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Sea Ice

Sea Ice Mixes it Up

Source: Manucharyan, G. E., and A. F. Thompson (2017), Submesoscale Sea Ice-Ocean Interactions. J. Geophys. Res. Oceans, 122, doi:10.1002/2017JC012895

Ice and Climate

Sea ice plays an important role in the climate system by modulating interactions between the ocean and atmosphere. For example, sea ice shields the surface ocean from winds and reflects a lot of sunlight due to its white color. Through mechanisms such as this, sea ice affects the ocean’s uptake of heat and nutrients. Therefore, accurately predicting sea ice extent and thickness is crucial to understanding and modeling global climate.

Current climate models do a bad job of accounting for sea ice processes, particularly on small spatial scales and in marginal ice zones, which are regions of the ocean only partially covered by ice. Therefore, a new study uses a complex model to examine how small-scale ocean currents and mixing impact the distribution of sea ice in marginal ice zones. This type of research will improve climate models by helping unravel how sea ice is influenced by (and influences) the ocean and atmosphere.

Making Waves

Taking measurements in the polar regions, where sea ice is common, is difficult due to the remote location and harsh weather. Because observational data is so sparse, a group of scientists used an ice-ocean model to investigate small-scale sea ice processes.

Scientists collecting data in a marginal ice zone. Source: Katrine Claassens via Wikimedia Commons.

In order to begin running a model, you must initialize it by specifying the starting values for certain variables such as temperature and salinity. The researchers in this study initialized their model with conditions that are common in marginal ice zones, allowing them to better represent processes in that environment.

One of the focuses of this study was to see how eddies, which are circular currents, impacted sea ice distribution. The results showed that sea ice tended to accumulate in cyclonic eddies (counterclockwise flowing in the northern hemisphere or clockwise flowing in the southern hemisphere). This is because cyclonic eddies are associated with piling up of water due to winds. These same winds move sea ice, causing it to build up as well. Furthermore, the transport of sea ice to warmer, previously ice-free surface waters resulted in cyclonic eddies also being regions of high heat transfer between the ocean and ice. Conversely, anticyclonic eddies (flowing in the opposite direction) typically showed lower concentrations of ice and had less heat transfer.

Satellite image of a marginal ice zone. Surface ice distribution clearly reflects circular flowing eddies. Source: NASA via Wikimedia Commons.

Another result from the model used by the researchers in this study was that most of the ice melt and growth occurred at the ice edges. Melting of sea ice adds freshwater to the ocean, while sea ice formation injects salt. By changing the salinity of the surface water, these processes in turn affect the stratification of the upper ocean since the density of seawater depends on the amount of salt in it. Therefore, this study found that there were lots of rapid changes in salinity near the ice edges, and these instabilities resulted in increased mixing and turbulence.

In other words, when trying to understand the distribution of sea ice in marginal ice zones, it is important to consider both the transport of ice by small-scale currents and eddies as well as the mixing that is generated by ice melt and growth at the ice edges. This mixing, triggered by the ice itself, can also in turn play a role in transporting the ice and controlling its distribution. There are many feedbacks like this in the climate system.

Average monthly Arctic sea ice extent from 1979-2012, showing a significant decrease over the past several decades. Source: NSIDC via Wikimedia Commons

On Thin Ice

Sea ice extent and concentration is driven by a complex combination of circulation and mixing, as well as interaction with the atmosphere. Understanding these processes is critical to improving climate models. Studies such as this are particularly relevant since the past few decades have seen dramatic declines in Arctic sea ice. As global ocean temperatures continue to increase, break up and melting of sea ice will actually cause marginal ice zones (regions of partial ice coverage) to expand. Therefore, accurately describing the dynamics in these regions, through studies like this one, will become increasingly important to future climate predictions.

Channing Prend
I’m a physical oceanography PhD student at Scripps Institution of Oceanography in La Jolla, California. I use a combination of numerical models, observations, and remote sensing to investigate the role of the ocean in climate. I’m particularly interested in Southern Ocean dynamics, including air-sea-ice interactions and physical controls on biogeochemistry.

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