Modelling the ocean circulation beneath the Ross Ice Shelf

first_imgWe applied a modified version of the Miami isopycnic coordinate ocean general circulation model (MICOM) to the ocean cavity beneath the Ross Ice Shelf to investigate the circulation of ocean waters in the sub-ice shelf cavity, along with the melting and freezing regimes at the base of the ice shelf. Model passive tracers are utilized to highlight the pathways of waters entering and exiting the cavity, and output is compared with data taken in the cavity and along the ice shelf front. High Salinity Shelf Water on the western Ross Sea continental shelf flows into the cavity along the sea floor and is transformed into Ice Shelf Water upon contact with the ice shelf base. Ice Shelf Water flows out of the cavity mainly around 180°, but also further east and on the western side of McMurdo Sound, as observed. Active ventilation of the region near the ice shelf front is forced by seasonal variations in the density structure of the water column to the north, driving rapid melting. Circulation in the more isolated interior is weaker, leading to melting at deeper ice and refreezing beneath shallower ice. Net melting over the whole ice shelf base is lower than other estimates, but is likely to increase as additional forcings are added to the model.last_img read more

Frost flowers on sea ice as a source of sea salt and their influence on tropospheric halogen chemistry

first_imgFrost flowers grow on newly-formed sea ice from a saturated water vapour layer. They provide a large effective surface area and a reservoir of sea salt ions in the liquid phase with triple the ion concentration of sea water. Recently, frost flowers have been recognised as the dominant source of sea salt aerosol in the Antarctic, and it has been speculated that they could be involved in processes causing severe tropospheric ozone depletion events during the polar sunrise. These events can be explained by heterogeneous autocatalytic reactions taking place on salt-laden ice surfaces which exponentially increase the reactive gas phase bromine (“bromine explosion”). We analyzed tropospheric bromine monoxide (BrO) and the sea ice coverage both measured from satellite sensors. Our model based interpretation shows that young ice regions potentially covered with frost flowers seem to be the source of bromine found in bromine explosion events.last_img

The radiative effect of a fir canopy on a snowpack

first_imgModels of snow processes in areas of possible large-scale change need to be site independent and physically based. Here, the accumulation and ablation of the seasonal snow cover beneath a fir canopy has been simulated with a new physically based snow–soil vegetation–atmosphere transfer scheme (Snow-SVAT) called SNOWCAN. The model was formulated by coupling a canopy optical and thermal radiation model to a physically based multilayer snow model. Simple representations of other forest effects were included. These include the reduction of wind speed and hence turbulent transfer beneath the canopy, sublimation of intercepted snow, and deposition of debris on the surface. This paper tests this new modeling approach fully at a fir site within Reynolds Creek Experimental Watershed, Idaho. Model parameters were determined at an open site and subsequently applied to the fir site. SNOWCAN was evaluated using measurements of snow depth, subcanopy solar and thermal radiation, and snowpack profiles of temperature, density, and grain size. Simulations showed good agreement with observations (e.g., fir site snow depth was estimated over the season with r2 = 0.96), generally to within measurement error. However, the simulated temperature profiles were less accurate after a melt–freeze event, when the temperature discrepancy resulted from underestimation of the rate of liquid water flow and/or the rate of refreeze. This indicates both that the general modeling approach is applicable and that a still more complete representation of liquid water in the snowpack will be important.last_img read more

