Subantarctic oceanography around New Zealand: preliminary results from an ongoing survey

An ongoing observational program focused on variability in the subantarctic currents and water masses around southern New Zealand was initiated in May 1998. This paper describes the preliminary results deduced from CTD (condu ctivity, temperature, depth) data collected during three hydrographic surve ys, and moored current and temperature records between 1998 and 1999. An ex tensive archived data set has also been analysed to provide a hydrographic climatology of the region. The low-frequency circulation within the subanta rctic zone is described, revealing previously unreported flow features: per sistent but weak anticyclonic and cyclonic circulations over the Campbell P lateau and a strong cyclonic flow around the western edge of the Bounty Tro ugh. Flow within the Subantarctic Front (SAF) is strong constrained by the New Zealand bathymetry; diverting to follow the south-eastern flanks of the Campbell Plateau and crossing to the Bounty Plateau, before separating to join the basin-scale circulation. The flow features deduced from recent dat a are consistent with the hydrographic climatology as well as mid-depth flo at trajectories. Subantarctic Mode Water (SAMW) is observed year-round over an extensive fraction of the subantarctic. zone: along the equatorward sid e of the SAF, over the Campbell Plateau, and flowing within the cyclonic ci rculation around the Bounty Trough. There is a marked cooling and freshenin g of SAMW between the deep water along the western flanks of Campbell Plate au and waters further east, consistent with a blocking of the eastward flow carrying warm and salty SAMW by the plateau. Earlier ideas that a substant ial volume of SAMW is formed over the Campbell Plateau by deep vertical mix ing (Heath 1981) are not substantiated by our data, which include seasonal observations of the upper water column.