Occurrence and controls on Dense Shelf Water Cascades around Australia.

Along Australian continental shelves, high evaporation during summer and cooling during winter result in a cross-shelf density gradient that drives gravity currents transporting denser water along the sea bed offshore. This process is defined as Dense Shelf Water Cascade (DSWC). Multi-year transects (192) of ocean glider data from eight contrasting regions around Australia confirmed the existence of DSWC as a regular occurrence during autumn and winter periods. The main parameters controlling DSWC were identified as buoyancy input (cross-shelf density gradient) and vertical mixing through wind and tidal action. To examine the spatial variability of DSWC along the Perth Metropolitan continental shelf region, a three-dimensional hydrodynamic model was used. The model, validated using field measurements, confirmed the presence of DSWC throughout the model domain. Although there was gradual cooling of coastal waters during autumn and winter, there were periods of rapid heat loss during the passage of storm systems. During these periods the cross-shelf density gradients were enhanced and generated strong DSWC. Onshore winds associated with cold fronts enhanced the DSWC. The field and numerical model results confirmed the cross-shelf density gradient as the dominant forcing mechanism for DSWC formation. The influence of tidal mixing was small even in regions of high tidal range compared to the cross-shelf density gradient. In contrast, wind effects had a strong influence through: (1) inhibiting DSWC through vertical mixing; and, (2) enhancing during onshore winds. DSWC play an important role in ecological and biogeochemical processes in Australian waters as a conduit to the transport of dissolved and suspended materials offshore.