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Following the southward progression of ice-shelf disintegration along the AP since the s, including the loss of Prince Gustav and Larsen A in and Larsen B in , the stability of LCIS would seem to be at risk.
In recent decades at least, LCIS, and particularly its northern sector, has exhibited a number of factors that are indicative of instability: Surface lowering and ice-shelf thinning on LCIS is a result of both firn air depletion and basal ice loss. The basal ice loss may be a result of reductions in ice accretion or of increases in melt or flow divergence; firn air may become depleted because of reductions in accumulation or because of enhanced surface melt and refreezing within the firn.
Recent trends in surface melt parameters on the Antarctic Peninsula, such as onset date, duration, or intensity, depend on the time period under consideration. When analyses include the first decade of the 21st century, during which time AP mean annual temperatures decreased , the trends based on PDDs remained positive but less steep.
The various studies do, however, reveal a large amount of interannual variability, with annual meltwater volume varying by a factor of 4 during the period from to During years in which surface melt periods are long and intense, widespread ponds may form on ice shelves.
Such ponds have been proposed as a trigger for ice-shelf break-up, either by enhancing the hydrofracture of existing crevasses or via the stresses induced by hydrostatic rebound following drainage , which may lead to runaway disintegration. A recent study based on a borehole drilled in Cabinet Inlet, profiles of temperature and density, ground-penetrating radar, firn density modelling, and satellite images identified a massive subsurface body of anomalously warm and dense ice.
This body of ice was interpreted to be the result of the intense melt and regular surface ponding that occurs in this area. Almost all surface meltwater that percolates down through the snow and firn refreezes in the firn and releases latent heat, thereby increasing the density and the temperature of the subsurface layers and changing the rheology of the ice shelf.
The increase in temperature reduces the viscosity of the ice allowing enhanced ice flow relative to colder ice, potentially increasing lateral rifting and leading to the possibility of ice-shelf break-up. The increase in density may, however, compensate for the temperature effect by increasing the fracture toughness and stabilizing the ice against crevassing. Without direct observations of density and temperature, modelling experiments designed to investigate the stability of the ice shelf must tune rheological parameters to minimize the misfit between modelled and observed velocity fields e.
In late we drilled four boreholes in addition to that reported by , consisting of two directly downstream from Cabinet Inlet and two downstream from Whirlwind Inlet Fig. Firn density may be derived from the luminosity of the recorded image and relies on an empirical relationship between image brightness and density, with lower reflectivity corresponding to denser ice.
The five borehole images are described in detail in and ; in summary, across the sites, four different ice types, or units, were identified on the basis of visual appearance, density, and refrozen ice content. Image thresholding was used to determine the proportion of ice within each unit that is composed of refrozen infiltration ice. The ice in U3 is homogeneous with only diffuse layering.
Extreme melt events are required to allow a sufficient quantity of mobile meltwater to percolate down and add to the previous upper surface of U3. U4 is present at the base of the CI-0 and WI-0 boreholes. In this study we use a flow-line model to investigate where and when various units of melt-affected ice observed within the Cabinet Inlet and four other boreholes originated, relate the origins to past local climate, and estimate how much of the ice shelf will have been affected by various intensities of surface melt.
Following and , a flow-line model was constructed for LCIS which simulates the advection and submergence of surface layers along trajectories passing through the borehole sites. We define trajectory as the surface route based on velocity information only. From any specified starting point, surface trajectories which allow the path length through and hence time spent in each grid cell to be determined were created.
The upstream and downstream limits of these trajectories were determined by the spatial extent of the velocity data. Next, a series of flow lines were initiated at selected points along the trajectories so that from these points the accumulated snow and ice thicknesses match the depths of the interfaces between the different units at each of the borehole sites. The grey shading represents the recorded luminosity of the ice, and the white profile represents the inferred density using the x -axis scale.
The coloured panels are the different ice type units referred to in the text. The figures are reproduced from where the methods are described in more detail.
