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

Abstract

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 (conductivity, temperature, depth) data collected during three hydrographic surveys, and moored current and temperature records between 1998 and 1999. An extensive archived data set has also been analysed to provide a hydrographic climatology of the region. The low‐frequency circulation within the subantarctic zone is described, revealing previously unreported flow features: persistent but weak anticyclonic and cyclonic circulations over the Campbell Plateau and a strong cyclonic flow around the western edge of the Bounty Trough. Flow within the Subantarctic Front (SAF) is strongly 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 data are consistent with the hydrographic climatology as well as mid‐depth float trajectories. Subantarctic Mode Water (SAMW) is observed year‐round over an extensive fraction of the subantarctic zone: along the equatorward side of the SAF, over the Campbell Plateau, and flowing within the cyclonic circulation around the Bounty Trough. There is a marked cooling and freshening of SAMW between the deep water along the western flanks of Campbell Plateau and waters further east, consistent with a blocking of the eastward flow carrying warm and salty SAMW by the plateau. Earlier ideas that a substantial volume of SAMW is formed over the Campbell Plateau by deep vertical mixing (Heath 1981) are not substantiated by our data, which include seasonal observations of the upper water column.