Experiments on the interaction of ice sheets with the polar oceans

Abstract

Antarctica and Greenland have been losing mass at an increasing rate over recent decades. The reducing volume of ice in Antarctica and Greenland has been a significant contribution to global sea level rise and will continue to be so in the future. Much of the mass loss occurs at the edge of the ice sheets where glaciers flow into the ocean. Interactions between the ice and the ocean are important in controlling the ablation rate of the glaciers. As such, there has been much recent work examining the response of ice shelves to changing ocean conditions. The majority of this work has used numerical models that allow a range of ocean conditions to be simulated. Here, we investigate the major ice-ocean interactions through idealized laboratory experiments.

Initially, the effect of fluid temperature on the ablation of a vertical ice wall is investigated. At the low temperatures and oceanic salinities that our experiments were conducted at, the temperature at the ice-fluid interface will be below 0 degrees Celsius and the interface salinity will be non-zero. Because of this, it is useful to consider a driving temperature defined as the difference between the fluid temperature and the freezing point at the fluid salinity. It is shown that the ablation rate increases like the driving temperature to the 4/3 power, while the interface temperature increases almost linearly with the driving temperature.

Ablation of an ice wall releases cold fresh water that rises up the ice face as a turbulent plume. This turbulent plume enhances the transport of heat and salt to the ice-fluid interface and helps to maintain ablation of the ice. The properties of the plume are investigated in detail and a model is developed that describes them.

The ocean around Antarctica and Greenland is generally stably stratified in salinity. The effect of stratification is investigated to examine the potential sensitivity of the ice sheets to changes in ambient fluid stratification. Regimes are found where small changes in the strength of stratification can lead to large changes in the ablation rate and the plume properties. This result highlights the possibility that weakening stratification, not just warming oceans, could lead to increased mass loss from the ice sheets.

In many locations around Greenland, plumes of freshwater are released at the base of the glacier. These subglacial plumes are modelled in the laboratory by releasing a two-dimensional freshwater plume at the base of the ice face. The additional source of buoyancy typically leads to significantly higher ablation rates and plume velocities, consistent with past numerical and observational studies.

These laboratory experiments represent an increasingly realistic model of the ice shelves around Antarctica and Greenland. Despite important physical processes still being excluded, the experiments present a useful and previously unavailable dataset with which numerical models can be tested and oceanographic field observations can be compared.