The same velocity dataset, smoothed with a low-pass 4. Annual means of surface mass balance SMB for the period — were calculated for a domain covering the AP and surrounding seas at a horizontal resolution of approximately 5. Despite this apparently improved dataset being available prior to final publication, we have not updated our analysis because the new dataset will not significantly impact on our results, discussion, or conclusions and is probably not an improvement for the context in which we are using it.
We converted surface mass fluxes to thickness using density derived from our borehole profiles from borehole CI along flow lines from Cabinet Inlet and WI along flow lines from Whirlwind Inlet. At each step along the flow line, after accumulating the appropriate amount of surface mass, the density was adjusted to the depth-mean density appropriate to the total thickness accumulated up to that point.
In this way we modelled the natural compression of accumulated firn as it was advected downstream. In order to predict the time evolution of the near-surface firn density profile at a single location within Cabinet Inlet we ran a one-dimensional firn densification and hydrology model FDM.
The model is driven by mass fluxes, wind speed, and surface temperature from RACMO2 and takes into account firn compaction, meltwater percolation, and refreezing. The shaded boundaries represent the uncertainty estimates as described in the text. Each flow line, triggered from a point along each of the Cabinet and Whirlwind Inlet trajectories, allows an age to be estimated for the transition or interface between the units observed in the borehole images and also for the bases of the boreholes Table.
The flow lines also pinpoint where on the ice-shelf surface the transition between units originated Fig. Uncertainties in dates and distances travelled may be a result of measurement velocity, density or model SMB error. The timescale is from the beginning of the trajectory. The borehole ice units correspond to those described in and are described in the text. No dates are calculated for the bases of CI-0 and WI-0 as they originate upstream of the available velocity data.
Depths of the unit interfaces observed in borehole images , and age of surface origin based on the flow-line modelling. Ages in parentheses are the uncertainty ranges. Throughout the time series the firn is interspersed with high-density layers caused by intermittent melt events.
A major melt event in — removed all firn, and the subsequent years of high melt combined with low accumulation left the dense ice within a metre of the surface until By , as reported in , a 2. Interfaces between the ice units identified in the borehole logs, when traced back to the surface using the flow-line model, may be interpreted in terms of either spatial or temporal changes in surface melt. For example, a transition from U1 a unit with only sporadic melt layers down into U2 a unit with a high proportion of melt—refreeze within a borehole might be a result of ice flow having passed from a region of high to low melt conditions or a result of a temporal switch from high to low surface melt conditions at the time the ice in transition was at the surface.
In discussing the transitions we refer to the upper and younger unit first and then the lower and older unit; for example, the boundary between U3 and U4 at CI-0 is referred to as U3—U4. Thus the flow-line model confirms that U4 in both CI-0 and WI-0 borehole logs is continental ice, which is in line with. Therefore, if basal crevasses are found only in continental or basal accreted ice, firn compaction under climate warming may not be a factor which would contribute to increasing penetration depths.
Time evolution of firn density as a function of depth for Cabinet Inlet, predicted by the firn density model. U3 ice, which by its nature would have required sufficient surface melt to allow percolation and refreezing in continuous vertical units, and probably also surface ponding, is only observed in the Cabinet Inlet boreholes Fig.
At CI-0, it lies beneath 2. The lower section of U3 at CI extends from We can use the lower boundaries of the U3 sections to estimate the earliest date at which surface ponds could have been forming, although the potentially mobile nature of large volumes of meltwater means that the actual origins of U3 could have been much later in time and farther downstream.
For the U3—U4 interface at CI-0 The FDM model Fig. There is ample evidence from station observations of significant warming in the AP region during the second half of the 20th century including the Orcadas and Vernadsky stations. Isotopic analyses of ice cores from Ferrigno on the coast of West Antarctica and Gomez on the south-western AP as well as of borehole temperatures on the Bruce Plateau also indicate a warming trend since the s.
The Southern Hemisphere annular mode SAM or the Antarctic oscillation index describes the difference in zonal-mean geopotential heights between mid- and high latitudes which drives the strength and latitude of the subpolar westerly winds.
