Turbulence in the sea ice impacted Southern Ocean and its implications for primary production and carbon export

Abstract

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 worlds 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 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. 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 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 is sensitive to sea ice, with downstream impacts on phytoplankton, the biological carbon pump and ultimately the upper cell of the meridional overturning circulation.