The impact of mesoscale jet activity on plankton heterogeneity and primary production : a numerical modelling study
The impact of mesoscale jet activity on plankton heterogeneity and primary production : a numerical modelling study
This thesis presents a study of the effect of a developing instability of an open ocean mesoscale jet on the local ecosystem. Previous observational and modelling studies of the physical characteristics of open ocean frontal jets show large vertical velocities. These can have consequences for the plankton community through both the upwelling of nutrients and the redistribution of biota in the water column. A coupled numerical physical ecosystem model is developed to investigate the effect of the dynamics of a frontal jet on the ecosystem.
The evolution of the instability of a mesoscale jet is found to give rise to both the vertical transport of nutrients into the euphotic zone and subduction of biota out of the euphotic zone. The upwelling of nutrients stimulates increases in primary production, with resulting increases in phytoplankton stocks. The increase in primary production can be locally of the order of 100%, and of the order of 10% when averaged over the frontal region. Biota subducted from the surface waters forms remnant populations of plankton below the euphotic zone destined for depletion. The action of upwelling and subduction introduces spatial heterogeneity in the plankton biomass and primary production at a variety of length scales. The length scales are of the order of a few kilometres for thin filaments and up to 50 km for coherent features.
The major reaction of the ecosystem is through the formation of coherent physical structures that act to transfer water parcels across the frontal jet and vertically along sloping isopycnals. The formation of the coherent features is found to be very dependent on the stratification associated with the front. It is suggested that the gradient in isopycnal thickness across the jet can play a large role in whether coherent features form and in determining the magnitude of cross frontal transport.
With increases in primary production occurring over several degrees of latitude and mostly resulting from increases in new production, frontal dynamics may make a significant contribution to the strength of the biological pump. Also with the relatively small scale of frontal features, these dynamic regions for the ecosystem are not able to be resolved in biogeochemical or climate models. With the aim of relating the effect of the frontal dynamics on the ecosystem to properties of the larger scale fields, a parameterisation of tracer fluxes is tested, based on the diffusion of isopycnal thickness. The evolution of along front average quantities is found to be well represented in the parameterisation model runs, and for the oligotrophic conditions at Bermuda in the summer months, the parameterised runs can also reproduce the increases in new production by considering vertical fluxes of a passive nitrate tracer.
University of Southampton
Spall, Steven Andrew
cc252ba6-f8b8-4494-8a69-11d2725af8fa
1997
Spall, Steven Andrew
cc252ba6-f8b8-4494-8a69-11d2725af8fa
Spall, Steven Andrew
(1997)
The impact of mesoscale jet activity on plankton heterogeneity and primary production : a numerical modelling study.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
This thesis presents a study of the effect of a developing instability of an open ocean mesoscale jet on the local ecosystem. Previous observational and modelling studies of the physical characteristics of open ocean frontal jets show large vertical velocities. These can have consequences for the plankton community through both the upwelling of nutrients and the redistribution of biota in the water column. A coupled numerical physical ecosystem model is developed to investigate the effect of the dynamics of a frontal jet on the ecosystem.
The evolution of the instability of a mesoscale jet is found to give rise to both the vertical transport of nutrients into the euphotic zone and subduction of biota out of the euphotic zone. The upwelling of nutrients stimulates increases in primary production, with resulting increases in phytoplankton stocks. The increase in primary production can be locally of the order of 100%, and of the order of 10% when averaged over the frontal region. Biota subducted from the surface waters forms remnant populations of plankton below the euphotic zone destined for depletion. The action of upwelling and subduction introduces spatial heterogeneity in the plankton biomass and primary production at a variety of length scales. The length scales are of the order of a few kilometres for thin filaments and up to 50 km for coherent features.
The major reaction of the ecosystem is through the formation of coherent physical structures that act to transfer water parcels across the frontal jet and vertically along sloping isopycnals. The formation of the coherent features is found to be very dependent on the stratification associated with the front. It is suggested that the gradient in isopycnal thickness across the jet can play a large role in whether coherent features form and in determining the magnitude of cross frontal transport.
With increases in primary production occurring over several degrees of latitude and mostly resulting from increases in new production, frontal dynamics may make a significant contribution to the strength of the biological pump. Also with the relatively small scale of frontal features, these dynamic regions for the ecosystem are not able to be resolved in biogeochemical or climate models. With the aim of relating the effect of the frontal dynamics on the ecosystem to properties of the larger scale fields, a parameterisation of tracer fluxes is tested, based on the diffusion of isopycnal thickness. The evolution of along front average quantities is found to be well represented in the parameterisation model runs, and for the oligotrophic conditions at Bermuda in the summer months, the parameterised runs can also reproduce the increases in new production by considering vertical fluxes of a passive nitrate tracer.
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Published date: 1997
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Local EPrints ID: 463232
URI: http://eprints.soton.ac.uk/id/eprint/463232
PURE UUID: 3e32053f-a760-4e62-8180-f11a16360ccd
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Date deposited: 04 Jul 2022 20:47
Last modified: 23 Jul 2022 01:09
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Author:
Steven Andrew Spall
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