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THE ICE SHELVES OF ANTARCTICA

Posted on: August 11, 2020

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Antarctica's Ross Ice Shelf, World's Largest, is Melting in a Way ...

Science of the Ross Ice Shelf

NASA space lasers to reveal new depths of planet's ice loss

 

THIS POST IS A CRITICAL REVIEW OF CLAIMS ABOUT UNUSUAL ICE SHELF MELT IN ANTARCTICA AND THE ATTRIBUTION OF THE MELT TO AGW CLIMATE CHANGE

 

 

PART-1: ATTRIBUTIONS OF ANTARCTIC ICE SHELF MELT TO AGW

  1. In the prior interglacial, the Eemian, the West Antarctic Ice Sheet (WAIS) had disintegrated and created a sudden and rapid sea level rise of 3 to 6 meters or perhaps as high as 5 to 9 meters. Details of this are reported in a related post [LINK] . In the AGW climate change era, these events have created an expectation of a similar fate for the WAIS with similar catastrophic sea level rise that may serve to motivate climate action in the form of moving the world’s energy infrastructure from fossil fuels to renewable energy. Recently, after 40 years of failed alarming forecasts of the imminent demise of the WAIS and a catastrophic sea level rise [LINK], this line of research was given a boost with the availability of satellite data from ICE-SAT-2 of ice conditions on Antarctica [LINK] . Here we describe the new findings reported from Ice-Sat-2 data in the new era of Antarctica and critical commentary with reference to the atmosphere bias of climate science in such evaluations that imply inadequate consideration of geological effects in West Antarctica, a region known to be geologically active [LINK] and a single minded obsession of describing all ice melt phenomena in terms of atmospheric composition and thereby related to fossil fuels and the need for climate action. Climate science is therefore best understood as anti fossil fuel activism [LINK]
  2. The new ice data from ICE-SAT-2 have rejuvenated the interest in the “Antarctica melting” line of research. This post is a review of a recent paper on this topic that we feel is representative of the current view of the ice melt impact of climate change on West Antarctica in the ICE-SAT-2 era. It should be noted that a geographical bias of this line of research that favors West Antarctica corresponds with high levels of geological activity in this region. These details of the WAIS are not considered in the interpretation of ice melt data provided by ICE-SAT-2. Here we offer a critical review of one such paper reviewed online by Phys.Org {Satellite record gives unprecedented view of Antarctic ice shelf melt pattern over 25 years, by Robert Monroe, University of California – San Diego} [LINK]  and published in Nature Geoscience:  CITATION:  Adusumilli, Susheel, et al. “Interannual variations in meltwater input to the Southern Ocean from Antarctic ice shelves.” Nature Geoscience (2020): ABSTRACT: The Antarctic ice sheet has been losing mass over past decades through the accelerated flow of its glaciers, conditioned by ocean temperature and bed topography. Glaciers retreating along retrograde slopes (that is, the bed elevation drops in the inland direction) are potentially unstable, while subglacial ridges slow down the glacial retreat. Despite major advances in the mapping of subglacial bed topography, significant sectors of Antarctica remain poorly resolved and critical spatial details are missing. Here we present a novel, high-resolution and physically based description of Antarctic bed topography using mass conservation. Our results reveal previously unknown basal features with major implications for glacier response to climate change. For example, glaciers flowing across the Transantarctic Mountains are protected by broad, stabilizing ridges. Conversely, in the marine basin of Wilkes Land, East Antarctica, we find retrograde slopes along Ninnis and Denman glaciers, with stabilizing slopes beneath Moscow University, Totten and Lambert glacier system, despite corrections in bed elevation of up to 1 km for the latter. This transformative description of bed topography redefines the high- and lower-risk sectors for rapid sea level rise from Antarctica; it will also significantly impact model projections of sea level rise from Antarctica in the coming centuries.

