The strongest ocean currents are heating up

Over the next 30 years, the intensity of heatwave events in some of the world’s strongest ocean currents is expected to be greater than the global average, according to Australian research.

Climate modelling from the ARC Centre of Excellence for Climate Extremes and CSIRO pinpoints areas that will experience above-average warming, including the Leeuwin Current and East Australian Current in Australian waters, the Gulf Stream (North Atlantic), the Kuroshio Current east of Japan and the Antarctic Circumpolar Current.

The research – published in the journal Nature Communications – also indicates that while heatwave days in these regions will increase in intensity, the number of them each year may lag behind the global average pace of increase.

“Previous studies were based on projections of global climate models whose spatial resolutions are too coarse to be practical for risk assessment and adaption planning for the local industry,” says lead author Hakase Hayashida, from the ARC Centre of Excellence for Climate Extremes.

“However, our results show there are remarkable regional differences, especially in areas where small-scale features play a fundamental role in the variability of the ocean dynamics.”

“For instance, we found intense marine heatwaves were more likely to form well off the coast of Tasmania, while along the Gulf Stream more intense marine heatwaves start to appear more frequently close to the shore, along the stretch of coastline from the [US] state of Virginia to [Canadian province] New Brunswick.”

To pinpoint these areas, Hayashida and colleagues used two near-global high-resolution simulations over current and future periods that could reproduce eddies – the circular movements of water causing whirlpools – 100km across, and from that generate boundary currents and fronts. All model projects were driven by the high-carbon-emission scenario.

Their simulations showed that more intense and frequent marine heatwaves appeared in every western boundary current they examined.

However, on the edge of these currents the eddies that spun off from the main flow created areas where the increase in the number of heatwave days was lower than the average. In some areas, the intensity even decreased.

The accuracy of the model was confirmed by comparing the model outputs with observations from 1982–2018. From this model, the researchers projected how marine heatwaves would alter with climate change up until 2050.

“Ideally, even higher resolutions (than 0.1 degree adopted in our high-resolution ocean model) are desirable from fisheries and aquaculture management perspectives, because these occur at a much finer scale,” Hayashida says.

The findings highlight that the effects of marine heatwave events – like many other aspects of the climate system – will not be felt the same everywhere, which has profound implications for fisheries and aquaculture.

“Seafood, for example, is very sensitive to ocean temperature changes. Extreme warming events such as marine heatwaves could damage those industries,” Hayashida says.

“Fisheries and aquaculture management would therefore want to know information such as where would be a good location to establish a new fish farm that has lesser influence of marine heatwave intensification.”

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