Papers by Charitha Pattiaratchi
Deep Sea Research Part II: Topical Studies in Oceanography, 2007
Within the coastal waters of the eastern Indian Ocean adjacent to Western Australia, we tested th... more Within the coastal waters of the eastern Indian Ocean adjacent to Western Australia, we tested the hypothesis that regenerated production (and, by inference, the microbial food web) would predominate in oligotrophic Leeuwin Current (LC) and offshore (OS) surface waters. Conversely, we expected that new production would be more important within the $5 times more productive shelf countercurrents (Ningaloo and Capes Currents; NC&CC) and the LC&OS deep chlorophyll maximum (DCM). Phytoplankton species composition and abundance were assessed using both light microscopy and chemotaxonomic methods, and isotopic nitrogen uptake experiments ( 15 NO 3 À , 15 NH 4 + ) were performed at trace (0.05 mM) and saturating (5.0 mM) levels. Phytoplankton community structure was statistically distinct between LC&OS and countercurrent regions. Picoplankton (unicellular cyanobacteria and prochlorophytes) accounted for a mean of 55-65% of pigment biomass in LC&OS waters, with haptophytes as the other primary contributor (21-32%). Conversely, within countercurrent and shelf regions, diatoms (up to 22%) and haptophytes (up to 57%) were more abundant, although cyanobacteria still played an important role (up to 40% of pigment biomass). Absolute NO 3 À uptake rates for all samples ranged between 0.5 and 7.1 nmol L À1 h À1 , and in countercurrent waters were not significantly different at the surface (3.072.1 nmol L À1 h À1 ; mean7SD) compared to the DCM (2.772.3 nmol L À1 h À1 ). However, in LC&OS waters, rates were significantly lower at the surface (1.270.7 nmol L À1 h À1 ) than the DCM (3.972.5 nmol L À1 h À1 ; p ¼ 0.05). These values represent conservative estimates for the region due to methodological difficulties encountered with nitrogen uptake experiments in these oligotrophic waters. In contrast with the distinct community composition between different water types, mean estimates of the f-ratio were similar across sampling depths and water types: 0.1770.07 at the surface and 0.1670.06 at the DCM of shelf countercurrent waters, and 0.1470.05 at the surface and 0.1470.09 at the DCM of LC&OS waters. These results demonstrate the importance of ammonium-based production in both oligotrophic LC&OS waters and the more productive upwelling-influenced countercurrents. They also highlight the utility of chemotaxonomic methods in studying pelagic ecosystem structure along the Gascoyne region of Western Australia. r
ABSTRACT This paper reviews development of a basin-scale observing system and regional/coastal ob... more ABSTRACT This paper reviews development of a basin-scale observing system and regional/coastal observing systems in the Indian Ocean, most of which are implemented by national or institutional projects/programs under international coordination by the Indian Ocean GOOS
Mixing in Estuaries and Coastal Seas
Coastal and Estuarine Studies, 1996
Wind and tidal mixing controls on stratification and dense water outflows in a large hypersaline bay
Journal of Geophysical Research: Oceans, 2015
Journal of Marine Science and Engineering, 2014
Results from a series of field experiments, conducted to investigate the influence of infragravit... more Results from a series of field experiments, conducted to investigate the influence of infragravity waves (from wave groups), ripple type and location relative to the breaker line on cross-shore suspended sediment flux close to the sea bed in nearshore environments, are presented. The field data were collected from Cable Beach (Broome) and Mullaloo Beach in Western Australia and Chilaw in Sri Lanka. These beaches experience different incident wave, tidal and morphological conditions, with Cable Beach having a 10-m spring tidal range, whilst the other two beaches have tidal ranges <1.0 m. Measurements included simultaneous records of surface elevation, two-dimensional horizontal current velocities and suspended sediment concentrations, together with half-hourly observations of the seabed topography. Although most of the data sets were obtained just outside of the surf zone, a few results from inside of the surf zone were also included. A significant correlation between wave groups and suspended sediment concentration was found at all of the measurement sites, either with or without bed ripples. The direction and magnitude of cross-shore suspended sediment flux varied with location with respect to the breaker line; however, other parameters, such as bed ripples and velocity skewness, could have influenced this result. In Broome, where the measurement location with respect to the breaker line varied with the tidal cycle, the cross-shore sediment flux due to swell waves OPEN ACCESS J. Mar. Sci. Eng. 2014, 2 569 was shoreward inside and just outside of the surf zone and seaward farther offshore of the breaker line. Further, sediment flux due to swell waves was onshore when the seabed was flat and offshore over post-vortex ripples. Sediment flux due to swell waves was onshore when the normalised velocity skewness towards the shore was high (positive); the flux was offshore when the skewness was lower, but positive, suggesting the influence of other parameters, such as ripples and grain size. The net cross-shore sediment flux was onshore when the Dean number was less than 1.67 and offshore when the Dean number was greater than 1.67. Nevertheless, the Dean number did not account for the influence of ripples or velocity skewness. The cross-shore sediment flux at the infragravity frequency was mainly offshore outside of the surf zone, whereas it varied between onshore and offshore inside of the surf zone.
