Detailed structure of the Subtropical Front over Chatham Rise, east of New Zealand

Abstract

A detailed description of the Subtropical Front (STF) over Chatham Rise is presented. Information is provided by conductivity‐temperature‐depth (CTD) profiler temperature and salinity transects, high‐resolution (3–4 km spacing), near‐synoptic expendable bathythermograph surveys, shipboard acoustic Doppler current profiler measurements, a 160 km constant depth undulator tow, and satellite SST measurements in austral autumn and spring (April and October) near 178°30′E. Detailed sections across the front reveal that the STF can be divided into northern and southern fronts (NSTF and SSTF) separated by a frontal zone, consistent with observations of the STF elsewhere. Comparison with a western CTD section along 176°E reveals that this structure breaks down close to the New Zealand landmass. The dominant impact of seasonally is the formation and decay of a seasonal thermocline of fairly constant depth over the entire region. Velocity measurements indicate eastward zonal flows on the northern and southern flanks of Chatham Rise. The flow over Chatham Rise is complicated by stirring processes but has a mean southward trend consistent with the less dense, warm, salty subtropical water (STW) overriding the more dense, cold, fresh subantarctic water (SAW) to the south. Horizontal‐length‐scale analyses of the temperature field for a constant depth tow at 21 m indicate a k−2.4 spectral falloff with no proof of preferential length scales. The temperature salinity gradients are largely density compensated in this mixed layer tow. The horizontal temperature standard deviation shows subsurface maxima concentrated at the locations of the NSTF and SSTF. Vertical length scales vary across the front, with large scales (80–100 m) in the STW north of the STF and smaller scales (20–40 m) in the subtropical frontal zone and SAW to the south. The surface vertical length scales are dominated by the presence of a seasonal thermocline.