Antarctic Bottom Water response to Varying Surface Fluxes

Antarctic Bottom Water (AABW) is one of the densest and most voluminous water masses of the global ocean. It forms the lower limb of the global overturning circulation and plays an important role in transporting carbon, heat and freshwater sequestered from the atmosphere to the deep ocean. Surface buoyancy fluxes modulate the production of AABW through the formation of Dense Shelf Water (DSW) on the Antarctic continental shelf. The DSW flows down the continental slope as an overflow, entraining ambient Circumpolar Deep Water (CDW), to form AABW. The AABW spreads through the abyssal ocean, influencing global deep stratification, water properties and circulation over centennial, and even millennial, time scales While surface fluxes play a key role in defining AABW production rates, the role of varying surface fluxes in influencing AABW properties and variability remains uncertain. Broad scale observational analysis of AABW processes is hindered by the extreme conditions particular to the Southern Ocean and Antarctic regions, and climate models struggle to accurately represent AABW formation processes. The difficulty climate models have in representing AABW formation originates from challenges in simulating DSW formation and the resultant overflow. Through both observational analysis and novel model development, this thesis provides insight into the role of varying surface fluxes in controlling AABW responses and feedbacks, and the limitations of climate models in representing such responses. A coarse resolution sector model of the Atlantic Ocean is developed to aid in testing the limitations of climate model representation of AABW formation. With realistic forcing and bathymetry, the sector model efficiently emulates climate model processes and allows AABW sensitivity to overflow parameterisations to be assessed. While AABW proves relatively insensitive to most current generation overflow parameterisations, understanding the importance of DSW formation in defining AABW's role in a changing climate remains an important challenge. Increased horizontal and vertical resolution allows the sector model to maintain DSW as the dominate mode of AABW formation. Under such formation conditions, the influence of varying surface buoyancy fluxes on DSW sourced AABW is assessed. Increased buoyancy fluxes decrease the cross-shelf exchange of DSW and CDW. The reduced exchange cools DSW and propagates changes to the abyssal ocean, driving a decadal scale variability of AABW. The role of surface buoyancy variations in driving the cross-shelf exchange and AABW production, is further revealed at seasonal time scales through an observational analysis of circulation on the Adelie Land continental shelf, East Antarctica. The seasonality of surface buoyancy fluxes leads to enhanced cross-shelf exchange of DSW and CDW in winter, at an order of magnitude larger than that in summer. The enhanced exchange sets up a cyclonic flow on the shelf and highlights the influence of buoyancy fluxes in controlling circulation on the continental shelf. The influence of surface buoyancy fluxes on AABW formation, shelf circulation and cross-shelf exchange, occurs through inclusion of DSW sourced AABW, a process absent from most climate models. Without correct representation of AABW formation mechanisms, climate models are missing key responses and feedbacks driven from changes in surface fluxes. On-going work into climate model development of AABW formation processes is thus essential to develop an increased understanding of AABW dynamics, variability and response to climate change

Sensitivity of Antarctic Bottom Water to changes in Surface Buoyancy Fluxes

The influence of freshwater and heat flux changes on Antarctic Bottom Water (AABW) properties are investigated within a realistic bathymetry coupled ocean–ice sector model of the Atlantic Ocean. The model simulations are conducted at eddy-permitting resolution where dense shelf water production dominates over open ocean convection in forming AABW. Freshwater and heat flux perturbations are applied independently and have contradictory surface responses, with increased upper-ocean temperature and reduced ice formation under heating and the opposite under increased freshwater fluxes. AABW transport into the abyssal ocean reduces under both flux changes, with the reduction in transport being proportional to the net buoyancy flux anomaly south of 60°S. Through inclusion of shelf-sourced AABW, a process absent from most current generation climate models, cooling and freshening of dense source water is facilitated via reduced on-shelf/off-shelf exchange flow. Such cooling is propagated to the abyssal ocean, while compensating warming in the deep ocean under heating introduces a decadal-scale variability of the abyssal water masses. This study emphasizes the fundamental role buoyancy plays in controlling AABW, as well as the importance of the inclusion of shelf-sourced AABW within climate models in order to attain the complete spectrum of possible climate change responses

The MISSE 7 Flexural Stress Effects Experiment After 1.5 Years of Wake Space Exposure

Low Earth orbit space environment conditions, including ultraviolet radiation, thermal cycling, and atomic oxygen exposure, can cause degradation of exterior spacecraft materials over time. Radiation and thermal exposure often results in bond- breaking and embrittlement of polymers, reducing mechanical strength and structural integrity. An experiment called the Flexural Stress Effects Experiment (FSEE) was flown with the objective of determining the role of space environmental exposure on the degradation of polymers under flexural stress. The FSEE samples were flown in the wake orientation on the exterior of International Space Station for 1.5 years. Twenty-four samples were flown: 12 bent over a 0.375 in. mandrel and 12 were over a 0.25 in. mandrel. This was designed to simulate flight configurations of insulation blankets on spacecraft. The samples consisted of assorted polyimide and fluorinated polymers with various coatings. Half the samples were designated for bend testing and the other half will be tensile tested. A non-standard bend-test procedure was designed to determine the surface strain at which embrittled polymers crack. All ten samples designated for bend testing have been tested. None of the control samples' polymers cracked, even under surface strains up to 19.7%, although one coating cracked. Of the ten flight samples tested, seven show increased embrittlement through bend-test induced cracking at surface strains from 0.70%to 11.73%. These results show that most of the tested polymers are embrittled due to space exposure, when compared to their control samples. Determination of the extent of space induced embrittlement of polymers is important for designing durable spacecraft

