. The complex distribution of fronts including the Subtropical 77
Convergence, Subantarctic Front, and Southland Front in the region southeast of New Zealand clearly will require a detailed reexamination. We thank the officers and crew of RIv Tangaroa for their cooperation in carrying out this experiment. The quality of the CTD measurements is due to the initiative and persistence of David Spell in diagnosing both hardware and software problems. The moorings could not have been deployed without the patience and care of Robert Still, Dennis Root, and Bruce Shakespeare. This experiment was supported by the New Zealand Oceanographic Institute of the Department of Scientific and Industrial Research, by the Environmental Research Laboratories of the National Oceanic and Atmospheric Administration, and by the Office for the International Decade of Ocean Exploration of the National Science Foundation under agreement OCE 77-22868 with Pacific Marine Environmental Laboratory and under grant OCE 77-22887 to Woods Hole Oceanographic Institution.
FDRAKE 77 Yelcho WORTH D. NOWLIN,JR
Department of Oceanography Texas A&M University College Station, Texas 77843 R. DALE PILLSBURY
School of Oceanography Oregon State University Corvallis, Oregon 97331 HELLMUTI-I A. SIEVERS
Instituto Hidrografico de la Armada Valparaiso, Chile In December 1977 the Chilean navy ship AGS Yelcho supported the last phase of the International Southern Ocean Study known as FDRAKE (First Dynamic Response and Kinematics Experiment). Hellmuth A. Sievers and eight other persons from the Department of the Chilean Naval Hydrographic Institute were aboard to assist in the program. The NSF Office of the International Decade of Ocean Exploration provided support for equipment and the participation of R. Dale Pillsbury (Oregon State University); Worth D. Nowlin, Jr., James Stasny, and Brady A. Elliott (Texas A&M University); and Earl Krause (Applied Physics Laboratory, University of Washington). The objectives of the cruise were: to recover deep pressure gages deployed during February 1977 on the north and south sides of Drake Passage; to install other pressure gages at these locations; to recover a cluster array of current/temperature recorders moored near 59°S.63°45'W. in the central passage during February 1977; and to deploy a single, year-long mooring in the central passage. 78
References
Bryden, H. L. In preparation. Poleward heat flux and conversion of available potential energy in Drake Passage. Deacon, G. E. R. 1937. Hydrology of the Southern Ocean. Discovery Reports, 15: 1-124. Heath, R A. 1976. Models of the diffusive-advective balance at the Subtropical Convergence. Deep-Sea Research, 23: 1153-1164. Masuzawa, J . 1969. Subtropical mode water. Deep-Sea Research, 16: 463-472. McCartney, M. S. 1977. Subantarctic mode water. In: A Voyage of Discovery (Supplement to vol. 24, Deep-Sea Research, M. Angel, ed.). pp. 103-119. Ridgway, N. M. 1975. Hydrology of the Bounty Island region (Memoir 75, New Zealand Oceanographic Institute). Worthington, L. V. 1959. The 18°-water in the Sargasso Sea. DeepSea Research, 5: 297-305.
The pressure measurements are part of a long term program to monitor pressure difference and to relate this difference to flow through the passage. Density profiles are made at times of installation and deployment of the gages. The principal objective of the cluster array was to determine the spatial variability of currents and thermal field at horizontal separations between 12 and 60 kilometers for the depth range 300-3,500 meters. This information was needed to design a major array to be deployed in Drake Passage during 1979 to monitor transport and to obtain data for special kinematics/dynamics experiments. A secondary objective of the cluster array was to obtain information to describe movements of sharp bands of current through the area and to compare local changes of temperature with horizontal advection of temperature gradients. Hydrographic data were taken before and after the deployment in support of this objective (see Whitworth, 1977). The cluster array and the new mooring in the central passage for 1978 extend time series of currents and temperatures at a location in the Antarctic Circumpolar Current to 4 years. These time series are being compared with variations in the geostrophic surface wind stress in the Southern Hemisphere. Departing Punta Arenas, Chile, on 4 December 1977, Yelcho proceeded to the position (56°32'S.67°01'W.) of the pressure gage at the northern side of Drake Passage and occupied a hydrographic station there in 500 meters water depth and another station nearby in 1,700 meters water depth. All current and temperature recorders from the five 1977 moorings clustered near 49°S.63°45'W. in the central Drake Passage were recovered. A total of 21 Aanderaa instruments had been deployed on the moorings at depths of 300, 500, 1,000, and 2,000 meters. Two moorings also had instruments at 3,500 meters, and two of the recorders at 300 meters were equipped with conductivity sensors. All but four recorders were operating when recovered. Three returned partial records of about 6 months, and one had failed soon after deployment. At the position of the central mooring, another mooring was deployed with current and temperature recorders at 500, 1,000, and 2,000 meters to operate until January 1979.
