New Zealand Journal of Marine and Freshwater Research abstracts

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Absolute geostrophic currents in the East Auckland Current region

P. J. H. Sutton

National Institute of Water and Atmospheric Research Limited
P.O. Box 14 901
Kilbirnie, Wellington
New Zealand
email: p.sutton@niwa.co.nz

T. K. Chereskin

Scripps Institution of Oceanography
University of California, San Diego
9500 Gilman Drive
La Jolla, CA 92093-0230
United States
email: tchereskin@ucsd.edu

Abstract   Absolute geostrophic currents are estimated for the full depth of the water column off the north-east coast of New Zealand for February 1998. This is achieved by objectively-mapping subsurface Acoustic Doppler Current Profiler (ADCP) data using horizontal length scales determined from repeat high-resolution Expendable Bathythermograph (XBT) data and enforcing horizontal non-divergence. The resulting 2-dimensional map of velocity at a single depth is used as a level of known motion. Velocities at all other depths are calculated by combining this known motion with the geostrophic shear calculated from hydrographic measurements. The flow field exhibits the expected large-scale features: the East Auckland Current (EAUC) and the North Cape Eddy. It also indicates that these flows were underestimated by the standard use of a 2000 m level of no motion, i.e., these features penetrate 2000 m. Flows at 2000 m are found to be as large as 30-40 cm/s with a mean of 17 cm/s. This is less than the maximum flows measured by deep current meters, but significantly higher than the mean. Comparisons of the ADCP-referenced surface flow field with the surface flow field calculated by combining climatology and TOPEX/Poseidon sea-surface height (SSH) show general qualitative agreement. The two independent flow estimates are similar in the core of the EAUC, with the differences being c. 10 cm/s, whereas the agreement further offshore is not as good.

Keywords   East Auckland Current; broadband Acoustic Doppler Current Profiler (ADCP); geostrophic currents

M01085 Received 19 October 2001; accepted 4 July 2002; Published 14 November
New Zealand Journal of Marine and Freshwater Research, 2002, Vol. 36: 751-762
0028-8330/02/3604-0751 $7.00 © The Royal Society of New Zealand 2002

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