The Ice–Ocean Boundary

1. Introduction

The ocean ice layer in polar regions is impacted by a complex and varying range of physical and thermodynamic processes. The ice itself is deformed and displaced by forces imposed by ocean surface currents and waves. Water temperature and salinity fluctuations act to modify the rheology and material properties of the ice while also driving ocean circulation at multiple scales. In turn, the ice insulates the ocean beneath it from the effects of wind, sun, and precipitation. The ice–ocean boundary is the primary interface at which these phenomena can be measured and modeled, and yet much about it remains unknown. Collectively, these processes play an important role in shaping the future climate of the Arctic, Antarctic, and ultimately the entire globe. It is thus of increasing importance that we fully understand them and develop the capability to predict their effects.

This Special Issue contributes to improving our knowledge of the ice–ocean boundary, offering a diverse sampling of innovative research efforts. It includes ten articles that examine the interactions between surface waves and ice, the thermodynamics of the ice–ocean interface, the determination of ice floe characteristics and distribution, and the broader effects of a changing global climate. The investigations range from theoretical and statistical analyses and modeling to direct measurements in laboratory or field experiments.

2. Wave–Ice Interactions

As mean global temperatures continue to climb, larger areas of the polar ocean surface are becoming ice-free for longer periods of each year. Wind forcing acts on the open water to produce more energetic waves, which then fracture and destroy still more surface ice, further expanding the ice-free regions. As civilian and military ship traffic continues to grow in the Arctic and Antarctic, it is increasingly important to fully understand these wave–ice processes and predict them as accurately as possible. In this Special Issue, five papers investigate the interactions between waves and ice using measurements, theory, or model simulations.

Johnson et al. [1] and Orzech et al. [2] each describe the use of innovative instruments and techniques to directly measure a range of properties of waves and ice, with the former acquiring data in the field and the latter in the laboratory. Johnson et al. develop and deploy a new type of inertial measurement sensor to track both flexural and gravity waves propagating through solid pack ice in the Beaufort Sea. They demonstrate how the measured wave properties can determine ice’s mechanical structure and behavior, producing a more complete characterization of ice rheology. These additional measurements can be useful both for guiding in situ data acquisition and instrument deployments during a field experiment and later in validating and improving model predictions of ice behavior. Working at a somewhat smaller scale, Orzech et al. describe the successful deployment and operation of a subsurface particle-imaging velocimetry (PIV) system beneath a broken ice layer in a laboratory wave tank maintained at sub-freezing temperatures. Through a calibrated series of wave simulations, they demonstrate the effectiveness of the PIV at measuring the small-scale ocean flow field directly under the ice. They perform an extended analysis of a selected test case, delineating the features of a fluid boundary layer near the water–ice interface.

In a companion paper to Orzech et al., Yu [3] performs a theoretical analysis of an idealized wave–ice boundary approximating that in a typical marginal ice zone, with the surface ice represented as a continuous, viscous layer above less-viscous water. The analysis demonstrates how the two different viscosity layers alter the phase relationship between horizontal and vertical fluid velocities to produce a secondary mean flow in the surface layer as well as a counterflow in the boundary layer immediately below it. The author finds that using a reasonable non-zero water eddy viscosity produces a better fit to measured wave attenuation rates in ice than assuming the water to be completely inviscid and proposes an empirical formula for wave attenuation that explicitly accounts for the ice thickness.

Tavakoli et al. [4] and Li et al. [5] employ model-based approaches to examine and predict wave interaction with surface ice from two different perspectives. In simulations with a computational fluid–solid dynamic model, including a viscoelastic representation of surface ice, Tavakoli et al. predict the attenuation and dispersion of surface waves in a consolidated ice layer. They validate their results against both lab and field data and demonstrate that their model correctly follows the scaling law. In a contrasting approach, Li et al. use a high-resolution coupled ice–ocean model to investigate the breakup and consolidation of sea ice during both the summer melt and winter freeze periods. They find that the physical action of the waves accelerates ice melting in the summer and increases ice concentration in the winter, also demonstrating that the thermodynamic effects of wave-induced mixing can act to oppose these trends in both seasons.

3. Thermodynamic Processes

Exchanges of heat between the ocean, ice, and atmosphere generally play a far more significant role than waves in determining the large-scale concentration and extent of the sea ice cover. While atmospheric cooling is most often the primary contributor to ice formation, ocean heat fluxes and solar radiation consistently act to weaken and melt the polar ocean surface layer. As global temperatures have increased, the former effect has diminished while the latter two have grown. This trend has been most noticeable at higher northern latitudes and has played a key role in the thinning and retreat of surface ice cover in the Arctic. In this Special Issue, these thermodynamic processes and their effects are investigated with three very different approaches.

