Regional sea level trends in New Zealand

Abstract

In terms of sea level data sets able to be used for long‐term sea level trend analysis, the Southern Hemisphere is a data sparse region of the world. New Zealand lies in this region, presently having four (major port) data sets used for such trend analysis. This paper describes the process followed to compute new sea level trends at another six ports, each with very discontinuous tide gauge records. In each case the tide gauge has previously only been used for precisely defining an historical local Mean Sea Level (MSL) datum. The process used involved a comparison of the old MSL datum with a newly defined datum obtained from sea level data covering the last decade. A simple linear trend was fitted between the two data points. Efforts were then made to assess possible bias in the results due to oceanographic factors such as the El Niño–Southern Oscillation (ENSO) cycle, and the Interdecadal Pacific Oscillation (IPO). This was done by taking the longer time series from the four major ports and assessing the spatially coherent variability in annual sea level using the dominant principal component from an empirical orthogonal function (EOF) analysis. The average relative sea level rise calculated from these six newly derived trends was 1.7 ± 0.1 mm yr−1, a result that is completely consistent with the analysis of the long‐term gauge records. Most importantly, it offers a relatively simple method of improving our knowledge of relative sea level trends in data sparse regions of the world.