Summary of the paper by Phys.Org:  A science team led by researchers at Scripps Institution of Oceanography at UC San Diego has created a detailed history of mass loss from Antarctica’s floating ice shelves. The researchers used a 25-year record of data from four separate European Space Agency satellite missions, NASA ice velocity data, and outputs from NASA computer models to find that these ice shelves have experienced a loss of nearly 4,000 gigatons since 1994—producing an amount of meltwater that can nearly fill the Grand Canyon—as a result of melting from increased heat in the ocean under the ice shelves. This is the most convincing evidence so far that long-term changes in the Southern Ocean are the reason for ongoing Antarctic ice loss,” said lead author Susheel Adusumilli. “It’s incredible that we are able to use satellites that orbit above the earth to see changes in regions of the ocean where even ships can’t go.” The NASA-funded study appears Aug. 10 in the journal Nature Geoscience and includes co-authors from NASA’s Goddard Space Flight Center, Earth and Space Research in Corvallis, Ore. and Colorado School of Mines. Detailed information on Antarctic ice shelves is hard to come by because of their vast size, and the difficulty for scientists to physically reach them. Satellites allow for year-round monitoring and are the only practical way to routinely collect information on Antarctic ice loss. For this study, the team used data from European Space Agency radar satellites, which send radio waves to the ground up to 20,000 times a second and measure the travel time of those waves as they bounce back to the satellite. Researchers can use that information to determine the precise height of land or ice. The result of analyzing these radar signals is the first-ever analysis of changes in melt of all Antarctic ice shelves, which collectively cover an area of 1.5 million square kilometers (580,000 square miles) – more than three times larger than California. The 25-year record showed that there is a lot of variation around Antarctica in the rate at which the ocean is melting the ice shelves, but in total, there is more loss than gain of ice shelf mass. Although ice shelf loss itself does not directly contribute to sea-level rise because ice shelves are already floating, ice shelves do act as a buffer to help slow the slide of ice sheets from land into the ocean, and when they become smaller this effect is weakened. If the West Antarctic Ice Sheet were to completely melt into the ocean, it would raise sea levels worldwide by around 3 meters (10 feet). Although that amount of melt is unlikely in the coming decades, even four inches of sea-level rise can double the frequency of flooding on the U.S. West Coast. The researchers also identified the depths in the ocean at which melting is occurring. This is important, they said, because increased melting of ice shelves has major environmental consequences beyond global sea-level rise. Melting ice produces water that is colder and fresher than the surrounding ocean. Depending on where this water ends up in the ocean, it can have a large effect on ocean circulation and climate around the globe. “We now have a continuous and detailed record of how all the ice shelves have changed since the mid 1990s, and where the meltwater has entered the ocean,” said Scripps Oceanography glaciologist Helen Amanda Fricker, a co-author. “This will allow us to decipher the atmospheric and ocean forces responsible for the changes, and how the meltwater affects the ocean, allowing us to improve models that predict future sea-level rise.”

 

