Chapter 1
Overview of sea ice growth and properties

Chris Petrich1 and Hajo Eicken2

1Northern Research Institute Narvik, Narvik, Norway

2University of Alaska Fairbanks, Fairbanks, AK, USA

1.1 Introduction

A recent, substantial reduction in summer Arctic sea ice extent and its potential ecological and geopolitical impacts generated a lot of attention in the media and among the general public. The satellite remote-sensing data documenting such recent changes in ice coverage are collected at coarse spatial scales (Chapter 9) and typically cannot resolve details finer than about 10km in lateral extent. However, many of the processes that make sea ice such an important aspect of the polar oceans occur at much smaller scales, ranging from the sub-millimetre to the metre scale. An understanding of how large-scale behaviour of sea ice monitored by satellite relates to and depends on the processes driving ice growth and decay requires an understanding of the evolution of ice structure and properties at these finer scales and this is the subject of this chapter.

The macroscopic properties of sea ice are of interest in many practical applications discussed in this book. They are derived from microscopic properties as continuum properties averaged over a specific volume (representative elementary volume) or mass of sea ice. This is not unlike macroscopic temperature and can be derived from microscopic molecular movement. The macroscopic properties of sea ice are determined by the microscopic structure of the ice, i.e. the distribution, size and morphology of ice crystals and inclusions. The challenge is to see both the forest (i.e. the role of sea ice in the environment) and the trees (i.e. the way in which the constituents of sea ice control key properties and processes). In order to understand and project how the forest will respond to changes in its environment, we have to understand the life cycle of its constituents, the trees. Here, we will adopt a bottom-up approach, starting with the trees, characterizing microscopic properties and processes and how they determine macroscopic properties, to lay the groundwork for understanding the forest. In using this approach, we will build up from the sub-millimetre scale and conclude with the larger scales shown in Figure 1.1.

Figure 1.1 Ice types, pack ice features and growth, melt and deformation processes.

Figure 1.2 Surface appearance and microstructure of winter lake ice (Imikpuk Lake, top, panels a-d) and sea ice (Chukchi Sea landfast ice, bottom, panels e-h) near Barrow, Alaska. The bright features apparent in the lake ice are cracks that penetrate all the way to the bottom of the ice cover (close to 1m thick), while the clear, uncracked ice appears completely black (a, top). (e) The sea ice surface photograph shows a network of brine channels that join into a few feeder channels. (b, c, f, g) Photographs of vertical thin sections from the two ice covers, with (b) and (f) recorded between crossed polarizers, highlighting different ice crystals in different colours. Panels (c) and (g) show the same section as (b) and (f) in plain transmitted light, demonstrating the effect of brine inclusions on transparency of the ice. (d, h) Photomicrographs showing the typical pore structure at a temperature of -5°C (lake ice) and -15°C (sea ice), with few thin inclusions along grain boundaries in lake ice (d) and a network of thicker brine inclusions in sea ice (h).

Sea ice would not be sea ice without salt. In fact, take away the salt and we are left with lake ice, differing in almost all aspects that we discuss in this chapter. The microscopic and macroscopic redistribution of ions opens the path to understanding all other macroscopic properties of sea ice. We will therefore start in Section 1.2 by looking at the influence of ions on ice growth at the scale of individual ice crystals, in sea ice growing under both rough and quiescent conditions. We will continue in Section 1.3 by looking at the dynamic feedback system between fluid dynamics and pore volume, both microscopically and at the continuum scale. We will point out that our knowledge is far from exhaustive in this fundamental aspect. However, armed with a basic understanding of crystal structure, phase equilibria and pore structure, we can shed light on ice optical, dielectric and thermal properties and macroscopic ice strength in Section 1.4. One of the most discussed aspects of sea ice is its presence or absence. We will look at the growth and energy budget of sea ice and touch on deformation and decay processes in Section 1.5.