Bend-Test Results of the MISSE 7 Flexural Stress Effects Experiment After 1.5 Years of Space Exposure

Low Earth orbital environmental exposure can cause degradation of exterior spacecraft materials. Radiation and thermal exposure often result in bond-breaking and embrittlement of polymers, reducing mechanical strength and structural integrity. The Flexural Stress Effects Experiment (FSEE) was flown with the objective of determining the role of space exposure on the degradation of polymers under flexural stress. The FSEE samples were flown in a wake orientation on the exterior of International Space Station for 1.5 years. Twenty-three polyimide and fluorinated polymers with various coatings were flown: 11 bent over a 0.375-inch diameter holder and 12 over a 0.25-inch diameter holder. A non-standard bend-test procedure was used to determine the surface strain at which embrittled polymers crack. None of the control samples cracked, even under surface strains up to 19.7%, although one coating cracked. Of the 10 flight samples tested, seven indicated increased embrittlement through bend-test cracking at surface strains from 0.65% to 8.11%. Therefore, most of the tested polymers were embrittled due to space exposure, when compared to their control samples. The samples flown over the 0.375-inch holder were more embrittled than those on the 0.25-inch holder. Determination of the extent of space induced embrittlement of polymers is important for designing durable spacecraft

Representing grounding line migration in synchronous coupling between a marine ice sheet model and a z-coordinate ocean model

Synchronous coupling is developed between an ice sheet model and a z-coordinate ocean model (the MITgcm). A previously-developed scheme to allow continuous vertical movement of the ice-ocean interface of a floating ice shelf (“vertical coupling”) is built upon to allow continuous movement of the grounding line, or point of floatation of the ice sheet (“horizontal coupling”). Horizontal coupling is implemented through the maintenance of a thin layer of ocean ( ∼ 1 m) under grounded ice, which is inflated into the real ocean as the ice ungrounds. This is accomplished through a modification of the ocean model’s nonlinear free surface evolution in a manner akin to a hydrological model in the presence of steep bathymetry. The coupled model is applied to a number of idealized geometries and shown to successfully represent ocean-forced marine ice sheet retreat while maintaining a continuous ocean circulation

Development of a novel tool for assessing coverage of implementation factors in health promotion program resources

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.Katharine B. Richardson Research Award at Children's Mercy Kansas CityNational Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health under Award Number F32DK115146Institute of Education Sciences and U.S. Department of Education by grant R305A15027

Glacier change along West Antarctica’s Marie Byrd Land Sector and links to inter-decadal atmosphere-ocean variability

Over the past 20 years satellite remote sensing has captured significant downwasting of glaciers that drain the West Antarctic Ice Sheet into the ocean, particularly across the Amundsen Sea Sector. Along the neighbouring Marie Byrd Land Sector, situated west of Thwaites Glacier to Ross Ice Shelf, glaciological change has been only sparsely monitored. Here, we use optical satellite imagery to track grounding-line migration along the Marie Byrd Land Sector between 2003 and 2015, and compare observed changes with ICESat and CryoSat-2-derived surface elevation and thickness change records. During the observational period, 33% of the grounding line underwent retreat, with no significant advance recorded over the remainder of the  ∼ 2200km long coastline. The greatest retreat rates were observed along the 650km-long Getz Ice Shelf, further west of which only minor retreat occurred. The relative glaciological stability west of Getz Ice Shelf can be attributed to a divergence of the Antarctic Circumpolar Current from the continental-shelf break at 135°W, coincident with a transition in the morphology of the continental shelf. Along Getz Ice Shelf, grounding-line retreat reduced by 68% during the CryoSat-2 era relative to earlier observations. Climate reanalysis data imply that wind-driven upwelling of Circumpolar Deep Water would have been reduced during this later period, suggesting that the observed slowdown was a response to reduced oceanic forcing. However, lack of comprehensive oceanographic and bathymetric information proximal to Getz Ice Shelf's grounding zone make it difficult to assess the role of intrinsic glacier dynamics, or more complex ice-sheet–ocean interactions, in moderating this slowdown. Collectively, our findings underscore the importance of spatial and inter-decadal variability in atmosphere and ocean interactions in moderating glaciological change around Antarctica

How accurately should we model ice shelf melt rates?

Assessment of ocean‐forced ice sheet loss requires that ocean models be able to represent sub‐ice shelf melt rates. However, spatial accuracy of modeled melt is not well investigated, and neither is the level of accuracy required to assess ice sheet loss. Focusing on a fast‐thinning region of West Antarctica, we calculate spatially resolved ice‐shelf melt from satellite altimetry and compare against results from an ocean model with varying representations of cavity geometry and ocean physics. Then, we use an ice‐flow model to assess the impact of the results on grounded ice. We find that a number of factors influence model‐data agreement of melt rates, with bathymetry being the leading factor; but this agreement is only important in isolated regions under the ice shelves, such as shear margins and grounding lines. To improve ice sheet forecasts, both modeling and observations of ice‐ocean interactions must be improved in these critical regions.This work was supported by Natural Environment Resources Council grant NE/M003590/1, and European Space Agency contracts CryoTop4000107394/12/I-NB and CryoTop Evolution 4000116874/16/I-N

Banner News

https://openspace.dmacc.edu/banner_news/1096/thumbnail.jp