ANTARCTICA GEOTHERMAL HEAT CURIE DEPTH

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PART-2: CRITICAL COMMENTARY

    1. CLAIM: “These ice shelves have experienced a loss of nearly 4,000 gigatons since 1994—producing an amount of meltwater that can nearly fill the Grand Canyon. RESPONSE: The total melt over 26 years of 4,000 gigatons is less than 2% of the weight of one large ice shelf. If only one ice shelf is melting, it melted at a rate of 0.077% per year and if that melt rate persists it will be gone in another thousand years or so. It is noted that ice shelf melt does not have a sea level rise interpretation. The melt rate appears to be rather insignificant and that insignificance is the likely reason that the Grand Canyon was invoked. This kind of data analysis made on behalf of the science of climate science by a scientific organization like Phys.Org is the real horror of climate change. 
    2. CLAIM: —as a result of melting from increased heat in the ocean under the ice shelves. RESPONSE:  It is a given that the melt implies the availability of heat but the more pertinent issue is the source of the heat energy, specifically, whether the source of the heat is anthropogenic global warming or whether it is natural as for example geothermal heat. The region under study is geologically active with significant geological features under the ice and under the water as seen in the bibliography below. As well, the location of the ice shelf melt events tend to favor locations known for geological activity and that in turn implies a role for geological heat sources. Several studies have identified hydrothermal vents as the heat source that warms the Deep Circumpolar Circulation that can transfer heat to the ice shelves as seen in the bibliography below. 
    3. CLAIMThis is the most convincing evidence so far that long-term changes in the Southern Ocean are the reason for ongoing Antarctic ice loss,”  RESPONSE:  The long term changes in the Southern Ocean in terms of climate change and warm water currents from the tropics may transfer some heat to the deep Antarctic Circumpolar Current but significant evidence is provided in the bibliography below (see for example the various works of Stephanie Downes) that a more credible and readily available source of heat that explains the relative warmth of the Circumpolar Current is the extensive  hydrothermal vent activity where the the Circumpolar Current crosses the mid-ocean ridge. Therefore, it is has not been established that the source of heat in the warming of the Circumpolar Current is anthropogenic global warming.  
    4. CLAIM:  The 25-year record showed that there is a lot of variation around Antarctica in the rate at which the ocean is melting the ice shelves, but in total, there is more loss than gain of ice shelf mass.  RESPONSEThat there is a lot of variation provides more support for local variable sources of geological heat (see bibliography below) as opposed to a uniform global source of heat in the form of anthropogenic global warming. 
    5. CLAIM: If the West Antarctic Ice Sheet were to completely melt into the ocean, it would raise sea levels worldwide by around 3 meters (10 feet). Although that amount of melt is unlikely in the coming decades, even four inches of sea-level rise can double the frequency of flooding on the U.S. West CoastRESPONSE: The continued saga in AGW climate change theory about a collapse of the WAIS and the consequent catastrophic sea level rise is derived not from Holocene [LINK] realities but from what happened in the previous interglacial, the Eemian [LINK]  when the climate was dramatically different from what we see in the Holocene. This line of illogical and unscientific climate fearology obsession with Eemian-like sea level rise caused by the disintegration of the WAIS [LINK] has now persisted for more than 40 years with failed forecast after failed forecast. Here we see that this comical chapter in climate science continues to this day unabated by the 100% failures of the past. In any case, ice melt in West Antarctica does not necessarily have a climate change interpretation because the region is geologically active in terms of the West Antarctic Rift system and the Marie Byrd mantle plume as described in a related post [LINK] and in the bibliography. The attribution of  ice melt events to anthropogenic global warming in such an active geological zone would require substantive unbiased empirical evidence to support the proposed causation mechanism.
    6. CLAIM We now have a continuous and detailed record of how all the ice shelves have changed since the mid 1990s, and where the meltwater has entered the ocean. This will allow us to decipher the atmospheric and ocean forces responsible for the changes, and how the meltwater affects the ocean, allowing us to improve models that predict future sea-level rise.”  RESPONSE:  Fully agree with this assessment. Now that you have the melt data, all  features of the region must be considered to determine how this melt occurs. Particularly so, it is imperative that the study of ice melt phenomena in this corner of the globe must not be studied exclusively in terms of anthropogenic global warming because these events are unique to this region that is also known for unique geological features that can provide the necessary heat balance for the observed melt. 

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kamis-byrd area

 

