Antarctic Bottom Water response to Varying Surface Fluxes
Date
2016
Authors
Snow, Kate
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Abstract
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.
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Keywords
Physical Oceanography, Antarctic Bottom Water, Global Overturning Circulation, Surface Buoyancy Fluxes
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Thesis (PhD)
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