Physical processes in Antarctica and the Southern Ocean are of great importance to the global climate system. This thesis considers two such processes, namely ice- ocean interaction in ice shelf basal crevasses and the conditional instability of frazil ice growth. It has been suggested that freezing within basal crevasses can act as a stabilising in uence on ice shelves, preventing their break up. Using Fluidity, a nite element ocean model, it is found that ocean circulation within a crevasse is highly dependent upon the amount of freezing in the crevasse. It is also found that frazil ice formation is responsible for the vast majority of freezing within a crevasse, and that there is a non linear relationship between the amount of supercooling in a crevasse and its freeze rate. The conditional instability of frazil ice growth is a little investigated mechanism of ice growth. Any frazil forming in the water column reduces the bulk density of a parcel of frazil-seawater mixture, causing it to rise. Due to the pressure-decrease in the freezing point, this causes more frazil to form, causing the parcel to accelerate, and so on. Numerical modelling nds that the instability does not operate in the presence of strong strati cation, high thermal driving (warm water), a small initial perturbation, high 'background' mixing or the prevalence of large frazil ice crystals. Given a large enough initial perturbation this instability could allow signi cant rates of ice growth even in water that is above the freezing point. The research presented in this thesis forms the material for two peer-reviewed publi- cations; 'Modelling ice ocean interactions in ice shelf basal crevasses' (Jordan et al., 2014) and 'On the conditional frazil ice instability in seawater' (Jordan et al., 2015).

Several attributes of the last British-Irish Ice Sheet (BIIS) make it a potentially important analogue to marine-based sectors of modern ice sheets, which are sensitive to oceanic and climatic changes and unstable during the current global climate warming. However, limitations on knowledge of the last BIIS's marine termini hinder the use of this ice mass in comparative assessments or modelling. This research addresses this critical knowledge gap through multiproxy examinations of BIIS marine landforms, sedimentation and ice-proximal palaeoenvironments. A multifaceted examination of the outer shelf (Porcupine Bank and Slyne Trough) using geomorphology, sedimentology and geochronology provides the first confirmation of BIIS shelf-wide extension during the last glaciation (Late Midlandian/Late Devensian) west of Ireland. Multivariate biofacies examinations from the outer shelf suggest the introduction of relatively warm and saline Atlantic water following BIIS decoupling and ice shelf formation. Shelfwide analyses of glacigenic deposits (till and glaciomarine) and landforms (moraines and grounding-zone wedges) provide the first estimates of BIIS marine-margin retreat rates and detailed reconstructions of BIIS marine-margin behaviour west of Ireland. These analyses reveal a major stillstand (<3,300 years) that punctuated overall retreat and deposited a large (~IS0-km long) grounding-zone wedge. Prior to this stillstand, the BIIS was stabilised by a buttressing ice shelf and its grounding line retreated at ~ 74 km/yr; following the stillstand and the loss of the buttressing ice shelf, the BIIS marine-margin retreated to the modern western Irish shoreline at an accelerated rate of ~ 113 m/yr. Radiocarbon analyses of glacitectonised glaciomarine sediment record the first evidence of an Irish ice mass readvancing offshore during the Nahanagan (Younger Dryas) Stadia. Shelf-wide palaeoenvironmental assessments using both novel and well-established multivariate and statistical techniques elucidate a previously unknown, likely climate-driven readvance (the Galway Lobe readvance) and suggest that initial BIIS marine-margin retreat was forced by climate or ocean warming.

