Spatial and Temporal Variability of Dense Shelf Water Cascades along the Rottnest Continental Shelf in Southwest Australia

Mahjabin, Tanziha; Pattiaratchi, Charitha; Hetzel, Yasha; Janeković, Ivica

doi:10.3390/jmse7020030

Cited by 5 publications

(10 citation statements)

References 53 publications

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“…The main parameters we used to analyse the LSB system were the wind vectors (10 m above mean sea level) and the surface currents. The same atmospheric and ocean model combination was used by Mahjabin et al [67] to examine dense water transport in the study region.…”

Section: Numerical Modelsmentioning

confidence: 99%

Dynamics of the Land–Sea Breeze System and the Surface Current Response in South-West Australia

Rafiq

Pattiaratchi

Janeković

2020

JMSE

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The land–sea breeze (LSB) system, driven by the thermal contrast between the land and the adjacent ocean is a widely known atmospheric phenomenon, which occurs in coastal regions globally. South-west Australia experiences a persistent and one of the strongest LSB systems globally with maximum wind speeds associated with the LSB system often exceeding 15 ms−1. In this paper, using field measurements and numerical simulations, we examine: (1) the local winds associated with the land–sea breeze with an emphasis on the ocean; and, (2) the response of the surface currents to the diurnal wind forcing. The measurements indicated that the wind speeds decreased between midnight and 0400 and increased rapidly after 1100, reaching maxima >10 ms−1 around 1800) associated with the sea breeze and decreased to midnight. Wind directions were such that they were blowing from south-east (120°) in the morning and changed to almost southerly (~200°) in the afternoon. Decomposition of the wind record to the diurnal and synoptic components indicated that the diurnal component of winds (i.e., LSB) was oriented along the south-west to north-east axis. However, the stronger synoptic winds were from the south-east to south quadrant and in combination with the LSB, the winds consisted of a strong southerly component. We examined the evolution, horizontal extent, and propagation properties of sea breeze fronts for characteristic LSB cycles and the sea breeze cell propagating offshore and inland. The results indicated that the sea breeze cell was initiated in the morning in a small area, close to 33° S, 115.5° E, with a width of ~25 km and expanded onshore, offshore and alongshore. The sea breeze cell expanded faster (30 kmh−1) and farther (120 km) in the offshore direction than in the onshore direction (10 kmh−1 and 30–40 km). Winds during the LSB cycle followed a counterclockwise rotation that was also reflected in the surface currents. The winds and surface currents rotated anticlockwise with the surface currents responding almost instantaneously to changes in wind forcing but were modified by topography. The diurnal surface currents were enhanced due to the resonance between the LSB forcing and the inertial response.

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Section: Numerical Modelsmentioning

confidence: 99%

Dynamics of the Land–Sea Breeze System and the Surface Current Response in South-West Australia

Rafiq

Pattiaratchi

Janeković

2020

JMSE

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“…These studies were limited to specific locations and did not examine the seasonal variability and its forcing mechanisms. In shallow continental shelf regions, the dynamical balance consists of buoyancy, Coriolis, and friction terms, and hence DSWC moves offshore at an angle to the coastline 23 . This paper used an extensive multi-year ocean glider data set obtained from 8 locations around the Australian continent under a range of incident tide and wind conditions that also enabled us to resolve seasonal variability.…”

Section: Discussionmentioning

confidence: 99%

“…In the relatively narrow shallow continental shelf regions considered in this study, the dynamical balance consists of buoyancy, Coriolis, wind and tidal induced vertical mixing and friction. This results in the DSWC being moved offshore and deflected to the left at some angle to the coastline 23 . Majority of the locations contained relatively uniform bathymetry with a slight offshore gradient.…”

Section: ( )mentioning

confidence: 99%

“…Circulation patterns resulting from interactions between these mechanisms together with complex topography of continental shelves are challenging to measure and to unravel. Numerical simulations that include idealised setups have been used to isolate the role of different processes and to develop predictive capability 22,23 . However, simple theories for important aspects of buoyancy-influenced coastal currents have been developed and evaluated with field campaigns and numerical models that have contributed to a broad understanding of coastal dynamics.…”

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confidence: 99%

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Occurrence and seasonal variability of Dense Shelf Water Cascades along Australian continental shelves

