Keith Nicholls

British Antarctic Survey | BAS · Polar Oceans

Thwaites Glacier represents 15% of the ice discharge from the West Antarctic Ice Sheet and influences a wider catchment1–3. Because it is grounded below sea level4,5, Thwaites Glacier is thought to be susceptible to runaway retreat triggered at the grounding line (GL) at which the glacier reaches the ocean6,7. Recent ice-flow acceleration2,8 and re...

Because ice shelves respond to climatic forcing over a range of time scales, from years to millennia, an understanding of their long-term history is critically needed for predicting their future evolution. We present the first detailed reconstruction of the Larsen C Ice Shelf (LCIS), eastern Antarctic Peninsula (AP), based on data from sediment cor...

The Retrospective Analysis of Antarctic Tracking Data (RAATD) is a Scientific Committee for Antarctic Research project led jointly by the Expert Groups on Birds and Marine Mammals and Antarctic Biodiversity Informatics, and endorsed by the Commission for the Conservation of Antarctic Marine Living Resources. RAATD consolidated tracking data for mul...

The overturning circulation of the global ocean is critically shaped by deep-ocean mixing, which transforms cold waters sinking at high latitudes into warmer, shallower waters. The effectiveness of mixing in driving this transformation is jointly set by two factors: the intensity of turbulence near topography and the rate at which well-mixed bounda...

Ocean‐driven basal melting of Amundsen Sea ice shelves has triggered acceleration, thinning, and grounding line retreat on many West Antarctic outlet glaciers. Here we present the first year‐long (2014) record of basal melt rate at sub‐weekly resolution from a location on the outer Pine Island Ice Shelf. Adjustment of the upper thermocline to local...

Calving of glacial ice into the ocean from the Greenland Ice Sheet is an important component of global sea level rise. The calving process itself is relatively poorly observed, understood, and modeled; as such, it represents a bottleneck in improving future global sea level estimates in climate models. We organized a pilot project to observe the ca...

The huge cavities beneath floating Antarctic ice shelves have only been explored recently by autonomous underwater vehicles (AUVs). Oceanic waters above the in situ freezing point melt those faces of marine-terminating glaciers and ice shelves with which they come into contact. This, in turn, impacts the dynamics of ice sheets as the reduction in b...

The population of Weddell seals in the southern Weddell Sea is unique in its proximity to the edge of the Antarctic continental shelf. The purpose of this study was to describe the post-moult distribution of the population, and to understand how this unique habitat is exploited. Twenty Conductivity-Temperature-Depth Satellite Relay Data Loggers (CT...

The ApRES (autonomous phase-sensitive radio-echo sounder) instrument is a robust, lightweight and relatively inexpensive radar that has been designed to allow long-term, unattended monitoring of ice-shelf and ice-sheet thinning. We describe the instrument and demonstrate its capabilities and limitations by presenting results from three trial campai...

Locally grounded features in ice shelves, called ice rises and rumples, play a key role buttressing discharge from the Antarctic Ice Sheet and regulating its contribution to sea level. Ice rises typically rise several hundreds of meters above the surrounding ice shelf; shelf flow is diverted around them. On the other hand, shelf ice flows across ic...

Diffusive convection-favorable thermohaline staircases are observed directly beneath George VI Ice Shelf, Antarctica. A thermohaline staircase is one of the most pronounced manifestations of double-diffusive convection. Cooling and freshening of the ocean by melting ice produces cool, freshwater above the warmer, saltier water, the water mass distr...

Antarctica's ice shelves are thinning at an increasing rate, affecting their buttressing ability. Channels in the ice shelf base unevenly distribute melting, and their evolution provides insight into changing subglacial and oceanic conditions. Here we used phase-sensitive radar measurements to estimate basal melt rates in a channel beneath the curr...

Turbulence profile measurements made on the upper continental slope and shelf of the southeastern Weddell Sea reveal striking contrasts in dissipation and mixing rates between the two sites. The mean profiles of dissipation rates from the upper slope are 1-2 orders of magnitude greater than the profiles collected over the shelf in the entire water...

Microstructure shear, temperature, and conductivity observations from a tethered profiler have been made beneath George VI Ice Shelf to examine processes driving vertical heat flux in the oceanic turbulent boundary layer. Such measurements at the ice-ocean interface within the cavity of an ice shelf are unprecedented, requiring the deployment of a...

The exchange between the open ocean and sub-ice shelf cavities is important to both water mass transformations and ice shelf melting. Here, the authors use a high-resolution (500 m) numerical model to investigate to which degree eddies produced by frontal instability at the edge of a polynya are capable of transporting dense high-salinity shelf wat...

To capture the austral summer to winter transition in water mass properties over the southern Weddell Sea continental shelf and slope region, 19 Weddell seals were tagged with miniaturized conductivity–temperature–depth sensors in February 2011. During the following 8 months the instruments yielded about 9000 temperature–salinity profiles from a pr...

Basal melting of Antarctic ice shelves plays an important role in ice sheet dynamics, as ice shelf loss allows the flow of inland glaciers to accelerate. Observed ice shelf thinning leads to suggestions of an increasing basal melt rate, yet given the inaccessibility of the ice shelf-ocean interface, the melt rate, vertical heat flux and processes t...

Sediment cores recovered after the break-up of the Larsen-B Ice Shelf show that it had been stable throughout the Holocene (past 11,500 years). This result led to the suggestion that recent ice shelf retreat on the Antarctic Peninsula (AP) was unprecedented, on Holocene timescales, and that we have entered a period of unparalleled climatic change....

the last decade, several hundred seals have been equipped with conductivity-temperature-depth sensors in the Southern Ocean for both biological and physical oceanographic studies. A calibrated collection of seal-derived hydrographic data is now available, consisting of more than 165,000 profiles. The value of these hydrographic data within the exis...

Observation of the thickness of Antarctic ice shelves is vital for predicting the rise in sea levels caused by melting of the world’s largest fresh water reservoir. This paper proposes a ground-based radar system using a MIMO phased array antenna layout to accurately monitor thickness changes and provide dimensional cross-range information of the i...

A model pulsed wavefield is presented that enables the behaviour of the associated wave dislocations to be computed exactly. It consists of three intersecting beams of plane-wave pulses. Because the results depend essentially only on the relative time delays of the three pulses at any given point, the computations have to be done only once, and the...

Two vertical ice temperature profiles from locations in the Antarctic Peninsula unaffected by meltwater are presented. A simple time-dependent heat diffusion-advection model is used to infer broad surface temperature variations in the Antarctic Peninsula over the century prior to the start of local meteorological records.Air temperature records fro...

Pulsed wavefields contain moving lines, called wave dislocations, where the amplitude is zero. As the lines move they sweep out surfaces called dislocation trajectories. The authors describe an experiment with ultrasound designed to test the theoretical prediction of Wright and Nye (1982) that, for small bandwidth, the trajectories are close to par...