Abstract Commonly used parameterization of mixed layer instabilities in general circulation models was developed for temperate oceans and does not take into account the presence of sea ice in any way. However, the ice–ocean drag provides a strong mechanical coupling between the sea ice and the surface ocean currents and hence may affect mixed layer restratification processes. Here we use idealized simulations of mixed layer instabilities to demonstrate that the sea ice dramatically suppresses the eddy-driven overturning in the mixed layer by dissipating the eddy kinetic energy generated during instabilities. Considering the commonly used viscous-plastic sea ice rheology, we developed an improvement to the existing mixed layer overturning parameterization, making it explicitly dependent on sea ice concentration. Below the critical sea ice concentration of about 0.68, the internal sea ice stresses are very weak and the conventional parameterization holds. At higher concentrations, the sea ice cover starts acting as a nearly immobile surface lid, inducing strong dissipation of submesoscale eddies and reducing the intensity of the restratification streamfunction up to a factor of 4 for a fully ice-covered ocean. Our findings suggest that climate projection models might be exaggerating the restratification processes under sea ice, which could contribute to biases in mixed layer depth, salinity, ice–ocean heat fluxes, and sea ice cover.
more »
« less
Heavy footprints of upper-ocean eddies on weakened Arctic sea ice in marginal ice zones
Arctic sea ice extent continues to decline at an unprecedented rate that is commonly underestimated by climate projection models. This disagreement may imply biases in the representation of processes that bring heat to the sea ice in these models. Here we reveal interactions between ocean-ice heat fluxes, sea ice cover, and upper-ocean eddies that constitute a positive feedback missing in climate models. Using an eddy-resolving global ocean model, we demonstrate that ocean-ice heat fluxes are predominantly induced by localized and intermittent ocean eddies, filaments, and internal waves that episodically advect warm subsurface waters into the mixed layer where they are in direct contact with sea ice. The energetics of near-surface eddies interacting with sea ice are modulated by frictional dissipation in ice-ocean boundary layers, being dominant under consolidated winter ice but substantially reduced under low-concentrated weak sea ice in marginal ice zones. Our results indicate that Arctic sea ice loss will reduce upper-ocean dissipation, which will produce more energetic eddies and amplified ocean-ice heat exchange. We thus emphasize the need for sea ice-aware parameterizations of eddy-induced ice-ocean heat fluxes in climate models.
more » « less- Award ID(s):
- 1829969
- NSF-PAR ID:
- 10381749
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 13
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract Marginal ice zones are composed of discrete sea‐ice floes, whose dynamics are not well captured by the continuum representation of sea ice in most climate models. This study makes use of an ocean large eddy simulation (LES) model, coupled to cylindrical sea‐ice floes, to investigate thermal and mechanical interactions between melt‐induced submesoscale features and sea‐ice floes, during summer conditions. We explore the sensitivity of sea‐ice melt rates and upper‐ocean turbulence properties to floe size, ice‐ocean drag, and surface winds. Under low wind conditions, upper ocean turbulence transports warm cyclonic filaments from the open ocean toward the center of the floes and enhances their basal melt. This heat transport is partially suppressed by trapping of ice within cold anticyclonic features. When winds are stronger, melt rates are enhanced by the decoupling of floes from the cold, melt‐induced lens underneath sea ice. Distinct dynamical regimes emerge in which the influence of warm filaments on sea‐ice melt is mitigated by the strength of ice‐ocean coupling and eddy size relative to floe size. Simple scaling laws, which may help parameterize these processes in coarse continuum‐based sea‐ice models, successfully capture floe melt rates under these limiting regimes.