PART-3: THE RELEVANT BIBLIOGRAPHY

  1. Fisher, Andrew T., et al. “High geothermal heat flux measured below the West Antarctic Ice Sheet.” Science advances 1.6 (2015): e1500093.  The geothermal heat flux is a critical thermal boundary condition that influences the melting, flow, and mass balance of ice sheets, but measurements of this parameter are difficult to make in ice-covered regions. We report the first direct measurement of geothermal heat flux into the base of the West Antarctic Ice Sheet (WAIS), below Subglacial Lake Whillans, determined from the thermal gradient and the thermal conductivity of sediment under the lake. The heat flux at this site is 285 ± 80 mW/m2, significantly higher than the continental and regional averages estimated for this site using regional geophysical and glaciological models. Independent temperature measurements in the ice indicate an upward heat flux through the WAIS of 105 ± 13 mW/m2. The difference between these heat flux values could contribute to basal melting and/or be advected from Subglacial Lake Whillans by flowing water. The high geothermal heat flux may help to explain why ice streams and subglacial lakes are so abundant and dynamic in this region.
  2. Herterich, Klaus. “A three-dimensional model of the Antarctic ice sheet.” Annals of glaciology 11 (1988): 32-35.  A preliminary version of a three-dimensional ice-sheet model for later use in climate models, but excluding ice shelves and basal sliding, is presented and applied to the Antarctic ice sheet. In the model, the three-dimensional fields of velocity and temperature are calculated in the coupled mode, and the temperature equation is integrated for 150 000 years; the shape of the Antarctic ice sheet remains fixed. The results from the model are consistent with a stationary state in the central parts of the Antarctic ice sheet, but not in marginal areas, where the flow in the model is too small. Including a parameterized form of basal sliding that is dependent on the water pressure is likely to improve the situation. [FULL TEXT]
  3. Engelhardt, Hermann. “Ice temperature and high geothermal flux at Siple Dome, West Antarctica, from borehole measurements.” Journal of Glaciology 50.169 (2004): 251-256.  A vertical temperature profile through the West Antarctic ice sheet (WAIS) at the summit of Siple Dome reveals an elevated geothermal flux. This could be the root cause for the existence of a dynamic ice-stream system in the WAIS. Siple Dome is still frozen on its bed, but adjacent ice streams have bed temperatures at the pressure-melting point of ice. Although present-day temperature increases due to climatic change do not have an immediate effect on the basal conditions that control the velocity of the ice, indirect effects like a rapid disintegration of the floating ice shelves or additional melt-water input at the surface could give rise to speed-up of the ice streams with an ensuing rise in sea level. Ongoing melt at the base of the ice and changes at the margins will allow continued rapid flow of the ice streams with a possibility of disintegration, within a relatively short period of time, of at least part of the WAIS. [FULL TEXT]
  4. Schroeder, Dustin M., et al. “Evidence for elevated and spatially variable geothermal flux beneath the West Antarctic Ice Sheet.” Proceedings of the National Academy of Sciences 111.25 (2014): 9070-9072.  Heterogeneous hydrologic, lithologic, and geologic basal boundary conditions can exert strong control on the evolution, stability, and sea level contribution of marine ice sheets. Geothermal flux is one of the most dynamically critical ice sheet boundary conditions but is extremely difficult to constrain at the scale required to understand and predict the behavior of rapidly changing glaciers. This lack of observational constraint on geothermal flux is particularly problematic for the glacier catchments of the West Antarctic Ice Sheet within the low topography of the West Antarctic Rift System where geothermal fluxes are expected to be high, heterogeneous, and possibly transient. We use airborne radar sounding data with a subglacial water routing model to estimate the distribution of basal melting and geothermal flux beneath Thwaites Glacier, West Antarctica. We show that the Thwaites Glacier catchment has a minimum average geothermal flux of ∼114 ± 10 mW/m2 with areas of high flux exceeding 200 mW/m2 consistent with hypothesized rift-associated magmatic migration and volcanism. These areas of highest geothermal flux include the westernmost tributary of Thwaites Glacier adjacent to the subaerial Mount Takahe volcano and the upper reaches of the central tributary near the West Antarctic Ice Sheet Divide ice core drilling site. [FULL TEXT]