Earth Surface Dynamics Discussions, 2014
Field measurements, collected at several low energy, microtidal beaches in southwestern Australia... more Field measurements, collected at several low energy, microtidal beaches in southwestern Australia were used to study the cross-shore transport and sediment resuspension over different sand ripple types. The measurements included simultaneous records of the water surface elevation, cross-shore current velocity, and suspended 5 sediment concentration, as well as free diver measurements of the ripple dimensions. The observed ripples were classified according to their geometry and sediment suspension patterns into six categories: flat bed, post-vortex ripples, two-dimensional (2-D) ripples, two/three-dimensional (2-D/3-D) ripples, three-dimensional (3-D) ripples, and cross ripples. Flat bed conditions were observed under the highest flow mobil-10 ity numbers. Post-vortex ripples were observed under slightly lower mobility numbers. The other ripple types occurred under low mobility numbers, with no significant difference in the mobility number among them. Two-dimensional ripples were observed more than the other ripple types in the presence of coarse grains. The suspended sediment concentration at ∼ 0.05 m above the bed was greater over steep ripples. The net 15 cross-shore suspended sediment flux close to the seabed (at ∼ 0.05 m) in the swell frequency band varied over the different ripples types: onshore over a flat bed, offshore over post-vortex ripples, onshore over 2-D and 2-D/3-D ripples, and offshore over 3-D ripples. The suspended sediment flux direction over the cross ripples varied between onshore and offshore.
WAIMOS Node Science and Implementation Plan (NSIP)
Challenges in transferring knowledge between scales in coastal sediment dynamics
Frontiers in Marine Science, 2015
B. Sc.(Hons.) Thesis, School of Environmental …, Jan 1, 2002
The CSIRO Mark 3 coupled Atmosphere-Ocean Global Climate Model was analysed to determine changes ... more The CSIRO Mark 3 coupled Atmosphere-Ocean Global Climate Model was analysed to determine changes to the regional oceanography of Western Australia, attributable to anthropogenic climate change under the (IPCC) A2 (greenhouse gas worst-case) scenario.
Changes to atmospheric and oceanographic circulation indicate a significant increase in heat content of the ocean, decreased salinity and increased atmosphere-ocean fluxes linked to global atmospheric heat content.
These changes agree with the findings of the IPCC resolving changes to terrestrial temperature and precipitation patterns, with mean global surface temperatures predicted to increase 1.4 - 5.8oC by 2100.
The 5 key regional currents, Leeuwin Current, West Australian Current, Indian Ocean Central Water intrusion, Leeuwin Undercurrent and the South Equatorial Current, are resolved by this model, which indicates significant changes to the oceanography of the region.
Inferred from the regional dataset, and in accordance with recent work on the topic, possible changing global ocean circulation is indicated by the model. This is expressed through the reproduction of the Indonesian throughflow, the only equatorial link between the 3 major global oceans. The model predicts a decrease to transport of this flow, inferring change to the global thermohaline circulation.
Changing patterns have been acknowledged, due to CO2 forced high-latitude freshening, leading to decreased salinity values. These, when coupled to increased heat content, lead to a more stratified ocean, reducing the density gradient driving the geostrophic thermohaline circulation.
Increased atmospheric temperature corresponds with greater fluxes into the ocean. These lead to a marked increase in heat content of the ’surface’ (top ∼500 m), also linked to a more stratified global ocean, reducing the density gradient driving thermohaline circulation. Advection of heat through the Indonesian throughflow, was not a significant cause of increased heat content for regional oceanography.
The dominance of Indian Ocean Central Water, a feature not previously acknowledged, appears responsible for a considerable increase in salinity values. Model output also indicates heat transport increases due to this source, suggesting this feature may be responsible for a considerable proportion of change in the region.
Changes to the currents replicate the patterns expressed in the temperature and salinity profiles for the regional ocean. These indicate decreased transport for the Indonesian throughflow and South Equatorial Currents. This flow too is the feeder flow driving the Leeuwin Current. Geopotential anomaly indicates a marginal increase in this gradient, supporting the concept of a more pronounced Leeuwin flow.
