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Plastic contamination of coastal seas causes ecological and economic concerns; particularly in nations reliant on marine ecosystems for livelihoods, industries, and tourism, such as Indonesia. Within the growing literature concerning plastic contamination at the sea surface, spatiotemporal variability has been recognised for some time. Yet, efforts to explain and integrate underlying processes causing this variability are still limited, when addressing potential impacts and mitigation. Using a regional ocean modelling system and Lagrangian particle tracking, the first aim of this study was to assess the seasonal and inter-annual variability of plastic pathways and densities emitted from the Top 20 rivers in Indonesia. The second aim was to test the efficiency of two hypothetical mitigation approaches for select Areas of Interest (AOIs): at the river source, or within the AOIs. We found that high accumulations within the archipelago overlap with identifiable convergent fronts, which varied both in location and magnitude depending on monsoon seasons and El Nino Southern Oscillation (ENSO). Furthermore, we found interannual differences in the amount of contributing rivers, contamination levels, and residence times of particles within our selected AOIs. These differences were not consistent between AOIs, where for example one area showed an order of magnitude increase in densities during an El Nino year, for another this was true during a neutral ENSO. In conclusion, the localised variability of efficiency in mitigation strategies underlines the need to account for spatiotemporal differences in informing management approaches. The drivers of this variability can be predictable in the short to long-term., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Sea-level oscillations and associated current variability, responding to meteorological forcing with periods in the range of 3-15 days (“weather-band”), are ubiquitous along continental shelves globally. However, the investigation of these weather-band sea-level (WBSL) variations over a long period (~10 years) and the understanding their forcing are limited, particularly along North-West Australia. The aims of this research were to: (1) investigate WBSL along North-West Australia from 2009 to 2018; (2) assess the different types of meteorological forcing that contribute to WBSL; and (3) evaluate different meteorological drivers that contribute to the generation of continental shelf waves (CSWs). These aims were achieved through the analysis of long-term sea level records from 7 tide gauge stations, between Port Hedland and Geraldton, separated by 1400 km, together with concurrent meteorological data. The most energetic weather events occurred during the cyclone season (Nov-Apr) and other strong weather events were present during austral winter, particularly along the southern stations. WBSL was linked to five different meteorological forcing conditions: tropical storms (cyclones and depressions); frontal systems, west coast trough and sub-tropical high pressure systems. Over the period of analysis, ~50% of WBSL were attributed to propagating signals and were identified as continental shelf waves (CSW). These waves had mean phase speeds of 4.92±0.6m/s within the study region. CSW’s occurred throughout the year with the most energetic being generated by tropical cyclones. Other WBSL signals were generated by large scale weather systems that resulted in simultaneous sea level changes over the study region., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)