Extreme ocean weather induced by upstream meandering of the East Australian Current 

2021 ◽  
Author(s):  
Christopher Chapman ◽  
Bernadette Sloyan ◽  
Madeleine Cahill
Keyword(s):  
East Coast ◽  
Complex Dynamics ◽  
Heat Waves ◽  
Small Scale ◽  
Western Boundary ◽  
East Australian Current ◽  
Boundary Current ◽  
Atmospheric Blocking ◽  
Australian Continent ◽  
Current Region

<p>We investigate Marine Heat Waves and Marine Cold Spells (MHWs/MCSs) along the east coast of the Australian continent, a western boundary current region with exceedingly complex dynamics. We provide evidence that episodic MWHs/MCSs along the south-east of the Australian continent are driven by upstream variations in the position, but not necessarily the strength, of the East Australian Current, and that these variations are, in turn, controlled by small-scale (100s of kilometers) eddies that propagate into the region from the east. These eddies are able to alternately 'shut-off' and `turn-on' the poleward transport of warm water by the boundary current in a manner analogous to atmospheric blocking. Precursors to these `blocks' are detectable as much as 60 days prior to the onset of an event. We will discuss the implications of our results for the early prediction of MHW/MCS events.</p>

Internal Wave Generation by a Combination of Tidal and Steady Currents

2021 ◽  
Author(s):  
Xiaolin Bai ◽  
Kevin Lamb ◽  
José da Silva
Keyword(s):  
Internal Wave ◽  
Wave Generation ◽  
Satellite Observations ◽  
Internal Tides ◽  
Small Scale ◽  
Western Boundary ◽  
Boundary Current ◽  
Reanalysis Dataset ◽  
Amazon Shelf ◽  
Internal Wave Generation

<p>In the presence of topography, two main contributors for internal wave energy are tide-topography interaction transferring energy from the barotropic tide to internal tides, and lee wave generation when geostrophic currents or eddying abyssal flows interact with topography. In the past few decades, many studies considered the respective contribution of the oscillating flows or steady background flows, but few investigations have considered both.  </p><p>In this talk, we consider the joint effects of tidal and steady currents to investigate internal wave generation and propagation on the Amazon shelf, a hotspot for internal solitary wave (ISW) generation. The Amazon Shelf is off the mouth of the Amazon River in the southwest tropical Atlantic Ocean, affected by strong tidal constituents over complex bottom bathymetry and a strong western boundary current, the North Brazilian Current (NBC). Both satellite observations and numerical modelling are used in this study. Satellite observations provide a clear visualization of the wave characteristics, such as temporal and spatial distributions, propagating direction and its relation to background currents. Based on parameters from satellite observations and reanalysis dataset, we set up a model to numerically investigate the dynamics of the ISW generation. We demonstrate that the small-scale topography contributes to a rich generation of along-shelf propagating ISW, which significantly contribute to the ocean mixing and potentially cause sediment resuspension. Moreover, the ISW-induced currents also contribute to the sea surface wave breaking as observed by satellite measurements. In addition, statistics based on a decade of satellite images and numerical investigations on seasonal variations of the ISWs and the NBC improve our understanding of the generation and evolution of these nonlinear internal waves in the presence of background currents.</p>

scholarly journals Investigating the Diversity of Marine Bacteriophage in Contrasting Water Masses Associated with the East Australian Current (EAC) System

Viruses ◽  
2020 ◽  
Vol 12 (3) ◽  
pp. 317 ◽  
Author(s):  
Amaranta Focardi ◽  
Martin Ostrowski ◽  
Kirianne Goossen ◽  
Mark V. Brown ◽  
Ian Paulsen
Keyword(s):  
Microbial Communities ◽  
Dna Sequences ◽  
Abiotic Factors ◽  
Western Boundary Current ◽  
Viral Diversity ◽  
Western Boundary ◽  
East Australian Current ◽  
Boundary Current ◽  
Regional Patterns ◽  
Marine Productivity

Virus- and bacteriophage-induced mortality can have a significant impact on marine productivity and alter the flux of nutrients in marine microbial food-webs. Viral mediated horizontal gene transfer can also influence host fitness and community composition. However, there are very few studies of marine viral diversity in the Southern Hemisphere, which hampers our ability to fully understand the complex interplay of biotic and abiotic factors that shape microbial communities. We carried out the first genetic study of bacteriophage communities within a dynamic western boundary current (WBC) system, the east Australian current (EAC). Virus DNA sequences were extracted from 63 assembled metagenomes and six metaviromes obtained from various depths at 24 different locations. More than 1700 bacteriophage genomic fragments (>9 kbps) were recovered from the assembled sequences. Bacteriophage diversity displayed distinct depth and regional patterns. There were clear differences in the bacteriophage populations associated with the EAC and Tasman Sea euphotic zones, at both the taxonomic and functional level. In contrast, bathypelagic phages were similar across the two oceanic regions. These data provide the first characterisation of viral diversity across a dynamic western boundary current, which is an emerging model for studying the response of microbial communities to climate change.

