Long‐Term Surface Current Variability Across the Continental Shelf and Slope.

Continental shelves are amongst the most dynamic and vulnerable regions in the oceans, and understanding their dynamics has implications for the transport of organisms and materials in both coastal and offshore zones. Determining the circulation patterns in systems experiencing high variability is crucial, but challenging as continuous and high‐resolution long‐term data sets are still scarce. In this paper, we use 9‐years (2010–2018) of High‐Frequency Radar surface current observations and satellite‐derived Sea Surface Temperature to investigate the spatiotemporal variability of surface currents on the offshore, shelf break and inner continental regions along the Wadjemup (Rottnest) Continental Shelf (WCS), South‐West Australia. We use the indigenous Noongar calendar that defines six‐seasons: Birak (DJ), Bunuru (FM), Djeran (AM), Makuru (JJ), Djilba (AS), and Kambarang (ON); to better represent the seasonal changes in local winds and surface circulation. The surface currents revealed the poleward‐flowing Leeuwin Current (LC), the equatorward‐flowing Capes Current (CC), their interactions, and eddy features (radii >10 km). The LC was the strongest (weakest) over Djeran/Makuru (Birak/Bunuru), whilst CC and offshore eddies were persistent from Kambarang‐Bunuru. Along WCS, CC and offshore eddies locally modulated LC strength and position, whereas the El Niño Southern Oscillation influenced its interannual variability. During El Niño (La Niña) years, LC was weakened (strengthened), whilst CC and offshore eddy activity were strengthened (weakened). Particularly during La Niña years, LC increased its cross‐shore migration and had an unseasonal peak in 2011. Our findings contribute to the understanding of the variability of boundary and coastal currents, and their local impacts on coastal circulation. Plain Language Summary: Variability in surface circulation influences the transport of suspended material and organisms. We investigated how patterns of surface currents change over intra‐seasonal, seasonal to interannual timescales across the Wadjemup (Rottnest) Continental Shelf using 9 years of High‐Frequency Radar maps of surface current and satellite Sea Surface Temperature (SST). The current patterns in the offshore, continental shelf break and the inner continental shelf were compared to local winds, sea level, and El Niño Southern Oscillation (ENSO) events to explain observed patterns. We adopted the 6 Noongar seasons to better represent winds and surface circulation variability. Results revealed distinct surface flow patterns along WCS: the poleward‐flowing Leeuwin Current (LC), the equatorward‐flowing Capes Current (CC) and associated eddy features. LC was the strongest (weakest) over Djeran (AM)/Makuru (JJ) (Birak (DJ)/Bunuru (FM)), whilst CC and offshore eddies were persistent during Kambarang (ON)‐Bunuru. The interannual variability of surface flows were modulated by ENSO. During El Niño (La Niña) years, LC was weakened (strengthened), whilst CC and offshore eddy activity were strengthened (weakened). Particularly during La Niña years, LC increased its cross‐shore migration and had an unseasonal peak in currents in 2011. Key Points: Long‐term HFR‐surface current observations revealed different flow regimes with strong spatiotemporal variability across the continental shelf and slopeSeasonal surface flow patterns reflected wind forcing related to local indigenous seasonsEl Niño Southern Oscillation events controlled the inter‐annual variability in the surface currents [ABSTRACT FROM AUTHOR]