Feature

Ice on the Ocean Wave

Scientists are learning more about why Antarctic sea ice is dramatically different from that found in the Arctic.

By Vernon A. Squire

There are distinct differences in the spread and type of ice encountered in the waters of the Antarctic and Arctic. In the Antarctic, sea ice spreads over 20 million square kilometres in September, shrinking back to only 4 million km² by February. It's considerably different in the Northern Hemisphere, where Arctic ice coverage changes by only 6 million km² from the March maximum to the August minimum. In contrast to the older ice in the north, Antarctic sea ice is usually less than a year old.

Why should the Antarctic sea ice canopy be more dynamic than that of the Arctic? And what other differences -- if any -- exist between the ice of the two oceans? Surely frozen sea water ought to be the same wherever it grows, or should it?

Undoubtedly the Southern Ocean is geographically very different from the Arctic Ocean. The Southern Ocean encircles the massive Antarctic continent and is unbounded to the north where it meets the almost unlimited reaches of the Atlantic and Pacific Oceans. The Arctic Ocean is more mediterranean -- an enclosed sea surrounded by land with limited access to the other oceans.

It is this geographical difference that leads to the dissimilarity in variability and to the large proportion of first-year ice in Antarctic waters. In the Arctic, significantly more ice survives the summer melt to become what is known as multi-year ice. Naturally multi-year ice is on average thicker than first-year ice, so we expect the sea ice of the Arctic to be thicker than that in Antarctica. But there is also another obvious difference between our two ice covers and it is again related to geography, or at least to the shelter offered by local land masses.

From above, the ice of the central Arctic looks like a continuous sheet, with its relief broken only by the occasional lead (or internal "river"), polynya (internal "lake"), or pressure ridge. The Antarctic, at least in its most seaward regions, is a jigsaw of pack ice of various floe sizes extending from the smallest pancakes near the ice edge up to vast ice floes within the interior.

Wave
Action

The distinction between central Arctic and Antarctic sea ice is due to ocean waves. Waves are the main shaper of the Antarctic sea ice mass, as they are for other marginal ice zones. In the Arctic Ocean, only very long waves are found, as those with shorter periods are diminished as they cross into the Arctic Basin. Such long waves have little effect on the sea ice canopy, which simply flexes gently with their passing.

Thousands of hectares of the Southern Ocean and the peripheral seas of the Arctic (such as the Greenland Sea, the Barents Sea, and the Bering Sea) form what is called a marginal ice zone. These zones are defined as the area outside and within the ice cover which is significantly affected by open ocean processes.

In the case of the Southern Ocean, the marginal ice zone may extend almost to the coast of the Antarctic continent, as open ocean processes in the Southern Ocean are particularly intense. For the Northern Hemisphere, the interior limit of the marginal ice zone is usually 100-200 kilometres from the ice edge. Ice in a marginal ice zone is often called pack ice.

Ocean waves, of course, are an open ocean process. As they enter an ice field, they are systematically damped by the sea ice in a manner which favours the penetration of long waves at the expense of short waves. The ice canopy acts to filter the incoming waves, with scattering, water turbulence, inelastic ice bending, and collisions between adjacent floes.

The waves affect the ice cover, travelling through the pack ice and bending each of the ice floes they encounter. If this bending is sufficient, the ice floe fractures, forming two or more smaller pieces. Whether this occurs or not depends on the height of the incoming waves. Observations suggest that wave break-up of pack ice is a common occurrence in the marginal ice zone, especially in the outer 50 kilometres.

We have developed a mathematical theory for single ice floes which successfully models break-up, as well as accurately predicting a floe's seakeeping behaviour, such as the way it heaves and rolls, and the way it transmits and reflects wave energy.

Now things get complicated. If the sea ice attenuates the waves and this attenuation depends on the shape of the ice, and if the ice is broken up by the waves, then a rather complex feedback process develops because the break-up is relentlessly changing the morphology which changes the damping which changes the break-up which changes the morphology, and so on.

