| Summary: | <p>Atlantic Water (AW) is the most significant source of oceanic heat in the Arctic Ocean,
and has played a key role in recent Arctic sea ice loss. Better understanding of the spatial and
temporal variability of AW heat and its future evolution is therefore key to understanding
future Arctic change.</p>
<p>I use observations and model data from CESM-LE (Community Earth System Model
Large Ensemble) to investigate 20th and 21st century AW layer changes. Observations show
that AW heat has had an increased influence on the upper ocean in the eastern Arctic, but
has become more isolated from the surface in the west. However, CESM-LE suggests that
this is due to a regional lag in response, and that the AW will warm, freshen and shoal
throughout the Arctic from the 2020s. These trends are anthropogenically forced. A shift
in atmospheric circulation likely plays an important role in these changes, with the resultant
atmospheric forcing from a weakened Beaufort High and expanded low over the eastern Arctic
enhancing AW transport from the Atlantic, and causing more AW to be advected into the
western Arctic.</p>
<p>Local processes such as mixing and cascading shelf flows also play an important role in
modifying AW properties, and buffer against high frequency AW temperature variability.</p>
<p>In CESM-LE, despite AW warming and freshening, the stability of the upper water column
increases in the future due to surface and halocline freshening. However, there is a shift to
alpha ocean stratification in the summer Barents Sea towards the end of the 21st century
which may have implications for AW heat loss and could occur in the Arctic basin itself in
the longer term.</p>
<p>This thesis highlights the importance of AW heat in governing how the future Arctic may
evolve in a changing climate, and the various processes that influence its fate.</p>
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