McCutchan and Johnson [6] provide a general overview of a broad range of laboratory experiments investigating ice melting in response to free and forced convective flows. Their review article encompasses almost 50 years of testing featuring laminar and turbulent flows with varied ice geometries, water salinity, and stratification, and includes an extensive appendix summarizing the configurations and measurements of all the experiments. They identify the most effective measurement techniques and discuss areas where further experimental investigations would yield the greatest benefit.

To explain and understand the evolution of consolidated ice ridges seen in measurements obtained off the west coast of the Svalbard archipelago, Marchenko [7] develops a new model of the thermodynamic evolution of such ice ridges for varied air temperatures and water heat flux rates. He finds that the consolidation or collapse of a given ridge is highly dependent on the initial porosity of the unconsolidated ice rubble prior to ridge formation, with the rubble consolidating for porosities under 0.2 and deteriorating for porosities over 0.3. The author further notes that the porosity of an ice ridge keel can be reduced (increasing the likelihood of consolidation) under conditions where ocean heat flux first produces melt water that penetrates the ice pores and is then later frozen by colder air temperatures.

A unique third perspective is offered by Bhargava and Echenique [8], who conduct an extensive statistical analysis of sea surface temperatures, ocean currents, and sea ice concentrations for a 20-year period and examine correlations among the datasets in both northern and southern hemispheres. The authors demonstrate a clear negative correlation between sea surface temperatures and sea ice concentrations in both regions and a positive correlation between ocean eddy kinetic energy and sea surface temperature in the northern hemisphere. They also find that, while temperatures have been consistently increasing in the north, the sea surface in the southern hemisphere has tended to oscillate between warmer and colder temperatures over the past two decades.

4. Ice Floe Characteristics and Distribution

Along its horizontal boundaries, the ocean surface ice layer is highly complex and varied, with individual ice floes ranging from O(1 m²) to O(1000 km²) in the area and ice thicknesses ranging from centimeters for pancake ice to over 8 m in some consolidated ice ridges [7]. To simplify the problem, this ice is often modeled as a continuous layer of varying viscosity and/or elasticity (as described above in Section 2). While these approaches can be very effective, the parametric representations that they employ must ultimately be configured to best fit the actual surface ice conditions in the field. Accurate and comprehensive measurements of the shapes, sizes, and distributions of ice floes, pack ice, and landfast ice throughout the polar regions are thus essential to better understand and model the effects of the widely varied ice conditions on rates of ice fracture, melting, and freeze-up, as well as ocean surface wave attenuation. In the field, such measurements are generally extracted using photographic or radar images obtained during overflights by aircraft or satellites. Two papers in this Special Issue address the acquisition of ice data from imagery.

Zvyagin et al. [9] use photogrammetry with orthorectification in a small-scale laboratory investigation of the breakup of a continuous ice layer impacted by a moving sloped structure. The rectified images allow the authors to distinguish between ice failure involving simple fracturing and more extreme failure that includes ice crushing. They find that simple fracturing tends to occur initially, with a fractured fragment acting as a spacer to transfer the contact force to the unbroken ice and thus maintain a “perfect fit” among the resulting ice pieces. However, as the ice rubble accumulates, this is eventually followed by the crushing failure of some pieces, which results in gaps between the ice that are instead occupied by slush and debris.

In contrast, working at much larger field scales, Wang et al. [10] use interferometric SAR data from the Sentinel 1A satellite to map the seaward boundary of a landfast ice layer along the coast of Finland. They introduce a new approach that utilizes coherence images to distinguish between fixed and moving ice, identifying smaller ice patches whose positions shift between the compared images. These patches are then excluded from the ice boundary determination. The authors demonstrate that their proposed approach maps the ice edge more accurately and efficiently than an alternative reference method based only on SAR amplitude.

5. Conclusions

As the Earth’s climate continues to rapidly evolve, interest in the Arctic and Antarctic regions is steadily growing, with increasing ship traffic and commercial development efforts in both areas. To ensure the safety of such ventures, avoid international conflicts, and better inform efforts to combat and adapt to global warming, a more comprehensive understanding of the physical processes linking ice and ocean is urgently necessary. The research efforts described in this Special Issue address a few of the many challenging questions associated with the ice–ocean boundary. These articles contribute significantly to advancing our knowledge of this complex environment.