THE MEAN AND VARIABILITY OF OCEAN CIRCULATION PAST NORTHERN NEW-ZEALAND - DETERMINING THE REPRESENTATIVENESS OF HYDROGRAPHIC CLIMATOLOGIES

Eastward flow in the Tasman Sea, from the separated East Australia Cur rent, reattaches to the shelf break near North Cape, New Zealand, and then continues alongshore to the southeast as the East Auckland Curren t. A series of three permanent warm core eddies occurs along the offsh ore side. The mean transport of the East Auckland Current is about 9 S v, with an additional 10 Sv or more of circulation in the eddies. An e xtensive hydrographic data set, archived broad scale expendable bathyt hermograph (XBT) data, two repeating high-resolution XBT transects, ne utrally buoyant float trajectories, and TOPEX altimetric data are used to estimate the temperature and absolute flow fields and to character ize variability. The aim is to examine the usefulness of time series i nformation and absolute velocity measurements in the interpretation of hydrographic Snapshots and climatologies, as well as to describe a re gion having intrinsic oceanographic interest and complexity. Issues of representativeness of the hydrographic data, of the magnitude and sca les of the underlying variability, of the existence of permanent fine- scale features, and of the appropriateness of deep reference levels ar e addressed directly. The relatively well sampled hydrographic climato logy is shown to contain the equivalent of as many as 10 independent r ealizations. Temperature errors, relative to the true mean, are typica lly a few tenths of a degree. Significant seasonal bias is identified in the surface layer, and interannual bias is seen in the position of an eddy near North Caps. The dynamic height field at 1000 dbar relativ e to 2000 dbar is similar to estimates based on float trajectories and the assumption of geostrophic dynamics. This study underlines the val ue of time series data in the interpretation of a hydrographic climato logy, in quantifying the errors in the estimated mean field as well as determining the magnitude and nature of variability. It also highligh ts the fact that the mean circulation of the oceans contains significa nt mesoscale structure, unnoticed in coarsely smoothed climatologies.