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The World War I book
Combining authoritative, exciting text and bold images, The World War I Book explores the historical background of the war, its causes, all of the key events across the major theatres of conflict, and its aftermath. Using the original, graphic-led approach of the series, entries profile more than 90 of the key events during and surrounding the conflict -- from the growing tensions between Europe's major powers to the assassination of Archduke Franz Ferdinand, the German invasion of Belgium, the endless slaughter in the trenches, the American entry into the war, the Russian Revolution, the Armistice, and the creation of the League of Nations.
BOOK
Convection-Driven Melting near the Grounding Lines of Ice Shelves and Tidewater Glaciers
Subglacial meltwater draining along the bed of fast-flowing, marine-terminating glaciers emerges at the grounding line, where the ice either goes afloat to form an ice shelf or terminates in a calving face. The input of freshwater to the ocean provides a source of buoyancy and drives convective motion alongside the ice–ocean interface. This process is modeled using the theory of buoyant plumes that has previously been applied to the study of the larger-scale circulation beneath ice shelves. The plume grows through entrainment of ocean waters, and the heat brought into the plume as a result drives melting at the ice–ocean interface. The equations are nondimensionalized by using scales appropriate for the region where the subglacial drainage, rather than the subsequent addition of meltwater, supplies the majority of the buoyancy forcing. It is found that the melt rate within this region can be approximated reasonably well by a function that is linear in ocean temperature, has a cube root dependence on the flux of subglacial meltwater, and has a complex dependency on the slope of the ice–ocean interface. The model is used to investigate variability in melting induced by changes in both ocean temperature and subglacial discharge for a number of realistic examples of ice shelves and tidewater glaciers. The results show how warming ocean waters and increasing subglacial drainage both generate increases in melting near the grounding line.
Journal Article
Shear, Stability and Mixing within the Ice-Shelf-Ocean Boundary Current
When the inclined base of an ice shelf melts into the ocean, it induces both a statically-stable stratification and a buoyancy-forced, sheared flow along the interface. Understanding how those competing effects influence the dynamical stability of the boundary current is the key to quantifying the turbulent transfer of heat from far-field ocean to ice. The implications of the close coupling between shear, stability and mixing are explored with the aid of a one-dimensional numerical model that simulates density and current profiles perpendicular to the ice. Diffusivity and viscosity are determined using a mixing length model within the turbulent boundary layer and empirical functions of the gradient Richardson number in the stratified layer below. Starting from rest, the boundary current is initially strongly stratified and dynamically stable, slowly thickening as meltwater diffuses away from the interface. Eventually, the current enters a second phase where dynamical instability generates a relatively well-mixed, turbulent layer adjacent to the ice, while beneath the current maximum, strong stratification suppresses mixing in the region of reverse shear. Under weak buoyancy forcing the timescale for development of the initial dynamical instability can be months or longer, but background flows, which are always present in reality, provide additional current shear that greatly accelerates the process. A third phase can be reached when the ice shelf base is sufficiently steep, with dynamical instability extending beyond the boundary layer into regions of geostrophic flow, generating a marginally-stable pycnocline through which the heat flux is a simple function of ice-ocean interfacial slope.
Journal Article
A Simple Model of the Ice Shelf–Ocean Boundary Layer and Current
Ocean-forced basal melting has been implicated in the widespread thinning of Antarctic ice shelves, but an understanding of what determines melt rates is hampered by limited knowledge of the buoyancy- and frictionally controlled flows along the ice shelf base that regulate heat transfer from ocean to ice. In an attempt to address this deficiency, a simple model of a buoyant boundary flow, considering only the spatial dimension perpendicular to the boundary, is presented. Results indicate that two possible flow regimes exist: a weakly stratified, geostrophic cross-slope current with upslope flow within a buoyant Ekman layer or a strongly stratified, upslope current with a weak cross-slope flow. The latter regime, which is analogous to the steady solution for a katabatic wind, is most appropriate when the ice–ocean interface is steep. For the gentle slopes typical of Antarctic ice shelves, the buoyant Ekman regime, which has similarities with the case of an unstratified density current on a slope, provides some useful insight. When combined with a background flow, a range of possible near-ice current profiles emerge as a result of arrest or enhancement of the upslope Ekman transport. A simple expression for the upslope transport can be formed that is analogous to that for the wind-forced surface Ekman layer, with curvature of the ice shelf base replacing the wind stress curl in driving exchange between the Ekman layer and the geostrophic current below.
Journal Article
West Antarctic Ice Sheet retreat in the Amundsen Sea driven by decadal oceanic variability
Mass loss from the Amundsen Sea sector of the West Antarctic Ice Sheet has increased in recent decades, suggestive of sustained ocean forcing or an ongoing, possibly unstable, response to a past climate anomaly. Lengthening satellite records appear to be incompatible with either process, however, revealing both periodic hiatuses in acceleration and intermittent episodes of thinning. Here we use ocean temperature, salinity, dissolved-oxygen and current measurements taken from 2000 to 2016 near the Dotson Ice Shelf to determine temporal changes in net basal melting. A decadal cycle dominates the ocean record, with melt changing by a factor of about four between cool and warm extremes via a nonlinear relationship with ocean temperature. A warm phase that peaked around 2009 coincided with ice-shelf thinning and retreat of the grounding line, which re-advanced during a post-2011 cool phase. These observations demonstrate how discontinuous ice retreat is linked with ocean variability, and that the strength and timing of decadal extremes is more influential than changes in the longer-term mean state. The nonlinear response of melting to temperature change heightens the sensitivity of Amundsen Sea ice shelves to such variability, possibly explaining the vulnerability of the ice sheet in that sector, where subsurface ocean temperatures are relatively high.
