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

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 southeasterly winds, local responses operated in conjunction with the regional “capping” mechanism described by Sharpies (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‐l winds. Vertical eddy diffusivity increased from 0.0015 to 0.04 m2 s‐1 as the cyclone strengthened.