Metadata record for data from ASAC Project 2960 See the link below for public details on this project Public The ocean's thermohaline circulation (THC) plays a fundamental role in global climate, transporting heat poleward and regulating the uptake of anthropogenic CO2. Multiple steady-states in the THC have been identified in the North Atlantic, including an "off" state where no deep water is formed, yet little is known regarding the possibility for multiple equilibria of the Southern Ocean THC. This study aims to (1) examine hysteresis behaviour and possible multiple equilibria of the Southern Ocean THC, and (2) quantify the role of the Southern Ocean THC by examining the difference between "on" and "off" states in various water-masses. Project objectives: The overarching goal of the proposed study is to explore the possibility of multiple steady-states of the Southern Ocean (SO) thermohaline circulation (THC) and to explore their role in the global climate system. Multiple steady-states in the ocean's THC have been identified in the Northern Hemisphere [e.g., Marotzke, 2000; Rahmstorf, 2002]. While substantial climate variability and change can be inferred from palaeoclimate data for the Southern Hemisphere, our understanding of the underlying physics of SO THC variability and the associated climate dynamics remains limited. It is also unclear how the Southern Ocean THC will change in the future. This study aims to: 1. Examine the hysteresis behaviour of the Southern Ocean thermohaline circulation in relation to surface freshwater forcing, both for AABW and AAIW, 2. Explore the possibility for multiple steady-states in the Southern Ocean THC, 3. Estimate how the present-day Southern Ocean THC may be changing in relation to this hysteresis diagram, and how this relates to global climate, and 4. Quantify the role of the present-day Southern Ocean THC by examining the difference between "on" and "off" states. Taken from the 2008-2009 Progress Report: Progress against objectives: Progress on this Antarctic Sciences project during 2008/2009 can be summarised as below. Each of the four main aims have been touched upon during the past 12 months, although the most significant progress has been against items 1, 3, and 4 as listed in Section 1.1 above. The existence of teleconnections of Southern Ocean freshwater anomalies to the North Atlantic THC was investigated, primarily in the context of past climates (Trevena, Sijp and England, 2008a). We found that a Southern Ocean freshwater pulse of comparable magnitude to meltwater pulse 1A, shuts down, instead of strengthens, NADW in a glacial climate simulation. Unlike a modern-day simulation, the glacial experiment is associated with a more fragile North Atlantic thermohaline circulation, whereby freshwater anomalies that propagate into the North Atlantic are able to dominate the bipolar density see-saw. The possibility for large-scale collapse and/or multiple steady-states in the Southern Ocean THC was also investigated using a coupled climate model of intermediate complexity. Also investigated was the impact of a slowdown of Antarctic Bottom Water (AABW) on regional Southern Hemisphere climate. This involved the gradual addition of meltwater anomalies to strategic locations of the Southern Ocean, then removal of these anomalies to explore whether the regional thermohaline circulation (THC) exhibits saddle-node instabilities (bifurcation points) as have been commonly found for the North Atlantic. We found that no stable AABW "off" state could persist, regardless of the freshwater anomaly imposed. We did, however, identify a significant impact on regional climate during the transient slow down of AABW (Trevena, Sijp and England, 2008b). In particular, during peak FW forcing, Antarctic surface sea and air temperatures decrease by a maximum of 2.5 degs C and 2.2 degs-C respectively. This is of a similar magnitude to the corresponding response in the North Atlantic. Taken from the 2009-2010 Progress Report: Progress against objectives: Progress on this Antarctic Sciences project during 2009/2010 can be summarised as below. Each of the four main aims have been touched upon during the past 12 months, although the most significant progress has been against items 2 and 4 as listed in Section 1.1 above. A large set of experiments were configured and analysed to examine Southern Ocean THC states in the global climate system. Specifically we conducted experiments using the Canadian University of Victoria Earth System Climate Model (the 'UVic' model) wherein the model is perturbed in some way to explore the possibility for multiple steady-states in the Southern Ocean THC. Where multiple steady states were obtained, the difference between "on" and "off" states was examined to quantify the role of the Southern Ocean THC in global climate. Three papers were published in the 2009/2010 period that were produced using support from this Antarctic Research project:- Sijp, W. P., M. H. England, and J.R. Toggweiler, 2009: Effect of ocean gateway changes under greenhouse warmth, J. Climate, 22, 6639-6652. In this study Southern Ocean gateway changes and the THC were examined under a suite of atmospheric CO2 levels, spanning pre-industrial (280 ppm) up to values relevant to the Eocene (1500 ppm). A markedly stronger gateway response is found under elevated CO2 levels, suggesting past work has underestimated the effects of gateway changes at the Oligocene-Eocene boundary. Sen Gupta, A., A. Santoso, A.S. Taschetto, C.C. Ummenhofer, J. Trevena and M.H. England, 2009: Projected changes to the Southern Hemisphere ocean and sea-ice in the IPCC AR4 climate models, J. Climate, 22, 3047-3078. In this study simulations of the Southern Ocean THC, water-masses, and mixed layer depth were examined and compared across a series of IPCC-class global climate models, under both present-day and climate change scenarios. Sijp, W. P. and M. H. England, 2009: The control of polar haloclines by along-isopycnal diffusion in climate models, J. Climate, 22, 486-498. In this study the ocean THC was shown to be sensitive to along-isopycnal diffusion rates in global climate models. This potentially impacts on past studies wherein multiple equilibria were obtained at unrealistic values of this mixing parameter.

