Browsing by Author "Nicholson, Sarah-Anne"

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    Intra-seasonal variability of Southern Ocean primary production: the role of storms and mesoscale turbulence
    (2016) Nicholson, Sarah-Anne; Lévy, Marina; Monteiro, Pedro; Swart, Sebastiaan; Vichi, Marcello
    The Southern Ocean is one of the stormiest places on earth; here strong mid-latitude storms frequently traverse large distances of this ocean. Underlying these passing storms, the Southern Ocean is characterized by having some of the highest eddy kinetic energy ever measured (eddies occupying the meso to sub-mesoscale). The presence of the passage of intense storms and meso to sub-mesoscale eddy variability has the potential to strongly impact the intraseasonal variability of the upper ocean environment where phytoplankton live. Yet, exactly how phytoplankton growth rates and its variability are impacted by the dominance of such features is not clear. Herein, lies the problem addressed by the core of this thesis, which seeks to advance the understanding of intra-seasonal variability of Southern Ocean primary production. The drivers of this intra-seasonal variability have been explored from two points of view: the local-scale and the remote-scale perspectives, with a suite of physicalbiogeochemical (NEMO-PISCES) numerical models of varying complexity. At the local-scale, these model experiments have suggested that intra-seasonal stormlinked physical supplies of dissolved iron (DFe) during the summer played a considerably more active and influential role in explaining the sustained summer productivity in the surface waters of the Southern Ocean than what was thought previously. This was through two important insights: 1. Storm-eddy interactions may strongly enhance the magnitude and extent of upperocean vertical mixing in both the surface mixed layer as traditionally understood as well as in the subsurface ocean. These two mixing regimes have different dynamics but act in concert to amplify the DFe fluxes to the surface ocean. 2. Storm initiated inertial motions may, through interaction with eddies, greatly reinforce w and thus, enhance the vertical advection of DFe to the surface ocean, an effect that may last several days after the storm. At the local-scale, such storm-eddy dynamics may greatly increase the intra-seasonal variability of primary production, a step towards helping to explain why this variability is so strong in large regions of the Southern Ocean. At the remote-scale, the cumulative impact of these short-term storm-eddy interactions have unexpected implications in respect of the larger-scale mean flow and its influence on the effectiveness of intra-seasonal forcing of DFe fluxes. This counter intuitive feedback is a reduced strength of the intra-seasonal variability in primary production despite what was shown at the local-scale. Moreover, the addition of storms intensified the main clockwise cell of the meridional overturning circulation particularly the downward branch thus, reducing DFe inventory from the upper-ocean. Such an impact could potentially be enhanced with increasing storm intensities as suggested by climate projections. Understanding these remote-scale and local-scale responses of primary productivity to storms and their interaction with the underlying ocean mesoscale turbulence may be key to better understanding the sensitivities of the carbon cycle to short-term variability and long-term trends in atmospheric forcing.
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    The circulation and thermal structure of False Bay: a process-oriented numerical modelling and observational study
    (2012) Nicholson, Sarah-Anne; Shillington, Frank; Waldron, Howard; Vietch, Jennifer
    False Bay, the largest bay in South Africa, lies between two of the world's most in-fluential current systems: the Agulhas and Benguela Current Systems. False Bay is situated at a biogeographic break (Pitcher et al., 2010), belonging to both the South-ern Benguela upwelling system and the Western Agulhas Bank. The oceanography of Southern Africa, particularly this region, is some of the most complex and highly variable in the world (Roberts, 2005). Owing to the surrounding steep orography and its position, the meteorology of the False Bay region is also complex, having wind con-ditions that are unique when compared to other regions of South Africa (Jury, 1991). It is under these special conditions that the circulation of False Bay is regarded as highly complex.
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    Turbulence in the sea ice impacted Southern Ocean and its implications for primary production and carbon export
    (2023) Nunes, Da Costa Isabelle; Ansorge, Isabel; Swart, Sebastiaan; Nicholson, Sarah-Anne
    The sea-ice impacted Southern Ocean, south of the Antarctic Circumpolar Current, is one of the most important regions on earth for the cycling of carbon and distribution of heat and freshwater around the globe. Here, along-isopycnal upwelling of warm, carbon-rich circumpolar deep water coincides with the annual growth and melt of Antarctic sea ice that represents one of the world's largest surface water transformations. The air-sea-ice buoyancy exchanges and biological processes that change the surface water properties therefore have global consequences, as they set the properties of downwelling intermediate waters that enter the upper branch of the global thermohaline circulation. The region hosts some of the largest uncertainties in global climate models. The reason for this stems from two sources. Firstly, the spatio-temporal resolution of global climate models is limited by computational constraints such that smaller scale processes need to be parameterized. Secondly, the challenges associated with making observations in or near sea ice and in the harsh and remote conditions of the Southern Ocean means that the region is sparsely sampled, and as such, the parameterizations of the small scale and turbulent terms in global climate models are validated based only on a few in situ samples. This thesis concerns the observation and interpretation of (sub)meso- to micro scale turbulence and its implications in the sea ice impacted Southern Ocean. I aimed to understand the 0.01-1 km scale physical and biological processes that drive changes in the properties of the upper ocean following sea ice melt, using groundbreaking sustained high temporal and spatial resolution observations made by gliders. There are three main findings. Firstly, we find that sea ice melt by introducing a lateral freshwater gradient enhances stirring of submesoscale flows (0.1-10 km) and therefore lateral variability in the upper ocean, but simultaneously constrains vertical fluxes between the ocean interior and surface by enhancing stratification. Secondly, turbulent diapycnal mixing and double diffusive convection (0.1-1 m scales) drive the warming of the subsurface winter water, therefore mediating fluxes between the ocean interior and surface. Finally, phytoplankton respond favourably to larger volume sea ice that enhances winter mixing of nutrients from the deep reservoir and to upper ocean stratification in the summer. The preliminary evidence from this study suggests that the resultant higher intensity phytoplankton bloom translates to enhanced short term carbon export but not necessarily long term export. Overall, we show, using observations, that the variability and transport of heat and freshwater flux in the sea ice impacted Southern Ocean are sensitive to sea ice, with downstream impacts on phytoplankton, the biological carbon pump and ultimately the upper cell of the meridional overturning circulation.