Future hot water drilling on Rutford Ice Stream 2004/05

first_imgIce streams are major drainage routes, through which much of the ice in Antarctica flows from the continent. They flow at speeds of up to two orders of magnitude greater that the rest of the ice sheet and are believed to rest on beds of soft, water-saturated sediments. As sliding and sediment deformation processes affect the ice stream dynamics, understanding them is essential to futurepredictions of ice sheet variations. Rutford Ice Stream provides one such example of a fast flowing glacier constrained by a deep bedrock trough and is one of a number of ice streams that drain the West Antarctic Ice Sheet into the southwestern Ronne Ice Shelf (Figure1). Extensivefieldwork has been conducted on this ice stream over the past 25 years with measurements of strain, velocity and elevation. Also, ice sounding radars and seismic techniques have been used to measure the ice thickness and examine the nature of the bed at several locations [Doake et al.,2001; Smith, 1997]. Many of these measurements have been made in an area approximately 40 km upstream of the grounding line. Here, the aim is to access the bed of Rutford Ice Stream at least twice through almost 2200 m of ice using a hot water drill. Ice cores will be retrieved from selected depths using a hot water ice core drill based on the Caltech design [Engelhardt et al., 2000] and samples of basal sediment will also be recovered. The hole will then be instrumented to measure basal sliding, bed and ice column deformation, basal water pressure and ice temperature. In situ optical images of the bed and any sediment within the ice may also be taken.last_img read more

Comparison of mid-Pliocene climate predictions from the HadAM3 and GCMAM3 General Circulation Models

first_imgThe mid-Pliocene warm period (ca. 3 to 3.3 million years ago) has become an important interval of time forpalaeoclimate modelling exercises, with a large number of studies published during the last decade. However,there has been no attempt to assess the degree of model dependency of the results obtained. Here we presentan initial comparison of mid-Pliocene climatologies produced by the Goddard Institute for Space Studies andHadley Centre for Climate Prediction and Research atmosphere-only General Circulation Models (GCMAM3and HadAM3). Whilst both models are consistent in the simulation of broad-scale differences inmid-Pliocenesurface air temperature and total precipitation rates, significant variation is noted on regional and local scales.There are also significant differences in the model predictions of total cloud cover. A terrestrial data/modelcomparison, facilitated by the BIOME 4 model and a new data set of Piacenzian Stage land cover [Salzmann, U.,Haywood, A.M., Lunt, D.J., Valdes, P.J., Hill, D.J., (2008). A new global biome reconstruction and data modelcomparison for the Middle Pliocene. Global Ecology and Biogeography 17, 432-447, doi:10.1111/j.1466-8238.2007.00381.x] and combined with the use of Kappa statistics, indicates that HadAM3-based biomepredictions provide a closer fit to proxy data in the mid to high-latitudes. However, GCMAM3-based biomes inthe tropics provide the closest fit to proxy data.last_img read more

Flow and retreat of the Late Quaternary Pine Island-Thwaites palaeo-ice stream, West Antarctica

first_imgMultibeam swath bathymetry and sub-bottom profiler data are used to establish constraints on the flow and retreat history of a major palaeo-ice stream that carried the combined discharge from the parts of the West Antarctic Ice Sheet now occupied by the Pine Island and Thwaites glacier basins. Sets of highly elongated bedforms show that, at the last glacial maximum, the route of the Pine Island-Thwaites palaeo-ice stream arced north-northeast following a prominent cross-shelf trough. In this area, the grounding line advanced to within similar to 68 km of, and probably reached, the shelf edge. Minimum ice thickness is estimated at 715 m on the outer shelf, and we estimate a minimum ice discharge of similar to 108 km(3) yr(-1) assuming velocities similar to today’s Pine Island glacier (similar to 2.5 km yr(-1)). Additional bed forms observed in a trough northwest of Pine Island Bay likely formed via diachronous ice flows across the outer shelf and demonstrate switching ice stream behavior. The “style” of ice retreat is also evident in five grounding zone wedges, which suggest episodic deglaciation characterized by halts in grounding line migration up-trough. Stillstands occurred in association with changes in ice bed gradient, and phases of inferred rapid retreat correlate to higher bed slopes, supporting theoretical studies that show bed geometry as a control on ice margin recession. However, estimates that individual wedges could have formed within several centuries still imply a relatively rapid overall retreat. Our findings show that the ice stream channeled a substantial fraction of West Antarctica’s discharge in the past, just as the Pine Island and Thwaites glaciers do today.last_img read more

