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Andrew F. Thompson
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Energetics of the Upper‐Ocean Under Sea Ice
Frictional Dissipation Versus Baroclinic Production
In polar regions, the presence of sea ice is known to reduce the ocean's eddy kinetic energy (EKE) by enhancing frictional dissipation at the surface. Here, using a coupled ocean—sea ice model, we discuss a mechanism that can instead increase EKE generation under sea ice and, in our simulations, compensates surface dissipation by 69% to 30% for sea ice concentrations varying between 30% and 100%. Mesoscale Ekman pumping and suction due to the sea ice cover give rise to vertical buoyancy fluxes in the core of mesoscale eddies, leading to rectified baroclinic energy production below the mixed layer. Additionally, in the shallow surface layer (~5 m), heterogeneous sea ice melt generates mixed layer eddies that enhance baroclinic production for conditions typical of the summer marginal ice zone (MIZ). EKE dissipation and production are both affected by the dynamics of the individual ice floes resolved in our model, which result in distinct patterns of sea ice aggregation around ocean eddies, a preference for anticyclonic floe rotation in the MIZ, and size-dependent melt. These results emphasize the tightly coupled nature of ocean—sea ice interactions, and the challenge in capturing them within coarse and continuum-based models.
Plain Language Summary
Eddies are swirling features of various sizes that are ubiquitous in the ocean. At the poles, these features may be damped by the presence of sea ice, which is a layer of frozen sea water that can form on the ocean surface and provide frictional damping to the underlying currents. This work uses numerical simulations representing a patch of the polar ocean to explore how pieces of ice influence ocean eddies, particularly when the sea ice cover is not fully compacted. The simulations show that sea ice indeed damps surface eddies, but also enhances their production rate by generating fine-scale circulation patterns in the upper ocean. These physics are likely not well represented by traditional climate models, which may bias sea ice melt rates and other critical polar ocean processes. This motivates the development of better numerical schemes that can help coarse models capture the detailed mechanisms noted in the high-resolution simulations used in this work. ...
Plain Language Summary
Eddies are swirling features of various sizes that are ubiquitous in the ocean. At the poles, these features may be damped by the presence of sea ice, which is a layer of frozen sea water that can form on the ocean surface and provide frictional damping to the underlying currents. This work uses numerical simulations representing a patch of the polar ocean to explore how pieces of ice influence ocean eddies, particularly when the sea ice cover is not fully compacted. The simulations show that sea ice indeed damps surface eddies, but also enhances their production rate by generating fine-scale circulation patterns in the upper ocean. These physics are likely not well represented by traditional climate models, which may bias sea ice melt rates and other critical polar ocean processes. This motivates the development of better numerical schemes that can help coarse models capture the detailed mechanisms noted in the high-resolution simulations used in this work. ...
In polar regions, the presence of sea ice is known to reduce the ocean's eddy kinetic energy (EKE) by enhancing frictional dissipation at the surface. Here, using a coupled ocean—sea ice model, we discuss a mechanism that can instead increase EKE generation under sea ice and, in our simulations, compensates surface dissipation by 69% to 30% for sea ice concentrations varying between 30% and 100%. Mesoscale Ekman pumping and suction due to the sea ice cover give rise to vertical buoyancy fluxes in the core of mesoscale eddies, leading to rectified baroclinic energy production below the mixed layer. Additionally, in the shallow surface layer (~5 m), heterogeneous sea ice melt generates mixed layer eddies that enhance baroclinic production for conditions typical of the summer marginal ice zone (MIZ). EKE dissipation and production are both affected by the dynamics of the individual ice floes resolved in our model, which result in distinct patterns of sea ice aggregation around ocean eddies, a preference for anticyclonic floe rotation in the MIZ, and size-dependent melt. These results emphasize the tightly coupled nature of ocean—sea ice interactions, and the challenge in capturing them within coarse and continuum-based models.