-
more » « less
Abstract The mechanisms that control the export of freshwater from the East Greenland Current, in both liquid and solid form, are explored using an idealized numerical model and scaling theory. A regional, coupled ocean–sea ice model is applied to a series of calculations in which key parameters are varied and the scaling theory is used to interpret the model results. The offshore ice flux, occurring in late winter, is driven primarily by internal stresses and is most sensitive to the thickness of sea ice on the shelf coming out of Fram Strait and the strength of alongshore winds over the shelf. The offshore liquid freshwater flux is achieved by eddy fluxes in late summer while there is an onshore liquid freshwater flux in winter due to the ice–ocean stress, resulting in only weak annual mean flux. The scaling theory identifies the key nondimensional parameters that control the behavior and reproduces the general parameter dependence found in the numerical model. Climate models predict that winds will increase and ice export from the Arctic will decrease in the future, both of which will lead to a decrease in the offshore flux of sea ice, while the influence on liquid freshwater may increase or decrease, depending on the relative changes in the onshore Ekman transport and offshore eddy fluxes. Additional processes that have not been considered here, such as more complex topography and synoptic wind events, may also contribute to cross-shelf exchange.
Significance Statement The purpose of this study is to provide a basic understanding of what controls the flux of sea ice and low-salinity water from the East Greenland shelf into the interior of the Greenland and Iceland Seas. This is a potentially important process since it has been shown that sufficient freshening of the surface waters in the interior of the Nordic seas can inhibit deep convection and the associated air–sea heat flux and water mass transformation. A combination of idealized computer models and basic theory indicates that the fluxes of liquid and solid freshwater are controlled by different mechanisms and occur at different times of the year. Accurate representation in climate models will require representation of small-scale processes such as mesoscale eddies and gradients of ice thickness across the shelf.
-
more » « less
Abstract Changes in the rate of ocean‐driven basal melting of Antarctica's ice shelves can alter the rate at which the grounded ice sheet loses mass and contributes to sea level change. Melt rates depend on the inflow of ocean heat, which occurs through steady circulation and eddy fluxes. Previous studies have demonstrated the importance of eddy fluxes for ice shelves affected by relatively warm intrusions of Circumpolar Deep Water. However, ice shelves on cold water continental shelves primarily melt from dense shelf water near the grounding line and from light surface water at the ice shelf front. Eddy effects on basal melt of these ice shelves have not been studied. We investigate where and when a regional ocean model of the Ross Sea resolves eddies and determine the effect of eddy processes on basal melt. The size of the eddies formed depends on water column stratification and latitude. We use simulations at horizontal grid resolutions of 5 and 1.5 km and, in the 1.5‐km model, vary the degree of topography smoothing. The higher‐resolution models generate about 2–2.5 times as many eddies as the low‐resolution model. In all simulations, eddies cross the ice shelf front in both directions. However, there is no significant change in basal melt between low‐ and high‐resolution simulations. We conclude that higher‐resolution models (<1 km) are required to better represent eddies in the Ross Sea but hypothesize that basal melt of the Ross Ice Shelf is relatively insensitive to our ability to fully resolve the eddy field.
-
more » « less
Abstract The Beaufort Gyre (BG) is a large anticyclonic circulation in the Arctic Ocean. Its strength is directly related to the halocline depth, and therefore also to the storage of freshwater. It has recently been proposed that the equilibrium state of the BG is set by the Ice‐Ocean Governor, a negative feedback between surface currents and ice‐ocean stress, rather than a balance between lateral mesoscale eddy fluxes and surface Ekman pumping. However, mesoscale eddies are present in the Arctic Ocean; it is therefore important to extend the Ice‐Ocean Governor theory to include lateral fluxes due to mesoscale eddies. Here, a nonlinear ordinary differential equation is derived that represents the effects of wind stress, the Ice‐Ocean Governor, and eddy fluxes. Equilibrium and time‐varying solutions to this three‐way balance equation are obtained and shown to closely match the output from a hierarchy of numerical simulations, indicating that the analytical model represents the processes controlling BG equilibration. The equilibration timescale derived from this three‐way balance is faster than the eddy equilibration timescale and slower than the Ice‐Ocean Governor equilibration timescales for most values of eddy diffusivity. The sensitivity of the BG equilibrium depth to changes in eddy diffusivity and the presence of the Ice‐Ocean Governor is also explored. These results show that predicting the response of the BG to changing surface forcing and sea ice conditions requires faithfully capturing the three‐way balance between the Ice‐Ocean Governor, wind stress, and eddy fluxes.
