Aspects of southern ocean transport and mixing

Damerell, Gillian (2012) Aspects of southern ocean transport and mixing. Doctoral thesis, University of East Anglia.

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Understanding and quantifying the circulation of the oceans and the driving
mechanisms thereof is an important step in developing models which can accurately
predict future climate change. In particular, model studies have shown that the
spatial variability of diapycnal diffusivity, which represents the rate at which deep
water returns to shallower depths by means of turbulent diapycnal mixing, is a
critical factor controlling the strength and structure of the circulation. Efforts are
therefore ongoing to measure diffusivity as extensively as possible, but temporal
variability in diffusivity has not been widely addressed.
Results from three Southern Ocean studies are presented in this thesis. Firstly,
a high resolution hydrographic survey carried out on the northern flank of the
Kerguelen Plateau identifies a complex meandering current system carrying a total
eastward volume transport of 174 ± 22 Sv, mostly associated with the blended
Subtropical Front/Subantarctic Front. Significant water mass transformation across
isopycnals is not required to balance the budgets in this region. Secondly,
results are presented which cast doubt on the advisability of using density
profiles acquired using Conductivity-Temperature-Depth instruments to estimate
diapycnal diffusivity (an attractive proposition due to low cost and widespread data
availability) in areas of weak stratification such as the Southern Ocean, because the
noise characteristics of the data result in inaccurate diffusivity estimates. Finally,
a method is developed for estimating diffusivity from profiles of velocity shear
acquired by moored acoustic Doppler current profilers. An 18-month time series
of diffusivity estimates is derived with a median of 3.3 × 10−4 m2 s−1 and a range
of 0.5 × 10−4 m2 s−1 to 57 × 10−4 m2 s−1. There is no significant signal at annual
or semiannual periods, but there is evidence of signals at periods of approximately
fourteen days (likely due to the spring-neaps tidal cycle), and at periods of 3.8 and
2.6 days most likely due to topographically-trapped waves propagating around the
local seamount. More widespread application of this method would allow for an
assessment of natural climate variability in diapycnal diffusivity.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Environmental Sciences
Depositing User: Users 2593 not found.
Date Deposited: 05 Dec 2013 14:16
Last Modified: 05 Dec 2013 14:16


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