What controls sea surface temperature variation at Leigh, New Zealand?

Abstract

The Leigh sea surface temperature (SST) time-series is one of the longest records of daily ocean temperatures in New Zealand. Established in 1967, this time-series not only provides great insight into temperature variation at Leigh, but its location on the north-east coast of New Zealand allows for the study of conditions within the Hauraki Gulf and around New Zealand’s Exclusive Economic Zone (EEZ). At a time where climate change is becoming a greater focus for scientists, policy-makers, and the public alike, studying the local impacts of large-scale atmospheric and oceanographic features such as El Niño Southern Oscillation (ENSO) is of great importance, especially so for the marine laboratory at Leigh. This thesis therefore investigates 43 years (1967-2009) of Leigh SSTs in order to determine the mechanisms that drive variation in ocean temperatures at Leigh. Previous studies (e.g. Stanton, 2001) have found that a link between ENSO, as measured by the Southern Oscillation Index (SOI), and New Zealand SSTs exists. The Leigh time-series confirms this with a highly significant correlation produced from running mean analysis of monthly means for Leigh SSTs and the SOI (r = 0.71, p = < 0.001). However, few studies have tried to explain exactly why Leigh SSTs correlate so well with the SOI. A correlation coefficient analysis between Leigh SSTs and satellite based measurements for New Zealand SSTs showed that Leigh is well correlated to the majority of SSTs in the New Zealand’s EEZ. The Leigh record is therefore used alongside other datasets to investigate the influences of mechanisms such as: heat flux, wind-driven Ekman transport and coastal upwelling/downwelling, Ekman pumping, and ocean advection; in order to explain the dominant controls on New Zealand SSTs. The results of these investigations discredit the roles of net heat flux and long-term coastal upwelling/downwelling in driving inter-annual variability in SST, find that ENSO forced alongshore (north-south) Ekman transport is strongly linked to Leigh SSTs, that ocean circulation associated with Ekman transport also corresponds with New Zealand SSTs, and that ocean advection has little influence on variability in Leigh SST. There is no simple answer to the title of this study. Multiple mechanisms are working across differing scales with varying degrees of influence to drive SST variation at Leigh and around New Zealand. However, the results from this study not only generate knowledge about how atmospheric and oceanic mechanisms influence coastal SSTs, but they also open up the possibility for future prediction of events at Leigh, within the wider Hauraki Gulf, and potentially throughout New Zealand’s EEZ.