  5. Begeman, Carolyn Branecky, Slawek M. Tulaczyk, and Andrew T. Fisher. “Spatially variable geothermal heat flux in West Antarctica: evidence and implications.” Geophysical Research Letters 44.19 (2017): 9823-9832.  Geothermal heat flux (GHF) is an important part of the basal heat budget of continental ice sheets. The difficulty of measuring GHF below ice sheets has directly hindered progress in the understanding of ice sheet dynamics. We present a new GHF measurement from below the West Antarctic Ice Sheet, made in subglacial sediment near the grounding zone of the Whillans Ice Stream. The measured GHF is 88 ± 7 mW m−2, a relatively high value compared to other continental settings and to other GHF measurements along the eastern Ross Sea of 55 mW m−2 and 69 ± 21 mW m−2 but within the range of regional values indicated by geophysical estimates. The new GHF measurement was made ~100 km from the only other direct GHF measurement below the ice sheet, which was considerably higher at 285 ± 80 mW m−2, suggesting spatial variability that could be explained by shallow magmatic intrusions or the advection of heat by crustal fluids. Analytical calculations suggest that spatial variability in GHF exceeds spatial variability in the conductive heat flux through ice along the Siple Coast. Accurate GHF measurements and high‐resolution GHF models may be necessary to reliably predict ice sheet evolution, including responses to ongoing and future climate change. [FULL TEXT]
  6. Carson, Chris J., et al. “Hot rocks in a cold place: high sub-glacial heat flow in East Antarctica.” Journal of the Geological Society 171.1 (2014): 9-12.  Numerical models are the primary predictive tools for understanding the dynamic behaviour of the Antarctic ice sheet. However, a key boundary parameter, sub-glacial heat flow, remains poorly constrained. We show that variations in abundance and distribution of heat-producing elements within the Antarctic continental crust result in greater and more variable regional sub-glacial heat flows than currently assumed in ice modelling studies. Such elevated heat flows would have a fundamental effect on ice sheet behaviour and highlight that geological controls on heat flow must be considered to obtain more accurate and refined predictions of ice mass balance and sea-level change[FULL TEXT]
  7. Larour, E., et al. “Ice flow sensitivity to geothermal heat flux of Pine Island Glacier, Antarctica.” Journal of Geophysical Research: Earth Surface 117.F4 (2012).  Model projections of ice flow in a changing climate are dependent on model inputs such as surface elevation, bedrock position or surface temperatures, among others. Of all these inputs, geothermal heat flux is the one for which uncertainty is greatest. In the area of Pine Island Glacier, Antarctica, available data sets differ by up to a factor of 2.5. Here, we evaluate the impact of such uncertainty on ice flow, using sampling analyses based on the Latin‐Hypercube method. First, we quantify the impact of geothermal heat flux errors on ice hardness, a thermal parameter that critically controls the magnitude of ice flow. Second, we quantify the impact of the same errors on mass balance, specifically on the mass flux advecting through thirteen fluxgates distributed across Pine Island Glacier. We contrast our results with similar uncertainties generated by errors in the specification of ice thickness. Model outputs indicate that geothermal heat flux errors yield uncertainties on ice hardness on the order of 5–7%, with maximum uncertainty reaching 15%. Resulting uncertainties in mass balance remain however below 1%. We discuss the uncertainty distribution and its relationship to the amount of heat available at the base of the ice sheet from friction, viscous and geothermal heating. We also show that comparatively, errors in ice thickness contribute more to model uncertainty than errors in geothermal heat flux, especially for fast‐flowing ice streams. [FULL TEXT]
  8. Thoma, Malte, et al. “Modelling circumpolar deep water intrusions on the Amundsen Sea continental shelf, Antarctica.” Geophysical Research Letters 35.18 (2008). [FULL TEXT]   Results are presented from an isopycnic coordinate model of ocean circulation in the Amundsen Sea, focusing on the delivery of Circumpolar Deep Water (CDW) to the inner continental shelf around Pine Island Bay. The warmest waters to reach this region are channeled through a submarine trough, accessed via bathymetric irregularities along the shelf break. Temporal variability in the influx of CDW is related to regional wind forcing. Easterly winds over the shelf edge change to westerlies when the Amundsen Sea Low migrates west and south in winter/spring. This drives seasonal on‐shelf flow, while inter‐annual changes in the wind forcing lead to inflow variability on a decadal timescale. A modelled period of warming following low CDW influx in the late 1980’s and early 1990’s coincides with a period of observed thinning and acceleration of Pine Island Glacier.
  9. Moffat, C., B. Owens, and R. C. Beardsley. “On the characteristics of Circumpolar Deep Water intrusions to the west Antarctic Peninsula continental shelf.” Journal of Geophysical Research: Oceans 114.C5 (2009).  [FULL TEXT]  Hydrographic and current velocity observations collected from March 2001 to February 2003 on the west Antarctic Peninsula shelf as part of the Southern Ocean Global Ecosystems Dynamics program are used to characterize intrusions of Upper Circumpolar Deep Water (UCDW) and Lower Circumpolar Deep Water (LCDW) onto the shelf and Marguerite Bay. UCDW is found on the middle and outer shelf along Marguerite Trough, which connects the shelf break to Marguerite Bay, and at another location farther south. UCDW intrudes in the form of frequent (four per month) and small horizontal scales (≈4 km) warm eddy‐like structures with maximum vertical scales of a few hundred meters. However, no evidence of UCDW intrusions was found in Marguerite Bay. LCDW was found in several deep depressions connected to the shelf break, including Marguerite Trough, forming a tongue of relatively dense water 95 m thick (on average) that reaches into Marguerite Bay through Marguerite Trough. A steady advective‐diffusive balance for the LCDW intrusion is used to make an estimation of the average upwelling rate and diffusivity in the deep layer within Marguerite Trough, which suggest the LCDW layer is renewed approximately every six weeks.
