Christina Hulbe

National School of Surveying, University of Otago

Dynamics of the marine ice sheet instability on Thwaites Glacier, West Antarctica

Rapid change now underway on in the Amundsen Sea sector of West Antarctica raises concern that a threshold for unstoppable grounding line retreat has been or is about to be crossed. The grounding line is transition between ice that is resting on the sea floor and ice that is floating in the ocean and mathematically, the instability is due to the inland-deepening bed of the glacier together with the power-law relationship between ice thickness and ice flux across the boundary. We use a high-resolution ice sheet model to examine the dynamics of self-sustained retreat on Thwaites Glacier by nudging the grounding line just past the point of instability. We find that by modifying surface slope in the region of the grounding line, the rate of the forcing dictates the rate of retreat, even after the external forcing is removed. Grounding line retreats that begin faster proceed more rapidly because the shorter time interval for the grounding line to erode into the grounded ice sheet means relatively thicker ice and larger driving stress upstream of the boundary. Retreat is sensitive to short-duration re-advances associated with reduced external forcing where the bathymetry allows re-grounding, even when an instability is invoked. The time and location of initiation is also sensitive to the roughness of the subglacial bed due to the flux-regulating effects of local high spots. These obstacles drive transient steepening in the vicinity of the grounding as it passes by and modify form drag in the grounded part of the system. All together, we show that small differences in forcing lead to large differences in retreat rate and ice discharge across the grounding line and the implication for future change is clear. If lower-end future warming scenarios are possible, then the sooner anthropogenic forcing is reduced, the slower the ice sheet response, even if a self-sustaining retreat has been initiated. Retreat may be inevitable past a certain dynamical threshold, but the rate at which the retreat proceeds is not.

Shaun Hendy

Department of Physics, University of Auckland

Molecular dynamics simulations for nano and microfluidic flows

Molecular dynamics is a numerical technique for simulating the evolution of many-body systems by solving Newton’s equations of motion for hundreds to hundreds of millions of particles (atoms or otherwise). In this talk I will illustrate its use for understanding nano and microfluidic flows. Even at scales where continuum descriptions of fluids are expected to hold, interfacial properties often have to be modelled in an ad hoc fashion, with model parameters that extracted from difficult to obtain experimental data. In molecular dynamics, however, these properties are an emergent feature of a simulation, arising naturally from the corresponding many-body interactions. While molecular dynamics has many practical shortcomings, including restriction to short simulation times and small system sizes, it allows for computational experiments that don’t require ad hoc modelling of chemical or interfacial phenomena. I will illustrate this by describing our recent work on the simulation of droplets moving on complex surfaces and the aggregation of Janus particles in uniform or shear flows.

Ashton Bradley

Department of Physics, University of Otago

Observing a quantum storm in a superfluid teacup

Atomic Bose-Einstein condensates (BECs) provide a uniquely controllable setting in which to study quantum fluid dynamics. In a stirred superfluid, quantized vortices typically proliferate, injecting linear and angular momentum into the fluid. In 1949, while studying the point-vortex model, Onsager predicted that confinement of quantum vortices can produce a surprising result: the possibility of vortices reaching negative temperatures. Negative temperature states contain significant energy, forming a collective storm of vortices circulating in the same direction: a giant vortex cluster. Vortex cluster states are the quantum analogue of the Great Red Spot, visible on the surface of Jupiter as a manifestation of classical fluid turbulence. I will describe our work on the theory of giant vortex clusters, and joint work with the BEC group at the University of Queensland to observe them for the first time in a quantum gas controlled by a digital micromirror device. Despite expectations that such high energy states should be unstable, we observe giant quantum vortex clusters with very long lifetimes. Our work confirms Onsager’s prediction after some 70 years, and opens the door to a new regime for quantum vortex matter at negative absolute temperatures, with implications for quantum turbulence, helium films, nonlinear optical materials, and fermi superfluids.

Julia Mullarney

Department of Earth & Ocean Sciences, University of Waikato

Reconciling multiple spatial and temporal scales: hydrodynamics within mangrove forests

Mangrove forests are highly productive ecosystems, which provide many physical, societal and ecological services in tropical and subtropical regions. Accurate prediction of the morphological evolution of these areas, in the face of global sea level rise and changes in sediment supply, requires understanding of interactions between vegetation elements, water flows, and sediment transport. This talk will provide an overview of hydrodynamics within mangrove forests. These salt-tolerant trees are characterised by complex aerial root systems (pneumatophores), which protrude many centimetres above the seabed. As the tide propagates into the forest, tide and wave energy is converted into dissipative wake-scale turbulence, through the processes of vortex shedding and eddy generation. We present unique small-scale field measurements, which reveal ‘hotspots’ of intense turbulence, with values sometimes reaching those observed in surf zones. Turbulence was particularly elevated in the fringing regions between mudflat and forest, although there existed substantial spatial variability. On the forest scale, the enhanced drag exerted on the water column by pneumatophores and tree trunks slows the flow and causes the water surface to tilt. We then observe a rotation of the obliquely incident flows toward an orientation nearly perpendicular to the vegetated/unvegetated boundary. The momentum balances governing the large-scale flow are assessed and indicate the relative importance of friction, winds and depth-averaged pressure forces. Drag coefficients were found to be 10–30 times greater than values usually observed for bottom friction, and the drag induced from pneumatophores was dominant relative to drag from the larger, but sparser, tree trunks.

The ability to predict turbulent stresses acting on the seabed and to formulate accurate drag parameterisations are essential to our ability to reliably model the interplay between vegetation and deposition and/or erosion. Knowledge of these processes is crucial for our understanding of the overall resilience of delta systems, which face an uncertain future worldwide.