Why researchers studied marine heat waves more than 200m below the ocean surface

Just as heat waves refer to a higher-than-average temperature prevailing on land, Marine Heat Waves (MHWs) are defined by the unusual warming of ocean waters.

In recent years, global warming has made MHWs more frequent and intense, but a new study now says this phenomenon is witnessed well beyond the surface.

MHWs have typically been measured by observing temperatures at the ocean surface. As one goes deeper, the amount of sunlight penetrating the water decreases. Coupled with high sea pressure, humans have found deep-ocean exploration extremely challenging.

The “twilight zone” of oceans, located between 200 and 1,000 metres, still has some visibility, but research on the domain is lacking. The study (‘Common occurrences of subsurface heatwaves and cold spells in ocean eddies’) published in Nature on Wednesday (October 16) saw researchers from China and Australia analyse MHWs and Marine Cold Spells in this region.

What have researchers found about heat waves in oceans?

Researchers found that MHWs deep in oceans may be “significantly under-reported”. Ming Feng, the Senior Principal Research Scientist at the Australian government agency CSIRO, was one of the study authors. In an article in The Conversation, he wrote about how in the deep ocean, atmospheric factors are not responsible for temperature changes (unlike MHWs). Instead, eddy currents play a major role.

Feng described them as “huge loops of swirling current, sometimes hundreds of kilometres across and reaching down over 1,000 metres”. Eddies carry warm or cold water across long distances.

To record the temperature changes at that depth, long-term moorings – measurement buoys suspended at depth – were deployed across the world’s oceans for the study. Argo floats, which are robotic divers that can go 2,000 metres deep and resurface, were used to sample temperature and salinity.

What is the relevance of the finding?

For one, it shows that global warming is impacting not just the surface ocean temperatures, but also the deep waters through eddies. Feng wrote, “Our research suggests eddy currents are acting to magnify the warming rates of marine heatwaves and the cooling rate of the cold spells. Warmer oceans overall are leading to stronger eddy currents”.

Extreme temperature changes in the temperature of the twilight zone are also a matter of concern, since many fish species and plankton reside here. Planktons form the base of the oceanic food chain and are a food source for small fish. “Heat and cold brought by eddies aren’t the only threat to the twilight zone. Marine heatwaves can lead to low oxygen levels in the water and reduced nutrients,” Feng also wrote.