ALBERT

Description: Mesoscale eddies are common physical features in the ocean that are important for the transport of heat and momentum, and for the distribution of biochemical materials in the ocean. The existence of an anomalous, poleward flowing surface current, the Leeuwin Current (LC), drives the highest eddy kinetic energy environment among all the global eastern boundary currents off Western Australia (WA). There is a limitation in comprehensive understanding of spatial and temporal variability, in particular, the formation and vertical structure of mesoscale eddies. Therefore, this study aimed to define the climatology and vertical structure of mesoscale eddies off WA. Eddy characteristics were derived off WA (10‒40ºS, 105‒130ºE) using a vector geometry-based eddy detection algorithm applied to daily satellite altimetry data over the period 1993-2020. The vertical structure of cyclonic eddies (CE) and anti-cyclonic eddies (AE) in the Perth canyon were examined using ocean glider data over the period 2010-2017 obtained from the Australian Integrated Marine Observing System (IMOS). The seasonal and inter-annual variability and the lifecycles of mesoscale eddies were documented off WA. Shorter-lived eddy formations (〈30 days) were concentrated along the boundaries of the LC with the CE (AE) along the offshore (inshore) boundary and were related to the generation of vorticity. In contrast to the general phenomenon, the concentrations of Chlorophyll were higher in AE compared to CE whilst both eddy types were hotspots of biological activity in the oligotrophic eastern Indian Ocean.

Description: Oceanographic observations has been traditionally undertaken using ships but the emergence of autonomous ocean gliders have provided an alternative measurement platform to acquire high spatial and temporal resolution data even during periods of extreme weather conditions. These data sets enable researchers to discover physical processes as well as document the natural variability of the ocean and coastal ecosystems. The Australian Integrated Marine Observation System (IMOS) ocean glider facility has been in operation for over 16 years and have completed more than 350 glider missions around Australia. One of the major highlights of the program was the discovery of Dense Shelf Water Transport (DSWT) around Australia that occurs when the density of the inner shelf water is higher compared to offshore and is transported along the sea bed across the continental shelf as a near bed dense water plume. The ocean glider data revealed that dense water plumes on the Australian continental shelves were a regular occurrence particularly during autumn and winter months. The extensive glider measurements revealed that the wind speed and direction play an important roles in the DSWT in addition to the cross-shelf density gradient. In recent years, there have been many high rainfall events around Australia that allowed the gliders to sample a number of river plumes and associated fronts across the country. Each of the river plumes exhibited contrasting dynamics with vertically well mixed fronts to typical river plumes. The roll-rate of the ocean glider was used to define the different mixing processes in these plumes.

Description: 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.

Description: Insights were obtained on the inter-connectivity of the upper Indian Ocean between Mascarene Plateau and east Africa by tracking 19 sea surface drifter (SSD) and four 15m depth SVP drogues during the Monaco Explorations 'Indian Ocean Expedition' (ME-IOE) of Oct-Nov 2022, an initiative endorsed by the Second International Indian Ocean Expedition. Drogues were deployed along a clockwise track between Mauritius and Seychelles, including clusters immediately west of Aldabra Atoll and skirting around eastern Mascarene Plateau. Interesting circulation features observed included those consistent with dynamics related to the westward flowing South Equatorial Current through the region, the South Equatorial Counter Current, and also meso-scale eddies and dynamical variability in the Mozambique Channel. There was sub-mesoscale eddying evident in some drogue patterns, and lateral divergences of flow passing through deep regions between banks: such as via the ~4000 m deep zone between the relatively shallow (〈100m) Saya de Malha and Nazareth banks. There were relatively rapid westward movements and eventual groundings along east Africa (Tanzania) of surface drogues after deployments off Aldabra Atoll: ie grounding in 〈 3 weeks, at ~ 1-2 m/s. This study complements Crochelet et al's recent inter-connectivity studies of the adjacent Indian Ocean region to the south of the ME-IOE. The results are used to consider the potential for viable ecological inter-connectivity between areas of spawning generation and potential downstream recruitment in the context of biological propagule transport (eg coral larvae). Postulations on transport of undesirable materials such as surface contaminants and debris (eg plastics) are also considered.

Description: Sri Lanka, located in the northern Indian Ocean with the Arabian Sea on its western side and the Bay of Bengal on its eastern side and experiences bi-annually reversing monsoon winds. This will explore the dynamics of the surface circulation and coastal upwelling in the waters around Sri Lanka, particularly along the southern coast, using satellite imagery and numerical simulations using the Regional Ocean Modelling System (ROMS). The results confirmed the presence of the reversing current system, between the equator and Sri Lanka, in response to the changing wind field: the eastward flowing Southwest Monsoon Current (SMC) during the Southwest (SW) monsoon and the westward flowing Northeast Monsoon Current (NMC) during the Northeast (NE) monsoon, respectively. Along the eastern and western coasts, during both monsoon periods, flow is southward converging along the south coast. During the SW monsoon, along the west coast, the Island deflects the eastward flowing SMC southward whilst along the east coast the southward flow results from the Sri Lanka Dome (SLD) recirculation. SLD is formed due to SMC interation with the Island. The major upwelling region was located along the south coast due to water convergence and subsequent divergence associated with the offshore transport of water. The location of the flow convergence and hence the upwelling centre was dependent on the relative strengths of wind driven flow along the east and west coasts: during the SW (NE) monsoon the flow along the western (eastern) coast was stronger migrating the upwelling centre to the east (west).

