Marine heatwaves around Aotearoa: La Niña and Climate Change

Figure 1: The bull kelp (Durvillaea spp) die-off observed at Punakaiki. March (left) and September (right) 2022. (Images credit: Department of Conservation)

El Niño Southern Oscillation (ENSO) global climate phenomenon has an important influence on climate and ocean dynamics around Aotearoa New Zealand. Between 2020 and 2023, the world experienced an unusually long period of La Niña conditions. During La Niña conditions ocean temperatures are expected to increase around Aotearoa. However, with the ongoing rising temperatures worldwide there is a new uncertainty regarding the local and regional impact and responses to ENSO events.  

In 2022, a marine heatwave (unusually warm ocean temperatures) in Aotearoa New Zealand killed 13,000 tonnes of salmon at a Malborough Sounds farm forcing the country's largest salmon producer to close farms and let go of staff. The oceanographic division of MetService has been undertaking marine heatwave analysis since 2020, developed through the Moana Project. Over 2021-2023, the twelve regions included in the Moana Project experienced marine heatwaves that last 100 days or longer, with Bay of Plenty holding the record with 381 days of continuous marine heatwave conditions.  

Over 2024, following the transition in 2023 to El Niño dominant conditions, our surrounding oceans have experienced fewer occurrences of marine heatwaves relative to the 2020-2023 period. However, last week NOAA announced that El Niño Southern Oscillation (ENSO) neutral conditions are currently present with a 70% chance of La Niña conditions favoured to develop during August-October and persist into late 2024 early 2025 (79% chance during Nov-Jan).  A switch to the predicted La Niña conditions would suggest that more - and probably stronger - marine heatwaves are to be expected later this year and through summer.  

Dr. Mireya Montano

Figure 2: Punakaiki Marine Reserve Heatwaves October 2021 - July 2023