Download Project EARTH-16-HLJ1 - Department of Earth Sciences

Project EARTH-16-HLJ1: What role does the ocean play in Arctic sea-ice decline?
Supervisor: Helen Johnson
Arctic sea ice retreat is a well-documented feature of recent climate change, with significant implications
for the planet as a whole due to the feedbacks on global temperature and on atmosphere and ocean
circulation. However, it is not clear what role the ocean plays. Warm water from the Atlantic is confined
below a strong halocline (salinity stratification) across much of the Arctic, preventing its heat from reaching
the sea ice above. The extent to which this will change as the sea-ice retreats and there is more turbulent
mixing is not clear.
This project will explore the role of the ocean in Arctic sea-ice decline. In particular, it will examine the
processes important in creating and maintaining the halocline, across a range of models, and how the
Arctic Ocean stratification is likely to change in the future. The student will begin by analysing data from
the control runs of a number of coupled climate models (such as those used in the IPCC’s Fifth Assessment
Report) to assess how well they represent the Arctic halocline and seasonal changes in stratification. The
particle-tracking package Ariane (www.univ-brest.fr/lpo/ariane/) will be used to explore the source of
waters making up the Arctic halocline in a high-resolution ocean model (ORCA12) as well as in a coupled
climate model. The relationship between changes in heat transport, heat content within the Atlantic water
layer, and sea-ice extent will also be identified.
The student will then use climate change simulations from models that both do and do not do a good job of
representing the observed sea-ice decline over recent years to investigate the changes in Arctic Ocean
stratification through time and the extent to which heat from the Atlantic water layer below the Arctic
halocline is implicated in the increased melting of sea-ice. Hypotheses to be explored include that (a) the
current generation of climate models are not sensitive enough to increased mixing associated with sea-ice
reduction, because vertical mixing in the models is already unrealistically high, and (b) there are negative
feedbacks in the system which may reduce any impact of ocean heat on ice melt as the sea-ice retreats.
This project will involve the statistical analysis and
interpretation of large geophysical datasets
including coupled atmosphere-ocean-sea-ice
general circulation models. Key processes and
feedbacks will be explored through the
development of simpler 1D and 2D numerical
models, and the application of fluid dynamics
theory. The project would best suit a strongly
motivated student with a solid background in
maths and physics and a desire to learn about
high latitude ocean dynamics. The student will
interact regularly with collaborators at the UK Met
Office, at the National Oceanography Centre in
Southampton and at Ifremer in Brest, France. The
student will be part of the Oxford Physical
Oceanography group, which straddles the gap
between the Earth Sciences and Physics
Departments. They will work closely with other
members of the group who are also involved in
investigations of high-latitude ocean dynamics
and climate. There may also be an opportunity to
participate in a research cruise should the student
wish.
The Arctic halocline: When sea-ice forms, it releases salt
into surface waters. These waters become denser and sink
to form the Arctic halocline – a layer of cold water that
acts as a barrier between sea-ice and deeper warmer
water that could melt the ice. (Illustration by Jayne
Doucette, WHOI)
Selected relevant references:
Stroeve, J. C., V. Kattsov, A. Barrett, M. Serreze, T. Pavlova, M. Holland, and W. N. Meier (2012), Trends in
Arctic sea ice extent from CMIP5, CMIP3 and observations, Geophys. Res. Lett., 39, L16502,
doi:10.1029/2012GL052676.
Proshutinsky, A., Y. Aksenov, J. Clement Kinney, R. Gerdes, E. Golubeva, D. Holland, G. Holloway, A. Jahn,
M. Johnson, E. Popova, M. Steele, and E. Watanabe. 2011. Recent advances in Arctic ocean studies
employing models from the Arctic Ocean Model Intercomparison Project. Oceanography 24(3):102–113,
http://dx.doi.org/10.5670/oceanog.2011.61.