New Zealand Journal of Marine and Freshwater Research abstracts
Features of 3-dimensional barotropic and baroclinic circulation in the
Hauraki Gulf, New Zealand
K. P. BLACK1
R. G. BELL
J. W. OLDMAN
National Institute of Water & Atmospheric
Research Ltd
P. O. Box 11 115, Hamilton
New Zealand
G. S. CARTER2
National Institute of Water & Atmospheric
Research Ltd
P. O. Box 8602, Christchurch
New Zealand
T. M. HUME
National Institute of Water & Atmospheric
Research Ltd
P. O. Box 11 115, Hamilton
New Zealand
1Present address: Department of Earth Sciences, University of
Waikato, Private Bag 3105, Hamilton, New Zealand. email:
kblack@waikato.ac.nz
2Present address: School of Oceanography, University of
Washington, Seattle, United States.
Abstract Predominant features of barotropic and baroclinic
circulation and mixing in the Hauraki Gulf on New Zealand's north-east coast
are described using measurements and 3-dimensional numerical model simulations.
Circulation in the Hauraki Gulf is strongly 3-dimensional with a primary
dynamical balance between surface wind stress and the associated pressure
gradients against the land. This leads to persistent up/downwelling and surface
manifestations in sea surface temperature patterns which are shown to vary
systematically and markedly with wind direction and stratification intensity. A
high degree of correspondence between a baroclinic numerical model and measured
temperature and nitrate concentrations indicated that many of the observed
spatial patterns could be largely explained by the interaction of the wind and
tidal circulation with the unique morphology of the Gulf. After strong
south-easterly winds, local responses operated in conjunction with the regional
"capping" mechanism described by Sharples (1997) of downwind surface water
intrusion from the shelf into the Gulf. However, the morphology acts to enhance
local upwelling causing bottom waters to be injected into the surface layers
which disrupts the "cap". The headlands and islands play an additional vertical
mixing role by presenting bathymetric variability leading to the formation of
upwelling jets in the core of eddies forming during ebb and flood tides. By
introducing bottom waters into the upper water column and acting to over-turn
the water body, up/downwelling is an important mechanism for mixing and
biological productivity which could vary systematically within the Gulf in
response to seasonal and interannual variability in the upwelling patterns.
Persistent south-east winds above a threshold of 10-12 m s-1 were
found to initiate breakdown of seasonal temperature stratification in the Gulf,
with complete breakdown after 3 days during a cyclone with 8-23 m
s-1 winds. Vertical eddy diffusivity increased from 0.0015 to 0.04
m2 s-1 as the cyclone strengthened.
Keywords north-east shelf; Hauraki Gulf; 3-dimensional
modelling; upwelling; tidal currents; internal tide; winds
M98075
Received 9 October 1998; accepted 1 July 1999
PDF file of entire paper: medium quality (8915K); (scanned from paper original: notes about this process)
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