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

Tropical cyclones can have serious impacts on marine and terrestrial environments, and on the organisms and communities that inhabit them. In the oceans, changes in sea surface temperature can disrupt biological processes and conditions that support life, destroy surface algae and other primary producers, impacting complex marine food chains, and damage coral reefs. Meanwhile, on land, heavy rains, strong winds, and storm surges can cause significant damage to property and infrastructure, as well as loss of life.

These natural phenomena are fueled by warm surface waters, as rising water vapor causes water droplets to condense, forming clouds and rain. This releases heat, further warming the atmosphere and causing the air to rise, bringing cooler air to the base, which we feel as strong winds. Therefore, as tropical cyclones move over land, they lose this initial source of energy and eventually dissipate.

The surface layer of the ocean is therefore particularly important. Recent research published in The Frontiers of Marine Science studied how the depth of the mixed layer (the deepest layer affected by surface turbulence and separating the colder ocean depths from atmospheric interactions) affects ocean temperatures and, subsequently, the formation of tropical cyclones.

To do this, Yalan Zhang of the National University of Defense Technology of China and his colleagues used models to simulate the influences of different depths of ocean mixed layers (2 m, 5 m, 10 m, 15 m, 20 m, 50 m and 100 m) on tropical cyclones in the western North Pacific over four days, in one and three dimensions. The first type of model focuses mainly on the influence of depth, while the second incorporates heat, salinity and the movement of water masses (e.g. upwelling).

The researchers found that the depth of the ocean mixed layer has only a small influence on the tropical cyclone’s track, with translation speeds being slower due to the shallower depth of the ocean mixed layer that moves the center of the tropical storm. However, they found a larger impact on the size and intensity of the event, peaking 72 to 84 hours after onset.

It is important to note that this phenomenon only occurs up to a depth of 15 m, after which the depth of the ocean mixed layer prior to the tropical cyclone has only a marginal influence on the destructiveness of the event. The destructive potential increased by 325.2% when the depth of the ocean mixed layer reached 5 m, decreasing to 50% at 15 m and to less than 15% at subsequent depths.

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

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

Additionally, the passage of multiple tropical cyclones in the same area can cause a deepening of the oceanic mixed layer, which may reduce their future activity in that region, although the time scales between events allowing this are still under study.

This research is important because global warming is likely to exacerbate the occurrence of tropical cyclones due to rising sea surface temperatures. The role of ocean mixed layer depth in modulating these phenomena is therefore crucial to understanding these marine phenomena and enabling populations to mitigate their devastation in vulnerable regions.

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