Surface and sub-surface hydrographic variability at the Prince Edward Islands: perspectives from the high resolution GLORYS model
The Prince Edward Islands (PEI), comprising of Marion and Prince Edward Island, are located in the direct path of the eastward flowing Antarctic Circumpolar Current, nestled within the Antarctic Polar Frontal Zone of the Southern Ocean. The islands are home to a multitude of species which are supported by a sensitive and complex oceanic environment. Understanding the mechanisms at work, which sustain this rich ecosystem is therefore imperative for both the ecological management of the PEIs, and for the possible prediction of future climate changedriven environmental impacts. The presence of a possible Taylor column has been suggested as a main driver in supporting and maintaining the PEI ecosystem. However, due to the remote and hostile environment in which the islands are located, in situ data collection has proven to be a challenging task, which is necessary to study the Taylor column and to understand the island's general hydrographic variability. This study thus makes use of available in situ, satellite, reanalysis and modelled bathymetry data, to compare against the GLORYSV12 model output. This was to determine how accurately the model could reproduce surface and subsurface variability of temperature, salinity, Sea Surface Height (SSH) and surface circulation at the PEIs, and to identify whether in situ, satellite and model conditions are suitable for the existence and evolution of a possible Taylor column at the islands. A clear overestimation of the geostrophic currents (up to 0.2 m s -1 ) and underestimation of SSH (up to 0.6 m) by GLORYS was observed. The spatial and temporal variability of temperature and salinity was captured by GLORYS throughout the entire water column, despite the differences between temperature (biases from -2 to 2 oC) and salinity (biases from -0.4 to 0.4 PSU). Additionally, GLORYS was also able to simulate all five water masses, known to occur within the PEI region, throughout time. When GLORYS was compared to single point in situ SST time series data, a seasonal bias was observed with GLORYS overestimating SST in late summer (January to March) and underestimating SST for the remainder of the year (May to December). However, statistically significant strong, positive correlations (r > 0.80, p < 0.001) and relatively low biases ( -0.50 to 0.10 oC) were still observed between GLORYS and the this in situ SST time series. Overall, this suggested that GLORYS reasonably captures temperature and salinity variability on a climatological-scale. However, when it comes to event-scale, the model fails to accurately reproduce specific mesoscale events, as observed in 2013, 2014 and 2015 when cyclonic and anticyclonic eddies, simulated by the model were not of the same size, intensity nor in the same location as observed by in situ CTD data. GLORYS and satellite data both successfully proved that conditions are suitable for the formation and persistence of a possible Taylor column/Taylor cone at the PEIs. This was concluded with the relatively low Rossby numbers (< 0.07), high Reynolds numbers (> 2000), Blocking parameters which did not exceed 0.2, appropriate Rossby radius of deformation values (< 1000 km for a barotropic ocean and < 24 km for a baroclinic ocean) and an anticyclonic flow pattern around the PEI plateau, all indicative of the fact that conditions, which are typical for the formation of Taylor columns/Taylor cones, occur at the PEIs
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