scholarly journals The historical development of physical research on the oceans bordering Southern Africa

1982 ◽  
Vol 1 (1) ◽  
pp. 42-52 ◽  
Author(s):  
J. R. E. Lutjeharms
Keyword(s):  
Ocean Circulation ◽  
Southern Africa ◽  
East Coast ◽  
Phenomenological Research ◽  
Historical Research ◽  
Western Boundary ◽  
Boundary Current ◽  
The West ◽  
Publication Rates ◽  
Circulation Patterns

Two archetypal ocean circulation patterns occur in the oceanic areas adjacent to southern Africa, namely, a western boundary current on the east coast and upwelling on the west coast. Investigation on the physical behaviour of these systems has passed through phases of exploratory, descriptive and phenomenological research. These historical research patterns are described briefly using mostly the publication rates on the physical oceanology of the areas in question.

Large-scale influence of the New Zealand seafloor topography on western boundary currents of the South Pacific Ocean.

1985 ◽  
Vol 36 (1) ◽  
pp. 1 ◽  
Author(s):  
RA Heath
Keyword(s):  
New Zealand ◽  
West Coast ◽  
East Coast ◽  
South Pacific ◽  
Surface Flow ◽  
Current Speed ◽  
Western Boundary ◽  
East Australian Current ◽  
South Pacific Ocean ◽  
Tasman Sea

The extensive New Zealand submarine platform lying approximately 1600 km east of Australia has a strong influence on the South Pacific circulation. Together with the Kermadec Ridge, it is the western boundary of the deep South Pacific Ocean with an associated deep western boundary current. Although New Zealand probably influences where the East Australian Current separates from the east Australian coast, at the latitude of northernmost New Zealand, the sloping seafloor on the New Zealand west coast does allow for a meridional flow there. However, the decrease in current speed with depth does decrease the influence of the bottom topography. The net result is that there is both an intensification of the zonal flow across the Tasman Sea at the latitude of northernmost New Zealand, the speed of which is enhanced by the flow over the extensive ridge system, and a general eastwards flow in the Tasman Sea over the latitudinal range of New Zealand, which feeds meridional flows on the New Zealand west coast. It is suggested that the general west to east flow past New Zealand restricts the westward propagation of second- and higher-order baroclinic Rossby waves with the result that, whereas the East Australian Current has rapid near-surface flow which decreases rapidly with depth in the upper 500 m, the surface flow on the east coast of New Zealand is less rapid and decreases more uniformly with depth. One possible consequence of the current speed change with depth is that the flow and eddies on the New Zealand east coast appear to be influenced by the bathymetry whereas the East Australian Current eddies are more a primary component of the current linked to instability in the current system.

Variability and meandering of the East Australian Current jet at 27oS

2021 ◽  
Author(s):  
Bernadette Sloyan ◽  
Christopher Chapman ◽  
Rebecca Cowley ◽  
Thomas Moore
Keyword(s):  
Continental Shelf ◽  
South Pacific ◽  
Sea Surface ◽  
Western Boundary ◽  
East Australian Current ◽  
Boundary Current ◽  
The South ◽  
Buoyancy Forcing ◽  
The Mean ◽  
Southward Flow

<p>The East Australian Current (EAC) is the complex and highly energetic western boundary current of the South Pacific Ocean gyre. Low frequency (>2 year) variability of the EAC reflects the changes in the wind and buoyancy forcing over the South Pacific. However, local and regional wind and buoyancy forcing drives higher frequency variability (< 1-2 year) of the EAC. Due to the narrow shelf, the EAC-jet  meandering has an immediate impact on the continental shelf circulation. Here we use the IMOS EAC mooring array between May 2015 to September 2019 and satellite observational data to quantify the quantify the EAC variability and assess the potential drives and impact of the on-shelf meandering of the EAC jet on the properties of the Coral and Tasman Seas.</p><p> </p><p>We find that there is considerable variability of Sea Surface Height (SSH) and Sea Surface temperature (SST) that at times co-vary, but at  other times the anomalies are opposed. We compare the surface anomalies with the EAC velocity and transport timeseries. The mean along-slope velocity vectors show poleward velocity dominates from 0-1500 m at the five mooring locations from the 500 m isobath to the deep abyssal basin with the strongest southward flow at the continental shelf. The variance ellipses show that the largest variability in EAC transport is in the along-shore direction. This indicates that the EAC variability is dominated by the movement of the EAC on- and off-shore. The EAC thus maintains its jet structure as it meanders onshore and offshore adjacent to the continental slope. While the mean along-shore velocity vectors provide a picture of the mean EAC, the time-series shows that the EAC has a complex and highly variable structure. Strong southward flow is associated with off-shore flow (positive across-slope velocity). While mostly measuring the EAC core we see times where the flow is northward (positive along-slope velocity). This northward velocity is due to the shelf flow extending from the coast to the shelf, and is generally associated with on-shore flow (negative across-slope velocity). These changes in the direction and strength of the velocity are driven by cyclonic eddies inshore of the jet, and have significant influence on the exchange between the open and shelf ocean.</p>

scholarly journals The Effects of Mesoscale Ocean–Atmosphere Coupling on the Large-Scale Ocean Circulation

