Dr David Gwyther¹, Dr Colette Kerry¹, Prof Moninya Roughan¹, Dr Shane Keating²

¹Coastal and Regional Oceanography Lab, School of Biological, Earth and Environmental Sciences, UNSW Sydney, Sydney, Australia, ²School of Mathematics and Statistics, UNSW Sydney, Sydney, Australia

Due to the rapid evolution, complex dynamics and baroclinic structure, circulation regimes such as Western Boundary Currents and the eddies they shed are high priorities for numerical estimation and forecasting. However, the ability to do so accurately is hamstrung by their very nature, and therefore requires dense, subsurface observations. Yet, the sparse sampling patterns of autonomous observation platforms leads to significant challenges in understanding the 3-dimensional structure of boundary currents and mesoscale eddies. Here we use Observing System Simulation Experiments to show that subsurface temperature observations in a high eddy kinetic energy region yield large improvement in representation of key East Australian Current (EAC) circulation features, both downstream and 600 km upstream of the observing location. These subsurface temperature observations (in concert with sea surface temperature and height measurements) are also critical for correctly representing ocean heat content along the length of the EAC. Furthermore, we find that a more poleward separation latitude leads to an EAC and eddy field that is represented with far reduced error, compared to when the EAC separates closer to the equator. Our results demonstrate the importance of subsurface observations for accurate state estimation of the EAC and ocean heat content that can lead to marine heatwaves. Further, we explore the improvement in the representation of mesoscale eddies and subsurface conditions with a spectrum of expendable bathythermograph (XBT) observations ranging in temporal resolution, from quarterly (matching current observing systems), monthly, fortnightly and weekly. These results provide suggestions for observing system design under different oceanographic regimes, for example, adaptive sampling, as well as suggesting at optimal repeat times for future XBT transects.

Biography:

David completed his PhD at the University of Tasmania in 2016, using ocean models to explore how the Southern Ocean melts the underside of Antarctic ice shelves. Following a postdoctoral position in ocean-ice shelf interactions, he transitioned to a more (sub)tropical research theme.