ORCID Profile
0000-0001-8362-3446
Current Organisations
British Antarctic Survey
,
Northumbria University
,
University of New South Wales
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Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-15264
Abstract: Through their role in buttressing upstream ice flow, Antarctic ice shelves play an important part in regulating future sea level change. Reduction in ice-shelf buttressing caused by increased ocean-induced melt along their undersides is now understood to be one of the key drivers of ice loss from the Antarctic Ice Sheet. However, despite the importance of this forcing mechanism, most ice-sheet simulations currently rely on simple melt-parametrisations of this ocean-driven process since a fully coupled ice-ocean modelling framework is prohibitively computationally expensive. Here, we provide an alternative approach that can capture the greatly improved physical description of this process provided by large-scale ocean-circulation models over currently employed melt-parameterisations, but with trivial computational expense.& This new method brings together deep learning and physical modelling to develop a deep neural network framework, MELTNET, that can emulate ocean model predictions of sub-ice shelf melt rates. We train MELTNET on synthetic geometries, using the NEMO ocean model as a ground-truth in lieu of observations to provide melt rates both for training and to evaluate the performance of the trained network. We show that MELTNET can accurately predict melt rates for a wide range of complex synthetic geometries, with a normalized root mean squared error of 0.11m/yr compared to the ocean model. MELTNET calculates melt rates several orders of magnitude faster than the ocean model and outperforms more traditional parameterisations for 96% of geometries tested. Furthermore, we find MELTNET's melt rate estimates show sensitivity to established physical relationships such as changes in thermal forcing and ice shelf slope. This study demonstrates the potential for a deep learning framework to calculate melt rates with almost no computational expense, that could in the future be used in conjunction with an ice sheet model to provide predictions for large-scale ice sheet models.
Publisher: Copernicus GmbH
Date: 12-01-2022
DOI: 10.5194/TC-2021-396
Abstract: Abstract. Through their role in buttressing upstream ice flow, Antarctic ice shelves play an important part in regulating future sea level change. Reduction in ice-shelf buttressing caused by increased ocean-induced melt along their undersides is now understood to be one of the key drivers of ice loss from the Antarctic Ice Sheet. However, despite the importance of this forcing mechanism most ice-sheet simulations currently rely on simple melt-parametrisations of this ocean-driven process, since a fully coupled ice-ocean modelling framework is prohibitively computationally expensive. Here, we provide an alternative approach that is able to capture the greatly improved physical description of this process provided by large-scale ocean-circulation models over currently employed melt-parameterisations but with trivial computational expense. We introduce a new approach that brings together deep learning and physical modelling to develop a deep neural network framework, MELTNET, that can emulate ocean model predictions of sub-ice shelf melt rates. We train MELTNET on synthetic geometries, using the NEMO ocean model as a ground-truth in lieu of observations to provide melt rates both for training and to evaluate the performance of the trained network. We show that MELTNET can accurately predict melt rates for a wide range of complex synthetic geometries and outperforms more traditional parameterisations for 95 % of geometries tested. Furthermore, we find MELTNET's melt rate estimates show sensitivity to established physical relationships such as a changes in thermal forcing and ice shelf slope. This study demonstrates the potential for a deep learning framework to calculate melt rates with almost no computational expense, that could in the future be used in conjunction with an ice sheet model to provide predictions for large-scale ice sheet models.
