Most viscous-plastic sea ice models use the elliptical yield curve. This yield curve has a fundamental flaw: it excludes acute angles between deformation features at high resolution. Conceptually, the teardrop and parabolic lens yield curves offer an attractive alternative. These yield curves feature a non-symmetrical shape, a Coulombic behavior for the low-medium compressive stress, and a continuous transition to the ridging-dominant mode. We show that the current formulation of the teardrop and parabolic lens viscous-plastic yield curves with normal flow rules results in negative or zero bulk and shear viscosities and, consequently, poor numerical convergence and representation of stress states on or within the yield curve. These issues are mainly linked to the assumption that the constitutive equation applicable to the elliptical yield curve also applies to non-symmetrical yield curves and yield curves with tensile strength. We present a new constitutive relation for the teardrop and parabolic lens yield curves that solves the numerical convergence issues naturally. Results from simple uni-axial loading experiments show that we can reduce the residual norm of the numerical solver with a smaller number of total solver iterations, resulting in significant improvements in numerical efficiency and representation of the stress and deformation field. These yield curves lead to smaller angles of failure, in agreement with theoretical predictions, and are good candidates to replace the elliptical yield curve in high-resolution pan-arctic sea ice simulation.

We implement a damage parametrization in the standard viscous-plastic sea ice model to disentangle its effect from model physics (visco-elastic or elasto-brittle vs. visco-plastic) on its ability to reproduce observed scaling laws of deformation. To this end, we compare scaling properties and multifractality of simulated divergence and shear strain rate (as proposed in SIREx1), with those derived from the RADARSAT Geophysical Processor System (RGPS). Results show that including a damage parametrization in the standard viscous-plastic model increases the spatial, but decreases temporal localization of simulated Linear Kinematic Features, and brings all spatial deformation rate statistics in line with observations from RGPS without the need to increase the mechanical shear strength of sea ice as recently proposed for lower resolution viscous-plastic sea ice models. In fact, including damage an healing timescale of $t_h=30\>$days and an increased mechanical strength unveil multifractal behavior that does not fit the theory. Therefore, a damage parametrization is a powerful tuning knob affecting the deformation statistics.

A document by Yuqing Liu. Click on the document to view its contents.

Comparing helicopter-borne surface temperature maps in winter and optical orthomosaics in summer from the year-long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, we find a strong geometric correlation between warm anomalies in winter and melt pond location the following summer. Warm anomalies are attributed to thinner snow and ice on level ice compared to the deformed ice in the surroundings or refrozen leads with only newly formed, thin ice. Warm surface temperature anomalies in January were 0.3 K to 2.5 K warmer on sea ice that later formed melt ponds. A one-dimensional steady-state thermodynamic model shows that the observed surface temperature differences are in line with the observed ice thickness and snow depth. We demonstrate the potential of seasonal prediction of summer melt pond location and coverage from winter surface temperature observations. A threshold-based classification achieves a correct classification for 41% of the melt ponds.

The impact of horizontal resolution on meridional Ocean Heat Transport (OHT) and sea ice in the Arctic is investigated using the GFDL CM2-O climate model suite (1°, 1/4°, and 1/10°) in both preindustrial control and climate change simulations. Results show an increase in OHT associated to a decrease in sea ice extent (SIE) in the Arctic on inter-annual and decadal time scales. This link, however, is not monotonic with spatial resolution. While OHT increases and SIE decreases from the Low to the Medium resolution, the reverse is true from the Medium to the High resolution. Differences in OHT and SIE between the three model configurations mostly arise from the preindustrial state. As the spatial resolution increases, the Irminger Current is favored at the expense of the North Atlantic Drift. This rerouting of water to the Western side of Greenland results in less heat delivered to the Arctic in the High resolution configuration than in its Medium counterpart. As a result, the Medium resolution configuration is in best agreement with observed SIE and Atlantic OHT. Concurrent with the change in the partitioning in volume is a change in deep convection centers from the Greenland-Irminger-Norwegian Seas in the Low resolution to the Labrador Sea in the Medium and High resolutions. Results suggest a coupling between OHT into the Arctic and deep convection in the North Atlantic.