2009 ◽  
Vol 22 (15) ◽  
pp. 4066-4082 ◽  
Author(s):  
Andrew Mc C. Hogg ◽  
William K. Dewar ◽  
Pavel Berloff ◽  
Sergey Kravtsov ◽  
David K. Hutchinson
Keyword(s):  
Spatial Scale ◽  
Ocean Circulation ◽  
Large Scale ◽  
Ocean Model ◽  
Small Scale ◽  
Western Boundary ◽  
Ekman Pumping ◽  
Boundary Current ◽  
Ocean Atmosphere ◽  
Gyre Circulation

Abstract Small-scale variation in wind stress due to ocean–atmosphere interaction within the atmospheric boundary layer alters the temporal and spatial scale of Ekman pumping driving the double-gyre circulation of the ocean. A high-resolution quasigeostrophic (QG) ocean model, coupled to a dynamic atmospheric mixed layer, is used to demonstrate that, despite the small spatial scale of the Ekman-pumping anomalies, this phenomenon significantly modifies the large-scale ocean circulation. The primary effect is to decrease the strength of the nonlinear component of the gyre circulation by approximately 30%–40%. This result is due to the highest transient Ekman-pumping anomalies destabilizing the flow in a dynamically sensitive region close to the western boundary current separation. The instability of the jet produces a flux of potential vorticity between the two gyres that acts to weaken both gyres.

Eddy variability east of Madagascar

2005 ◽  
Vol 363 (1826) ◽  
pp. 77-79 ◽  
Author(s):  
G. D. Quartly ◽  
J. J. H. Buck ◽  
M. A. Srokosz
Keyword(s):  
High Resolution ◽  
Numerical Model ◽  
East Coast ◽  
Sea Surface ◽  
Western Boundary ◽  
Biological Productivity ◽  
Boundary Current ◽  
Dynamic Height ◽  
Current Flows ◽  
Height Data

A strong but variable western boundary current flows south along the east coast of Madagascar, and at the island's southern end it interacts with eddies propagating zonally from the east. These two routes of variability are compared using altimetric sea–surface–height data and dynamic height from a high–resolution numerical model. The effects on biological productivity are also discussed.

scholarly journals Estimating the Velocity and Transport of Western Boundary Current Systems: A Case Study of the East Australian Current near Brisbane

2018 ◽  
Vol 35 (6) ◽  
pp. 1313-1329 ◽  
Author(s):  
N. V. Zilberman ◽  
D. H. Roemmich ◽  
S. T. Gille ◽  
J. Gilson
Keyword(s):  
High Resolution ◽  
Time Scales ◽  
Low Frequency ◽  
Volume Transport ◽  
Western Boundary ◽  
Present Calculation ◽  
East Australian Current ◽  
Boundary Current ◽  
Heat Budgets

AbstractWestern boundary currents (WBCs) are highly variable narrow meandering jets, making assessment of their volume transports a complex task. The required high-resolution temporal and spatial measurements are available only at a limited number of sites. In this study a method is developed for improving estimates of the East Australian Current (EAC) mean transport and its low-frequency variability, using complementary modern datasets. The present calculation is a case study that will be extended to other subtropical WBCs. The method developed in this work will reduce uncertainties in estimates of the WBC volume transport and in the interannual mass and heat budgets of the meridional overturning circulations, improving our understanding of the response of WBCs to local and remote forcing on long time scales. High-resolution expendable bathythermograph (HR-XBT) profiles collected along a transect crossing the EAC system near Brisbane, Australia, are merged with coexisting profiles and parking-depth trajectories from Argo floats, and with altimetric sea surface height data. Using HR-XBT/Argo/altimetry data combined with Argo trajectory-based velocities at 1000 m, the 2004–15 mean poleward alongshore transport of the EAC is 19.5 ± 2.0 Sv (1 Sv ≡ 106 m3 s−1) of which 2.5 ± 0.5 Sv recirculate equatorward just offshore of the EAC. These transport estimates are consistent in their mean and variability with concurrent and nearly collocated moored observations at 27°S, and with earlier moored observations along 30°S. Geostrophic transport anomalies in the EAC system, including the EAC recirculation, show a standard deviation of ±3.1 Sv at interannual time scales between 2004 and 2015.