Publisher: American Geophysical Union (AGU)
Date: 2016
DOI: 10.1002/2015JC011486
Publisher: American Geophysical Union (AGU)
Date: 07-2020
DOI: 10.1029/2019JC015889
Publisher: Copernicus GmbH
Date: 27-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-5388
Abstract: & & The climate of the polar regions is characterized by large fluctuations and has experienced dramatic changes over the past decades. In particular, the patterns of changes in sea ice and ice sheet mass are complex in the Southern Hemisphere. The Antarctic Ice Sheet has also lost mass in the past decades, especially in Western Antarctica, with a spectacular thinning and weakening of ice shelves, i.e., the floating extensions of the grounded ice sheet. Despite recent advances in observing and modelling the Antarctic climate, the mechanisms behind this long-term mass loss remain poorly understood because of the limited amount of observations and the large biases of climate models in polar regions, in concert with the large internal variability prevailing in the Antarctic. Among all the processes involved in the mass variability, changes in the general atmospheric circulation of the Southern Hemisphere may have played a substantial role. One of the most important atmospheric modes of climate variability in the Southern Ocean is the Southern Annular Mode (SAM), which represents the position and the strength of the westerly winds. During years with a positive SAM index, lower sea level pressure at high latitudes and higher sea level pressure at low latitudes occur, resulting in a stronger pressure gradient and intensified Westerlies. However, the current knowledge of the impact of these fluctuations of the Westerlies on the Antarctic cryosphere is still limited. Over the past few years, some efforts investigated the impact of the SAM on the Antarctic sea ice and the surface mass balance of the ice sheet from an atmosphere-only perspective. Recently, a few oceanic studies have focused on the local impact of SAM-related fluctuations on the ice-shelf basal melt in specific regions of Antarctica, particularly Western Antarctica. However, to our knowledge, there is no such study at the scale of the whole Antarctic continent. In this study, we performed idealized experiments with a pan-Antarctic regional ice-shelf cavity-resolving ocean - sea-ice model for different phases of the SAM. We show that positive (negative) phases lead to increased (decreased) upwelling and subsurface ocean temperature and salinity close to ice shelves. A one-standard-deviation increase of the SAM leads to a net basal mass loss of 40 Gt yr& sup& -1& /sup& , with strong regional contrasts: increased melt in the Western Pacific and Amundsen-Bellingshausen sectors and the opposite response in the Ross sector. Taking these as a baseline sensitivity, we estimate last millennium and end-of-21& sup& st& /sup& -century ice-shelf basal melt changes due to SAM of -60.7 Gt yr& sup& -1& /sup& and 1.8 to 26.8 Gt yr& sup& -1& /sup& (depending on the emission scenario considered), respectively, compared to the present.& & & & & / &
Publisher: Copernicus GmbH
Date: 27-03-2022
DOI: 10.5194/EGUSPHERE-EGU22-1163
Abstract: & & Ocean mixing around Antarctica is a key process that influences the vertical distributions of heat and nutrients, affecting glacier and ice shelf retreats, sea ice formation and marine productivity, with implications for regional ecosystems, global sea level and climate. Here we show that collapsing glacier fronts associated with calving events trigger internal tsunamis, the propagation and breaking of which can lead to significant mixing. Observations of one such event at the West Antarctic Peninsula, during which 3-20 megatonnes of ice were discharged to the ocean, reveal rapidly-elevated internal wave kinetic energy and upper-ocean shear, with strong homogenisation of the water column. Scaling arguments indicate that, at the West Antarctic Peninsula, just a few such events per summer would make this process comparable in magnitude to winds, and much more significant than tides, in driving shelf mixing. We postulate that this process is likely relevant to all regions with calving marine-terminating glaciers, including also Greenland and the Arctic. Glacier calving is expected to increase in a warming climate, likely strengthening internal tsunamigenesis and mixing in these regions in the coming decades.& &
Publisher: American Geophysical Union (AGU)
Date: 15-10-2021
DOI: 10.1029/2020JD034391
Abstract: The importance of resolving mesoscale air‐sea interactions to represent cyclones impacting the East Coast of Australia, the so‐called East Coast Lows (ECLs), is investigated using the Australian Regional Coupled Model based on NEMO‐OASIS‐WRF (NOW) at resolution. The fully coupled model is shown to be capable of reproducing correctly relevant features such as the seasonality, spatial distribution and intensity of ECLs while it partially resolves mesoscale processes, such as air‐sea feedbacks over ocean eddies and fronts. The mesoscale thermal feedback (TFB) and the current feedback (CFB) are shown to influence the intensity of northern ECLs (north of ), with the TFB modulating the pre‐storm sea surface temperature (SST) by shifting ECL locations eastwards and the CFB modulating the wind stress. By fully uncoupling the atmospheric model of NOW, the intensity of northern ECLs is increased due to the absence of the cold wake that provides a negative feedback to the cyclone. The number of ECLs might also be affected by the air‐sea feedbacks but large interannual variability h ers significant results with short‐term simulations. The TFB and CFB modify the climatology of SST (mean and variability) but no direct link is found between these changes and those noticed in ECL properties. These results show that the representation of ECLs, mainly north of , depend on how air‐sea feedbacks are simulated. This is particularly important for atmospheric downscaling of climate projections as small‐scale SST interactions and the effects of ocean currents are not accounted for.
