Submitted as: development and technical paper
23 Aug 2022
Submitted as: development and technical paper | 23 Aug 2022
Status: this preprint is currently under review for the journal GMD.

How does 4DVar data assimilation affect the vertical representation of mesoscale eddies? A case study with OSSEs using ROMS v3.9

David E. Gwyther1, Shane R. Keating2, Colette Kerry1, and Moninya Roughan1 David E. Gwyther et al.
  • 1Coastal and Regional Oceanography Lab, School of Biological, Earth and Environmental Sciences, UNSW Sydney, Sydney, NSW, Australia
  • 2School of Mathematics and Statistics, UNSW Sydney, Sydney, NSW, Australia

Abstract. Accurate estimates and forecasts of ocean eddies in key regions such as Western Boundary Currents are important for weather and climate, biology, navigation and search and rescue. The dynamic nature of mesoscale eddies requires data assimilation to produce accurate eddy timings and locations in ocean model simulations. However, data assimilating models are rarely assessed below the surface due to a paucity of observations, hence it is not clear how data assimilation impacts the subsurface eddy structure. Here, we use a suite of Observing System Simulation Experiments to show how the subsurface representation of eddies is changed within data assimilating simulations even when assimilating nearby observations. We examine in detail two possible manifestations of how the data assimilation process impacts 3-dimensional eddy structure, namely, by producing overly active baroclinic instability and through inaccurate vertical mode structure. Therefore in DA simulations, subsurface temperature structures can be too deep and too warm, particularly in dynamic eddy features. Our analyses demonstrate the need for further basic research in ocean data assimilation methodologies to improve representation of subsurface ocean structure.

David E. Gwyther et al.

Status: open (until 02 Nov 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

David E. Gwyther et al.

David E. Gwyther et al.


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Short summary
Ocean eddies are important for weather, climate, biology, navigation, and search and rescue. Since eddies change rapidly, models that incorporate or assimilate observations are required to produce accurate eddy timings and locations, yet the model accuracy is rarely assessed below the surface. We use a unique type of ocean model experiment to assess 3-dimensional eddy structure in the East Australian Current and explore two pathways in which this subsurface structure is being degraded.