Such a process is difficult but not impossible to model, particularly if one assumes that the system eventually ends up in a steady state -- that the ice cover turns into a permanent distribution of floe sizes and thicknesses. In reality, no steady state ice distribution is ever reached, which is a shame as the mathematics are quite pleasant.

Important Implications

Far from being an esoteric academic problem of little relevance to New Zealand, this topic has some significance. The sea ice just to the south of us covers 20 million km², or 74 times the area of New Zealand. That's a lot of sea ice, and it naturally has a profound effect on global -- and our -- climate, as sea ice changes affect the relationship between the atmosphere and the underlying ocean.

Since waves alter sea ice morphology, and sea ice influences climate, then ocean waves at least partly drive climate. Of course since it is the wind which generates the waves in the first place, it all gets rather complicated.

But why else is the topic of wave propagation through pack ice important? Ships operate in areas covered with sea ice. In the Antarctic, these are mainly supply and tourist ships, but in the sub-Arctic tankers routinely plough their way through ice-infested seas. There have been accidents due to ships underestimating the severity of the ocean waves in marginal ice zones.

Finally, and fortunately only in the Arctic for now and in the foreseeable future, the demands of hydrocarbon exploration in waters where pack ice is present are becoming more ambitious as the industry tries to lengthen its operating season. The notion of a caisson or pipeline failure or a blow-out underneath ice where waves are present is highly distressing, as none of the conventional methods could be used to coagulate the oil or to limit the spread of the slick.

Shore-Fast Sea Ice

Skirting the coast of the Antarctic and Arctic and sub-Arctic landmasses, and lingering in bays and harbours through winter into early spring, are vast sheets of shore-fast sea ice. These sheets are characterized by their uniformity and, when away from topographic features, by their lack of flaws such as cracks and ridges. Our own Antarctic station, Scott Base, on Ross Island in McMurdo Sound, sits alongside more than 2,000 km² of shore-fast ice for much of the year.

In many cases, it is ocean waves which destroy these ice sheets catastrophically with the onset of spring. Incoming waves travel beneath the ice, causing it to flex to and fro as they pass. Standing on the ice you can sometimes actually see the wave fronts coming towards you -- a potentially dangerous situation, as it can indicate break-up of the ice.

The first attempt to model mathematically waves bending shore-fast ice was published in 1887 but, despite various attempts in the intervening years, it is only recently that the problem has finally been cracked. Dr Colin Fox of the University of Auckland and I have been able to construct a precise mathematical model to represent ocean waves entering and travelling within shore fast ice.

The model successfully explains many observations, including what happens in the seas off the ice edge, how and why break-up occurs, and the existence of a critical angle beyond which ocean waves cannot penetrate the ice sheet. The explanation of the break-up phenomenon is most satisfying, as it has worried me for many years why cracks always seem to form at roughly the same distance apart, regardless of the wave periods in the incoming sea.

Ice Factory

The new theory predicts this, showing that whereas crack separation is strongly dependent on ice thickness, it is only weakly dependent on period. It is an important result because the break-up of fast ice serves as an ice factory, supplying floes for the marginal ice zone. If crack separation depends mainly on ice thickness then there ought to be a verifiable relationship between floe diameter and thickness. We hope that this is the case.

The most exciting outcome of a theoretical model is its validation by experiment. This was attempted last November when a team from the Universities of Otago and Auckland joined forces with the New Zealand Institute for Water and Atmospheric Research in McMurdo Sound.

The aim was to use a novel seastate microwave radar to record the incoming directional wave structure of the ocean, while simultaneously measuring the ice-coupled waves response using strain gauges and accelerometers. The experiment was successful, and the data is still being processed.

Although primarily environmental, the modelling and experiments have wider application to such issues as wave power and floating breakwaters. With some modification, the theory can even be applied to the break-up of supertankers in the open ocean, as these huge vessels bend significantly and, in certain unfavourable seas, may resonate themselves to destruction.

Professor Squire holds the Chair of Applied Mathematics at the University of Otago.