Publisher: American Geophysical Union (AGU)
Date: 02-2021
DOI: 10.1029/2020JC016550
Abstract: Recent work on the Filchner‐Ronne Ice Shelf (FRIS) system has shown that a redirection of the coastal current in the southeastern Weddell Sea could lead to a regime change in which an intrusion of warm Modified Circumpolar Deep Water results in large increases in the basal melt rate. Work to date has mostly focused on how increases in the Modified Circumpolar Deep Water crossing the continental shelf break leads directly to heat driven changes in melting in the ice‐shelf cavity. In this study, we introduce a Weddell Sea regional ocean model configuration with static ice shelves. We evaluate a reference simulation against radar observations of melting, and find good agreement between the simulated and observed mean melt rates. We analyze 28 sensitivity experiments that simulate the influence of changes in remote water properties of the Antarctic Slope Current on basal melting in the FRIS. We find that remote changes in salinity quasi‐linearly modulate the mean FRIS net melt rate. Changes in remote temperature quadratically vary the FRIS net melt rate. In both salinity and temperature perturbations, the response is rapid and transient, with a recovery time‐scale of 5–15 years dependent on the size/type of perturbation. We show that the two types of perturbations lead to different changes on the continental shelf, and that ultimately different factors modulate the melt rates in the FRIS cavity. We discuss how these results, are relevant for ocean hindcast simulations, sea level, and melt rate projections of the FRIS.
Publisher: Wiley
Date: 18-01-2021
Publisher: Copernicus GmbH
Date: 07-02-2023
Abstract: Abstract. Through their role in buttressing upstream ice flow, Antarctic ice shelves play an important part in regulating future sea-level change. Reduction in ice-shelf buttressing caused by increased ocean-induced melt along their undersides is now understood to be one of the key drivers of ice loss from the Antarctic ice sheet. However, despite the importance of this forcing mechanism, most ice-sheet simulations currently rely on simple melt parameterisations of this ocean-driven process since a fully coupled ice–ocean modelling framework is prohibitively computationally expensive. Here, we provide an alternative approach that is able to capture the greatly improved physical description of this process provided by large-scale ocean-circulation models over currently employed melt parameterisations but with trivial computational expense. This new method brings together deep learning and physical modelling to develop a deep neural network framework, MELTNET, that can emulate ocean model predictions of sub-ice-shelf melt rates. We train MELTNET on synthetic geometries, using the NEMO ocean model as a ground truth in lieu of observations to provide melt rates both for training and for evaluation of the performance of the trained network. We show that MELTNET can accurately predict melt rates for a wide range of complex synthetic geometries, with a normalised root mean squared error of 0.11 m yr−1 compared to the ocean model. MELTNET calculates melt rates several orders of magnitude faster than the ocean model and outperforms more traditional parameterisations for 96 % of geometries tested. Furthermore, we find MELTNET's melt rate estimates show sensitivity to established physical relationships such as changes in thermal forcing and ice-shelf slope. This study demonstrates the potential for a deep learning framework to calculate melt rates with almost no computational expense, which could in the future be used in conjunction with an ice sheet model to provide predictions for large-scale ice sheet models.
Publisher: Springer Science and Business Media LLC
Date: 22-06-2022
DOI: 10.1038/S43247-022-00458-X
Abstract: The Southern Hemisphere cryosphere has recently shown regionally-contrasted responses to climate change, in particular to the positive phases of the Southern Annular Mode. However, the understanding of the impacts of this mode on ice-shelf basal melt at a circum-Antarctic scale is still limited. Here, we performed idealized experiments with a pan-Antarctic regional ice-shelf cavity-resolving ocean—sea-ice model for different phases of the Southern Annular Mode. We show that positive phases lead to increased upwelling and subsurface ocean temperature and salinity close to ice shelves, while the opposite occurs for negative phases. A one-standard-deviation increase of the Southern Annular Mode leads to a net basal mass loss of 40 Gt yr −1 , with strong regional contrasts: increased ice-shelf basal melt in the Bellingshausen and Western Pacific sectors and the opposite response in the Amundsen sector. Estimates of 1000–1200 and 2090–2100 ice-shelf basal melt changes due to the Southern Annular Mode are −86.6 Gt yr −1 and 55.0 to 164.9 Gt yr −1 , respectively, compared to the present.
