Intra-seasonal variability of Southern Ocean primary production: the role of storms and mesoscale turbulence

Doctoral Thesis


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University of Cape Town

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.