Ice shelf – ocean boundary interaction
Thermodynamic ablation of ice in contact with the ocean remains poorly understood. Consequently the main topic of this thesis is the interplay between the ice shelf-ocean boundary and the mechanisms driving ice shelf ablation. The rate of ablation is determined by the temperature, salinity and speed of the water that impinges the ice, and how this water transports heat to and from the ice-water interface. Both of these components are explored here as they are the key to understanding the long-term stability of Antarctica’s ice shelves. The appearance of warm water in the Ross Ice Shelf cavity has recently been observed. To explain the process, we propose a novel mechanism for ice shelf front circulation. Direct contact of warm Antarctic Surface Water with the frontal wall of an ice shelf generates meltwater that rises along the vertical face and leads to formation of a ‘wedge’ of fresher water in front of the ice shelf, immediately adjacent to the wall. This wedge allows isopycnals to curve gently downwards, creating an efficient conduit by which warm surface waters enter the ice shelf cavity. Based on observational data from the Ross Sea and the Ross Ice Shelf cavity we demonstrate that the freshwater wedge is a pervasive feature of the Ross Sea in summer. Consequences of the wedge circulation depend upon the details of thermodynamic interaction of inflowing ocean water and the ice shelf. We argue that classifications of ablation, initially developed for vertical interfaces, can be applied to the more common horizontal ice-ocean interfaces, typical of the base of an ice shelf. Four ablation types are defined based on two thermal regimes (melting and dissolving) and two turbulent flux regimes (shear-controlled and buoyancy-controlled regimes). We present the dominant processes expected next to ice interfaces of different orientations, and classify observational, modelling and theoretical studies of the ice-ocean boundary. We apply this classification to sets of observations from the Ross and Ronne Ice Shelf cavities. Based on the ablation classification, we test ice-ocean ablation parameterisations for the most common ablation type: shear-controlled dissolving. To quantify uncertainties in ice shelf boundary parameterisations in an idealised ice shelf-ocean model, we undertake numerical experiments featuring 16 different formulations from the literature. Significant variability is found: the mean ablation rate in warm cavity scenarios varies between 2.1 and 4.7 m year-1, and in cold cavity scenarios between 0.03 and 0.17 m year-1. The different behaviours relate to differences in modelled ocean boundary layer stratification, highlighting an existing observational uncertainty of ice- ocean boundary layer processes. Recognising that warm surface water can easily enter the Ross Ice Shelf cavity, understanding how this heat is translated into basal ablation is a challenge. New approaches to ablation parameterisations have been presented, and existing ablation parameterisations have been tested against in situ observations.
Advisor: Langhorne, Patricia; Williams, Michael; Robinson, Natalie
Degree Name: Doctor of Philosophy
Degree Discipline: Physics
Publisher: University of Otago
Keywords: ablation; ice-ocean modelling; ice shelf cavity observations; cryosphere; ice melting
Research Type: Thesis