Physical and chemical signatures of a developing anticyclonic eddy in the Leeuwin Current, eastern Indian Ocean

A multidisciplinary cruise aboard the R/V Southern Surveyor was conducted in May 2006 to sample a developing anticyclonic eddy of the Leeuwin Current off Western Australia. The eddy formed from a meander of the Leeuwin Current in mid-April 2006 and remained attached to the current until mid-August. In this study, a combination of satellite data (altimeter, sea surface temperature, and chlorophyll a) and shipboard measurements (acoustic Doppler current profiler and conductivity-temperature-depth) were used to characterize the physical and chemical signatures of the eddy. The temperature-salinity properties of the mixed layer waters within the anticyclonic eddy and on the shelf were both connected to that of the Leeuwin Current, indicating the water mass in the eddy is mainly derived from the Leeuwin Current and the modified Leeuwin Current water on the shelf. Above the salinity maximum near the eddy center, there was a regionally significant concentration of nitrate (>0.9 μmol L-1), and the maximum (2 μmol L-1) was at 150 in depth, below the photic zone. Nitrification within the eddy and/or local upwelling due to the forming eddy could be responsible for this high concentration of nitrate near the eddy center which potentially makes the eddy a relatively productive feature in the Leeuwin Current.

[1]  Gilles Reverdin,et al.  Global high-resolution mapping of ocean circulation from TOPEX/Poseidon and ERS-1 and -2 , 2000 .

[2]  G. Cresswell,et al.  Observations of a south-flowing current in the southeastern Indian Ocean , 1980 .

[3]  S. Pesant,et al.  Food web structure in two counter-rotating eddies based on δ15N and δ13C isotopic analyses , 2007 .

[4]  B. Warren.  Transindian hydrographic section at Lat. 18°S: Property distributions and circulation in the South Indian Ocean , 1981 .

[5]  S. Pesant,et al.  Contrasting the vertical differences in the phytoplankton biology of a dipole pair of eddies in the south-eastern Indian Ocean , 2007 .

[6]  J. Dunn,et al.  A mixed-layer nutrient climatology of Leeuwin Current and Western Australian shelf waters: Seasonal nutrient dynamics and biomass , 2006 .

[7]  P. Brickley,et al.  Impact of Haida Eddies on chlorophyll distribution in the Eastern Gulf of Alaska , 2005 .

[8]  L. Beckley,et al.  Ichthyoplankton assemblage structure in two meso-scale Leeuwin Current eddies, eastern Indian Ocean , 2007 .

[9]  C. Fandry,et al.  Characteristics of two counter-rotating eddies in the Leeuwin Current system off the Western Australian coast , 2007 .

[10]  T. Mooreb,et al.  The Leeuwin Current and its eddies : An introductory overview , 2007 .

[11]  L. Twomey,et al.  Variability in nitrogen uptake and fixation in the oligotrophic waters off the south west coast of Australia , 2007 .

[12]  G. Meyers,et al.  Do Eddies Play a Role in the Momentum Balance of the Leeuwin Current , 2005 .

[13]  P. L. Traon,et al.  AN IMPROVED MAPPING METHOD OF MULTISATELLITE ALTIMETER DATA , 1998 .

[14]  Nick Caputi,et al.  Ocean currents and the larval phase of Australian western rock lobster, Panulirus cygnus , 2001 .

[15]  R. W. Griffiths,et al.  The Mesoscale Structure of the Leeuwin Current: A Comparison of Laboratory Models and Satellite Imagery , 1991 .

[16]  S. Libes.  An introduction to marine biogeochemistry , 1992 .

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

[18]  Susan Wijffels,et al.  Annual and interannual variations of the Leeuwin Current at 32°S , 2003 .

[19]  Simón Ruiz,et al.  Diagnostic analysis of the 3D ageostrophic circulation from a multivariate spatial interpolation of CTD and ADCP data , 2001 .

[20]  R. McCauley,et al.  Eddy formation through the interaction between the Leeuwin Current, Leeuwin Undercurrent and topography , 2007 .

[21]  Fangxin Fang,et al.  Anatomy of three warm-core Leeuwin Current eddies , 2003 .

[22]  L. Beckley,et al.  Larval fish assemblages and water mass structure off the oligotrophic south‐western Australian coast , 2007 .

[23]  Kosei Sasaoka,et al.  Bering Sea cyclonic and anticyclonic eddies observed during summer 2000 and 2001 , 2002 .

[24]  John Marra,et al.  Phytoplankton variability off the Western Australian Coast : Mesoscale eddies and their role in cross-shelf exchange , 2007 .

[25]  M. Pedder,et al.  Errors in dynamical fields inferred from oceanographic cruise data: Part II. The impact of the lack of synopticity , 2005 .

[26]  Patrice Klein,et al.  Impact of sub-mesoscale physics on production and subduction of phytoplankton in an oligotrophic regime , 2001 .

[27]  M. Lynch,et al.  The Hillarys Transect (1): Seasonal and cross-shelf variability of physical and chemical water properties off Perth, Western Australia, 1996-98 , 2006 .

[28]  J. Holton.  An introduction to dynamic meteorology , 2004 .

[29]  M. Pedder,et al.  Errors in dynamical fields inferred from oceanographic cruise data. Part I. The impact of observation errors and the sampling distribution , 2005 .

[30]  A one-dimensional simulation of biological production in two contrasting mesoscale eddies in the south eastern Indian Ocean , 2007 .