Indian Ocean Dipole Effects on the Wave Climate of New Zealand

Is the El Nino-Southern Oscillation (ENSO) really the great villain of the Southwestern Pacific Ocean? ENSO effects are correlated to many remarkable events across the globe, and hence, governments and local authorities have focused their attention on this oscillation mode. However, other climate patterns could play a similar role in certain areas and have not been considered in such detail. The modulation of the wave climate of New Zealand by the Indian Ocean Dipole (IOD) is investigated in this work using results of a 45-year (1957-2002) wave hindcast and the wavelet spectral analysis. Firstly, a cluster analysis was conducted in order to separate regions with different wave climates. Then, the modelled significant wave height (Hs) was correlated with the Dipole Mode Index (DMI) at the central geographical location of each cluster using the wavelet technique. Strong correlations of the DMI with Hs were observed in all clusters. The strongest correlation was found in the western cluster with cycles of approximately four years from the early 1970s to late 1990s. Combined effects resultant from opposite phases of the IOD and ENSO led to larger waves during extreme events of the modes, such as in 1972 and 1997. This shows the importance of taking into consideration a range of atmospheric oscillations in the assessment of wave conditions. Finally, the IOD negative phase is shown to have an effect on the wave climate to the north of New Zealand, whereas the rest of the country is more affected by the positive phase of the mode.