  10. Wåhlin, A. K., et al. “Inflow of warm Circumpolar Deep Water in the central Amundsen shelf.” Journal of Physical Oceanography 40.6 (2010): 1427-1434. [FULL TEXT]   The thinning and acceleration of the West Antarctic Ice Sheet has been attributed to basal melting induced by intrusions of relatively warm salty water across the continental shelf. A hydrographic section including lowered acoustic Doppler current profiler measurements showing such an inflow in the channel leading to the Getz and Dotson Ice Shelves is presented here. The flow rate was 0.3–0.4 Sv (1 Sv ≡ 106 m3 s−1), and the subsurface heat loss was estimated to be 1.2–1.6 TW. Assuming that the inflow persists throughout the year, it corresponds to an ice melt of 110–130 km3 yr−1, which exceeds recent estimates of the net ice glacier ice volume loss in the Amundsen Sea. The results also show a 100–150-m-thick intermediate water mass consisting of Circumpolar Deep Water that has been modified (cooled and freshened) by subsurface melting of ice shelves and/or icebergs. This water mass has not previously been reported in the region, possibly because of the paucity of historical data.
  11. Dinniman, Michael S., John M. Klinck, and Walker O. Smith Jr. “A model study of Circumpolar Deep Water on the West Antarctic Peninsula and Ross Sea continental shelves.” Deep Sea Research Part II: Topical Studies in Oceanography 58.13-16 (2011): 1508-1523.  [FULL TEXT]   Transport of relatively warm, nutrient-rich Circumpolar Deep Water (CDW) onto continental shelves around Antarctica has important effects on physical and biological processes. However, the characteristics of the CDW along the shelf break, as well as what happens to it once it has been advected onto the continental shelf, differ spatially. In the present study high resolution (4–5 km) regional models of the Ross Sea and the West Antarctic Peninsula coastal ocean are used to compare differences in CDW transport. The models compared very well with observations from both regions. Examining the fluxes not only of heat, but also of a simulated “dye” representing CDW, shows that in both cases CDW crosses the shelf break in specific locations primarily determined by the bathymetry, but eventually floods much of the shelf. The frequency of intrusions in Marguerite Trough was ca. 2–3 per month, similar to recent mooring observations. A significant correlation between the along shelf break wind stress and the cross shelf break dye flux through Marguerite Trough was observed, suggesting that intrusions are at least partially related to short duration wind events. The primary difference between the CDW intrusions on the Ross and west Antarctic Peninsula shelves is that there is more vigorous mixing of the CDW with the surface waters in the Ross Sea, especially in the west where High Salinity Shelf Water is created. The models show that the CDW moving across the Antarctic Peninsula continental shelf towards the base of the ice shelves not only is warmer initially and travels a shorter distance than that advected towards the base of the Ross Ice Shelf, but it is also subjected to less vertical mixing with surface waters, which conserves the heat available to be advected under the ice shelves. This difference in vertical mixing also likely leads to differences in the supply of nutrients from the CDW into the upper water column, and thus modulates the impacts on surface biogeochemical processes.
  12. Steig, Eric J., et al. “Tropical forcing of Circumpolar Deep Water inflow and outlet glacier thinning in the Amundsen Sea Embayment, West Antarctica.” Annals of Glaciology 53.60 (2012): 19-28. [FULL TEXT]   Outlet glaciers draining the Antarctic ice sheet into the Amundsen Sea Embayment (ASE) have accelerated in recent decades, most likely as a result of increased melting of their ice-shelf termini by warm Circumpolar Deep Water (CDW). An ocean model forced with climate reanalysis data shows that, beginning in the early 1990s, an increase in westerly wind stress near the continental shelf edge drove an increase in CDW inflow onto the shelf. The change in local wind stress occurred predominantly in fall and early winter, associated with anomalous high sea-level pressure (SLP) to the north of the ASE and an increase in sea surface temperature (SST) in the central tropical Pacific. The SLP change is associated with geopotential height anomalies in the middle and upper troposphere, characteristic of a stationary Rossby wave response to tropical SST forcing, rather than with changes in the zonally symmetric circulation. Tropical Pacific warming similar to that of the 1990s occurred in the 1940s, and thus is a candidate for initiating the current period of ASE glacier retreat.
