The latest NIWA climate assessment shows that sea surface temperatures in coastal waters around New Zealand are well above average. Marine heatwave conditions are already occurring in parts of the Tasman Sea and the ocean around New Zealand and looking to become the new normal.
What's in a name
Currently, marine heatwaves are defined as periods that last for five or more days with temperatures warmer than the 90th percentile based on a 30-year historical baseline. Given we are likely to experience many more such events as the oceans continue to warm, it is time to understand and categorise the intensity of marine heat.
The names Hurricane Katrina, tropical cyclone Giselle (which sank the ferry Wahine 50 years ago), tropical cyclone Winston give a malevolent personality to geophysical phenomena. Importantly they get graded into categories, so we can rapidly assess their potential impact.
An Australian team has developed a classification scheme for marine heatwaves. The team used an approach similar to that used for hurricanes and cyclones – changing conditions can be slotted into to a sequence of categories.
At the moment it looks like we are in marine heat wave category one conditions, but potentially entering category two if it continues to warm
The Great Barrier Reef has already been hit hard by a succession of marine heatwave events, bleaching the iconic corals and changing the structure of the ecosystem it supports.
Further south, off Tasmania's east coast, a number of species that normally occur in tropical waters have extended their range further south. A number of fish species, lobster and octopus species have also taken up residence along the Tasmanian coast, displacing some of the species that call this coast home. Mobile species can escape the warmer temperatures, but sedentary plants and animals are hardest hit.
As important as it is to identify a marine heatwave at the time, reliable predictions of developing conditions would help fishers, aquaculture companies and local authorities – and in fact anyone living and working around the ocean.
One of the important points to keep in mind is that when we are at the beach, we are sampling only the surface temperature. The same is true of satellites – they monitor less than the top millimetre of the ocean.
Sea surface temperatures are several degrees above normal at the moment. But in deeper waters, because of the high heat content of water, even a tenth of a degree is significant. Temperature in the deeper ocean is monitored by a network of moored buoys on and off the continental shelf along the Australian coast. New Zealand has almost nothing that would be comparable.
Measuring temperature in real time
What we can look to, in the absence of moored buoys, is a fleet of ocean robots that monitor temperature in real time. Argo floats drift with ocean currents, sink to two kilometres every ten days and then collect data as they return to the surface.
These data allowed us to identify that the 2017/18 marine heatwave around New Zealand remained shallow. Most of the warmer water was in the upper 30 metres. Looking at the present summer conditions, one Argo robot off New Zealand’s west coast shows it is almost four degrees above normal in the upper 40 metres of the ocean. On the east coast, near the Chatham Islands, another float shows warmed layers to 20 metres deep. To the south, the warming goes deeper, down to almost 80 metres.
The research on categories of marine heatwaves shows we will have to keep shiftingwhat we regard as a heat wave as the ocean continues to warm. None of this should come as a surprise. We have known for some time that the world’s oceans are storing most of the additional heat and the impacts of a warming ocean will be serious.