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

A database of storm surge events was constructed for two sites in south-eas tern Tauranga Harbour, New Zealand, for the period 1960-mid 1998. Storm sur ge events were defined as occasions when the residual level between the pre dicted high tide level and recorded water level exceeded 10 cm. The residua l was determined at high tide only (every 12.4 h), with 954 storm surge eve nts found over the 38.4-year period analysed. The magnitude and frequency o f storm surge events varied considerably between 1960 and 1997, with a mark ed shift evident c. 1976. The period from 1976 to 1997 corresponded to a re duced storm surge frequency and magnitude, compared to the period 1960-76. Wavelet analysis of 125 years of wind storm annual frequencies showed stron g fluctuations at inter-decadal periods. Therefore, it is suggested that th e frequency of storm surges varies in response to a coherent inter-decadal oscillation in surface temperature over the Pacific Ocean, known as the Int er-decadal Pacific Oscillation (IPO), that reversed phase c. 1976. The El N ino Southern Oscillation (ENSO) also affected the number of days exceeding the storm surge threshold per year, with La Nina events being associated wi th more storm surge: days. The presence of significant decadal variations i ndicates that annual exceedence probability distributions may misrepresent the storm surge hazard. The available data indicates that there are extende d 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 ha ve largely been based on data obtained during a period of reduced hazard. C onditions that were associated with larger and more frequent storm surges d uring 1960-76 may be expected to prevail again over the next few decades.