Winter Warmth Beneath the Waves: Marine Heatwaves Grip New Zealand’s Coasts
While frost coats paddocks and snow dusts the Southern Alps, an invisible heatwave is simmering beneath New Zealand’s coastal waters. Unintuitively for mid-winter, large swathes of the country’s marine environment are experiencing marine heatwave (MHW) conditions, with sea surface temperatures (SSTs) significantly above seasonal norms.
According to the MetService led Moana Project’s marine heatwave forecast, parts of the South Island and eastern North Island are currently seeing SSTs exceeding the 90th percentile of long-term averages—meeting the scientific threshold for a marine heatwave. These anomalies are not just statistical quirks; they’re part of a broader trend that’s reshaping Aotearoa’s marine ecosystems.
“Winter used to be a time when the ocean cooled and marine life got a seasonal reset,” says Dr. Carine Costa, a physical oceanographer with MetOcean Solutions. “Now, we’re seeing persistent warmth that’s disrupting that cycle.”
Figure 1(below): Moana Project’s heatwave forecast
Sea surface temperatures forecasted for the upcoming 7 days.
Difference between forecasted sea surface temperatures and the long-term average for this time of year. Red shading indicates that the ocean is currently warmer than normal; blue that it is colder.
The science behind the surge
Marine heatwaves are defined as prolonged periods—typically five days or more—of abnormally high sea temperatures (1). While they’ve become more common during summer months, their appearance in winter is a stark signal of a warming ocean. NIWA’s seasonal outlook confirms that SSTs around New Zealand have remained 0.5°C to 1.5°C above average through autumn and into winter, with hotspots persisting off the east coast of both islands.
This warmth is being driven by a combination of residual heat from previous seasons, weakened oceanic currents, and atmospheric patterns that favour calm, clear conditions—allowing the ocean’s surface to absorb and retain more solar energy.
Ripple effects on ecosystems and industries
The implications are far-reaching. Warmer waters can stress cold-adapted species, alter migration patterns, and trigger harmful algal blooms. A recent Fisheries New Zealand report highlights that marine heatwaves have already impacted species like green-lipped mussels, snapper, and king salmon, with potential knock-on effects for commercial fisheries and aquaculture.
With increased and more intense marine heat waves, previous techniques to fisheries management and catch levels may no longer be effective for some species and stocks. As coastal and ocean temperatures rise, wild fisheries should expect warmer water species to become more abundant, dominant, and widely distributed. Temperature-sensitive species may migrate south into colder seas. In aquaculture, MHWs can result in higher mortality and revenue losses. While ocean temperatures are the catalyst, other factors such as oxygen levels, food availability and the status of the entire ecosystem and food web play an important role in the health of our oceans (2).
Watching anomalies
As seas are getting warmer a forecast developed by the Moana Project will be shortly available to the public on SwellMap.com, showing the difference between forecasted and average sea surface temperatures for New Zealand waters.
This 7-day forecast shows a map of the difference between forecasted and average temperatures for that specific day of the year, with red colouring indicating warmer than usual conditions and blue colouring colder than usual conditions. Users can click on the map to see the exact temperature differences for any location.
Figure 2 (Above): The SwellMap sea temperature anomaly at surface map’ showing difference between actual and climatological temperatures.
A glimpse of the future
Climate models suggest that marine heatwaves will become more frequent, intense, and longer-lasting as global temperatures rise. Under high-emission scenarios, New Zealand could see up to 150 additional marine heatwave days per year by the end of the century.
For now, the winter warmth serves as a sobering reminder: climate change isn’t just a summer story. It’s unfolding year-round, above and below the surface.
Dr Brett Beamsley
Dr. Carine Costa
(1) Hobday et al. (2016)