Ocean temperature climate off north-east New Zealand

The ocean temperature field off the north-east coast of New Zealand is stud ied to quantify the annual cycle and reveal the intra- and inter-annual var iability. The data used are repeat expendable bathythermograph (XBT) sectio ns between Auckland and either Suva or Honolulu which have been collected q uarterly since 1986. These sections give temperature measurements between t he surface and 800 m and Auckland and 30 degreesS from 1986 to August 1999. The mean and annual cycle are compared with those from the NOAA World Ocea n Atlas (WOA98). The results are similar; however WOA98 lacks the horizonta l resolution to fully discern the East Auckland Current and North Cape Eddy , while the XBT analysis lacks the temporal resolution to discern higher fr equency intra-annual signals. The temperature variability in the mixed laye r is dominated by the annual cycle, which accounts for 80-90% of the varian ce. The amplitude of the annual cycle diminishes rapidly with depth, from 2 .8 degreesC at the surface, to c. 0.1 degreesC at 180 m. The phase of the a nnual cycle is retarded with depth, with peak temperatures occurring in Feb ruary at the surface and in June/July at 180 m. Removing the annual cycle f rom the time series reveals the more subtle inter- and intra-annual variabi lity. This variability is of the order of 1 degreesC in the upper 50 m, dec reasing to 0.3 degreesC at 400-500 m. The surface layer was cold between 19 91 and 1994 (c. 0.7 degreesC cooler than average), and 0.7 degreesC warmer than average in 1999. The deeper ocean shows a different signal, being up t o 0.3 degreesC cooler in 1990-92, 0.3 degreesC warmer in 1998, and c. 0.2 d egreesC warmer than average in 1999. The inter-annual mixed layer variabili ty is highly correlated with the Southern Oscillation Index and also with i nter-annual terrestrial air temperature and wind measurements from northern New Zealand. In contrast, at higher intra-annual frequencies, the mixed la yer variability is not correlated with air and wind measurements. At these higher frequencies, the air temperature is better correlated with the sea s urface temperature (SST) than with the bulk mixed layer temperature.