Dense Shelf Water Cascades along the North-West Australian Continental Shelf

摘要

High evaporation during the summer and cooling during winter along the coastal regions of the Australian north-west shelf (NWS) results in a cross-shelf density gradient. This drives a gravitational circulation with the offshore transport of higher density water along the sea bed, defined as Dense Shelf Water Cascades (DSWC). Ocean glider data available from the Integrated Marine Observing System (IMOS) were used to measure cross-shelf density gradients under varying wind and tide conditions along the Kimberley and Pilbara regions of the North-West Australian shelf. Analysis of 41 transects from 26 missions of high spatial and temporal resolution data collected between 2011 and 2015 confirmed that DSWC occur on a regular basis during the winter months, mainly due to cooling of the coastal waters that were pre-conditioned with higher salinity resulting from evaporation during the summer months. The cross-shelf transects indicated that both temperature and salinity contributed to the dense water formation. The dense water flow along the sea bed was identified to depths of up to 150m. The strongest density gradient calculated in the Kimberley was 5.123×10~(-6) kgm~(-4), whilst for Pilbara it was 14.23×10~(-6) kgm~(-4). The temporal variability and controlling mechanisms of the DSWC were investigated using data from current meter moorings deployed on shelf regions of the Kimberley and Pilbara. Although these two regions are macro-tidal and are subject to wind mixing, the vertical temperature stratification and monthly mean cross-shore velocity profiles indicated the presence of cascades during winter months. It is shown that even in the presence of high tidal mixing DSWCs persist due to the strength of the cross-shelf density gradient