Abstract:
The occurrence of marine heatwaves (MHWs) has increased globally, with further increases in frequency and intensity projected under anthropogenic climate change. Most research undertaken on MHWs focuses on recent events globally or in large regions such as Australia, the North Pacific, and South America, with smaller regions such as New Zealand largely overlooked. This research aims to evaluate the atmospheric and oceanic processes driving MHW events near New Zealand, including recent severe MHWs, such as the one in 2015/16. Marine heatwaves can impact glacier ice melting over the Southern Alps, influence marine species/ecosystems within New Zealand waters, and hinder regional economies (i.e. wine-grape maturation). This study focuses on three oceanic regions surrounding the country by analysing observations within three 5°x5° areas Northeast, West and Southeast of New Zealand.
Overall, the most severe events surrounding New Zealand occurred between 2016-2020 with anomalous temperatures up to ~2.4 °C warmer than normal. The Northeast exhibited the most prolonged duration and cumulative intensity of any MHW event during 1998, whereas for the other regions, the largest MHW events occurred during 2016-2019. Results show evidence of atmospheric influence on the intensity and duration of MHWs surrounding New Zealand: the top ten MHWs within each region occurred during a high-pressure system, and wind speeds were lower than average. Climate modes, such as the Southern Annular Mode (SAM) and ENSO (El Niño–Southern Oscillation), may be linked to the duration and intensity of certain events. The SAM index is further from average than the ENSO index for the top ten events around New Zealand. This agrees with previous studies of MHWs in the Tasman Sea that show a positive phase of SAM influenced the MHW during the 2017/18 summer. The Northeast region patterns are mainly linked to strong El Niño phases, especially the 1998 event, which proved to be the strongest event across all regions. Deeper ocean temperatures surrounding New Zealand also showed periods of warmer temperatures during the top MHWs within each of the regions. Further, the interaction between atmospheric and oceanic processes is shown to impact specific events' duration and intensity.