Quantitative mapping of the East Australian Current encroachment using time series Himawari-8 sea surface temperature data

The East Australian Current (EAC) is a complex and highly dynamic western boundary current (WBC) component of the South Pacific Gyre. The interaction between the EAC and the continental shelf significantly influences the coastal ocean dynamics and the marine ecosystem off the southeast coast of Australia. In this study, we attempted to map the spatial structure of the sea surface temperature (SST) signature of the EAC system using Himawari-8 SST data, and to investigate the spatial and temporal evolution of the EAC encroachment for the period July 2015 to Sept 2017. The very high spatiotemporal resolution of Himawari-8 SST images has enabled the mapping of the highly dynamic EAC using a Topographic Position Index (TPI) method. The results demonstrated that the TPI mapping was able to capture the complex spatial structure of the EAC system, based on which the spatial variability and temporal evolution of EAC’s meanders, offshoots, separation, eddies, and extensions along its path have been clearly documented. Using an eddy-resolving, data assimilating numerical ocean model, we were able to relate the EAC's SST signature with its surface current speed signature, with an overall area ratio of overlap ~60% between the two signatures. Temporal wavelet analyses indicated that the variability of the EAC encroachment is dominated by an eddy-shedding timescale, being 65~80 days upstream and 90~100 days downstream of the EAC separation point. In the upstream, the EAC’s proximity to the coast also exhibited seasonal cycle in the 2 year (26 months) study period, with the encroachment being stronger during austral summer and weaker during austral winter. Off the eastern coast of Tasmania, powerful summer pulses of the EAC encroachment were observed, which is an expression of the seasonality of the EAC boundary flow in this region. Upstream, we believe that the “encroach and accelerate” mode of the EAC would drive coastal upwelling which can occur all year round with local maximum typically in summer. Downstream, the “encroach and decay” mode of the western arm of the EAC or its eddies is less likely to induce and sustain bottom water uplift. The seasonal current sheer previously observed upstream on the continental shelf is believed to be a combined effect of the stronger EAC encroachment and the enhanced temperature difference between shelf and EAC waters (i.e. thermal wind effect) in summer compared to winter.

Citation: Xie, S., Huang, Z., Wang, X. H., & Leplastrier, A. (2020). Quantitative mapping of the East Australian Current encroachment using time series Himawari‐8 sea surface temperature data. Journal of Geophysical Research: Oceans, 125, e2019JC015647. https://doi.org/10.1029/2019JC015647