Mahjabin

Pattiaratchi

Hetzel

2020

Sci Rep

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occurrence and seasonal variability of Dense Shelf Water cascades along Australian continental shelves tanziha Mahjabin ✉ , charitha pattiaratchi & Yasha Hetzel transport of water between the coast and the deeper ocean, across the continental shelf, is an important process for the distribution of biota, nutrients, suspended and dissolved material on the shelf. presence of denser water on the inner continental shelf results in a cross-shelf density gradient that drives a gravitational circulation with offshore transport of denser water along the sea bed that is defined as Dense Shelf Water Cascade (DSWC). Analysis of field data, collected from multiple ocean glider data missions around Australia, confirmed that under a range of wind and tidal conditions, DSWC was a regular occurrence during autumn and winter months over a coastline spanning > 10,000 km. It is shown that even in the presence of relatively high wind-and tidal-induced vertical mixing, DSWcs were present due to the strength of the cross-shelf density gradient. the occurrence of DSWc around Australia is unique with continental scale forcing through air-sea fluxes that overcome local wind and tidal forcing. it is shown that DSWc acts as a conduit to transport suspended material across the continental shelf and is a critical process that influences water quality on the inner continental shelf. Globally, the coastal ocean is the receiving basin for input of suspended and dissolved matter that includes nutrients, biota and pollutants and represents an important component of the ocean environment, connecting the terrestrial system to the deeper ocean 1. Transport processes that contribute to the exchange of water between the coast and deeper regions: cross-shelf exchange, has been defined as one of the central problems in coastal physical oceanography 2. Majority of inputs to the coastal zone are through fresh water sources such as rivers and anthropogenic discharges. As these inputs are usually buoyant, they are transported along the ocean surface layer subject to the prevailing hydrodynamic regime and are easily identified in the field and in satellite imagery. If the spatial scale is unrestricted by coastal features and under relatively low mixing conditions, river plumes can influence the hydrodynamics to hundreds of kilometres from the source region 3,4. These systems have been subject to many investigations over the past decades being defined as river plumes or Regions of Freshwater Influence (ROFI) where the hydrodynamic regime is governed by the positive density gradient between lower salinity riverine water and higher salinity oceanic water 3. In ROFI, buoyancy input from freshwater sources exceeds that from changes in heat and freshwater fluxes across the ocean surface. Regions that experience a Mediterranean climate (dry hot summers, mild cooler winters and low precipitation) are subject to high rates of evaporation and negligible river input resulting in a net loss of fresh water from the coastal region. Therefore, coastal water bodies become mo...

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“…The study region is located at 32 • S, close to the critical latitude (30 • S), where the inertial period is 24 h. Because the diurnal sea breeze system also has a ∼ 24 h period, resonance occurs, which generates near-inertial waves (Mihanović et al, 2016;Chen, 2017). Field measurements revealed that near-inertial waves force the thermocline to oscillate at a diurnal timescale with a vertical excursion > 50 m (Mihanović et al, 2016;Chen, 2017). This vertical excursion of the thermocline causes the thermocline to migrate along the continental slope on a diurnal timescale.…”

Section: Discussionmentioning

confidence: 99%

Seasonal and inter-annual variability of water column properties along the Rottnest continental shelf, south-west Australia

et al. 2019

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A multi-year ocean glider dataset, obtained along a representative cross-shelf transect along the Rottnest continental shelf, south-west Australia, was used to characterise the seasonal and inter-annual variability of water column properties (temperature, salinity, and chlorophyll fluorescence distribution). All three variables showed distinct seasonal and inter-annual variations that were related to local and basin-scale ocean atmosphere processes. Controlling influences for the variability were attributed to forcing from two spatial scales: (1) the local scale (due to Leeuwin Current and dense shelf water cascades, DSWC) and (2) the basin scale (El Niño-Southern Oscillation, ENSO, events). In spring and summer, inner-shelf waters were well mixed due to strong wind mixing, and deeper waters ( > 50 m) were vertically stratified in temperature that contributed to the presence of a subsurface chlorophyll maximum (SCM). On the inner shelf, chlorophyll fluorescence concentrations were highest in autumn and winter. DSWCs were also the main physical feature during autumn and winter. Chlorophyll fluorescence concentration was higher closer to the seabed than at the surface in spring, summer, and autumn. The seasonal patterns coincided with changes in the wind field (weaker winds in autumn) and air-sea fluxes (winter cooling and summer evaporation). Inter-annual variation was associated with ENSO events. Lower temperatures, higher salinity, and higher chlorophyll fluorescence (> 1 mg m −3 ) were associated with the El Niño event in 2010. During the strong La Niña event in 2011, temperatures increased and salinity and chlorophyll fluorescence decreased (< 1 mg m −3 ). It is concluded that the observed seasonal and inter-annual variabilities in chlorophyll fluorescence concentrations were related to the changes in physical forcing (wind forcing, Leeuwin Current, and air-sea heat and moisture fluxes).

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Spatial and Temporal Variability of Dense Shelf Water Cascades along the Rottnest Continental Shelf in Southwest Australia

Cited by 5 publications

References 53 publications

Dynamics of the Land–Sea Breeze System and the Surface Current Response in South-West Australia

Dynamics of the Land–Sea Breeze System and the Surface Current Response in South-West Australia

Occurrence and seasonal variability of Dense Shelf Water Cascades along Australian continental shelves

Seasonal and inter-annual variability of water column properties along the Rottnest continental shelf, south-west Australia

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