Over the last three decades, Antarctic Peninsula Ice Shelves have shown a pattern of sustained retreat, often ending in catastrophic and rapid breakup. This study provides a detailed analysis of the structures and dynamics of Bach, George VI and Stange Ice Shelves, situated on the southwest Antarctic Peninsula, to determine their current and future stability on the premise that glaciological changes can be identified well in advance of final breakup phases. Spatial extent and glaciological surface features were mapped for each ice shelf from 1973 to 2010 using optical and radar satellite images to assess their structural stability, structural evolution and historical dynamics. A combination of InSAR and feature tracking methods has been used to assess the recent dynamic configurations of the ice shelves from 1989 to 2010, with repeat ICESat measurements used to evaluate their vertical changes from 2003 to 2008. On Bach Ice Shelf, the formation of two large fractures near the ice front is linked to widespread thinning (~2 ma-1) and sustained retreat (~360 km2). It is postulated that iceberg calving along these fractures will alter the frontal geometry sufficiently to promote enhanced, irreversible retreat within the next decade. On George VI Ice Shelf, acceleration is observed at both ice fronts linked to a release of back-stresses through continued ice loss (1995 km2 in total). The most significant changes are recorded at its southern ice front, with ice flow accelerating up to 360% between ca. 1989 and ca. 2010, coupled with widespread rifting and a mean thinning rate of 2.1 ma-1. On Stange Ice Shelf, shear-induced fracturing was observed between two flow units, also linked to widespread thinning (~4.2 ma-1) illustrating a response of southwest Antarctic Peninsula Ice Shelves within the proposed limit of viability. A semi-quantitative assessment reveals that the southern margin of George VI Ice Shelf is most susceptible to rapid retreat, whilst its northern ice front, Bach Ice Shelf and the northern front of Stange Ice Shelf are more vulnerable than those situated on the east Antarctic Peninsula.

The sea ice microstructure is permeated by millimetre to micrometre sized inclusions filled with concentrated seawater-derived brine. It is within these brines that the in-situ chemical and biological reactions occur. The brines are confined to a temperature-dependent composition, becoming more concentrated and reducing in volume with decreasing temperature. Upon sufficient cooling the coupled effects of lower temperature and higher salinity results in the brine exceeding the solubility of a mineral, which precipitates. Given the complex composition of seawater, there are several minerals that can exceed saturation within the polar temperature spectrum, each with their own dynamics and environmental significance. This thesis investigates mirabilite (Na2SO4 10H2O), gypsum (CaSO4 2H2O) and hydrohalite (NaCl 2H2O). The small crystal size ( m), temperature dependence, and solubility of these minerals acts to limit the scope for studying their existence and behaviour in sea ice with field experiments. For these reasons, their dynamics have been investigated in a laboratory setting using synchrotron X-ray powder distraction experiments, and measurements of mineral solubility in solutions representative of sea ice brines at thermal equilibrium. The experimental observations are supplemented with model predictions, and together are used to provide a comprehensive assessment of the existence, role and e ects of mineral precipitation in sea ice. Mirabilite and hydrohalite are found to cause substantial changes to brine composition and the sea ice microstructure, and are observed to interact in accordance with equilibrium crystallisation. The precipitation of mirabilite is also found to have implications for the measurement of sea ice brine salinity. In contrast, the solubility of gypsum displays complex dynamics between 0.2 and -25 °C, and is shown to be highly dependent upon the SO²⁻₄ concentration, resulting in the processes of mirabilite precipitation and dissolution controlling the fate of gypsum in sea ice.

No description available.

Satellite observations play an important role in ice identification services because they are cost effective and efficient compared to extensive field campaigns. Radar data are extensively used to derive information about sea ice extent and move- ment. In the first part of this thesis I adapt a semi-automated algorithm, originally developed by Silva (2006) to track large icebergs in Antarctic waters, to track the movement of ice in the Northern Hemisphere. In addition to the move from Antarctic to Arctic waters, the algorithm is adapted to track sea ice rather than icebergs, with an attendant change in the shape of the tracked objects and their radar backscatter characteristics. The algorithm development is focused on the identification of appropriate image segmentation, brightness thresholding, and shape parameters appropriate to the identification and tracking of sea ice floes throughout the year. These developments are tested on images from a variety of locations, and from different SAR sensors. Recent literature documents the warming of the Arctic region (Alexandrov et al., 2004; Serreze et al., 2007) with an accompanying decline in sea ice cover (Kwok and Rothrock, 2009). The identification of ice extent and movement is an important tool in the study of climate variability (Spreen et al., 2006; Bochert, 1999); for example the magnitude of the sea ice flux through the Fram Strait is a measure of net ice production in the Arctic Ocean (Widell et al., 2003). The Fram Strait is of key importance for the export of ice from the Arctic (Kwok and Rothrock, 1999; Kwok et al., 2004) and well known for the presence of strong surface currents (Dickson et al., 2007; Fahrbach et al., 2001). In the second part of the thesis I investigate the competing influences of atmospheric and oceanographic forcings on ice export through the Fram Strait. The focus is on the western (Greenland) side of the strait between 79 - 81 ◦N. This area is within the East Greenland Current and also covers the boundary between fast ice and drift ice. The East Greenland Current, coupled with the prevailing northerly wind, is the main driver for ice export through the Fram Strait. On shorter temporal resolution ice movement is seen to be governed by the winds. Where the temporal resolution is greater than 1-2 days the influence of the East Greenland Current becomes more dominant and overall movement is towards the south. My results suggest that the prevailing wind speed and direction have a key impact on the rate of ice export through the Fram Strait. A period in which the wind forcing is in agreement with the East Greenland Current will see greater ice export than a period in which the two are acting in opposite directions.