Streaming flow of an Antarctic Peninsula palaeo-ice stream, both by basal sliding and deformation of substrate

first_imgAcoustic sub-bottom profiler surveys on the northeast Antarctic Peninsula shelf indicate that parts of the seabed are underlain by an acoustically transparent layer that is thin on the inner shelf and becomes thicker and more extensive towards the outer shelf. Sedimentological and geophysical data are combined to construct a bed model where streaming ice flow, by both deformation and basal sliding, took place within cross-shelf troughs. The model suggests only limited deformation contributed to fast flow on the inner shelf, i.e. in the onset zone of ice streaming, where the bed was predominantly underlain by a stiff till. Thus, fast ice flow in this area might have been by basal sliding, with deformation confined to discontinuous patches of soft till <40 cm thick. Towards the middle and outer shelf, extensive, thick sequences of soft till suggest a change in the dominant subglacial process towards widespread deformation. This downstream change from basal sliding to subglacial deformation is manifest in the transition from stiff-till dominance to soft-till dominance, while a downstream increase in ice flow velocity is evident from the complex geomorphic imprint on the inner shelf evolving to the more restricted set of bedforms on the outer shelf.last_img read more

Survey of Saturn Z-mode Emission

first_imgBecause of the role of Z-mode emission in the diffusive scattering and resonant acceleration of electrons, we conduct a survey of intensity in the Saturn inner magnetosphere. Z-mode is primarily observed as “5 kHz” narrowband emission in the lower density regions where the ratio of cyclotron to plasma frequency, fc/fp > 1 to which we limit this study. This occurs at Saturn along the inner edge of the Enceladus torus near the equator and at higher latitudes. We present profiles and parametric fits of intensity as a function of frequency, radius, latitude, and local time. The magnetic field intensity levels are lower than chorus, but the electric field intensities are comparable. We conclude that Z-mode wave-particle interactions may make a significant contribution to electron acceleration in the inner magnetosphere of Saturn, supplementing acceleration produced by chorus emission.last_img

Intense winter surface melt on an Antarctic ice shelf

first_imgThe occurrence of surface melt in Antarctica has hitherto been associated with the austral summer season, when the dominant source of melt energy is provided by solar radiation. We use in‐situ and satellite observations from a previously unsurveyed region to show that events of intense surface melt on Larsen C Ice Shelf occur frequently throughout the dark Antarctic winter, with peak intensities sometimes exceeding summertime values. A regional atmospheric model confirms that, in the absence of solar radiation, these multi‐day melt events are driven by outbreaks of warm and dry föhn wind descending down the lee side of the Antarctic Peninsula mountain range, resulting in downward turbulent fluxes of sensible heat that drive sustained surface melt fluxes in excess of 200 W m−2. From 2015 to 2017 (including the extreme melt winter of 2016), ∼23% of the annual melt flux was produced in winter, and spaceborne observations of surface melt since 2000 show that wintertime melt is widespread in some years. Winter melt heats the firn layer to the melting point up to a depth of ∼3 m, thereby facilitating the formation of impenetrable ice layers, and retarding or reversing autumn and winter cooling of the firn. While the absence of a trend in winter melt is consistent with insignificant changes in the observed southern hemisphere atmospheric circulation during winter, we anticipate an increase in winter melt as a response to increasing greenhouse gas concentration.last_img read more

Changes in penguin distribution over the Antarctic Peninsula and Scotia Arc (Version 2.0)

first_imgRobust evidence highlights major shifts in the abundance and distribution of penguins (Adélie, chinstrap, gentoo, emperor, king, and macaroni) breeding on the Antarctic Peninsula and across the Scotia Arc in FAO statistical SubAreas 48.1 to 48.4. In SubArea 48.1, Adélies and chinstraps have declined throughout most of the western Antarctic Peninsula (WAP) to the north of Marguerite Bay. Adélies are stable or increasing in Marguerite Bay and to the south, and stable or increasing in the eastern Antarctic Peninsula. By contrast, gentoos on the WAP (48.1) and at the South Orkney Islands (48.2) are increasing and expanding their breeding range southwards; elsewhere, their populations are highly variable but not trending significantly. In SubArea 48.3, macaronis have experienced substantial declines while kings have increased. In SubArea 48.4, chinstraps and Adélies are stable. These findings highlight considerable spatial heterogeneity in species trends, and the importance of up to date census data to underpin work to predict and monitor future trends.last_img read more