Long-term warming and record-breaking marine heatwaves in the Hauraki Gulf, northern New Zealand

As global oceans warm, long-term temperature records are critical in understanding and interpreting warming trends and the significance of marine heatwaves (MHWs) in coastal environments. Daily measurements of sea surface temperature (SST) have been taken since 1967 at the Leigh Marine Laboratory, Hauraki Gulf, northern New Zealand. We analyse long-term trends in SST anomalies and MHWs at Leigh from 1967 to 2023. We find a significant long-term warming trend in annual SST anomaly, with warming evident in autumn and winter, but not summer and spring. SST has been consistently and anomalously warm over the last decade, with 2022 being the warmest year in the 57-year record and having record high temperatures for six months. There were no long-term trends in the number of MHW days and cumulative intensity of MHWs annually, but break point analysis revealed a significant and sharp increase in both metrics since 2012. 2022 had the greatest number of MHW days (313 days) on record, including the two longest and cumulatively intense MHWs which had numerous unpredicted impacts on coastal ecosystems. The long-term data analysed here provides a unique long-term perspective on warming trends in the Hauraki Gulf and demonstrates the unprecedented nature of recent MHWs.