Scientists study how to predict marine heatwaves

THURSDAY, MAY 16, 2019

Scientists have taken a step closer to predicting marine heatwaves with new
NIWA-led research finding a link between their formation and the length of time sea temperatures are warmer than normal.

For the past two summers, the Tasman Sea has experienced a marine heatwave, where periods of extremely warm sea surface temperatures persisted for a prolonged period of time and extended thousands of kilometres.

They are potentially devastating for marine ecosystems but are expected to become more frequent under a range of climate change scenarios. Recent studies have shown they also have a strong influence on climate, temperature and rainfall patterns.

NIWA scientist Dr Erik Behrens is the lead author of a new research paper showing that ocean currents in the Tasman Sea are controlling the oceanic heat content and marine heat waves in the Tasman Sea. The paper has just been published in the journal Frontiers in Marine Science, which has dedicated the entire issue to marine heatwave findings.

Dr Behrens says marine heatwaves are difficult to predict because there are numerous factors influencing how and when they occur.
However, using scientific models in combination with observations from satellites and autonomous ocean drifters, he was able to show a relationship between changes in heat content in the Tasman Sea and the size and intensity of marine heatwaves over years and decades.

“Measuring ocean heat content is like measuring fever but in the ocean and tells you a lot about its wellbeing. If the ocean is already warm, the likelihood for marine heatwaves is enhanced because less surface heating is required to drive temperature extremes.”

Dr Behrens’ research showed heat fluctuations in the Tasman Sea are dominated by more warm water being transported from the subtropics by the East Australian Current and Tasman Front with the heat content acting as a preconditioner for the development of marine heatwaves.

“The flow of warm water from the subtropics varies a lot, which results in warmer and colder years. But on average the Tasman Sea is a region of oceanic heat convergence, meaning that average surface heat fluxes are directed toward the atmosphere, balancing the oceanic heat convergence. On average around 67% of the ocean heat transported from the subtropics into the Tasman Sea warms the atmosphere” Dr Behrens said.

There are at least two different mechanisms affecting the occurrence of marine heatwaves in the Tasman Sea: heat content which is persistent and enables long-term predictability, and winds and surface heating which is harder to predict, since they vary on shorter time scales.

“Periods with enhanced heat content or when ocean heat content increases rapidly, increase the likelihood of marine heatwaves developing. However, if winds are strong during the same time then the marine heat waves are milder.”

Marine heatwave conditions continued in the Tasman Sea throughout April. NIWA forecasters predict these warmer than average seas may lead to a reduction in the intensity and duration of cold spells over winter.

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