The Canada Basin has exhibited a significant trend toward a fresher surface layer and thus a more stratified upper ocean over the past three decades. Here, we explore the extent to which the Community Earth System Model (CESM) accurately simulates the observed surface freshening and seasonal processes that contribute to the freshening. We examine 30 simulations from CESM1 (used in the IPCC AR5), 3 simulations from CESM2 (IPCC AR6), and ocean observations from 1975 and 2006-2012. In contrast to the observations, the models simulate salinity profiles that show relatively little variation between 1975 and 2012. We demonstrate that this bias can be partly attributed to the model’s tendency to mix freshwater too deep, creating a surface layer that is saltier than observed. The results provide insight for climate model improvement that could have wide-reaching implications because upper-ocean stratification influences the vertical transport of heat and nutrients.

High-resolution viscous-plastic (VP) sea ice models reproduce the narrow deformations lines observed in the Arctic sea ice, called the Linear Kinematic Features (LKFs). Recent studies showed that standard VP models overestimate the intersection angles between the LKFs when compared to observations. We investigate fracture angles in a uniaxial compression test and two different VP rheology. The first one uses an elliptical yield curve and a normal flow rule. In contrast, the second rheology uses a different elliptical plastic potential that creates a non-normal flow rule. Results show that the non-normality of the flow rule changes the angles of fracture. This new rheology can create fracture angles as low as 22º when the rheology with normal flow rule is limited to angles above 30º. A newly adapted theory – based on one developed from granular material observations – predicts the modeled fracture angles accurately. Using a non-normal flow rule takes longer to solve numerically, but allow reductions of the fracture angle to values within the range of satellite observations and decouples the angle of fracture from the shape of the yield curve.

In recent years, there has been a significant sea ice retreat in the Pacific sector of the Arctic. One possible cause is the increase in ocean heat flux amplified by the ice-albedo feedback. This paper looks at vertical ocean heat transport from waters of Pacific origin and solar heat into the mixed layer and their impact on the sea ice mass balance in the Community Earth System Model - Large Ensemble (CESM-LE). To this end, we focus on two specific periods with observational hydrographic data from the Arctic Ice Dynamics Joint Experiment (1975-76) and Ice-Tethered Profiler (2004-2018). A comparison between simulated and observed salinity and potential temperature profiles highlights two key model biases in all ensemble members: an absence of Pacific Waters in the water column and a deepening of the winter mixed layer in opposition to observations that show a reduction in depth of the mixed layer and a stronger increase in stratification. Results from a one-dimensional vertical heat budget show that remnant solar heat trapped beneath the halocline is mostly ventilated to the surface by mixing before the following melt season, while vertical advection associated with Ekman pumping, even in early fall when the winds are strong and the pack-ice weak, only has a small effect on the vertical heat transport. Furthermore, we estimate from the 1D heat budget a reduction of 1.4 m winter ice growth over three years (the residence time of ice in the Beaufort Gyre) associated with the missing Pacific Waters.

Over the past decades, Arctic sea ice has declined in thickness and extent and is shifting towards a seasonal ice regime, with accelerated ice drift and an increase in the seasonal ice zone. The changing Arctic ice cover will impact the trans-border exchange of sea ice between the Exclusive Economic Zones (EEZs) of the Arctic nations, with important implications for ice-rafted contaminant transport. To investigate projected changes to transnational ice exchange, we use the Lagrangian Ice Tracking System (LITS) to follow ice floes from the location of their formation to where they ultimately melt. We apply this tool to output from two ensembles of the Community Earth System Model (CESM): the CESM Large Ensemble, which uses a high emission scenario (RCP8.5) that leads to over 4°C global warming by 2100, and the CESM Low Warming ensemble, with reduced emissions that lead to a stabilized warming of 2°C by 2060. We also use the National Snow and Ice Data Center Polar Pathfinder and Climate Data Record products to evaluate the fidelity of the CESM present-day tracking simulations. Transnational ice exchange is well represented in CESM except for ice traveling from Russia to Norway, with twice as much ice following this pathway compared to observations. Initial results suggest this might be due to a combination of internal variability and speed biases in the observational data. The CESM projects that by mid-century, transnational ice exchange will expand, with a large increase in the fraction of transnational ice originating from Russia and the Central Arctic. As the seasonal ice zone grows, ice floes accelerate and transit times decrease, eventually cutting off ice exchange between longer pathways. By the end of the 21st century, we see a large impact of the emission scenario on ice exchange: consistent ice-free summers under the high emission scenario act to reduce the total fraction of transnational ice exchange compared to mid-century. The low emission scenario on the other hand continues to see an increase in transnational ice exchange by 2100. Under both scenarios, all pathways have decreased to average transit times of less than 2 years, compared to a maximum of 6 years under present-day conditions and 3 years by mid-century, effectively bringing the Arctic nations closer together.

