Three-dimensional structure of sub-mesoscale eddies on the continental shelf

Abstract. Sub-mesoscale eddies are oceanic features crucial for energy transfer processes, transport of heat and biogeochemical tracers, and for promoting diversity in marine ecosystems. Their small length scales (<20 km) and short lifespans (O~days), make their characterization challenging using field measurements. In this paper, we combined remote sensing, ocean glider, shipborne, and mooring data to investigate the internal structure and dynamical properties of sub-mesoscale eddies and peddies ('petite' eddies; diameters ≤10 km extending through the water column) along the Wadjemup (Rottnest) Continental Shelf (WCS), Western Australia. Data collected in August 2010 showed an anti-cyclonic mesoscale eddy spawning several cyclonic, cold core sub-mesoscale eddies that promoted upwelling (vertical extension ~185 m) with maxima in chlorophyll concentration at the centre. Results suggested that sub-mesoscale eddies drive chlorophyll advection either at (in) their periphery (centre) of cyclonic eddies, or at their interfaces. In general, cyclonic eddies promoted upwelling and anti-cyclonic eddies promoted downwelling; however, our data also showed that upwelling conditions could also occur in anti-cyclonic eddies. Winds were found to affect the internal structure and lifecycle of sub-mesoscale eddies, particularly under strong wind (speeds > 7 ms-1) conditions, through upwelling and/or vertical mixing. In the presence of dense shelf water outflows along the continental shelf, sub-mesoscale eddies were found to trap, upwell, and transport denser nearshore waters off the continental shelf. This unique dataset provided a detailed picture of the three-dimensional structure of sub-mesoscale eddies, including the characterization of sub-surface eddies, and contributes to the understanding of their role in chlorophyll advection in shelf and coastal environments.