ANTARCTIC JOURNAL
Interaction of the Antarctic Circumpolar Current with topography south of New Zealand R. A. HEATH
New Zealand Oceanographic Institute Wellington, New Zealand H. L. BRYDEN
Department of Physical Oceanography Woods Hole Oceanographic Institution Woods Hole, Massachusetts 02543
Wearn, R. B., and P. K. Park. 1975. Contributions of ARA Islas Orcadas to FDRAKE 1975. AntarcticJournal of the US., 10(4): 141-142. Wust, C. 1936. Schictung und Zirkulation der Atlantischen Ozeans. Wissenschafihiche Ergebnisse der Deutschen Atlantischen Expedition "Meteor" 1925-27. Bd. VI, Tell 1,2, Die Stratosphare.
S. P. HAYES
Pacific Marine Environmental Laboratory National Oceanic and Atmospheric Administration Seattle, Washington 98105 The cruise southeast of New Zealand by the iiv Tangaroa from 4 to 24 April 1978 (figure 1) marked the start of concentrated International Southern Ocean Studies (iSos) designed to investigate the interaction of the Antarctic Circumpolar Current with the topography of the Macquarie Ridge/Campbell Plateau region south of New Zealand and the formation of subantarctic mode water on the Campbell Plateau. The purpose of this initial experiment was to study the spatial and temporal variability of low frequency motions
FIgure 1. Cruise track of niv Tan geroa 4-24 April 1978, shown on a bathymetric chart of the region southeast of New Zealand. Also shown are cm station and mooring positions and dynamic height (0-1,000 meters) contours drawn from the CTD data. 76
ANTARCTIC JOURNAL
and their effects on the mean circulation and to study the late winter formation of subantarctic mode water (McCartney, 1977), which is hypothesized to be similar to the formation of subtropical mode water south and east of the Gulf Stream and Kuroshio (Masuzawa, 1969; Worthington, 1959). Participating in the Tangaroa cruise were Harry Bryden from Woods Hole Oceanographic Institution, Ron Heath and Bruce Shakespeare from the New Zealand Oceanographic Institute, Dennis Root and Robert Still from Oregon State University, and David Spell from the Pacific Marine Environmental Laboratory. During the cruise, six moorings equipped with Aanderaa current meters measuring temperature, pressure, and current speed and direction were deployed: five in a cluster array near 49°30'S. 170°W. on the abyssal plain of the southwestern Pacific basin (figure 2), and one at 51°10'S.175°E. on the slope of the Campbell Plateau. In addition, one drifting buoy provided by George Cresswell (Commonwealth Scientific and Industrial Organization, Cronulla, Australia) was launched at 50°30'S.176°40'W. and will be tracked by satellite on its journey across the Pacific. Near the cluster array, 13 CTD (conductivity-temperature-depth) stations to 1,500 meters depth were made to determine the spatial scales of the density structure. A larger scale survey of 26 CTD stations (figure 1) was made to define the broad scale density structure and associated circulation southeast of New Zealand.
500
The mooring on the slope of Campbell Plateau was equipped with five thermistor chains as well as two current meters to study the winter overturning of subantarctic mode water and its relationship to the production of antarctic intermediate water (McCartney, 1977). The CTD station near this mooring (figure 3) revealed a 100-meter-deep mixed layer with temperature and salinity values similar to those of subantarctic mode water. That such a deep mixed layer was present in early autumn suggests that this is a region of active formation of subantarctic mode water and that the formation may take place throughout the year rather than only during late winter. 34.20 34.40
SALINITY 34.80 35.00 35.20
34.60
TEMPERATURE
2.00 4.00 6.00 8.00 10.00
N
1000
M 4900\_4 ^
W
1000
\92000
91000
2m
4000 4900
I: \
1100'I km
J
Depth Scale
10 km Horizontal le
Figure 2. Positions of current meters in the cluster array near 49030'S.1700W. From the cluster array measurements and the smaller scale CTD survey, the dynamics of low frequency motions or eddies will be studied and the effects of the eddies on the mean circulation will be estimated. We plan to investigate whether the eddies are freely propagating waves, atmospherically forced waves, or locally generated eddies growing in a baroclinic instability process. Following analysis of Drake Passage measurements (Bryden, in preparation), we will estimate the rate of energy exchange between eddies and mean circulation. October 1978
26.00
27.00 SIGMA-T
28100
Figure 3. Temperature and salinity v. pressure from the CTD station near the thermistor chain mooring at 51010'S.1750E. Preliminary analysis of the CTD data showed that the cluster array was set in a strong frontal region where the dynamic height between 1,000 meters depth and the surface changed by 10 dynamic centimeters over about 15 nautical miles. Because the CTD data in this region revealed significant amounts of subtropical water intermixed in subantarctic water, this front was probably the southeastward extension of the subtropical convergence located by Ridgway (1975). Within this front there was a pronounced tongue of water with a salinity maximum at a depth of about 150 meters. The presence of such a tongue extending southward from the Subtropical Convergence was first observed by Deacon (1937) and more recently has been analyzed by Heath (1976). The Subantarctic Front within which the thermistor chain mooring was located passes south of the cluster array. The complex distribution of fronts including the Subtropical 77
Convergence, Subantarctic Front, and Southland Front in the region southeast of New Zealand clearly will require a detailed reexamination. We thank the officers and crew of RIv Tangaroa for their cooperation in carrying out this experiment. The quality of the CTD measurements is due to the initiative and persistence of David Spell in diagnosing both hardware and software problems. The moorings could not have been deployed without the patience and care of Robert Still, Dennis Root, and Bruce Shakespeare. This experiment was supported by the New Zealand Oceanographic Institute of the Department of Scientific and Industrial Research, by the Environmental Research Laboratories of the National Oceanic and Atmospheric Administration, and by the Office for the International Decade of Ocean Exploration of the National Science Foundation under agreement OCE 77-22868 with Pacific Marine Environmental Laboratory and under grant OCE 77-22887 to Woods Hole Oceanographic Institution.