Plain Language Summary
Eddies are swirling features of various sizes that are ubiquitous in the ocean. At the poles, these features may be damped by the presence of sea ice, which is a layer of frozen sea water that can form on the ocean surface and provide frictional damping to the underlying currents. This work uses numerical simulations representing a patch of the polar ocean to explore how pieces of ice influence ocean eddies, particularly when the sea ice cover is not fully compacted. The simulations show that sea ice indeed damps surface eddies, but also enhances their production rate by generating fine-scale circulation patterns in the upper ocean. These physics are likely not well represented by traditional climate models, which may bias sea ice melt rates and other critical polar ocean processes. This motivates the development of better numerical schemes that can help coarse models capture the detailed mechanisms noted in the high-resolution simulations used in this work.
Plain Language Summary
Eddies are swirling features of various sizes that are ubiquitous in the ocean. At the poles, these features may be damped by the presence of sea ice, which is a layer of frozen sea water that can form on the ocean surface and provide frictional damping to the underlying currents. This work uses numerical simulations representing a patch of the polar ocean to explore how pieces of ice influence ocean eddies, particularly when the sea ice cover is not fully compacted. The simulations show that sea ice indeed damps surface eddies, but also enhances their production rate by generating fine-scale circulation patterns in the upper ocean. These physics are likely not well represented by traditional climate models, which may bias sea ice melt rates and other critical polar ocean processes. This motivates the development of better numerical schemes that can help coarse models capture the detailed mechanisms noted in the high-resolution simulations used in this work.
Marginal ice zones are composed of individual sea ice floes, whose breakage and melt influence its dynamical behavior. These processes are not well represented by global or regional climate models due to the continuum approximations and uncertainties regarding forcing, data resolution and parameterizations used for sea ice. Here, we use a Discrete Element Model (DEM) coupled to a slab thermodynamic ocean to investigate how breakage and melt processes impact the decay of summer sea ice. The DEM is calibrated using MODIS satellite imagery and reanalysis data within the Arctic Ocean's Baffin Bay during June–July 2018. The sensitivity of the sea ice decay is evaluated by varying the solar heating, the ice/ocean heat exchange parameter, and a prescribed floe breakage rate. For the parameter regime that best fits observations, the ratio of mass loss of resolved floes (diameter (Formula presented.) 2 km) due to breakage versus melt is 0.47, and oceanic versus solar melt is 0.46. The rate at which resolved floes lose mass is most sensitive to the breakage rate, as compared to the solar and oceanic melt parameters. The number decay of the largest floes (D (Formula presented.) 21 km) is controlled by breakage, whereas the decay of smaller floes (D = 2–21 km) depends strongly on lateral and basal melt. Inferences from this exploration of the parameter space may help motivate more accurate parameterizations of the floe size distribution evolution in climate models.
...
Marginal ice zones are composed of individual sea ice floes, whose breakage and melt influence its dynamical behavior. These processes are not well represented by global or regional climate models due to the continuum approximations and uncertainties regarding forcing, data resolution and parameterizations used for sea ice. Here, we use a Discrete Element Model (DEM) coupled to a slab thermodynamic ocean to investigate how breakage and melt processes impact the decay of summer sea ice. The DEM is calibrated using MODIS satellite imagery and reanalysis data within the Arctic Ocean's Baffin Bay during June–July 2018. The sensitivity of the sea ice decay is evaluated by varying the solar heating, the ice/ocean heat exchange parameter, and a prescribed floe breakage rate. For the parameter regime that best fits observations, the ratio of mass loss of resolved floes (diameter (Formula presented.) 2 km) due to breakage versus melt is 0.47, and oceanic versus solar melt is 0.46. The rate at which resolved floes lose mass is most sensitive to the breakage rate, as compared to the solar and oceanic melt parameters. The number decay of the largest floes (D (Formula presented.) 21 km) is controlled by breakage, whereas the decay of smaller floes (D = 2–21 km) depends strongly on lateral and basal melt. Inferences from this exploration of the parameter space may help motivate more accurate parameterizations of the floe size distribution evolution in climate models.