No abstract

occurrence and seasonal variability of Dense Shelf Water cascades along Australian continental shelves tanziha Mahjabin ✉ , charitha pattiaratchi & Yasha Hetzel transport of water between the coast and the deeper ocean, across the continental shelf, is an important process for the distribution of biota, nutrients, suspended and dissolved material on the shelf. presence of denser water on the inner continental shelf results in a cross-shelf density gradient that drives a gravitational circulation with offshore transport of denser water along the sea bed that is defined as Dense Shelf Water Cascade (DSWC). Analysis of field data, collected from multiple ocean glider data missions around Australia, confirmed that under a range of wind and tidal conditions, DSWC was a regular occurrence during autumn and winter months over a coastline spanning > 10,000 km. It is shown that even in the presence of relatively high wind-and tidal-induced vertical mixing, DSWcs were present due to the strength of the cross-shelf density gradient. the occurrence of DSWc around Australia is unique with continental scale forcing through air-sea fluxes that overcome local wind and tidal forcing. it is shown that DSWc acts as a conduit to transport suspended material across the continental shelf and is a critical process that influences water quality on the inner continental shelf. Globally, the coastal ocean is the receiving basin for input of suspended and dissolved matter that includes nutrients, biota and pollutants and represents an important component of the ocean environment, connecting the terrestrial system to the deeper ocean 1. Transport processes that contribute to the exchange of water between the coast and deeper regions: cross-shelf exchange, has been defined as one of the central problems in coastal physical oceanography 2. Majority of inputs to the coastal zone are through fresh water sources such as rivers and anthropogenic discharges. As these inputs are usually buoyant, they are transported along the ocean surface layer subject to the prevailing hydrodynamic regime and are easily identified in the field and in satellite imagery. If the spatial scale is unrestricted by coastal features and under relatively low mixing conditions, river plumes can influence the hydrodynamics to hundreds of kilometres from the source region 3,4. These systems have been subject to many investigations over the past decades being defined as river plumes or Regions of Freshwater Influence (ROFI) where the hydrodynamic regime is governed by the positive density gradient between lower salinity riverine water and higher salinity oceanic water 3. In ROFI, buoyancy input from freshwater sources exceeds that from changes in heat and freshwater fluxes across the ocean surface. Regions that experience a Mediterranean climate (dry hot summers, mild cooler winters and low precipitation) are subject to high rates of evaporation and negligible river input resulting in a net loss of fresh water from the coastal region. Therefore, coastal water bodies become mo...

Along the majority of Australian shallow coastal regions, summer evaporation increases the salinity of shallow waters, and subsequently in autumn/winter, the nearshore waters become cooler due to heat loss. This results in the formation of horizontal density gradients with density increasing toward the coast that generates gravity currents known as dense shelf water cascades (DSWCs) flowing offshore along the sea bed. DSWCs play important role in ecological and biogeochemical processes in Australian waters through the transport of dissolved and suspended materials offshore. In this study a numerical ocean circulation model of Rottnest continental shelf, validated using simultaneous ocean glider and mooring data, indicated that the passage of cold fronts associated with winter storms resulted in rapid heat loss through evaporative cooling. These conditions resulted in enhancement of the DSWCs due to modifications of the cross-shelf density gradient and wind effects. Specifically, onshore (offshore) directed winds resulted in an enhancement (inhibition) of DSWCs due to downwelling (vertical mixing). Consequently, the largest DSWC events occurred during the cold fronts when atmospheric temperatures reinforced density gradients and onshore winds promoted downwelling that enhanced DSWCs. Advection of DSWCs was also strongly influenced by the wind conditions, with significantly more transport occurring along-shelf compared to cross-shelf.

No abstract