FDRAKE 77 Yelcho WORTH D. NOWLIN,JR
Department of Oceanography Texas A&M University College Station, Texas 77843 R. DALE PILLSBURY
School of Oceanography Oregon State University Corvallis, Oregon 97331 HELLMUTI-I A. SIEVERS
Instituto Hidrografico de la Armada Valparaiso, Chile In December 1977 the Chilean navy ship AGS Yelcho supported the last phase of the International Southern Ocean Study known as FDRAKE (First Dynamic Response and Kinematics Experiment). Hellmuth A. Sievers and eight other persons from the Department of the Chilean Naval Hydrographic Institute were aboard to assist in the program. The NSF Office of the International Decade of Ocean Exploration provided support for equipment and the participation of R. Dale Pillsbury (Oregon State University); Worth D. Nowlin, Jr., James Stasny, and Brady A. Elliott (Texas A&M University); and Earl Krause (Applied Physics Laboratory, University of Washington). The objectives of the cruise were: to recover deep pressure gages deployed during February 1977 on the north and south sides of Drake Passage; to install other pressure gages at these locations; to recover a cluster array of current/temperature recorders moored near 59°S.63°45'W. in the central passage during February 1977; and to deploy a single, year-long mooring in the central passage. 78
References
Bryden, H. L. In preparation. Poleward heat flux and conversion of available potential energy in Drake Passage. Deacon, G. E. R. 1937. Hydrology of the Southern Ocean. Discovery Reports, 15: 1-124. Heath, R A. 1976. Models of the diffusive-advective balance at the Subtropical Convergence. Deep-Sea Research, 23: 1153-1164. Masuzawa, J . 1969. Subtropical mode water. Deep-Sea Research, 16: 463-472. McCartney, M. S. 1977. Subantarctic mode water. In: A Voyage of Discovery (Supplement to vol. 24, Deep-Sea Research, M. Angel, ed.). pp. 103-119. Ridgway, N. M. 1975. Hydrology of the Bounty Island region (Memoir 75, New Zealand Oceanographic Institute). Worthington, L. V. 1959. The 18°-water in the Sargasso Sea. DeepSea Research, 5: 297-305.
The pressure measurements are part of a long term program to monitor pressure difference and to relate this difference to flow through the passage. Density profiles are made at times of installation and deployment of the gages. The principal objective of the cluster array was to determine the spatial variability of currents and thermal field at horizontal separations between 12 and 60 kilometers for the depth range 300-3,500 meters. This information was needed to design a major array to be deployed in Drake Passage during 1979 to monitor transport and to obtain data for special kinematics/dynamics experiments. A secondary objective of the cluster array was to obtain information to describe movements of sharp bands of current through the area and to compare local changes of temperature with horizontal advection of temperature gradients. Hydrographic data were taken before and after the deployment in support of this objective (see Whitworth, 1977). The cluster array and the new mooring in the central passage for 1978 extend time series of currents and temperatures at a location in the Antarctic Circumpolar Current to 4 years. These time series are being compared with variations in the geostrophic surface wind stress in the Southern Hemisphere. Departing Punta Arenas, Chile, on 4 December 1977, Yelcho proceeded to the position (56°32'S.67°01'W.) of the pressure gage at the northern side of Drake Passage and occupied a hydrographic station there in 500 meters water depth and another station nearby in 1,700 meters water depth. All current and temperature recorders from the five 1977 moorings clustered near 49°S.63°45'W. in the central Drake Passage were recovered. A total of 21 Aanderaa instruments had been deployed on the moorings at depths of 300, 500, 1,000, and 2,000 meters. Two moorings also had instruments at 3,500 meters, and two of the recorders at 300 meters were equipped with conductivity sensors. All but four recorders were operating when recovered. Three returned partial records of about 6 months, and one had failed soon after deployment. At the position of the central mooring, another mooring was deployed with current and temperature recorders at 500, 1,000, and 2,000 meters to operate until January 1979.
ANTARCTIC JOURNAL
