Tropical and extratropical-origin storm wave types and their influence on the East Australian longshore sand transport system under a changing climate

Tropical expansion is potentially an amplifier of coastal change in the subtropics, through directional wave climate shifts. The storm wave climate and directional wave power distribution along the Southeast Australian Shelf (SEAS) is investigated with respect to tropical extent. Forty years of storm wave observations from nine midshelf wave buoys are evaluated using synoptic storm wave typing. A robust latitudinal and along-shelf gradient in storm wave types and wave propagation patterns exists. The tropical origin storms produce a shore-normal propagation pattern along the SEAS, reduce the connectivity of coastal compartments through minor headland bypassing events or episodically reversing the net northward transport. In contrast, the extratropical origin storms produce a shore-oblique propagation pattern from the Southern Tasman to the Coral Sea, and are an important control on the connectivity of regional longshore sand transport through episodic major headland bypassing events between compartments, and the maintenance of down-drift coastlines in dynamic equilibrium. Future climate change projections indicate that the recent trend in the expansion of the latitudinal extent of the tropics in the south-west Pacific region will continue throughout this century. The combined impacts of a projected 2.5° poleward shift on the storm wave climate is a significant reduction in net northward longshore sand transport and the efficiency of headland bypassing events. On the North and Central Coasts of New South Wales we project a ∼30% reduction in longshore sand transport for the dominant extratropical-origin storm events, together with a ∼5% increase in reversed (net southward) longshore sand transport for tropical-origin storm events.