These dense pools of shelf water have increasingly been revealed with the high-resolution repeat sampling possible with ocean gliders.
The Winter Cascade was first described by Godfrey et al, 1980, with reference to Bass Strait. The mechanism is simple. Sea temperatures are significantly warmer than air temperatures in winter so the ocean loses heat to the atmosphere.
Cooled surface water sinks as a consequence of its density increasing. In shallow water, where there is no deep reservoir of heat, the whole water column can cool down nearly to air temperature.
Once dense enough, the cold coastal water slides down the sloping sea floor as a ‘gravity current’ to form a distinct layer underneath the warmer offshore waters.
Four recent Slocum glider missions around Australia (Spencer Gulf, Two Rocks in WA, the Kimberley and Yamba NSW) have all just recently encountered dense shelf water pools.
It's not always clear-cut how to distinguish the winter cascade from upwelling but distinguishing features of winter cascade are that:
- the coldest, saltiest water is at the coast,
- it usually occurs on a wide shallow shelf and
- it is found after cold winds occur, typically off the land, rather than from the upwelling-favourable direction.