Oceanography of the Subtropical Convergence Zone around southern New Zealand

Seawaters of the Subtropical Convergence Zone around the south of New Zealand were surveyed concurrently by two research ships in May 1989. The general surface features of the region are depicted in remotely sensed images of sea-surface temperatures obtained at the time of the survey, and also from preceding years. Shipboard data, obtained underway and on station, are used to describe the water masses and frontal structures. Short-term variability is discussed where stations were re-occupied after an interval of several days. Apositive gradient of nutrient concentration was observed when proceeding from subtropical to subantarctic waters. This general trend is modified in coastal regions by freshwater run-off, and by mixing processes in open ocean waters. It is suggested that possible mechanisms for the nutrient enrichment of surface waters, leading to enhanced biological productivity, are horizontal advection, and uplifting of subsurface waters-coupled with deep mixing events or influenced by the continental slope. It seems that all of these processes may be variable and sporadic; no sustained process such as upwelling was observed in the region.