The Arctic frequently experiences temperatures that support the formation of mixed-phase clouds that contain supercooled liquid droplets and ice crystals. The composition of such clouds plays a crucial role in the region’s energy balance and climate system. Clouds with more liquid last longer and reflect more sunlight than those with more ice crystals.

With Arctic warming, meteorologists have been interested in determining the effect of rising temperatures on cloud composition and its broader effect on the region. Climate models generally predict that as the Arctic warms, clouds in the region will contain more liquid water and less ice, since warmer temperatures typically suppress the formation of ice crystals.

However, cloud formation is also influenced by the presence of aerosols which act as seeds, both for the condensation of liquid droplets and the formation of ice crystals.

In a study published in Communications Earth & Environment, on 18 September 2024, researchers led by Associate Professor Yutaka Tobo from the National Institute of Polar Research, Japan, investigated the relationship between rising surface air temperatures and aerosols known as ice-nucleating particles (INPs), which are known to promote ice crystal formation in clouds.

They found that surface warming in the Arctic leads to an increase in snow and ice-free areas, which release higher amounts of active INPs. These INPs can induce ice formation in clouds, reducing the liquid water content in mixed-phase clouds and potentially accelerating further warming.

“We found that the INPs tended to increase exponentially with rising surface air temperatures when the temperatures rose above 0°C and snow/ice-free barren areas and vegetated areas appeared in Svalbard, a region experiencing warming five to seven times faster than the global average,” says Associate Professor Tobo.

The observations are based on year-round measurements of INPs taken during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) project period from September 2019 to early-October 2020 at the Zeppelin Observatory in Svalbard. To identify the number of INPs, the researchers collected aerosol samples and used an established droplet-freezing method, where the samples were exposed to cold conditions to see if they could form ice.

They observed that the number of INPs increased during the warmer months (mid-April to September), when surface air temperatures were above 0°C. Using scanning electron microscopy with energy-dispersive X-ray analysis, they found that the INPs observed in the warmer months were mainly mineral dust and carbonaceous particles, resembling microorganisms or plant debris.

So, where did these aerosols come from? The Normalized Difference Vegetation Index (NDVI) data, which indicates vegetation density, revealed that in the summer, about 35% of Svalbard had positive NDVI values between 0 and 0.5, indicating snow-free barren areas like glacial outwash plains and vegetated areas with grasses, mosses, and lichens. The findings suggest that the INPs are the dust and biological organic aerosols, such as microorganisms or plant debris, released from these regions.

The findings are concerning because winter warming trends are even more severe than in summer, with temperatures increasing by more than 2°C per decade in Svalbard. As snow and ice-free areas become more common in the Arctic winter in the coming decades, INP emissions will likely increase, changing the composition of mixed-phase clouds.

“Our results suggest the possibility that the supply of highly active INPs from high-latitude terrestrial sources will increase in response to the projected surface warming, and thus, this effect needs to be considered in climate models to improve our understanding of the phase transition scenario of Arctic mixed-phase clouds,” concludes Associate Professor Tobo.

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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