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Mary L. Batteen and Martin J. Rutherford

Abstract

A high resolution, multilevel, primitive equation (PE) model is used to investigate the generation and stability of the Leeuwin Current and eddies off the west coast of Australia. Two numerical experiments are conducted to investigate the roles of the Indian Ocean temperature field and the North West (NW) Shelf waters in generating both the current and eddies. In the first experiment an alongshore temperature gradient, typical of the Indian Ocean temperature field, is imposed, while in the second experiment the additional effects of the NW Shelf waters are considered. In the first experiment, the meridional Indian Ocean temperature gradient is sufficient to drive a poleward surface flow (the Leeuwin Current) and an equatorial undercurrent. The surface flow is augmented by onshore geostrophic flow and accelerates downstream. In the second experiment, the inclusion of the NW Shelf waters completely dominates in the NW Shelf equatorial source region. The effects of the NW Shelf waters weaken away from the source region but they continue to augment the Indian Ocean forcing, resulting in a stronger flow along the entire coastal boundary. The currents generated by the model agree well with the Leeuwin Current Interdisciplinary Experiment (LUCIE) current meter observations obtained during the austral fall and winter, when the Leeuwin Current is observed, and other recent modeling studies.

The current is unstable and has significant mesoscale variability. The current forced only by the alongshore temperature gradient is unstable toward the poleward end of the model domain. In this region, barotropic instability tends to dominate over baroclinic instability. When the NW Shelf waters are added to force the current, eddies are generated near the source of these waters (in the equatorward end of the model domain) through barotropic instability of the current. Farther dowmstream, the NW Shelf waters add strong baroclinicity, which weakens poleward, to the current. Eddies are subsequently generated downstream from the NW Shelf region through both baroclinic and barotropic instability processes.

Several scales of eddies are found to be dominant. The forcing by the Indian Ocean leads to eddy growth on scales around 150 km. With the inclusion of the NW Shelf waters, the wavelength associated with mesoscale variability is around 180 km. Both of these length scales are close to the wavelength associated with a low-mode Rossby radius of deformation. The eddies generated by the model compare well with available observations.

This study shows that the Leeuwin Current can be successfully modeled using a PE model forced by the mean climatology. Consistent with the findings of recent modeling studies, a shelf is not required to produce and maintain the current. The mesoscale feature which have been missing from previous modeling studies are produced by the model and at scales comparable with available observations.

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Mary L. Batteen, Martin J. Rutherford, and Eric J. Bayler

Abstract

A high-resolution, multilevel, primitive equation model is initialized with climatological data to investigate the combined effects of wind and thermal forcing on the ocean circulation off Western Australia during the austral fall and winter, corresponding to the period of strongest flow for the anomalous Leeuwin Current. This process-oriented study builds on previous modeling studies, which have elucidated the role of thermal forcing in the generation of the Leeuwin Current and eddies, by including the additional effects of wind forcing for the eastern boundary current region off Western Australia. The ocean circulation is generated by the model using a combination of density forcing from the climatological Indian Ocean thermal structure, the influx of warm low-salinity waters from the North West (NW) Shelf, and the climatological wind stress. In the first experiment (case 1), forcing by the Indian Ocean and wind stress are imposed, while in the second experiment (case 2), the additional effects of the North West (NW) Shelf waters are considered. In the absence of the NW Shelf waters (case 1), geostrophic flow, driven by the Indian Ocean thermal gradient, dominates the wind forcing at the poleward end of the domain and establishes an equatorward undercurrent and a poleward surface current (the Leeuwin Current), which accelerates poleward into the prevailing wind. Wind-forcing effects are discernible only offshore at the equatorward end of the region. The inclusion of NW Shelf waters (case 2) completely dominates the wind forcing at the equatorward end of the model. The effects of the NW Shelf waters weaken away from the source region but they continue to augment the Indian Ocean forcing, resulting in a stronger flow along the entire coastal boundary.

The ocean circulation also has significant mesoscale variability. In the first experiment, both the Indian Ocean thermal structure and wind forcing lead to the dominance of barotropic (horizontal shear) instability over baroclinic (vertical shear) instability. In the second experiment, the NW Shelf water add baroclinicity, which weakens poleward, to the Leeuwin Current and locally increase the barotropic instability near their source. Away from the source waters, where there is a mixed instability, the combined effect of the Indian Ocean thermal structure and wind forcing is stronger than the NW Shelf waters and leads to a dominance of barotropic over baroclinic instability. Several scales of eddies are found to be dominant. The forcing by the Indian Ocean and wind stress (case 1) leads to an eddy wavelength of ∼330 km. With the inclusion of the NW Shelf waters (case 2), the wavelengths associated with mesoscale variability are ∼150 and 330 km, consistent with observed eddy length scales.