Journal Article
West Antarctic ice loss influenced by internal climate variability and anthropogenic forcing
Recent ice loss from the West Antarctic Ice Sheet has been caused by ocean melting of ice shelves in the Amundsen Sea. Eastward wind anomalies at the shelf break enhance the import of warm Circumpolar Deep Water onto the Amundsen Sea continental shelf, which creates transient melting anomalies with an approximately decadal period. No anthropogenic influence on this process has been established. Here, we combine observations and climate model simulations to suggest that increased greenhouse gas forcing caused shelf-break winds to transition from mean easterlies in the 1920s to the near-zero mean zonal winds of the present day. Strong internal climate variability, primarily linked to the tropical Pacific, is superimposed on this forced trend. We infer that the Amundsen Sea experienced decadal ocean variability throughout the twentieth century, with warm anomalies gradually becoming more prevalent, offering a credible explanation for the ongoing ice loss. Existing climate model projections show that strong future greenhouse gas forcing creates persistent mean westerly shelf-break winds by 2100, suggesting a further enhancement of warm ocean anomalies. These wind changes are weaker under a scenario in which greenhouse gas concentrations are stabilized.
Journal Article
Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf
The ice shelf buttressing Antarctica’s Pine Island Glacier has been melting rapidly. Observations taken between 1994 and 2009 show that meltwater production has increased by about 50% since 1994, as a result of a stronger circulation below the ice shelf.
In 1994, ocean measurements near Antarctica’s Pine Island Glacier showed that the ice shelf buttressing the glacier was melting rapidly
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. This melting was attributed to the presence of relatively warm, deep water on the Amundsen Sea continental shelf. Heat, salt and ice budgets along with ocean modelling provided steady-state calving and melting rates
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,
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. Subsequent satellite observations and modelling have indicated large system imbalances, including ice-shelf thinning and more intense melting, glacier acceleration and drainage basin drawdown
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,
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,
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,
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,
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. Here we combine our earlier data with measurements taken in 2009 to show that the temperature and volume of deep water in Pine Island Bay have increased. Ocean transport and tracer calculations near the ice shelf reveal a rise in meltwater production by about 50% since 1994. The faster melting seems to result mainly from stronger sub-ice-shelf circulation, as thinning ice has increased the gap above an underlying submarine bank on which the glacier was formerly grounded
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. We conclude that the basal melting has exceeded the increase in ice inflow, leading to the formation and enlargement of an inner cavity under the ice shelf within which sea water nearly 4 °C above freezing can now more readily access the grounding zone.
Journal Article
Two-timescale response of a large Antarctic ice shelf to climate change
A potentially irreversible threshold in Antarctic ice shelf melting would be crossed if the ocean cavity beneath the large Filchner–Ronne Ice Shelf were to become flooded with warm water from the deep ocean. Previous studies have identified this possibility, but there is great uncertainty as to how easily it could occur. Here, we show, using a coupled ice sheet-ocean model forced by climate change scenarios, that any increase in ice shelf melting is likely to be preceded by an extended period of reduced melting. Climate change weakens the circulation beneath the ice shelf, leading to colder water and reduced melting. Warm water begins to intrude into the cavity when global mean surface temperatures rise by approximately 7 °C above pre-industrial, which is unlikely to occur this century. However, this result should not be considered evidence that the region is unconditionally stable. Unless global temperatures plateau, increased melting will eventually prevail.
New simulations find that one of Antarctica’s largest ice shelves, the Filchner–Ronne, may be less vulnerable to climate change than previously thought. Melting of the ice shelf initially decreases for many decades, and only increases when global warming exceeds approximately 7 °C.
Journal Article
Star Stream Velocity Distributions in Cold Dark Matter and Warm Dark Matter Galactic Halos
The dark matter subhalos orbiting in a galactic halo perturb the orbits of stars in thin stellar streams. Over time, the random velocities in the streams develop non-Gaussian wings. The rate of velocity increase is approximately a random walk at a rate proportional to the number of subhalos, primarily those in the mass range ≈106−7 M ⊙. The distribution of random velocities in long streams is measured in simulated Milky Way–like halos that develop in representative warm dark matter (WDM) and cold dark matter (CDM) cosmologies. The radial velocity distributions are well modeled as the sum of a Gaussian and an exponential. The resulting Markov Chain Monte Carlo fits find Gaussian cores of 1−2 km s−1 and exponential wings that increase from 3 km s−1 for 5.5 keV WDM, 4 km s−1 for 7 keV WDM, to 6 km s−1 for a CDM halo. The observational prospects to use stream measurements to constrain the nature of galactic dark matter are discussed.
Journal Article
Strong Sensitivity of Pine Island Ice-Shelf Melting to Climatic Variability
Pine Island Glacier has thinned and accelerated over recent decades, significantly contributing to global sea-level rise. Increased oceanic melting of its ice shelf is thought to have triggered those changes. Observations and numerical modeling reveal large fluctuations in the ocean heat available in the adjacent bay and enhanced sensitivity of ice-shelf melting to water temperatures at intermediate depth, as a seabed ridge blocks the deepest and warmest waters from reaching the thickest ice. Oceanic melting decreased by 50% between January 2010 and 2012, with ocean conditions in 2012 partly attributable to atmospheric forcing associated with a strong La Niña event. Both atmospheric variability and local ice shelf and seabed geometry play fundamental roles in determining the response of the Antarctic Ice Sheet to climate.
Journal Article