  13. Mashayek, A., et al. “The role of the geothermal heat flux in driving the abyssal ocean circulation.” Geophysical Research Letters 40.12 (2013): 3144-3149[FULL TEXT]  The results presented in this paper demonstrate that the geothermal heat flux (GHF) from the solid Earth into the ocean plays a non‐negligible role in determining both abyssal stratification and circulation strength. Based upon an ocean data set, we show that the map of upward heat flux at the ocean floor is consistent (within a factor of 2) with the ocean floor age‐dependent map of GHF. The observed buoyancy flux above the ocean floor is consistent with previous suggestions that the GHF acts to erode the abyssal stratification and thereby enhances the strength of the abyssal circulation. Idealized numerical simulations are performed using a zonally averaged single‐basin model which enables us to address the GHF impact as a function of the depth dependence of diapycnal diffusivity. We show that ignoring this vertical variation leads to an under‐prediction of the influence of the GHF on the abyssal circulation. Independent of the diffusivity profile, introduction of the GHF in the model leads to steepening of the Southern Ocean isopycnals and to strengthening of the eddy‐induced circulation and the Antarctic bottom water cell. The enhanced circulation ventilates the GHF derived heating to shallow depths, primarily in the Southern Ocean.
  14. Stewart, Andrew L., and Andrew F. Thompson. “Eddy‐mediated transport of warm Circumpolar Deep Water across the Antarctic shelf break.” Geophysical Research Letters 42.2 (2015): 432-440. [FULL TEXT]  The Antarctic Slope Front (ASF) modulates ventilation of the abyssal ocean via the export of dense Antarctic Bottom Water (AABW) and constrains shoreward transport of warm Circumpolar Deep Water (CDW) toward marine‐terminating glaciers. Along certain stretches of the continental shelf, particularly where AABW is exported, density surfaces connect the shelf waters to the mid-depth Circumpolar Deep Water offshore, offering a pathway for mesoscale eddies to transport CDW directly onto the continental shelf. Using an eddy‐resolving process model of the ASF, the authors show that mesoscale eddies can supply a dynamically significant transport of heat and mass across the continental shelf break. The shoreward transport of surface waters is purely wind driven, while the shoreward CDW transport is entirely due to mesoscale eddy transfer. The CDW flux is sensitive to all aspects of the model’s surface forcing and geometry, suggesting that shoreward eddy heat transport may be localized to favorable sections of the continental slope.
  15. Downes, Stephanie M., et al.  [LINK] “The transient response of southern ocean circulation to geothermal heating in a global climate model.” Journal of Climate 29.16 (2016): 5689-5708.  Model and observational studies have concluded that geothermal heating significantly alters the global overturning circulation and the properties of the widely distributed Antarctic Bottom Water. Here two distinct geothermal heat flux datasets are tested under different experimental designs in a fully coupled model that mimics the control run of a typical Coupled Model Intercomparison Project (CMIP) climate model. The regional analysis herein reveals that bottom temperature and transport changes, due to the inclusion of geothermal heating, are propagated throughout the water column, most prominently in the Southern Ocean, with the background density structure and major circulation pathways acting as drivers of these changes. While geothermal heating enhances Southern Ocean abyssal overturning circulation by 20%–50%, upwelling of warmer deep waters and cooling of upper ocean waters within the Antarctic Circumpolar Current (ACC) region decrease its transport by 3–5 Sv (1 Sv = 106 m3 s−1). The transient responses in regional bottom temperature increases exceed 0.1°C. The large-scale features that are shown to transport anomalies far from their geothermal source all exist in the Southern Ocean. Such features include steeply sloping isopycnals, weak abyssal stratification, voluminous southward flowing deep waters and exported bottom waters, the ACC, and the polar gyres. Recently the Southern Ocean has been identified as a prime region for deep ocean warming; geothermal heating should be included in climate models to ensure accurate representation of these abyssal temperature changes.
  16. Barnes, Jowan M., et al. “Idealised modelling of ocean circulation driven by conductive and hydrothermal fluxes at the seabed.” Ocean Modelling 122 (2018): 26-35[FULL TEXT]   Geothermal heating is increasingly recognised as an important factor affecting ocean circulation, with modelling studies suggesting that this heat source could lead to first-order changes in the formation rate of Antarctic Bottom Water, as well as a significant warming effect in the abyssal ocean. Where it has been represented in numerical models, however, the geothermal heat flux into the ocean is generally treated as an entirely conductive flux, despite an estimated one third of the global geothermal flux being introduced to the ocean via hydrothermal sources.

