On the East Australian Current Encroachment : Remote Sensing, Quantitative Mapping and Spatio-temporal Variability

The East Australian Current (EAC) is a highly dynamic western boundary current of the South Pacific Gyre. The EAC frequently encroaches shoreward, drives upwelling, changes coastal bio-physical dynamics, and thus exerts significant impacts on coastal marine ecosystem. This thesis aims to provide a first quantitative and systematic study on the EAC encroachment off southeast Australia. First, a quantitative mapping method of the EAC is developed using remotely sensed Sea Surface Temperature (SST) data and a Topographic Position Index (TPI) based image processing technique. The validation using Bluelink ReANalysis model data suggested good reliability of our mapping results, based on which direct measurement of EAC encroachment is for the first time made possible. A study using EAC maps generated from 6-day composited Himawari-8 SST data has provided new insights into the EAC encroachment. Along the coast of New South Wales (NSW), large-scale and high-frequency EAC intrusion was observed, being every 60-80 days upstream (30-32ºS) and every 90-100 days downstream (33-35ºS), which is associated with the EAC’s intrinsic oscillation and eddy shedding. Downstream, the EAC intrusion exhibits a 20-day longer period and a quasi-double amplitude. Such dephasing is due to abrupt change of regime at the EAC separation point (32-33ºS). Higher-frequency (every 16-32 days) and smaller-magnitude EAC intrusion was observed along the entire NSW coast (28-37ºS), which is associated with EAC’s meanders and frontal eddies. In the Extension Zone (37.30-44ºS), we observed maximum EAC intrusion in summer, which is an expression of a seasonal boundary flow off eastern Tasmania. Further, using 26-year AVHRR SST data, seasonality of the EAC intrusion off north NSW has been quantified and analyzed. The results show that the EAC moves closer to the coast in summer than in winter, which is due to its seasonal shift (upstream of 29º40'S) and seasonal widening (downstream of 29º40'S). These findings provide new understandings of the seasonal upwelling and shelf circulation observed off north NSW. Finally, with HF radar and mooring data, we performed an accurate daily tracking of a rapid EAC intrusion event and monitored its impacts on the shelf water off Coffs Harbor. During the event, the EAC was highly dynamic with considerable daily onshore/offshore movement (~5km/day). Bottom ocean temperature and surface current speed on the shelf varied linearly with the EAC-to-coast distance.