ALBERT

Description: Adriatic and Ionian seas are Mediterranean sub-basins linked through the Bimodal Oscillating System mechanism responsible for decadal reversals of the Ionian basin-wide circulation. Altimetric maps showed that the last cyclonic mode started in 2011 but unexpectedly in 2012 reversed to anticyclonic. We related this "premature" inversion to the extremely strong winter in 2012, which caused the formation of very dense Adriatic waters, flooding Ionian flanks in May and inverting the bottom pressure gradient. Using Lagrangian float measurements, the linear regression between the sea surface height and three isopycnal depths suggests that the southward deep-layer flow coincided with the surface northward geostrophic current and the anticyclonic circulation regime. Density variations at depth in the northwestern Ionian revealed the arrival of Adriatic dense waters in May and maximum density in September. Comparison between the sea level height in the northwestern Ionian and in the basin centre showed that in coincidence with the arrival of the newly formed Adriatic dense waters the sea level was lowered in the northwestern flank, inverting the surface pressure gradient. Toward the end of 2012, the density gradient between the basin flanks and its centre went to zero, coinciding with the weakening of the anticyclonic circulation and eventually with its return to the cyclonic pattern. Thus, the premature and transient reversal of Ionian surface circulation originated from the extremely harsh winter in the Adriatic, resulting in the formation and spreading of highly dense bottom waters. The present study highlights the remarkable sensitiveness of the Adriatic–Ionian BiOS to climatic forcing.

Description: Adriatic and Ionian Seas are Mediterranean sub-basins linked through the Bimodal Oscillating System mechanism responsible for decadal reversals of the Ionian basin-wide circulation. Altimetric maps showed that the last cyclonic mode started in 2011 but unexpectedly in 2012 reversed to anticyclone. We related this "premature" inversion to extremely strong winter in 2012, which caused the formation of very dense Adriatic waters, flooding Ionian flanks in May and inverting the bottom pressure gradient. Using Lagrangian float measurements, the linear regression between the sea surface height and three isopycnal depths suggests that the southward deep-layer flow coincided with the surface northward geostrophic current and the anti-cyclonic circulation regime. Density variations at depth in the north-western Ionian revealed the arrival of Adriatic dense waters in May and maximum density in September. Comparison between the sea level height in the north-western Ionian and in the basin centre showed that in coincidence with the arrival of the newly formed Adriatic dense waters the sea level lowered in the north-western flank inverting the surface pressure gradient. Toward the end of 2012, the density gradient between the basin flanks and its centre went to zero, coinciding with the weakening of the anticyclonic circulation and eventually with its return to the cyclonic pattern. Thus, the premature and transient reversal of Ionian surface circulation originated from the extremely harsh winter in the Adriatic, resulting in the formation and spreading of highly dense bottom waters. The present study highlights the remarkable sensitiveness of the Adriatic–Ionian BiOS to climatic forcing.

Description: Previous studies have demonstrated that the salinity in the Levantine basin depends on the intensity of the Atlantic water (AW) inflow. Moreover, its spreading eastward (to the Levantine basin) or northward (to the Ionian Sea) is determined by the Ionian circulation pattern, i.e. by the Adriatic–Ionian Bimodal Oscillating System (BiOS) mechanism. The aim of this paper is to relate salinity variations in the Levantine basin to the salt content variability in the core of the Levantine Intermediate Water (LIW) passing through the Sicily Channel (SC) and its possible impact on the Western Mediterranean Transition – WMT (i.e. the sudden salinity and temperature increase in the deep layer of the Algero-Provençal subbasin occurring since 2004). From the historical data set MEDAR/MEDATLAS in the Levantine and northern Ionian, we present evidence of decadal occurrences of extreme salinities associated with the varying influx of AW over the last 60 yr. Furthermore, we show that the salinity variations in the two subbasins are out of phase. High-salinity episodes in the Levantine are a pre-conditioning for the potential occurrence of the events like the Eastern Mediterranean Transient (EMT). Cross-correlation between the salinity time series in the Levantine basin and in the SC suggests that the travel time of the LIW is between 10 and 13 yr. Comparing the timing of the salinity increase associated with the WMT and the salinity in the LIW core in the SC, we estimate that the total time interval needed for the signal propagating from the Levantine to reach the deep mixed layers of the Algero-Provençal subbasin is about 25 yr. We also showed that the extra salt input from the eastern Mediterranean contribute up to about 60% to the salt content increase in the bottom layer of the western Mediterranean.

Description: Time scales and modes of variability of the flow in the water column in the Northern Adriatic Sea for late summer 2002 are described based on current record from a single bottom-mounted ADCP in the shallow-water area in front of the Venice Lagoon. The time averaged flow was directed 277° E (CCW), roughly aligned with the coastline, with typical magnitudes in the range 4–6 cm/s and a limited, not significant clockwise veering with depth. Tidal forcing was weak and mainly concentrated in the semidiurnal frequency band, with a barotropic (depth-independent) structure. On a diurnal time scale, tidal signal was biased by the sea-breeze regime and was characterized by a clockwise veering with depth according to the Ekman spiral. A complex EOF analysis on the velocity profile time series extracted two dominant spatial modes of variability, which explained more than 90% of the total variance in the current field. More than 78% of the total variance was accounted for by the first EOF mode, with a barotropic structure that contained the low-frequency components and the barotropic tidal signal at semidiurnal and diurnal frequencies. The second mode had a baroclinic structure with a zero-crossing at mid-depth, which was related with the response of the water column to the high-frequency wind-driven diurnal sea breeze variability. The response of low-passed non-tidal currents to local wind stress was fast and immediate, with negligible temporal lag up to mid-depth. Currents vectors were pointing to the right of wind stress, as expected from the surface Ekman veering, but with angles smaller than the expected ones. A time lag in the range 10 to 11 h was found below 8 m depth, with current vectors pointing to the left of wind stress and a counterclockwise veering towards the bottom. The delay was consistent with the frictional adjustment time scale describing the dynamics of a frictionally dominated flow in shallow water, thus suggesting the importance of bottom friction on the motion over the entire water column.

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: 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.

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: 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.