The mid-Pliocene Warm Period (mPWP, 3.264 to 3.025 Myr ago) has been extensively studied through the use of general circulation models (GCMs). Whilst the output from these simulations replicates closely many of the patterns of the climate of the interval indicated by proxy data, at northern high latitudes the reconstructed proxy data temperatures exceed the model temperatures by over 15˚C for some sites. This data-model discrepancy highlights the importance of focusing on model representation of processes that strongly affect the northern high latitude climates. Arctic sea ice exerts a strong influence on the Arctic climate, largely due to the ice-albedo feedback mechanism, and by creating an insulating layer between the ocean and the atmosphere. Interest in Arctic sea ice and its representation in climate models has been enhanced in recent years due to the rapid decline in the September minimum sea ice extent that has been observed since the advent of satellite observations in 1979. This thesis describes the results from simulations of the mPWP with the GCM HadCM3, focusing on the simulated Arctic temperatures and sea ice. A change to the parameterisation of sea ice albedo is implemented in the model, based on recent observations of changes in the albedo of Arctic sea ice. The results show mean annual surface air temperature (SAT) increases of up to 6˚C, and mean annual sea surface temperature (SST) increases of up to 2˚C, and the disappearance of Arctic sea ice in some summer months, but very small changes in the discrepancy between the model and proxy data temperatures. The sensitivity of simulated Arctic sea ice to orbital forcings and atmospheric CO2 in HadCM3 is also explored, with the results suggesting that changes in orbital forcing are sufficient to change the simulated mid-Pliocene Arctic from perennial to seasonal sea ice, unless combined with lower CO2 concentrations. Changes to orbits and CO2 are also combined with the alternative albedo parameterisation, and further data-model comparisons are performed, with the results continuing to show cooler model temperatures, but with a reduced gap. Also shown are the simulated Arctic sea ice outputs from eight different GCMs as part of the Pliocene Modelling Intercomparison Project (PlioMIP). The comparison demonstrates the model dependency on the simulation of Arctic sea ice, as only half of the models simulate perennial Arctic sea ice in the mid-Pliocene. The dominant influences on the sea ice simulation in the ensemble are also discussed.

This thesis aims to improve our understanding of the fundamental processes affecting the growth of sea ice in the polar oceans in order to improve climate models. Newly formed sea ice contains a significant amount of salt as liquid brine in the interstices of an ice matrix. My focus is on one of the processes by which the salt content of sea ice decreases, namely convective desalination, which is also often called gravity drainage by geophysicists. Modelling convective desalination requires an understanding not only of the thermo-dynamics of sea-ice growth but also of its internal fluid dynamics. This thesis considers a class of physical systems called mushy layers, of which sea ice is an example. Mushy layers are multi-component systems consisting of a porous matrix of solid phase whose interstices contain the same substance in the liquid phase. I develop a mathematical description of these systems in terms the of mushy-layer equations and explore the appropriate boundary conditions at a mush-liquid interface. I develop a simple Chimney-Active-Passive (CAP) model of convection in mushy layers for arrays of liquid chimneys in two and three dimensions. This allows the interstitial fluid flow and salt flux from the mushy layer to be determined in terms of the dimensionless parameters of the system. I discuss important mathematical and physical aspects of the CAP model. I then explain the physics of gravity drainage from sea ice, elucidating the connection between downward flow through liquid brine channels (chimneys) and a convective upwelling in the rest of the ice that is sustained by horizontal density differences and provides the fluid to replace that which drains from the ice. I use the CAP model to determine the convective upwelling velocity mathematically, deriving a new, physical parameterization of gravity drainage. I test my predictions by investigating previous laboratory observations of the propagation of dye fronts. Finally, I take a one-dimensional, thermodynamic sea-ice model of the kind currently used in coupled climate models and parameterize convective desalination using the CAP model. The parameterization allows determination of physical properties and salt fluxes from sea ice dynamically, corresponding to the calculated, evolving salinity of the sea ice, in contrast to older, established models that prescribe a fixed salinity. I find substantial differences compared to previous models, particularly in terms of predicted salt fluxes from sea ice. I explain the likely implications and potential advantages of my parameterization for climate models.