Prediction of future climate change requires knowledge of past changes. Polar snow forms an archive of environmental conditions that is accessible by drilling and analysing ice cores. This project uses ice core data to reconstruct records, including past temperature and atmospheric composition, to improve understanding of the climate system. Report from the 2007/2008 season This proposal encompasses the laboratory-based component of ice core research at the Australian Antarctic Division. The project is principally focused on analysis of currently archived ice core material but will include analysis of new cores (to be collected in future field activities that will be the subject of separate research proposals through the duration of the project). This work is conducted as part of the ACE-CRC (Antarctic Climate and Ecosystems Cooperative Research Centre). The overall general aim for this AAS project is to understand past climate variability and change, through the study of Antarctic ice cores. More specifically, this research explores the role of Antarctica in hemispheric and global climate, with particular emphasis on climate variability and change in the Southern Ocean, mid-latitudes, and the Australian sector. To effectively achieve this aim, we have defined four research questions, broadly based on a separation at different temporal and spatial scales: 1. What do high resolution comparisons of instrumental climate data and ice cores reveal about calibration of ice core signals and underlying mechanisms? 2. What is the spatial and temporal variability in climate across the wider East Antarctic region in the last few centuries, particularly spanning the onset of anthropogenic influence? How is this connected with overall variability in the Antarctic, and the Southern Ocean, particularly the Australian sector? 3. What changes and modes of variability are seen in Holocene Antarctic and Southern Ocean climate from high resolution ice cores? 4. What climate changes were seen in coastal Antarctica through the last glacial and deglaciation, and how does the timing compare with other records, especially the Northern Hemisphere records? Feeding into these research questions are a number of specific scientific objectives (listed below, with clearly identified methodology to achieve outcomes). These objectives address issues essential to a number of research fields across the Australian Antarctic program (see 3.1.3), and have been identified through knowledge gained from the earlier AAS project 757 and the scientific literature (discussed in more detail in section 3.1.2). Research will use high-resolution ice core studies as a tool to probe climate variability on timescales from seasonal through to millennial. This ability to access very high resolution climate records through ice cores is of major importance because it is the only means of calibrating the ice core recorder against observed meteorology. Also, the seasonal- to interannual-timescales capture climate variability that is not readily probed in other records. The high snow accumulation on Law Dome, combined with a 1.2km thick ice sheet, provides a unique high resolution record of the Holocene and access to the last Glacial-interglacial cycle. The main objectives are listed below, with a brief explanation of the methodology employed to achieve these objectives: - Extend the time-series of ice core chemical and physical measurements - Focussed on East Antarctic sites, (particularly Law Dome). The length and resolution of records so far obtained will be increased and the range of measured parameters increased. This includes from the DSS core: completion of a full 90 thousand year record of trace ion data to accompany the completed d18O isotope series; high-resolution (subannual) series for trace ions and d18O over the last 2000 years; new measurements including d13CH4 (and potentially dCH3D) in collaboration with University of Colorado, NIWA and CSIRO and deuterium excess measurements using new mass-spectrometry facilities. - Calibrate ice core measurements against instrumental records - Calibrate ice core measurements against meteorological, and other proxy series, in order to better understand the climate signals in ice cores and to provide new proxies. This work will use ultra-high resolution data, especially through the period of instrumental overlap (for Antarctic records, this period covers the nearly 50 years since the first IGY). The study is expected to draw data from a field activity in 2008/09 summer in conjunction with IPY, which has a 'special observing period' for tracking airmasses arriving at ice core sites. - Investigate modes of climate variability - Investigate the strength, variability and alteration in modes of variability for specific climate processes, especially to examine any recent changes in these from Holocene background. In particular, processes or indices that will be explored include ENSO, the Southern Annular Mode, sea-ice extent, decadal variability in coupled ocean-atmosphere modes such as the Antarctic Circumpolar Wave (White and Peterson, 1996), atmospheric circulation indices (e.g. stratospheric markers such as nitrate or beryllium-10 and dust or trace-metal variations). - Examine response and sensitivity to forcing variations and explore mechanisms - This includes studies of: insolation links to climate variability, the timing and magnitude of major volcanic events, and variations associated with atmospheric composition changes (the '8200 BP' event, deglacial interhemispheric climate variations and abrupt changes in the last glacial). - Improve the understanding of the Antarctic climate system using multiple records - Explore relationships between the high resolution ice core records and other ice cores including the Antarctic interior to better understand both