Abstract:

Western boundary currents (WBCs), such as the Kuroshio, East Australian Current and the Gulf Stream are typically characterised by a poleward jet and a complex field of eddies. WBCs play an important role in distributing heat and biota, and influence weather and climate.

Many WBCs exhibit a subsurface velocity maximum, but the dynamics behind this feature remain largely unexplained. Using observations and models of the East Australian Current, a comprehensive explanation for this aspect of WBCs is presented. Two factors are important. First, the EAC is a narrow current, carrying low-density water poleward. This results in horizontal density gradients with a different sign on either side of the jet, negative onshore and positive offshore. According to the thermal wind relation, this produces vertical gradients in the poleward current that are surface intensified onshore and subsurface intensified offshore. Second, the winds over the shelf are mostly downwelling favourable, drawing surface waters onshore. This aligns the region of positive horizontal density gradients with the EAC core, producing a subsurface velocity maximum. The subsurface velocity maximum might play a crucial role in triggering baroclinic instability, which in turn could lead to the formation of mesoscale eddies.

The eddy fields of WBCs are further complicated by the continuous development, evolution, and merging of these eddies. Subsurface observations of merging eddies are rare, but data from Argo floats captured a remarkable event in the Tasman Sea, where two anticyclonic eddies merged. In this event, the smaller eddy spiralled around the larger one and sank to a depth of 1000 meters. For the first time, the theory of eddy merging has been compared to oceanic observations, demonstrating a striking agreement. These findings suggest that merged eddies may be more common than previously believed, with significant implications for heat and fresh water transport, ocean productivity and the transfer of energy across different scales.

Biography:

Prof. Peter Oke earned his PhD in physical oceanography from the School of Mathematics at the University of New South Wales in 1998. Following his PhD, he completed a postdoctoral fellowship at Oregon State University, where he worked on coastal ocean data assimilation. He returned to the University of New South Wales for another postdoctoral position, this time in climate dynamics. In 2002, Peter joined CSIRO, where he developed the data assimilation system that underpins Australia’s global ocean forecasting and reanalysis system. Peter has worked at CSIRO for over 20 years, working on many projects involving ocean modelling, data assimilation, ocean observing, fisheries, and climate. 

Peter was a long-standing member of the GODAE OceanView and OceanPredict Science Team, and he was the founding Co-Chair of a task team focused on observing system evaluation under GODAE OceanView. Currently, Peter leads the Australian Argo Program, serves as an Executive Member of the International Argo Steering Team, Co-Chairs the Global Synthesis and Observations Panel under CLIVAR, and is a member of the Ocean Observations, Physics, and Climate Panel under the WMO.

Peter has published over 100 peer-reviewed papers, which include descriptions of new methods and techniques in ocean data assimilation, as well as numerous studies on ocean dynamics. He is best known for his contributions to understanding the East Australian Current and for his advancements in ensemble data assimilation.

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