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Diana Greenslade, Mark Hemer, Alex Babanin, Ryan Lowe, Ian Turner, Hannah Power, Ian Young, Daniel Ierodiaconou, Greg Hibbert, Greg Williams, Saima Aijaz, João Albuquerque, Stewart Allen, Michael Banner, Paul Branson, Steve Buchan, Andrew Burton, John Bye, Nick Cartwright, Amin Chabchoub, Frank Colberg, Stephanie Contardo, Francois Dufois, Craig Earl-Spurr, David Farr, Ian Goodwin, Jim Gunson, Jeff Hansen, David Hanslow, Mitchell Harley, Yasha Hetzel, Ron Hoeke, Nicole Jones, Michael Kinsela, Qingxiang Liu, Oleg Makarynskyy, Hayden Marcollo, Said Mazaheri, Jason McConochie, Grant Millar, Tim Moltmann, Neal Moodie, Joao Morim, Russel Morison, Jana Orszaghova, Charitha Pattiaratchi, Andrew Pomeroy, Roger Proctor, David Provis, Ruth Reef, Dirk Rijnsdorp, Martin Rutherford, Eric Schulz, Jake Shayer, Kristen Splinter, Craig Steinberg, Darrell Strauss, Greg Stuart, Graham Symonds, Karina Tarbath, Daniel Taylor, James Taylor, Darshani Thotagamuwage, Alessandro Toffoli, Alireza Valizadeh, Jonathan van Hazel, Guilherme Vieira da Silva, Moritz Wandres, Colin Whittaker, David Williams, Gundula Winter, Jiangtao Xu, Aihong Zhong, and Stefan Zieger
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Diana Greenslade, Mark Hemer, Alex Babanin, Ryan Lowe, Ian Turner, Hannah Power, Ian Young, Daniel Ierodiaconou, Greg Hibbert, Greg Williams, Saima Aijaz, João Albuquerque, Stewart Allen, Michael Banner, Paul Branson, Steve Buchan, Andrew Burton, John Bye, Nick Cartwright, Amin Chabchoub, Frank Colberg, Stephanie Contardo, Francois Dufois, Craig Earl-Spurr, David Farr, Ian Goodwin, Jim Gunson, Jeff Hansen, David Hanslow, Mitchell Harley, Yasha Hetzel, Ron Hoeke, Nicole Jones, Michael Kinsela, Qingxiang Liu, Oleg Makarynskyy, Hayden Marcollo, Said Mazaheri, Jason McConochie, Grant Millar, Tim Moltmann, Neal Moodie, Joao Morim, Russel Morison, Jana Orszaghova, Charitha Pattiaratchi, Andrew Pomeroy, Roger Proctor, David Provis, Ruth Reef, Dirk Rijnsdorp, Martin Rutherford, Eric Schulz, Jake Shayer, Kristen Splinter, Craig Steinberg, Darrell Strauss, Greg Stuart, Graham Symonds, Karina Tarbath, Daniel Taylor, James Taylor, Darshani Thotagamuwage, Alessandro Toffoli, Alireza Valizadeh, Jonathan van Hazel, Guilherme Vieira da Silva, Moritz Wandres, Colin Whittaker, David Williams, Gundula Winter, Jiangtao Xu, Aihong Zhong, and Stefan Zieger

Abstract

The Australian marine research, industry, and stakeholder community has recently undertaken an extensive collaborative process to identify the highest national priorities for wind-waves research. This was undertaken under the auspices of the Forum for Operational Oceanography Surface Waves Working Group. The main steps in the process were first, soliciting possible research questions from the community via an online survey; second, reviewing the questions at a face-to-face workshop; and third, online ranking of the research questions by individuals. This process resulted in 15 identified priorities, covering research activities and the development of infrastructure. The top five priorities are 1) enhanced and updated nearshore and coastal bathymetry; 2) improved understanding of extreme sea states; 3) maintain and enhance the in situ buoy network; 4) improved data access and sharing; and 5) ensemble and probabilistic wave modeling and forecasting. In this paper, each of the 15 priorities is discussed in detail, providing insight into why each priority is important, and the current state of the art, both nationally and internationally, where relevant. While this process has been driven by Australian needs, it is likely that the results will be relevant to other marine-focused nations.

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