Publisher: American Geophysical Union (AGU)
Date: 12-2017
DOI: 10.1002/2017JC013311
Publisher: American Association for the Advancement of Science (AAAS)
Date: 25-11-2022
Abstract: Ocean mixing around Antarctica exerts key influences on glacier dynamics and ice shelf retreats, sea ice, and marine productivity, thus affecting global sea level and climate. The conventional paradigm is that this is dominated by winds, tides, and buoyancy forcing. Direct observations from the Antarctic Peninsula demonstrate that glacier calving triggers internal tsunamis, the breaking of which drives vigorous mixing. Being widespread and frequent, these internal tsunamis are at least comparable to winds, and much more important than tides, in driving regional shelf mixing. They are likely relevant everywhere that marine-terminating glaciers calve, including Greenland and across the Arctic. Calving frequency may change with higher ocean temperatures, suggesting possible shifts to internal tsunamigenesis and mixing in a warming climate.
Publisher: American Geophysical Union (AGU)
Date: 02-2018
DOI: 10.1002/2017JC013412
Publisher: Ovid Technologies (Wolters Kluwer Health)
Date: 15-02-2022
Abstract: While peripheral artery disease (PAD) is associated with increased cardiovascular morbidity with mortality remaining high and challenging to predict, accurate understanding of serial PAD‐specific health status around the time of diagnosis may prognosticate long‐term mortality risk. Patients with new or worsening PAD symptoms enrolled in the PORTRAIT Registry across 10 US sites from 2011 to 2015 were included. Health status was assessed by the Peripheral Artery Questionnaire (PAQ) Summary score at baseline, 3‐month, and change from baseline to 3‐month follow‐up. Kaplan‐Meier using 3‐month landmark and hierarchical Cox regression models were constructed to assess the association of the PAQ with 5‐year all‐cause mortality. Of the 711 patients (mean age 68.8±9.6 years, 40.9% female, 72.7% white mean PAQ 47.5±22.0 and 65.9±25.0 at baseline and 3‐month, respectively), 141 (19.8%) died over a median follow‐up of 4.1 years. In unadjusted models, baseline (HR, 0.90 per‐10‐point increment 95% CI, 0.84–0.97 P =0.008), 3‐month (HR [95% CI], 0.87 [0.82–0.93] P .001) and change in PAQ (HR [95% CI], 0.92 [0.85–0.99] P =0.021) were each associated with mortality. In fully adjusted models including combination of scores, 3‐month PAQ was more strongly associated with mortality than either baseline (3‐month HR [95% CI], 0.85 [0.78–0.92] P .001 C‐statistic, 0.77) or change (3‐month HR [95% CI], 0.79 [0.72–0.87] P .001). PAD‐specific health status is independently associated with 5‐year survival in patients with new or worsening PAD symptoms, with the most recent assessment being most prognostic. Future work is needed to better understand how this information can be used proactively to optimize care.
Publisher: Authorea, Inc.
Date: 12-08-2023
DOI: 10.22541/ESSOAR.169186334.40961666/V1
Abstract: Central to improving our understanding of ocean temperature change on Antarctica’s continental shelf is a better understanding of how the ocean circulation drives the onshore flux of warm deep waters across the shelf break. This study uses a primitive equation ocean model to explore how the circulation regime and changes in surface stress influence the temperature structure on Antarctica’s shelf seas. As the shelf temperature changes are largely driven by ocean circulation changes, understanding these becomes our focus. A simple barotropic model is used to describe the linear theory of the difference between throughflow and gyres regimes, and their expected response to changes in forcing. This theory informs our understanding of the barotropic circulation response of the primitive equation model where a momentum budget confirms that over the simulated equilibrated timescales with surface forcing changes, the response is first-order linear. Consistent with previous findings, we find that climate change projection-like wind shifts (stronger westerlies that shift south) have a direct influence on Ekman processes across the shelf break and upwell warmer waters onto the shelf. We also find that the circulation regime (throughflow or gyre – determined by basin geometry), influences the mean shelf temperature and how susceptible the existing shelf temperatures are to changes in surface stress. While the throughflow regime can experience a complete transition in on-shelf temperatures when the transition between westerly and easterly winds shifts southward, we find relatively modest bottom intensified warming at the Ice Front in a gyre regime.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Christopher Bull.