ASPI

The Strategist

By Quentin Comminsoli*

29 January 2025

Source: https://www.aspistrategist.org.au/marine-heatwaves-a-rising-challenge-for-naval-warfare/

We now know that rising sea temperatures will affect sonar performance, sometimes greatly affecting submarines’ ability to find ships and other submarines, and ships’ ability to find them. This leaves us wondering about the specific effects of another phenomenon: marine heatwaves, which can create large and sudden changes in temperatures.

Navies need to know how these events will affect them. It’s a high priority for research.

The effect of rising ocean temperatures on sound propagation and therefore sonar performance was revealed in a landmark study published in the Texas National Security Review in the spring of 2024. In colder waters, such as those in the North Atlantic near the Bay of Biscay, passive sonar ranges could shrink dramatically due to changes in thermal stratification and salinity. In one example for that location, detection range would fall from 60 kilometres to less than 20 kilometres. 

But in warmer, shallower waters in the Western Pacific, sonar effectiveness could rise, making North Korean submarines, for example, more detectable.

Marine heatwaves are defined as periods when sea temperatures exceed the seasonal 90th percentile for at least five consecutive days. Unlike gradual warming, heatwaves create sudden and intense thermal anomalies that disrupt the ocean’s thermal layers, presumably affecting acoustic wave propagation, perhaps strongly. 

Moreover, these events can extend to several hundred meters in depth, and their frequency has more than doubled since 1982. In fact, Australia will experience intense marine heatwaves in the years ahead, say Australian government researchers.

Thermal layers act as a natural barrier in the ocean. They reflect sound waves, somewhat protecting submarines below them from detection by sonars above. When the water warms, such as during marine heatwaves, these layers can shift unpredictably or weaken. Shallow submarines become easier to detect, for example.

Conceivably, an otherwise undetectable submarine would become detectable. From the point of view of the Royal Australian Navy, for example, it may be a Chinese submarine that can now be observed and tracked. Or it could be an Australian one that’s now tracked by a Chinese sonar—in another submarine, in a ship, or mounted on the sea floor.

Either way, navies had better find out what the effect will be.

Marine heatwaves are not just increasing in frequency; they are intensifying due to rising global temperatures and shifting climate patterns. For example, during the El Nino episode of 2015 and 2016, thermal anomalies in the western tropical Pacific reached depths of 150 metres, with deviations of up to 8.9 degrees C. These anomalies disrupted subsurface conditions for months, a phenomenon likely to recur with even greater intensity in the near future.

The Western Pacific, a location of global trade routes and intense submarine operations, is particularly susceptible.

Naval planners must account for marine heatwaves, just as they are beginning to do for gradual ocean warming. Such tools as Mercator Ocean International’s marine heatwave bulletins provide data on sea temperature anomalies. Navies use sophisticated oceanographic monitoring systems, but these are not specifically designed for predicting marine heatwaves.

Also, there is evidence that marine heatwaves at deep levels are underreported, raising doubts about the reliability of civilian prediction models.

Technological solutions are available to address some data-collection challenges. Autonomous underwater gliders equipped with thermometers and salinity sensors can provide near-instantaneous data on thermal anomalies, so their data can be used for planning before a ship or submarine enters a zone. 

Sensor networking and quantum sensing technologies offer promising avenues for monitoring effects of marine heatwaves on acoustic detection; operators can respond with sonar adjustment to mitigate effects.

Uncertainty over the sonar effects of marine heatwaves is amplifying strategic risks: operations plans may prove to be ineffective during marine heatwaves, or the other side’s operations may become suddenly and unexpectedly more effective.

The study of marine heatwaves will require a multidisciplinary approach, combining climate science, oceanography and military strategy. The result will be, first, an understanding of the events and their effects then, second, adaptation of technologies and tactics to cope with them.

* Quentin Comminsoli is a Sciences Po Aix graduate in geostrategy with a strong interest in climate security. He has lived in New Caledonia for 10 years.

Image of RAN submarine HMAS Waller: Jarryd Capper via Department of Defence.