As the sea-ice modeling community is shifting to advanced numerical frameworks, developing new sea-ice rheologies, and increasing model spatial resolution, ubiquitous deformation features in the Arctic sea ice are now being resolved by sea-ice models. Initiated at the Forum for Arctic Modelling and Observational Synthesis (FAMOS), the Sea Ice Rheology Experiment (SIREx) aims at evaluating current state-of-the-art sea-ice models using existing and new metrics to understand how the simulated deformation fields are affected by different representations of sea-ice physics (rheology) and by model configuration. Part I of the SIREx analysis is concerned with evaluation of the statistical distribution and scaling properties of sea-ice deformation fields from 35 different simulations against those from the RADARSAT Geophysical Processor System (RGPS). For the first time, the Viscous-Plastic (and the Elastic-Viscous-Plastic variant), Elastic-Anisotropic-Plastic, and Maxwell-Elasto-Brittle rheologies are compared in a single study. We find that both plastic and brittle sea-ice rheologies have the potential to reproduce the observed RGPS deformation statistics, including multi-fractality. Model configuration (e.g. numerical convergence, atmospheric forcing, spatial resolution) and physical parameterizations (e.g. ice strength parameters and ice thickness distribution) both have effects as important as the choice of sea-ice rheology on the deformation statistics. It is therefore not straightforward to attribute model performance to a specific rheological framework using current deformation metrics. In light of these results, we further evaluate the statistical properties of simulated Linear Kinematic Features (LKFs) in a SIREx Part II companion paper.

Simulating sea-ice drift and deformation in the Arctic Ocean is still a challenge because of the multi-scale interaction of sea-ice floes that compose the Arctic sea ice cover. The Sea Ice Rheology Experiment (SIREx) is a model intercomparison project formed within the Forum of Arctic Modeling and Observational Synthesis (FAMOS) to collect and design skill metrics to evaluate different recently suggested approaches for modeling linear kinematic features (LKFs) and provide guidance for modeling small-scale deformation. In this contribution, spatial and temporal properties of LKFs are assessed in 36 simulations of state-of-the-art sea ice models and compared to deformation features derived from RADARSAT Geophysical Processor System (RGPS). All simulations produce LKFs, but only very few models realistically simulate at least some statistics of LKF properties such as densities, lengths, or growth rates. All SIREx models overestimate the angle of fracture between conjugate pairs of LKFs and LKF lifetimes pointing to inaccurate model physics. The temporal and spatial resolution of a simulation and the spatial resolution of atmospheric forcing affect simulated LKFs as much as the model’s sea ice rheology and numerics. Only in very high resolution simulations (≤2\,km) the concentration and thickness anomalies along LKFs are large enough to affect air-ice-ocean interaction processes.

The seasonal halocline impacts the exchange of heat, energy, and nutrients between the surface and the deeper ocean, and it is changing in response to Arctic sea ice melt over the past several decades. Here, we assess seasonal halocline formation in 1975 and 2006-2012 by comparing daily, May to September, below-ice salinity profiles collected in the Canada Basin. We evaluate differences between the two time periods using a one-dimensional (1D) bulk model to quantify differences in freshwater input and vertical mixing. The 1D model metrics indicate that two separate factors contribute similarly to stronger stratification in 2006-2012 than in 1975: (1) larger surface freshwater input and (2) less vertical mixing of that freshwater. The first factor is mainly important in August-September, consistent with a longer melt season in recent years. The second factor is mainly important from June until mid-August, when similar levels of freshwater input in 1975 and 2006-2012 are mixed over a different depth range, resulting in different stratification. These results imply that decadal changes to ice-ocean dynamics, in addition to freshwater input, significantly contribute to the stronger seasonal stratification in 2006-2012 than in 1975. The findings highlight the need for near-surface process studies to elucidate the roles of lateral processes and ice-ocean momentum exchange on vertical mixing.