Description: Storm surges (non-tidal water level) are a hazard, which result in coastal inundation, erosion, and possible loss of lives and is usually defined as the difference between the observed water level and the predicted tide. But there are many different physical processes that contribute to storm surge. Tropical storms (cyclones, hurricanes, typhoons) are among the most energetic forcing agents for the coastal ocean. Physical processes that influence the non-tidal water level associated with storms systems can persists for up to 14 days, beginning 3–4 days prior to storm landfall and ceasing up to 10 days after landfall. There is an additional contribution due to the influence of surface gravity waves (wave set-up). The storms also generate long waves with periods in the order of hours to days, which influence the water levels and currents both locally and many thousands of kilometres away. The components of a storm surge include: (1) forerunner, an increase in the mean water level up to several days prior to storm landfall; (2) meteotsunami; (3) continental shelf seiches; (4) edge waves with periods of ~six hours, that move both directions along the coast; and, (5) continental shelf waves, which propagate in a single direction with the coast on their left (right) in the southern (northern) hemisphere, with the restoring force being the Coriolis force. In this presentation, we use field measurements and numerical modelling from Western Australia (North West Shelf and south-west) to identify these processes and define their contribution to the storm surge.

Description: In the eastern margin of the Indian Ocean, anomalous poleward-flowing Leeuwin Current (LC) interacts with the local circulation and topographic features along its path promoting instabilities and a highly energetic eddy field. Eddies play a major role on transferring physical and biogeochemical properties over a wide range of spatial and temporal scales. In particular, sub-mesoscale eddies (SME) are believed to intensify vertical fluxes and are crucial for energy transport. However, as continuous high spatial and temporal resolution data are essential for SME characterization due to their short lifespans and length scale, they are still poorly understood. Thus, this research aims to analyse the spatiotemporal variability of sub-mesoscale eddies distribution and characteristics as a response of LC dynamics along the Rottnest Continental Shelf (ROT). We applied an eddy detection and tracking algorithm to long-term (2010-2018) surface current observations obtained using High-Frequency Radar. LC interactions with the Capes Current and offshore eddies promoted zones with high horizontal shear that were linked to SME generation regions. Counter-clockwise (AC) and clockwise (C) SME were prevalent at the eastern and western boundaries of the LC, respectively, remaining close to their generation spots with 25-50h lifespans. Most of AC (C) SME were formed in August (September). AC eddies generation hotspots migrated meridionally with season, whilst C eddies were clustered in a preferential location, but migrated counter-clockwise over the seasons. The analysis of the long-time series provided detailed information of the generation and lifespans of sub-meso scale eddies and their variability in time in the study region.

Description: An eruption of the Hunga Tonga-hunga Ha’apai Volcanic Eruption on 15 January 2012 caused tsunami waves, observed globally in tide gage records. In this study we investigate the origin of tsunami waves through the analysis of tide gauge records and numerical simulations in the Indo-Pacific region. The atmospheric pressure wave (Lamb wave) due the eruption was recorded in meteorological stations across Australia. The moving pressure jump was recorded as a ~6.5-hPa jump at the Norfolk Island; ~3.5 hPa at Broken Hill, NSW and Perth and was estimated to be travelling at ~340 ms–1. In the deep ocean this allows for Proudman resonance between the ocean and atmosphere generating a meteotsunami propagating westward across the Indian Ocean. Tide gauges located across the whole Indian Ocean basin recorded the meteotsunami generated by the pressure jump. There were two distinct signal on the tide gauge records. The first signal was recorded in advance of the predicted volcanic tsunami along the east coast of Australia and was due to the meteotsunami. The arrival of the second signal corresponded to tsunami wave generated by the displacement of the volcano travelling as a ‘free’ wave. These findings were confirmed by numerical simulations using an ocean circulation model that incorporated a moving atmospheric pressure jump travelling at a speed of ~ 340 ms–1. The long ocean waves were amplified due to Proudman resonance in the deep ocean, where the water was deeper than 5000 m. The model predicted of the meteotsunami corresponded well with observations.

Description: Surface current observations, using surface drifters, have been made for more than 2000 years with the earliest measurements made using visual sightings of natural and/or man-made floating objects within sight of land or from an anchored ship that served as a reference. The development of the GPS allowed for more accurate position fixing through satellites but was of limited the accuracy (~100m). The selective availability was removed in May 2000 that allowed for higher resolution position fixing (~10m). At this time, developing low-cost drifters to measure surface currents was possible. Over this period more than 500 drifters have been deployed. This talk will detail the how the drifters have been developed as different ‘off the shelf’ technologies have become available and the scientific discoveries through data analysis with an emphasis on low cost. Initial deployments, using ‘surf zone’ drifters were used to examine nearshore processes such as rip and longshore currents and dispersion. With the availability of ‘SPOT’ satellite trackers extended deployments in the open ocean was possible. In the past 3 years, more than 150 drifters have been deployed in the north-west shelf region that allowed for the understanding of surface circulation in the eastern tropical Indian Ocean. Analysis of these drifter tracks reveal major forcing mechanisms in the region through tides, winds (inertial currents), tropical cyclones and meso-scale eddies. An interesting result is that dispersion studies indicated that irrespective of the scales of the motion (nearshore, open ocean), dispersion follows the 4/3 law proposed by Richardson (1926).

Description: 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.