Vertical mixing processes in Intermittently Closed and Open Lakes and Lagoons, and the dissolved oxygen response
Estuarine, Coastal and Shelf …, 2006
Intermittently Closed and Open Lakes and Lagoons (ICOLLs) are located on micro-tidal coasts (max.... more Intermittently Closed and Open Lakes and Lagoons (ICOLLs) are located on micro-tidal coasts (max. tidal range < 2 m) in temperate regions where the annual rainfall is non-seasonal. ICOLLs are generally shallow (<5 m depth) and are closed to the ocean due to the formation of an entrance bar for the majority of the year, when rainfalls are low. After periods of heavy rainfall, the super elevated water levels result in the natural or artificial breaching of the entrance bar.Due to their small size and absence of significant river inflows, ICOLLs exhibit strong temporal variations in their vertical density gradients, which can result in episodic density stratification. Such episodic stratification events may result in deterioration of the water quality including toxic algal blooms. This paper presents the results of field studies undertaken to determine the physical processes governing vertical mixing/stratification in ICOLLs and their implications on dissolved oxygen dynamics. Data from two contrasting ICOLLs located along the south-eastern coastline of Australia; (a) Wamberal Lagoon a small, shallower (∼2 m max. depth) frequently open ICOLL; and, (2) Smiths Lake, a larger, deeper (∼5 m max. depth) infrequently open ICOLL, are presented.The results indicated that Wamberal Lagoon was susceptible to periods of stratification during both the closed and the open states. During the closed state, periods of rainfall, low wind and/or high solar insolation led to short (<3 days) and irregular stratification events, whilst during the open state, stratification events occurred through a combination of rainfall, low winds and variations in tidal mixing. There was a tendency for dissolved oxygen to decrease, in the bottom waters, when the Buoyancy Frequency was >0.1 s−1. Smiths Lake demonstrated higher vertical stability and exhibited a tendency for persistent stratification, during both the closed and open states, primarily due to solar insolation (closed state) and gravitational circulation (open state), respectively. The persistent stratification maintained a vertical gradient in dissolved oxygen between the surface and bottom layers. However, tidal pumping associated with fortnightly tides appears to promote isolation of the bottom waters, causing the dissolved oxygen rates to temporarily decrease (for approximately 5 days) during the neap cycle.
Tidal signatures in modern and ancient sediments. IAS Special Publication No. 24, 1995
An explanation for sediment dispersal on tidally-dominated continental shelves, known as the bedl... more An explanation for sediment dispersal on tidally-dominated continental shelves, known as the bedload parting zone (BLP) model, is reviewed in terms of controlling processes and the nature of associated sediment deposits. The facies occur in relation to decreasing tidal bottom stress, from tidally scoured bed to mobile sand sheet, ending with a muddy sand facies at the lowest tidal energy. A review of available data on the distribution of these facies, shows that they are developed in relation to zones of local maximum bed stress, induced by either standing wave nodal points (type A) or geomorphic constrictions (type B). Around the western European continental shelf, 22 different locations are associated with local bottom stress maxima (type A or B).
A model is proposed for the development of scour zones as a function of sediment supply, with consideration given to the movement of facies boundaries through time in relation to the bottom stress maxima. Where sand is plentiful, the transport pattern is dominated by linear sandbanks and associated dunes; these delimit a mutually evasive transport system. Incipient scour zones are those which occupy a significant (~50%) portion of the channel width. Partial scour zones occupy nearly the entire channel, except for deposits along the margins, which link the flanking depositional areas. Complete scour zones separate totally the mobile sand deposits located on opposite flanks of a channel, with no significant marginal sand deposits occurring. Only 11 of the 22 zones of local maximum bottom stress correlate with "complete" scour zones.
BLP's are classified as a special case of scour zone, in which a local bottom stress maxima coincides with a divergent pattern in sand transport (inferred from bedform asymmetries). A minimum scale of about 15-20km is proposed for a "complete" BLP, which separates flanking depositional areas, on the basis of the tidal excursion length of a sand grain. The model proposed for scour zone development applies also to BLP's, suggesting that only 3 of the 6 BLP's previously identified for the west European shelf are at a "complete" stage of development.
