A numerical modeling study of the East Australian Current encircling and overwashing a warm-core eddy

[1] Warm-core eddies (WCEs) often form in the meanders of Western Boundary Currents (WBCs). WCEs are frequently overwashed with less dense waters sourced from the WBC. We use the Regional Ocean Modelling System to investigate the ocean state during the overwashing of one such WCE in October 2008 in the East Australian Current (EAC). Comparisons of model outputs with satellite sea surface temperature and vertical profiles show that the model provides a realistic simulation of the eddy during the period when the EAC encircled and then overwashed the eddy. During the encircling stage, an eddy with closed circulation persisted at depth. In the surface EAC water entered from the north, encircled the eddy and exited to the east. The overwashing stage was initiated by the expulsion of cyclonic vorticity. For the following 8 days after the expulsion, waters from the EAC washed over the top of the eddy, transferring heat and anticyclonic vorticity radially-inward. After approximately one rotation period of overwashing, the eddy separated. The overwashing creates a two-layer system that forms a subsurface maximum velocity at the interface of the two layers. Analysis of water mass properties, Eulerian tracer dynamics, and Lagrangian particle tracks show that the original eddy sinks 10–50 m during the overwashing period. Overwashing has been observed in many WBCs and occurs in most WCEs in the western Tasman Sea.

[1]  C. Nilsson,et al.  The formation and evolution of East Australian current warm-core eddies , 1980 .

[2]  E. F. Bradley,et al.  Bulk parameterization of air‐sea fluxes for Tropical Ocean‐Global Atmosphere Coupled‐Ocean Atmosphere Response Experiment , 1996 .

[3]  John Wilkin,et al.  Modes of mesoscale sea surface height and temperature variability in the East Australian Current , 2007 .

[4]  C. P. Duncan.  An eddy in the subtropical convergence southwest of South Africa , 1968 .

[5]  F. Schott,et al.  Eddies in the North Brazil Current retroflection region observed by Geosat altimetry , 1993 .

[6]  S. Diamond,et al.  Effect of Surface , 1982 .

[7]  J. S. Godfrey,et al.  Seasonal cycle of the East Australian Current , 1997 .

[8]  T. Moore,et al.  Nutrient supply to anticyclonic meso-scale eddies off western Australia estimated with artificial tracers released in a circulation model , 2009 .

[9]  W. Dewar,et al.  Alignment of lenses: laboratory and numerical experiments , 1994 .

[10]  D. M. Nelson,et al.  Enhanced near-surface nutrient availability and new production resulting from the frictional decay of a Gulf Stream warm-core ring , 1989 .

[11]  C. Parker.  Gulf stream rings in the Sargasso Sea , 1971 .

[12]  B. Hamon.  The East Australian Current , 1965 .

[13]  M. Baird,et al.  Modelling coastal connectivity in a Western Boundary Current: Seasonal and inter-annual variability , 2011 .

[14]  Su Jilan,et al.  The calculation of Kuroshio current structure in the East China Sea—early summer 1986 , 1988 .

[15]  R. Reynolds,et al.  The NCEP/NCAR 40-Year Reanalysis Project , 1996, Renewable Energy.

[16]  R. Parker,et al.  Are warm-core eddies unproductive? , 1980, Nature.

[17]  Nicholas R. Bates,et al.  Eddy/Wind Interactions Stimulate Extraordinary Mid-Ocean Plankton Blooms , 2007, Science.

[18]  A. Bakun,et al.  Fronts and eddies as key structures in the habitat of marine fish larvae: opportunity, adaptive response and competitive advantage , 2006 .

[19]  R. Flather,et al.  Results from a storm surge prediction model of the north-west European continental shelf for April, November and December, 1973 , 1976 .

[20]  B. Hamon.  The East Australian current, 1960–1964 , 1965 .

[21]  James C. McWilliams,et al.  Mesoscale to Submesoscale Transition in the California Current System. Part II: Frontal Processes , 2008 .

[22]  G. Cresswell.  Physical Evolution of Tasman Sea Eddy J , 1983 .

[23]  G. Hallegraeff,et al.  Phytoplankton pigments, species and light climate in a complex warm-core eddy of the East Australian current , 1987 .

[24]  D. Nof,et al.  The Sinking of Warm-Core Rings , 1988 .

[25]  D. Griffin,et al.  The effect of surface flooding on the physical-biogeochemical dynamics of a warm-core eddy off southeast Australia , 2011 .

[26]  M. Kahru,et al.  Eddies enhance biological production in the Weddell‐Scotia Confluence of the Southern Ocean , 2007 .

[27]  D. Vaudrey,et al.  Biological significance of surface flooding in warm-core ocean eddies , 1982, Nature.

[28]  C. Langdon,et al.  Seasonal variations in the phytoplankton biomass and productivity of a warm-core Gulf Stream ring , 1985 .

[29]  N. White,et al.  Satellite Altimetry for Geodetic, Oceanographic, and Climate Studies in the Australian Region , 2011 .

[30]  G. Mellor,et al.  Development of a turbulence closure model for geophysical fluid problems , 1982 .

[31]  Dennis A. Hansell,et al.  Response to Comment on "Eddy/Wind Interactions Stimulate Extraordinary Mid-Ocean Plankton Blooms" , 2008, Science.

[32]  Alexander F. Shchepetkin,et al.  The regional oceanic modeling system (ROMS): a split-explicit, free-surface, topography-following-coordinate oceanic model , 2005 .

[33]  M. D. Sikirić,et al.  A new approach to bathymetry smoothing in sigma-coordinate ocean models , 2009 .

[34]  Glenn R. Flierl,et al.  Frictionally induced circulations and spin down of a warm‐core ring , 1985 .

[35]  A. Oort.  Computations of the Eddy Heat and Density Transports Across the Gulf Stream , 2015 .

[36]  Yih Yang,et al.  Taiwan Current (Kuroshio) and Impinging Eddies , 1999 .

[37]  D. Chapman.  Numerical Treatment of Cross-Shelf Open Boundaries in a Barotropic Coastal Ocean Model , 1985 .

[38]  W. Emery,et al.  Variability and forcing of the East Australian Current , 2005 .

[39]  James C. McWilliams,et al.  A method for computing horizontal pressure‐gradient force in an oceanic model with a nonaligned vertical coordinate , 2003 .

[40]  P. Oke,et al.  An avenue of eddies: Quantifying the biophysical properties of mesoscale eddies in the Tasman Sea , 2012 .

[41]  J. S. Godfrey,et al.  The Separation of the East Australian Current , 1980 .

[42]  J. Church,et al.  Eddy shedding and energy conversions in the East Australian Current , 2006 .

[43]  A Modeling Study of the Climatological Current Field and the Trajectories of Upwelled Particles in the East Australian Current , 2002 .

[44]  George L. Mellor,et al.  The Pressure Gradient Conundrum of Sigma Coordinate Ocean Models , 1994 .

[45]  R. Coleman,et al.  Decadal variability of East Australian Current transport inferred from repeated high-density XBT transects, a CTD survey and satellite altimetry , 2008 .