The sensitivity of Southern Ocean biogeochemical cycles to vertical mixing
File(s)
Author(s)
Ellison, Elizabeth
Type
Thesis
Abstract
The Southern Ocean (SO) plays a critical role in global overturning circulation and biogeochemical
cycles, connecting the world’s major ocean basins and facilitating the upwelling of
deep waters. Understanding the physical processes occurring in the SO is crucial to understand
and model our climate system. While surface-intensified turbulent mixing significantly affects
biogeochemical cycles, the importance of variations in the weaker background mixing below
the mixed layer is generally overlooked. Background subsurface mixing rates in the SO vary
greatly in space and time. The processes known to induce ocean background turbulence are
typically not represented in models, and there is great uncertainty in the parameterisation of
this mixing. This thesis investigates the sensitivity of biogeochemical tracers to altered SO
diapycnal mixing.
Firstly, we show that changes to SO mixing over a modest range alter Atlantic biogeochemical
tracer distributions over short and long timescales. On annual to decadal timescales changes
are dominated by the direct impact of changes in diapycnal mixing on tracer fluxes, while on
centennial timescales they are dominated by the mixing-induced variations in the advective
transport of the tracers by large-scale circulation.
Secondly, we examine changes to SO biogeochemistry on sub-decadal timescales. We show that
increasing the background mixing significantly alters SO air-sea CO2 fluxes and phytoplankton
productivity. These could alter atmospheric CO2 concentration over longer timescales.
Finally, when using our current best estimates of spatially varying SO diapycnal mixing rates,
we find no reduction in the misfit between modelled and observed tracer concentrations compared
to using a spatially homogeneous mixing profile.
This work demonstrates the importance of accurately representing spatiotemporally variable
local and non-local SO mixing processes. It is essential to simulate biogeochemical cycles on
decadal timescales and to represent the indirect impact of mixing-induced changes to circulation
on biogeochemical cycles on longer timescales.
cycles, connecting the world’s major ocean basins and facilitating the upwelling of
deep waters. Understanding the physical processes occurring in the SO is crucial to understand
and model our climate system. While surface-intensified turbulent mixing significantly affects
biogeochemical cycles, the importance of variations in the weaker background mixing below
the mixed layer is generally overlooked. Background subsurface mixing rates in the SO vary
greatly in space and time. The processes known to induce ocean background turbulence are
typically not represented in models, and there is great uncertainty in the parameterisation of
this mixing. This thesis investigates the sensitivity of biogeochemical tracers to altered SO
diapycnal mixing.
Firstly, we show that changes to SO mixing over a modest range alter Atlantic biogeochemical
tracer distributions over short and long timescales. On annual to decadal timescales changes
are dominated by the direct impact of changes in diapycnal mixing on tracer fluxes, while on
centennial timescales they are dominated by the mixing-induced variations in the advective
transport of the tracers by large-scale circulation.
Secondly, we examine changes to SO biogeochemistry on sub-decadal timescales. We show that
increasing the background mixing significantly alters SO air-sea CO2 fluxes and phytoplankton
productivity. These could alter atmospheric CO2 concentration over longer timescales.
Finally, when using our current best estimates of spatially varying SO diapycnal mixing rates,
we find no reduction in the misfit between modelled and observed tracer concentrations compared
to using a spatially homogeneous mixing profile.
This work demonstrates the importance of accurately representing spatiotemporally variable
local and non-local SO mixing processes. It is essential to simulate biogeochemical cycles on
decadal timescales and to represent the indirect impact of mixing-induced changes to circulation
on biogeochemical cycles on longer timescales.
Version
Open Access
Date Issued
2023-07
Date Awarded
2023-12
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Mashayek, Ali
Sponsor
Engineering and Physical Sciences Research Council
Publisher Department
Civil and Environmental Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)