The Arctic region plays an important role in the global climate system through various feedbacks, involving surface albedo, oceanic deep-water formation, and sea surface salinity, which can amplify climate variability and change. We investigate the exploitation of data collected by the first Earth-orbiting laser altimeter carried onboard ICESat over the sea-ice covered regions of the Arctic Ocean. We extract parameters associated with the study of the polar climate system including the time-varying component of sea surface topography and sea ice freeboard. We assess an existing method for the retrieval of Arctic sea surface height from ICESat data. We present an alternative method for sea surface height retrieval, based on surface reflectivity and analysis of parameters associated with the shape of the received echo. This method aims to discriminate echoes originating over leads or thin ice. We provide the first maps of Arctic sea surface height as derived from ICESat. We examine the accuracy of our results through comparisons with independent sea surface height estimates derived from ENVISAT radar altimetry. We demonstrate the use of sea surface height data for oceanographic and geodetic applications in the Arctic Ocean. We derive an ICESat mean sea surface which, when combined with the recently developed Arctic hybrid geoid model, can be used to analyse mean dynamic ocean topography. In addition we investigate the use of ICESat sea surface height measurements to map marine gravity anomalies up to the limit of coverage at 86 N. By combining ICESat surface elevation measurements with sea surface height estimates, we derive sea ice freeboard throughout the Arctic up to 86 N. We compare our results to coincident estimates of sea ice freeboard from ENVISAT. Finally, we explore the feasibility of combining satellite laser and radar altimetric measurements of sea ice freeboard to measure the depth of snow loading on sea ice.

Arctic sea ice loss is a robust feature of observations and of climate model projections. Amplified winter lower tropospheric warming in the Arctic relative to the global mean is associated with this ice loss. Many recent studies have addressed the possible effects of these changes on the midlatitude atmospheric circulation, particularly in the North Atlantic. These studies suggest responses including an equatorward jet shift, a negative annular mode response and changes in Rossby wave behaviour. However, there is disagreement on the magnitude, significance and even sign of these responses. Previous studies have shown the advantages of model hierarchies for understanding the atmosphere. In this thesis, experiments are conducted in HadGAM1 with simplified lower boundary conditions. Two sets of experiments are conducted, one in a zonally symmetric aquaplanet and the other in a configuration with representative northern hemisphere land masses. A wide range of sea ice profiles are imposed. The dominant response to ice removal in an aquaplanet is an equatorward jet shift, consistent with previous work. This response is moderate in magnitude for ice which does not exceed 65◦ latitude, but strongly nonlinear for greater ice extents. The zonal mean response is qualitatively similar in the asymmetric configuration, but the nature of the asymmetric response shows sensitivity to the exact ice edge location. These results have implications for understanding the impact of sea ice anomalies in past as well as present climates. Changes in surface temperature gradients, including from Arctic amplification, could affect midlatitude climate even if circulation changes are small. In particular, changes in thermal advection could alter midlatitude temperature variability and extremes. In this thesis a multiple regression model is used to investigate projected monthly temperature variance changes in a recent single model ensemble. Many robust changes, including reduced winter temperature variance in Europe, are consistent with the effect of changes in the seasonal mean temperature gradient alone.