Journal of Experimental Marine Biology and Ecology, Feb 28, 2013
Understanding the movements of turtle hatchings is essential for improved understanding of disper... more Understanding the movements of turtle hatchings is essential for improved understanding of dispersal behaviour and ultimately survivorship, life history strategies and population connectivity. Yet investigation of in-water movement has been hampered by the small size of hatchlings relative to the size of available tracking technologies. This has resulted in the use of labour intensive visual tracking methods, or active tracking methods with high transmitter to body weight ratios. These methods are confounded by the presence of the observer, the size of the tag, usual small treatment sample sizes and studies that are constrained to daylight hours when turtles hatch predominantly at night. Passive acoustic monitoring using new miniature tags can overcome these limitations. We tested the effectiveness of active and passive acoustic tracking in monitoring turtle hatchling movement in order to measure the influence of artificial light on newly hatched turtles once they enter the water. A Vemco VR2W Positioning System (VPS) comprising an array of 18 VR2W receivers was deployed in the surf zone to detect signals from acoustic-coded transmitters (1.14 ± 0.06% of body mass) attached to 26 flatback turtle hatchlings released into the array. A total of 1328 detections were recorded for 22 hatchlings with turtles spending a mean of 16.63 ± 5.89 min in the array. The test detection range for this technology in the surf-zone was 50-100 m and was influenced by wave noise and shallow deployment. Cyclonic conditions hampered the experiment and resulted in an inconclusive test of light effects. Three additional instrumented flatback hatchlings were followed in a small boat using a mobile acoustic receiver and directional hydrophone up to 2 km from shore. Passive acoustic monitoring is a viable technology for tracking small marine animals and removes many of the confounding effects of other telemetry methods. It has great potential to examine natural and anthropogenic factors influencing orientation and behaviour during a crucial stage in turtle life historytheir initial movement from the beach through predator-rich, near shore waters. While the data obtained by passive acoustic monitoring is limited in its spatio-temporal coverage, being constrained by the size of the array, active acoustic tracking can be applied over larger scales. Such studies will be particularly important for assessing the impacts of anthropogenic pressures that have changed the natural light, noise or wave environments and for providing behavioural data to improve and validate bio-physical models of the migration and dispersal of young turtles.
TSUNAMIS–THEIR CAUSES AND EFFECTS
Performance of an artificial surfing reef: Cable Station, Western Australia
Estimates of sand transport under waves and currents
The cables artificial surfing reef, Western Australia
Observing the oceans with gliders: techniques for data visualisation and analysis
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Papers by Charitha Pattiaratchi
Changes to atmospheric and oceanographic circulation indicate a significant increase in heat content of the ocean, decreased salinity and increased atmosphere-ocean fluxes linked to global atmospheric heat content.
These changes agree with the findings of the IPCC resolving changes to terrestrial temperature and precipitation patterns, with mean global surface temperatures predicted to increase 1.4 - 5.8oC by 2100.
The 5 key regional currents, Leeuwin Current, West Australian Current, Indian Ocean Central Water intrusion, Leeuwin Undercurrent and the South Equatorial Current, are resolved by this model, which indicates significant changes to the oceanography of the region.
Inferred from the regional dataset, and in accordance with recent work on the topic, possible changing global ocean circulation is indicated by the model. This is expressed through the reproduction of the Indonesian throughflow, the only equatorial link between the 3 major global oceans. The model predicts a decrease to transport of this flow, inferring change to the global thermohaline circulation.
Changing patterns have been acknowledged, due to CO2 forced high-latitude freshening, leading to decreased salinity values. These, when coupled to increased heat content, lead to a more stratified ocean, reducing the density gradient driving the geostrophic thermohaline circulation.
Increased atmospheric temperature corresponds with greater fluxes into the ocean. These lead to a marked increase in heat content of the ’surface’ (top ∼500 m), also linked to a more stratified global ocean, reducing the density gradient driving thermohaline circulation. Advection of heat through the Indonesian throughflow, was not a significant cause of increased heat content for regional oceanography.
The dominance of Indian Ocean Central Water, a feature not previously acknowledged, appears responsible for a considerable increase in salinity values. Model output also indicates heat transport increases due to this source, suggesting this feature may be responsible for a considerable proportion of change in the region.
Changes to the currents replicate the patterns expressed in the temperature and salinity profiles for the regional ocean. These indicate decreased transport for the Indonesian throughflow and South Equatorial Currents. This flow too is the feeder flow driving the Leeuwin Current. Geopotential anomaly indicates a marginal increase in this gradient, supporting the concept of a more pronounced Leeuwin flow.
A model is proposed for the development of scour zones as a function of sediment supply, with consideration given to the movement of facies boundaries through time in relation to the bottom stress maxima. Where sand is plentiful, the transport pattern is dominated by linear sandbanks and associated dunes; these delimit a mutually evasive transport system. Incipient scour zones are those which occupy a significant (~50%) portion of the channel width. Partial scour zones occupy nearly the entire channel, except for deposits along the margins, which link the flanking depositional areas. Complete scour zones separate totally the mobile sand deposits located on opposite flanks of a channel, with no significant marginal sand deposits occurring. Only 11 of the 22 zones of local maximum bottom stress correlate with "complete" scour zones.
BLP's are classified as a special case of scour zone, in which a local bottom stress maxima coincides with a divergent pattern in sand transport (inferred from bedform asymmetries). A minimum scale of about 15-20km is proposed for a "complete" BLP, which separates flanking depositional areas, on the basis of the tidal excursion length of a sand grain. The model proposed for scour zone development applies also to BLP's, suggesting that only 3 of the 6 BLP's previously identified for the west European shelf are at a "complete" stage of development.