Mean and variability in the wairarapa and Hikurangi Eddies, New Zealand

The mean and variability of the circulation off the east coast of the North Island, New Zealand are investigated using shipboard conductivity-temperature-depth (CTD) and satellite altimeter data collected between 1993 and 2003. The altimeter data are used to adjust the in situ observations for the mesoscale eddy variability before computing the 11-year mean in dynamic height. Mean dynamic height shows two anticyclonic eddies, centred near 178.3° E, 41.2° S and 176.2° E, 42.4° S. These locations are consistent with previous historical observations of the Wairarapa and Hikurangi Eddies, respectively. A long-term trend in both in situ and satellite data shows that dynamic height rose at an average rate of up to 2 dyn cm year(-1) in the centre of the Wairarapa Eddy which is consistent with a strengthening of the eddy over the 11 years. The satellite data show periodic shedding of the Wairarapa Eddy from near East Cape at a rate of between two and three eddies per year. Thus, rather than indicating a permanent stationary eddy, the mean eddy reflects a region where the eddies tend to stall out or merge with the previous eddy. Often, the eddies will continue up the Hikurangi Trough so that the Hikurangi Eddy can be regarded as an older Wairarapa Eddy.