1.1.1 Lake ice versus sea ice

Ice in a small lake tends to form before coastal sea ice at a similar location. This is largely explained by the fact that, in contrast to freshwater, the temperature of maximum density of seawater is not above the freezing point. If a freshwater body is cooled from above then the water body undergoes convective overturning until the temperature reaches +4°C, after which the coldest water stays at the surface where it is cooled rapidly. Hence, ice formation starts relatively early in the season but progresses slowly as the underlying water mass is still above freezing. The situation is different if strong winds continuously overturn the water (e.g. in big lakes), or if ice grows from seawater. In these cases, the entire mixed layer has to be cooled to the freezing point before ice formation sets in. Once this happens, however, thickening progresses relatively quickly.

Figure 1.3 Crystal structure of ice Ih (from Weeks & Ackley, 1986). The -axis is indicated at left and right, and the centre panels correspond to a view along (top) and normal (bottom) to the -axis.

Salt further impacts ice microstructure. The photographs in Figure 1.2 show the surface of snow-free lake ice and sea ice in spring near Barrow, Alaska. Despite comparable thickness and growth conditions, lake ice, transparent, appears much darker than sea ice, which scatters light. This is also expressed in a large difference in albedo (the fraction of the incident short-wave radiation reflected from a surface; Section 1.4), such that more than three-quarters of the incoming short-wave irradiative flux penetrates the lake ice surface into the underlying water, compared with less than half for a sea ice cover. This has substantial consequences for the heat budget of the ice cover and the water beneath. The fact that sea ice albedo is typically higher than open water albedo by a factor of up to 10 gives rise to the so-called ice-albedo feedback: a perturbation in the surface energy balance resulting in a decreased sea ice extent due to warming may amplify, as the ice cover reduction increases the amount of solar energy absorbed by the system (Chapter 4; Curry et al., 1995; Perovich et al., 2007). For low-albedo lake ice, this effect is less pronounced. What causes these contrasts? As the thin-section photographs in Figure 1.2 demonstrate, lake ice is nearly devoid of millimetre and sub-millimetre liquid inclusions, whereas sea ice can contain more than 10mm-3. The inclusions scatter light due to a contrast in refractive index (Section 1.4). This explains both the high albedo and lack of transparency of thicker sea ice samples.

The crystal microstructure differs between lake ice and sea ice. Lake ice grows with a planar liquid-solid interface rather than a lamellar interface, as is the case of sea ice. In sea ice, brine is trapped between the lamellae at the bottom of the ice, allowing for retention of between 10% and 40% of the ions between the ice crystals. While the differences in bulk ice properties, such as albedo and optical extinction coefficient, are immediately obvious from these images, the physical features and processes responsible for these differences only reveal themselves in the microscopic approach, as exemplified by the thin-section images depicting individual inclusions (Figure 1.2). In the sections that follow, we will consider in more detail how microstructure and microphysics are linked to sea ice growth and evolution, and how both in turn determine the properties of the ice cover as a whole.

1.2 Ions in the water: sea ice microstructure and phase diagram

1.2.1 Crystal structure of ice Ih

The characteristic properties of sea ice and its role in the environment are governed by the crystal lattice structure of ice Ih, in particular its resistance to the incorporation of sea salt ions. Depending on pressure and temperature, water ice can appear in more than 15 different modifications. At the Earth's surface, freezing of water under equilibrium conditions results in the formation of the modification ice Ih, with the 'h' indicating crystal symmetry in the hexagonal system. Throughout this chapter, the term 'ice' refers to ice Ih.

Water molecules (H2O) in ice are arranged tetrahedrally around each other, with a six-fold rotational symmetry apparent in the so-called basal plane (Figure 1.3). This is why snowflakes have six-fold symmetry. The principal crystallographic axis [referred to either as the corresponding unit vector (0001) or simply as the c-axis] is normal to the basal plane and corresponds to the axis of maximum rotational symmetry (Figure 1.3). The interface of the basal plane is smooth at the molecular level. The basal plane is spanned by the crystal a-axes, and the crystal faces perpendicular to this plane...