EASTERN AUSTRALIAN COASTAL BEHAVIOUR IN RESPONSE TO EXTREME STORM CLIMATE BETWEEN 1600-1900 AD, DETERMINED FROM A COUPLED CLIMATE RECONSTRUCTION AND COASTAL MORPHODYNAMIC APPROACH

Abstract:

A sustained morphodynamic reorganisation of the east Australian coast occurred over a large latitudinal gradient from subtropical Queensland (S 25°) to mid-latitude Bass Strait (S 40°) between ~1600 to 1900 CE. These changes indicate that a large-scale shift in the modal climate occurred together with changes in extreme storm frequency or clustering of East Coast Cyclones (ECC), when compared to the past century. ECC are complex subtropical weather systems that form off the east coast of Australia and/or travel parallel to the coast of Australia from south-east Queensland to Victoria.

We investigate coastal evolution and the associated climate drivers using a novel combination of methods, including: LIDAR DEM and field mapping of coastal geology; a decadal-scale climate reconstruction of sea-level pressure, marine windfields, and paleo-storm synoptic type and frequency, using a paleoclimate data assimilation approach; together with wave transformation and coastal planform modelling for paleo-wave directions, and historical bathymetry. We present the morphodynamic response to changes in directional wave power, by linking the paleo-windfield reconstruction to wave transformation models.

The combined methodology has illuminated the ‘ultimate’ storm impacts not seen in the past century, and defines the multi-decadal coastal system response and recovery to extreme storm sequences. Increased embaymentisation and anticlockwise rotation of embayed and barrier coast planform geometry; shifts in barrier-estuary-inlet configuration; and a ubiquitous foredune transgression, are shown to have occurred between ~1600 to 1800 CE. This was in response to a poleward shift in the subtropics and frequency of tradewind-driven wave climate, and tropical-origin storms. From 1800 to 1900 CE, an equatorward shift in the subtropics, and clustering of extratropical-origin storms drove an increase in the shoreface-littoral sediment budget and a clockwise coastline progradation. This energetic, oblique directional wave climate, delivered higher than modern across-shoreface and alongshore sand transport rates. The 20^th century decline in storm wave energy and implied reduction in across-shoreface sand transport is a significant driver of modern shoreline behavior along the Australian east coast.