    A modelling study is presented which investigates the sensitivity of the geothermally forced circulation to the way heat is supplied to the abyssal ocean. An analytical two-dimensional model of the circulation is described, which demonstrates the effects of a volume flux through the ocean bed. A simulation using the NEMO numerical general circulation model in an idealised domain is then used to partition a heat flux between conductive and hydrothermal sources and explicitly test the sensitivity of the circulation to the formulation of the abyssal heat flux. Our simulations suggest that representing the hydrothermal flux as a mass exchange indeed changes the heat distribution in the abyssal ocean, increasing the advective heat transport from the abyss by up to 35% compared to conductive heat sources. Consequently, we suggest that the inclusion of hydrothermal fluxes can be an important addition to course-resolution ocean models.

  17. Downes, Stephanie M., et al. “Hydrothermal heat enhances abyssal mixing in the Antarctic Circumpolar Current.” Geophysical Research Letters 46.2 (2019): 812-821. Upwelling in the world’s strongest current, the Antarctic Circumpolar Current, is thought to be driven by wind stress, surface buoyancy flux, and mixing generated from the interaction between bottom currents and rough topography. However, the impact of localized injection of heat by hydrothermal vents where the Antarctic Circumpolar Current interacts with mid‐ocean ridges remains poorly understood. Here a circumpolar compilation of helium and physical measurements are used to show that while geothermal heat is transferred to the ocean over a broad area by conduction, heat transfer by convection dominates near hydrothermal vents. Buoyant hydrothermal plumes decrease stratification above the vent source and increase stratification to the south, altering the local vertical diffusivity and diapycnal upwelling within 500 m of the sea floor by an order of magnitude. Both the helium tracer and stratification signals induced by hydrothermal input are advected by the flow and influence properties downstream.

 

 

 

 

 

2 Responses to "THE ICE SHELVES OF ANTARCTICA"

Point 5 about the WAIS melting is just so on point and well worded.

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  • chaamjamal: Thank you very much for this informative andcrelevant comment.
  • philohippous: https://polarbearscience.com is run by Susan J. Crockford. She has been studying them in detail for many years. She was the first to contest a fals
  • chaamjamal: https://principia-scientific.com/japan-reveals-first-uv-lamp-to-safely-kill-coronavirus/
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