Seasonal and inter-annual variability of SST off the east coast of the South Island, New Zealand

This study examined the temporal variability of sea surface temperature (SST) of three water masses east of the South Island, New Zealand, where two of these water masses are part of the global Subtropical Front (STF). The STF is well defined and can be detected easily using remote sensing Advanced Very High Resolution Radiometer (AVHRR) SST data. Ten sites were used from a ten year AVHRR SST data set and they were compared with a manually sampled 45 year SST data set from Portobello Marine Laboratory, Dunedin. This investigation decomposed each SST series into a periodic seasonal component and an interannual trend. The latter component was further decomposed into low and mid-frequency bands using smoothing techniques. The results showed that the periodic component of offshore waters were in phase and reached their maximum temperature 10 days after the inshore neritic waters. The amplitudes of seasonal variations of Subantarctic and Subtropical water masses were similar while the neritic waters showed considerably higher seasonal variation. El Niño has a clear effect and lowers the temperatures of all water masses by 1.4ºC at Portobello and 2.2-3.4ºC for all offshore waters.