Late Quaternary changes in the Westerly Winds over the Southern Ocean

Lead Research Organisation: NERC British Antarctic Survey
Department Name: Science Programmes

Abstract

Public summary

Climate changes have been attributed to the increasing concentrations of greenhouse gases (e.g. CO2) in our atmosphere. These measured increases in atmospheric CO2 are partly controlled by changes in the ability of the world's oceans to absorb CO2 at the surface (e.g. via diffusion and the biological pump) versus release of old carbon to the atmosphere from deep ocean reservoirs (e.g. via upwelling and out gassing).

Of the world's oceans, the Southern Ocean has been identified (by models) as playing a major role in modulating global atmospheric CO2, particularly on glacial-interglacial timescales. This is because surface nutrients are high indicating their incomplete utilisation by the biological pump, and because wind driven changes in ocean circulation can bring old carbon stored in deep ocean reservoirs to the surface.

Changes in the strength of the Southern Hemisphere Westerly Winds (SHW) influence Southern Ocean circulation and control how much of this carbon rich deep water reaches the ocean surface. Thus any change in the strength or position of the SHW such as the recently observed intensification of the winds, could influence whether the Southern Ocean acts as a net source or sink of atmospheric CO2.

At present our understanding of past changes in the SHW is based mainly on geological proxy records from South America, one record from South Africa, and two from New Zealand. With the exception of Campbell Island there are no studies of changes in the SHW in the Southern Ocean where the core of the SHW wind belt is located. This lack of spatial resolution has been identified by Stager et al. (2012) as a major limitation in our understanding of past climate. As a result, although present General Circulation Models use a variety of processes (biology, ocean chemistry, and ocean physics), they either fail to produce the magnitude of past atmospheric CO2 variations or do not agree with geologic field data.

Here we propose to substantially improve the spatial resolution of the geological data by generating proxy records in each of the three major sectors of the Southern Ocean, focusing on sub-Antarctic islands situated in the core belt of the SHW. We apply a novel diatom proxy for past wind strength independently controlled by a range of standard sedimentological and biogeochemical proxies. The new proxy is based on the direct transfer of sea spray across the islands by wind, and its effect on the salinity of west coast lakes and ponds. This works on sub-Antarctic islands where there is a marked west-east conductivity gradient in water bodies across the island. This conductivity gradient determines which diatom communities are present in the lakes. Once this diatom- conductivity relationship is established quantitatively, the subfossil diatom communities deposited in radiocarbon dated sediment cores can be used to reconstruct changes in conductivity through time, and hence past relative wind strength.

We have demonstrated that this approach works at Macquarie Island and we have also tested its feasibility at Marion and Campbell Islands. This proposal is for funding to support further work on Campbell and Marion Islands, and in the Cape Horn archipelago.

To interpret our data we will carry out a series of General Circulation Model runs to explore the long term changes in SHW strength and the processes driving them, by taking advantage of the new Paleoclimate Modelling Intercomparison Project 3 (PMIP3) AOGCM (climate model) simulations. These experiments will allow comprehensive model-observation evaluation of the new proxy wind strength reconstructions.

Ultimately this work will help provide improved boundary conditions for models which simulate the impact of past changes in wind strength on the upwelling of deep ocean carbon reservoirs, and improve our understanding of the relationship between past changes in global atmospheric CO2 and temperature.

Planned Impact

1. Specific users who might benefit from the economic and societal outcomes of the research of this work and how they will benefit

Target audiences who will benefit from this research are: palaeoclimate scientists, climate modellers and Quaternary scientists (the main beneficiaries of the research as detailed in the 'Academic beneficiaries' section) as well as stakeholders (with a focus on policymakers) and the general public (with an emphasis on young people).

The main users of the research outside of the immediate research community will be policy-makers and their science advisors (in the UK and internationally, for example through the Scientific Committee on Antarctic Research, the Antarctic Treaty System, and IPCC) who use evidence on climate change to gauge the severity of anthropogenically enhanced global warming and develop an appropriate level of response. If for example we were to demonstrate that the recorded changes in the Southern Hemisphere Westerly Winds are unprecedented in the last few millennia, it would increase the urgency for political action proposed by this user group. A related user group are the environmental (and potentially the 'climate sceptic') pressure groups who develop campaign strategies based (partly) on recent science evidence. Communication with these users is important to avoid misunderstanding and potential mis-use of the science.

2. Evidence of engagement with potential users of the research prior to submitting the proposal
We have a long track record of engaging with researchers working on Southern Hemisphere climate reconstructions and interpreting these within a global context. Examples include our editorship/participation in large syntheses of the science and impacts of climate changes such as:

Turner, J., Bindschadler, R.A., Convey, P., Di Prisco, G., Fahrbach, E., Gutt, J., Hodgson, D.A., Mayewski, P.A. and Summerhayes, C.P. (Editors), 2009. Antarctic Climate Change and the Environment. Scientific Committee on Antarctic Research, ISBN 978-0-948277-22-1, Cambridge, 526 pp.

Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report, Working Group II 2013: Impacts, Adaptation, and Vulnerability. Chapter 28 - Polar Regions

We also have engaged with researchers outside of the immediate research community including policy-makers and their science advisors. For example: talks at UK Government Departments (e.g. Business Innovation and Skills, Hodgson May 2012), and representation through IPCC authorship (Hodgson, ongoing). In addition, talks to groups and young people at schools in the Cambridge area are already highlighting this area of research (Roberts - STEM Ambassadors programme activities).
 
Description Fieldwork has been completed for this award. Most of the analyses have now been completed. Remaining research is being carried out by a researcher who has moved to part time working - effectively extending the time required to complete. Six publications can be attributed to this grant. A further four are in preparation. Our research is of broad interest to Geoscientists and the wider scientific community as it provides evidence of a long term link between the the strength of the Southern Hemisphere Westerly Winds (SHW) and the performance of the Southern Ocean as a global CO2 sink. Specifically, by extending data analysis beyond the instrumental record, it supports the original Le Quéré, hypothesis of 'Saturation of the Southern Ocean CO2 sink due to recent climate change' Science 316, 1735 - 1738 (2007), rather than the Landschützer, paper on 'The reinvigoration of the Southern Ocean carbon sink' Science 349, 1221-1224 (2015). Our main conclusion is that on multi-decadal to millennial timescales, further increases in wind strength from enhanced latitudinal temperature gradients will lead to faster accumulation of CO2 in the atmosphere (This key finding is reported in Nature Geoscience https://www.nature.com/articles/s41561-018-0186-5).

Another major finding is that the Southern Hemisphere Westerly Winds are closely associated with Southern Hemisphre continental droughts (South America, South Africa and Australia/New Zealand). This is being prepared for publication in 2019.
Exploitation Route The findings of this project will be used in Global Climate Models (GCM's) to determine whether the Southern Ocean acts as a net source or sink of atmospheric CO2 relative to changes in the strength and position of the Southern Hemisphere westerly winds.
Sectors Environment

URL https://www.bas.ac.uk/project/late-quaternary-changes-in-the-westerly-winds-over-the-southern-ocean/
 
Description The findings of this project will inform the next generation of IPCC models and help governments make well-informed choices about adapting to climate change
First Year Of Impact 2016
Sector Environment