Features of 3-dimensional barotropic and baroclinic circulation in the Hauraki Gulf, New Zealand

Predominant features of barotropic and baroclinic circulation and mixing in the Hauraki Gulf on New Zealand's north-east coast are described using mea surements and 3-dimensional numerical model simulations. Circulation in the Hauraki Gulf is strongly 3-dimensional with a primary dynamical balance be tween surface wind stress and the associated pressure gradients against the land. This leads to persistent up/downwelling and surface manifestations i n sea surface temperature patterns which are shown to vary systematically a nd markedly with wind direction and stratification intensity. A high degree of correspondence between a baroclinic numerical model and measured temper ature and nitrate concentrations indicated that many of the observed spatia l patterns could be largely explained by the interaction of the wind and ti dal circulation with the unique morphology of the Gulf After strong southea sterly winds, local responses operated in conjunction with the regional "ca pping" mechanism described by Sharpies (1997) of downwind surface water int rusion from the shelf into the Gulf. However, the morphology acts to enhanc e local upwelling causing bottom waters to be injected into the surface lay ers which disrupts the "cap". The headlands and islands play an additional vertical mixing role by presenting bathymetric variability leading to the f ormation of upwelling jets in the core of eddies forming during ebb and flo od tides. By introducing bottom waters into the upper water column and acti ng to over-turn the water body, up/downwelling is an important mechanism fo r mixing and biological productivity which could Vary systematically within the Gulf in response to seasonal and interannual variability in the upwell ing patterns. Persistent south-east winds above a threshold of 10-12 m s(-1 ) were found to initiate breakdown of seasonal temperature stratification i n 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 m(2 ) s(-1) as the cyclone strengthened.