Seasonal, interannual, and decadal variability of storm surges at Tauranga, New Zealand

Abstract

A database of storm surge events was constructed for two sites in south‐eastern Tauranga Harbour, New Zealand, for the period 1960‐mid 1998. Storm surge events were defined as occasions when the residual level between the predicted high tide level and recorded water level exceeded 10 cm. The residual was determined at high tide only (every 12.4 h), with 954 storm surge events found over the 38.4‐year period analysed. The magnitude and frequency of storm surge events varied considerably between 1960 and 1997, withamarked shift evident c. 1976. The period from 1976 to 1997 corresponded to a reduced storm surge frequency and magnitude, compared to the period 1960–76. Wavelet analysis of 125 years of wind storm annual frequencies showed strong fluctuations at inter‐decadal periods. Therefore, it is suggested that the frequency of storm surges varies in response to a coherent inter‐decadal oscillation in surface temperature over the Pacific Ocean, known as the Inter‐decadal Pacific Oscillation (IPO), that reversed phase c. 1976. The El Nino Southern Oscillation (ENSO) also affected the number of days exceeding the storm surge threshold per year, with La Nina events being associated with more storm surge days. The presence of significant decadal variations indicates that annual exceedence probability distributions may misrepresent the storm surge hazard. The available data indicates that there are extended periods when the IPO increases the hazard, and others when the hazard is decreased. Existing analyses of storm surge hazard for the Bay of Plenty have largely been based on data obtained during a period of reduced hazard. Conditions that were associated with larger and more frequent storm surges during 1960–76 may be expected to prevail again over the next few decades.