Tropical cyclone intensity exacerbated by increasing depth of ocean mixed layer, finds study

Tropical cyclones can have severe consequences for both the marine and terrestrial environments, as well as the organisms and communities who inhabit them. In the oceans, there can be alterations in sea surface temperature that disrupt biological processes and hospitable conditions for life, the devastation of surface algae and other primary producers, which impacts complex marine food chains, as well as damaging coral reefs. Meanwhile, on land, the heavy rainfall, strong winds and storm surges can lead to significant damage to property and infrastructure, as well as loss of lives.

These natural phenomena are powered by warm surface waters, as the rising water vapor causes condensation of water droplets, and thus cloud formation and rain. This releases heat, warming the atmosphere further and causing the air to continue to rise, bringing in cooler air towards the base, which we experience as strong winds. Consequently, as tropical cyclones move over land they lose this initial energy source and eventually dissipate.

Therefore, the surface layer of the ocean is particularly important. Recent research published in Frontiers in Marine Science has investigated how the depth of the mixed layer (the deepest layer affected by surface turbulence and separating cooler ocean depths from atmospheric interactions) impacts ocean temperatures, and subsequently tropical cyclone formation.

To do so, Yalan Zhang, of China’s National University of Defense Technology, and colleagues used models to simulate different ocean mixed layer depth (2 m, 5 m, 10 m, 15 m, 20 m, 50 m and 100 m) influences on tropical cyclones in the western North Pacific over four days, in both one and three dimensions. The former model type focuses mostly on the influence of depth, while the latter incorporates heat, salinity and water mass movement (for example, upwelling).

The researchers found that ocean mixed layer depth only has a small influence on the track the tropical cyclone takes, with slower translation speeds resulting from shallower ocean mixed layer depth moving the center of the tropical storm. However, they discovered a greater impact on the size and intensity of the event, reaching its peak 72 to 84 hours after initiation.

Importantly, this is only the case up to 15 m water depth, after which the ocean mixed layer depth prior to the tropical cyclone has marginal influence on the destructiveness of the event. The destructive potential increased 325.2% when the ocean mixed layer depth reached 5 m, reducing to 50% at 15 m and below 15% at depths thereafter.

This is because surface winds bring cold water from below the ocean mixed layer depth when it is shallower than 15 m, which decreases the temperature of the upper ocean. In fact, the scientists suggest 75% to 90% of sea surface cooling can be attributed to turbulence from wind-induced vertical shear (the change in wind speed and direction with altitude).

However, as the ocean mixed layer depth increases beyond this threshold point of 15 m, the effect of surface winds on sea surface temperature cooling is reduced, leading to increasing surface temperatures below the tropical cyclones, therefore fueling their development.

Furthermore, the passage of multiple tropical cyclones through the same area can cause the ocean mixed layer depth to deepen, which may reduce their future activity in that region, though the timescales between events to allow this are still being studied.

This research is significant, as global warming is likely to exacerbate tropical cyclone occurrences due to rising sea surface temperatures, so the role of ocean mixed layer depth in modulating these is paramount to understanding these phenomena of the marine realm and allowing populations to mitigate against their devastation in vulnerable regions.

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