scholarly journals Transport variability over the Hawkesbury Shelf (31.5–34.5°S) driven by the East Australian Current

PLoS ONE ◽  
2020 ◽  
Vol 15 (11) ◽  
pp. e0241622
Author(s):  
Nina Ribbat ◽  
Moninya Roughan ◽  
Brian Powell ◽  
Shivanesh Rao ◽  
Colette Gabrielle Kerry
Keyword(s):  
Western Boundary Current ◽  
Western Boundary ◽  
East Australian Current ◽  
Boundary Current ◽  
High Productivity ◽  
Shelf Circulation ◽  
Southeastern Australia ◽  
Temperature Regimes ◽  
Tasman Sea ◽  
Shelf Region

The Hawkesbury Bioregion located off southeastern Australia (31.5–34.5oS) is a region of highly variable circulation. The region spans the typical separation point of the East Australian Current (EAC), the western boundary current that dominates the flow along the coast of SE Australia. It lies adjacent to a known ocean warming hotspot in the Tasman Sea, and is a region of high productivity. However, we have limited understanding of the circulation, temperature regimes and shelf transport in this region, and the drivers of variability. We configure a high resolution (750m) numerical model for the Hawkesbury Shelf region nested inside 2 data assimilating models of decreasing resolution, to obtain the best estimate of the shelf circulation and transport over a 2-yr period (2012–2013). Here we show that the transport is driven by the mesoscale EAC circulation that strengthens in summer and is related to the separation of the EAC jet from the coast. Transport estimates show strong offshore export is a maximum between 32-33oS. Median offshore transports range 2.5–8.4Sv seasonally and are a maximum during in summer driven by the separation of the EAC jet from the coast. The transport is more variable downstream of the EAC separation, driven by the EAC eddy field. Onshore transport occurs more frequently off Sydney 33.5–34.5oS; seasonal medians range -1.7 to 2.3Sv, with an onshore maximum in winter. The region is biologically productive, and it is a known white shark nursery area despite the dominance of the oligotrophic western boundary current. Hence an understanding of the drivers of circulation and cross-shelf exchange is important.

scholarly journals Structure, variability, and dynamics of the West Greenland Boundary Current System

2022 ◽  
Author(s):  
◽  
Astrid Pacini
Keyword(s):  
Current System ◽  
Meridional Overturning Circulation ◽  
Labrador Sea ◽  
Small Scale ◽  
Western Boundary ◽  
Coastal Current ◽  
Boundary Current ◽  
The West ◽  
West Greenland ◽  
Atlantic Origin

The ventilation of intermediate waters in the Labrador Sea has important implications for the strength of the Atlantic Meridional Overturning Circulation. Boundary current-interior interactions regulate the exchange of properties between the slope and the basin, which in turn regulates the magnitude of interior convection and the export of ventilated waters from the subpolar gyre. This thesis characterizes theWest Greenland Boundary Current System near Cape Farewell across a range of spatio-temporal scales. The boundary current system is composed of three velocity cores: (1) the West Greenland Coastal Current (WGCC), transporting Greenland and Arctic meltwaters on the shelf; (2) the West Greenland Current (WGC), which advects warm, saline Atlantic-origin water at depth, meltwaters at the surface, and newly-ventilated Labrador Sea Water (LSW); and (3) the Deep Western Boundary Current, which carries dense overflow waters ventilated in the Nordic Seas. The seasonal presence of the LSW and Atlantic-origin water are dictated by air-sea buoyancy forcing, while the seasonality of the WGCC is governed by remote wind forcing and the propagation of coastally trapped waves from East Greenland. Using mooring data and hydrographic surveys, we demonstrate mid-depth intensified cyclones generated at Denmark Strait are found offshore of the WGC and enhance the overflow water transport at synoptic timescales. Using mooring, hydrographic, and satellite data, we demonstrate that the WGC undergoes extensive meandering due to baroclinic instability that is enhanced in winter due to LSW formation adjacent to the current. This leads to the production of small-scale, anticyclonic eddies that can account for the entirety of wintertime heat loss within the Labrador Sea. The meanders are shown to trigger the formation of Irminger Rings downstream. Using mooring, hydrographic, atmospheric, and Lagrangian data, and a mixing model, we find that strong atmospheric storms known as forward tip jets cause upwelling at the shelfbreak that triggers offshore export of freshwater. This freshwater flux can explain the observed lack of ventilation in the eastern Labrador Sea. Together, this thesis documents previously unobserved interannual, seasonal, and synoptic-scale variability and dynamics within the West Greenland boundary current system that must be accounted for in future modeling.

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