the spatial structure of the Antarctic climate system (including teleconnections), and the interpretation of the ice cores themselves. - Contribute to Antarctic mass-balance and sea-level rise - Derive records of accumulation input and variability for the last 100-200 years at sites in eastern Wilkes Land and for the last 90 thousand years at Law Dome. These records contribute to understanding Antarctic mass-balance and sea-level impacts. - Develop and maintain facilities and expertise for analysis of ice cores - Continue to develop and maintain facilities and expertise for analysis of ice cores and related climate studies. The facilities supported by this project provide a core capacity for downstream analysis and interpretation of Australian field studies, by the AAD, and also by collaborative partners in CSIRO, University of Newcastle, Curtin University of Technology as well as several important international partnerships. In the last 12 months, the project has predominantly been in a laboratory/measurement phase and so progress is predominantly against the first and last objectives at 1.1 (Extend time series, Develop facilities). The isotope and trace chemistry records for the Law Dome cores are being extended and in-filled where gaps occur. The time series have been extended. Most measurements have been undertaken using recently drilled new core material (DSS0506 from AAS2384), as this is providing an opportunity to derive new series (deuterium) and check existing data for inter-core fidelity. New core material which brings records up to January 2008 has been analysed and the data are being combined with other cores to provide continuous series. For the interpretive objectives, progress consists predominantly of results that have so far been presented at various meetings. We now have new data that strongly mitigate against the "EPICA hypothesis" that posits that sea-salts in ice cores (particularly inland cores) are specifically connected with sea-ice extent. We are able to quantify the degree of influence of sea-ice surface as a source of salt and demonstrate that it decreases with distance from the coast. We have further investigated the snowfall accumulation at Law Dome and are probing links seen to rainfall in Southwest Western Australia. We have also investigated subannual variations in snowfall accumulation and find that winter accumulation variability dominates the annual signal. We have new results from very high resolution studies of beryllium-10 which demonstrate a shorter atmospheric residence time for this cosmogenically produced species than has been accepted. This work has potential to improve the use of beryllium-10 as a proxy for solar variability and has implications for understanding of atmospheric transport. Taken from the 2008-2009 Progress Report: Progress against objectives: This year's activities have been focused upon data generation and also with associated fieldwork for AAS 3025 (Aurora Basin North Ice Core Drilling). A deliberate slowing of progress on AAS 757 this year was planned because of a large investment of personnel time toward AAS 3025, however good progress has nevertheless been made. While the intention of AAS 3025 was to generate data within an independent project, field constraints forced a change to theatre of operations - providing core material that now fits within the scope of this project. This fieldwork produced ~130m of core from a new site on Law Dome (DSSW10k), extensions of the record at Law Dome Summit South (10m), new cores on the lower Totten Glacier (~17m) and Totten-Law Dome Trench (~15m) and Mill Is (~17m). Analysis of these cores within AAS 757 has already commenced. The DSSW10k core provides a new ~250 year record from Law Dome that will be useful in its own right, but will provide an opportunity to test both deposition processes and ice core proxy fidelity. The core comes from a location only 10km from the main coring site, but it has only half the snow accumulation rate. Comparison of the records will allow testing of the influence of snowfall rate on preservation of ice core signals. The shallow cores at Totten Glacier and Mill Is are the first records from these locations and will permit assessment of the suitability of these sites for deeper drilling. The Totten cores may also shed light on recent accumulation changes in a location where substantial surface lowering is occurring. Most of the non-field activities are directed at the first and last objectives at 1.1 (Extend time series, develop facilities), although some significant work has also been conducted towards the second and third objectives (calibration of ice core records against instrumental records, and investigating modes of variability). This has been through further investigation of the modes of variability the linking Law Dome snow accumulation with rainfall in southwest Western Australia. Calibration with both ERA-40 and NCEP reanalysis data sets and investigation of links with meridional circulation have brought this work to the point where a manuscript has been submitted on the topic. Emerging work from a recently commenced PhD student is expanding the record of water isotopes from Law Dome cores, in particular providing a time series of deuterium excess. Early results are suggestive of a new finding in which major volcanic eruptions leave a differential signal in isotopes of hydrogen and oxygen, possibly due to stratospheric oxidation processes. Work is underway to test this. Other new work toward synthesis objectives includes an interdisciplinary study tying the ice core record of methanesulphonic acid into a larger consideration of seasonal phytoplankton stress and solar irradiance. A manuscript reporting this has also been submitted. This project wound up in 2012, and was replaced by other ice-coring projects.