In the late s the SAM entered a phase of increasing positive indices, particularly in summer and fall , indicating a strengthening of the Antarctic circumpolar vortex, bringing strong westerly air flow to the AP. At meteorological stations to the north-east of the AP, positive SAM indices in summer and autumn are correlated with high air temperatures. A period of intense surface melt in —, generated by unusually high frequencies of north-westerly winds, probably triggered the break-up of Larsen B Ice Shelf.
We therefore propose, on the basis of the borehole evidence, the flow-line model results for the U3—U1 interface at CI and climatology that the ponding which has led to the formation of the upper sections of U3 has become a feature of Cabinet Inlet only during the second half of the 20th century and maybe not until the early s.
Either we are underestimating the SMB in the model or there is a significant amount of lateral influx of meltwater, or both, in the region upstream of CI Both the modelled depth of the U4 continental ice at CI and the comparison of the U4 origin with the interferometric grounding line suggest SMB in Cabinet Inlet may be underestimated; from a modelling perspective it is the regions closest to the mountains and downstream of the dominant zonal wind component which may be most affected by poor topographic resolution.
Lateral influx of meltwater is also a real possibility in this locality. By analogy with observations within the percolation zone of the Greenland Ice Sheet , the formation of spatially discontinuous impermeable near-surface layers of ice following melt—refreeze events would facilitate horizontal flow of meltwater along and across the troughs in which the melt ponds form.
Vertical infiltration at the boundaries of the ice barriers would result in a horizontally heterogeneous distribution of U3 type bodies. The importance of horizontal liquid water transport and its dependence on surface slope on the Greenland Ice Sheet is emphasized by. As described in , borehole CI-0 was drilled into a melt-pond trough which might be expected to contain a local concentration of infiltration ice. The existence of a deep section of U3 at CI between This warming interval repeatedly produced temperatures equivalent to those seen in the latter half of the 20th century and may have conditioned the ice in Cabinet Inlet to a density at which surface ponding could occur, in a manner similar to that which has probably been occurring since the recent AP warming which began in the mids.
In other words, we find that the formation of a deep U3 section at CI coincided with an anomalously warm period during the first half of the 18th century. Unit 2 is only seen at the base of the borehole at CI Fig. U2 is not as homogeneous as U3, but it does contain evidence of intense melt and densification exceeding that expected from compaction metamorphism alone. U2 at the base of CI was therefore probably also generated by the 18th century warming discussed earlier in connection with U3 although U2 is not as dense and bubble-free as U3.
Along the trajectory originating in Whirlwind Inlet, it is only as we reach WI that we predict borehole sampling of ice as old as the U2 and U3 units at the bases of the boreholes at CI and CI, respectively. That we do not sample any U2 or U3 ice at WI is compatible with the clear north-to-south decrease in summer melt duration currently observed on LCIS.
We have used a flow-line model to trace the surface origins of distinct units of ice observed in boreholes across LCIS. From boreholes imaged along the Cabinet Inlet flow line we can deduce that warming from the midth century preconditioned the surface of the ice shelf within the inlet until it became sufficiently impermeable to support surface ponds. The earliest possible date for 20th century pond formation, based on the Firn density modelling starting from indicates that the surface was able to support ponds by , and satellite observations confirm that ponds were present by the late s.
Intense melt on the northern part of LCIS, and probably ponding within Cabinet Inlet, also occurred during the 18th century, corresponding with an earlier period of warming over the AP identified in ice-core temperature reconstructions. Unit 3 ice at the base of the CI borehole was forming up until and U2 at the base of the CI borehole up until The pattern of melt reflected in the borehole logs, including the absence of U2 and U3 down flow of Whirlwind Inlet, suggests that past as well as recent melt is reflected in the current spatial distribution of firn air content.
Below this depth the shelf ice must consist either of continental ice or accreted marine ice. This vertical heterogeneity has implications for determining the resistance of the shelf to fracture, with it being the meteoric ice that exhibits the least resistance to rifting. The previous AP warm period in the 18th century is captured in the stratigraphy of the shelf and is further evidence of the link between atmospheric warming and the collapse or decay of eastern AP ice shelves throughout the Holocene.