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The Leeuwin Current and Its Eddies: an Introductory Overview

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The Leeuwin Current and Its Eddies: an Introductory Overview ARTICLE IN PRESS Deep-Sea Research II 54 (2007) 789–796 www.elsevier.com/locate/dsr2 The Leeuwin Current and its eddies: An introductory overview A.M. Waitea,Ã, P.A. Thompsonb, S. Pesanta,c,d, M. Fenge, L.E. Beckleyg, C.M. Dominguesb, D. Gaughanj, C.E. Hansona,k, C.M. Hollh,i, T. Koslowe, M. Meulenersa, J.P. Montoyah, T. Mooreb, B.A. Muhlingg, H. Patersona,e,f, S. Renniea, J. Strzeleckid,e, L. Twomeya,l aSchool of Environmental Systems Engineering, University of Western Australia, 35 Stirling Hwy, Crawley, 6009 WA, Australia bMarine and Atmospheric Research, CSIRO, GPO Box 1538, Hobart, TAS, Australia cCNRS, Laboratoire d’Oce´anographie de Villefranche, 06230 Villefranche-sur-Mer, France dUniversite´ Pierre et Marie Curie-Paris 6, Laboratoire d’Oce´anographie de Villefranche, 06230 Villefranche-sur-Mer, France eMarine and Atmospheric Research, CSIRO, Floreat, WA, Australia fSchool of Animal Biology, University of Western Australia, Crawley, 6009 WA, Australia gSchool of Environmental Science, Murdoch University, South St., Murdoch, 6150 WA, Australia hSchool of Biology, Georgia Institute of Technology, Atlanta, GA, USA iOceanic Institute, 41-202 Kalaniana’ole Highway, Waimanalo, HI 96795, USA jDepartment of Fisheries, P.O Box 20, North Beach, WA 6920, Australia kCentre for Ecosystem Management, Edith Cowan University, 100 Joondalup Drive, Joondalup, WA 6027, Australia lSchool of Biological Sciences and Biotechnology, Murdoch University, South St., Murdoch, 6150 WA, Australia Received 22 December 2005; received in revised form 18 December 2006; accepted 24 December 2006 Available online 21 June 2007 Abstract The Leeuwin Current (LC) is an anomalous poleward-flowing eastern boundary current that carries warm, low-salinity water southward along the coast of Western Australia. We present an introduction to a new body of work on the physical and biological dynamics of the LC and its eddies, collected in this Special Issue of Deep-Sea Research II, including (1) several modelling efforts aimed at understanding LC dynamics and eddy generation, (2) papers from regional surveys of primary productivity and nitrogen uptake patterns in the LC, and (3) the first detailed field investigations of the biological oceanography of LC mesoscale eddies. Key results in papers collected here include insight into the source regions of the LC and the Leeuwin Undercurrent (LUC), the energetic interactions of the LC and LUC, and their roles in the generation of warm-core (WC) and cold-core (CC) eddies, respectively. In near-shore waters, the dynamics of upwelling were found to control the spatio-temporal variability of primary production, and important latitudinal differences were found in the fraction of production driven by nitrate (the f-ratio). The ubiquitous deep chlorophyll maximum within LC was found to be a significant contributor to total water column production within the region. WC eddies including a single large eddy studied in 2000 contained relatively elevated chlorophyll a concentrations thought to originate at least in part from the continental shelf/shelf break region and to have been incorporated during eddy formation. During the Eddies 2003 voyage, a more detailed study comparing the WC and CC eddies illuminated more mechanistic details of the unusual dynamics and ecology of the eddies. Food web analysis suggested that the WC eddy had an enhanced ‘‘classic’’ food web, with more ÃCorresponding author. Tel.: +61 8 6488 3082; fax: +61 8 6488 1015. E-mail address: [email protected] (A.M. Waite). 0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.12.008 ARTICLE IN PRESS 790 A.M. Waite et al. / Deep-Sea Research II 54 (2007) 789–796 concentrated mesozooplankton and larger diatom populations than in the CC eddy. Finally, implications for fisheries management are addressed. r 2007 Elsevier Ltd. All rights reserved. Keywords: Mesoscale eddy; Leeuwin Current; Leeuwin Undercurrent; Primary production; f-ratio; Chlorophyll a 1. Introduction (Letelier et al., 2000; Garcon et al., 2001) by two primary mechanisms, ‘‘eddy pumping’’ of nutrients The Leeuwin Current (LC) off Western Australia upwards from deep to shallow waters by cyclonic is anomalous in that it is the only poleward-flowing eddies (McGillicuddy and Robinson, 1997; Brze- eastern boundary current globally. It therefore zinski et al., 1998; Siegel et al., 1999) and transport carries warm, low-salinity water southward along of nutrients and productivity offshore from coastal, the coast of Western Australia (Cresswell and frontal or upwelling areas by either cyclonic or Golding, 1980; Pearce, 1991) in contrast to other anticyclonic eddies (Garcia-Gorriz and Carr, 2001; eastern boundary currents where strong upwelling is Lima et al., 2002; Whitney and Robert, 2002). typical. The physical dynamics of the LC have been Productivity enhancement due to eddy pumping is studied over the past two decades (e.g., Godfrey and generally seen in the centre of upwelling cyclonic Ridgway, 1985; Thompson, 1987; Feng et al., 2005). eddies and at the periphery of convergent antic- Although the dynamics of the LC are critical to yclonic eddies (Oschlies and Garcon, 1998; Morda- regional fisheries such as the economically impor- sova et al., 2002). tant Western Rock Lobster, Panulirus cygnus Very few LC eddies have been sampled and (Caputi et al., 2003), the biological impact of the studied by field-going oceanographers, so before the LC has only recently been studied (Griffin et al., investigations presented here, relatively little was 2001; Hanson et al., 2005a, b). In general, the known about their influence on the regional interaction between ocean physics and biology in biogeochemistry and ecology. SeaWiFS ocean- the LC region is understudied in comparison with colour imagery suggested that anticyclonic eddies other boundary current systems. of the LC had higher near-surface concentrations of The strength of the LC varies seasonally, flowing chlorophyll a than adjacent waters, and that the strongly in the austral winter when southerly winds time of maximum concentration was mid-winter, are weakest (e.g., Cresswell, 1991; Fieux et al., 2005) when the LC is most intense (Griffin et al., 2001). and the alongshore pressure gradient strengthens Cresswell and Griffin (2004), however, found that (Potemra, 2001; Godfrey and Ridgway, 1985). fluorescence was a local minimum in the center of Interannual variation in LC strength is related to an anticyclonic LC eddy south of our study region. the El Nin˜o Southern Oscillation: stronger flow Papers in this volume represent the first collection occurs during La Nin˜a years and weaker during El of intensive multidisciplinary investigations of the Nin˜o years (Pearce and Phillips, 1988; Feng et al., LC and its associated mesoscale eddies. Our aim is 2003; Li and Clarke, 2004). As the current strength- to characterize the dynamics and productivity of the ens both annually and interannually, a significant LC in some detail, with special emphasis on portion of LC water feeds into the mesoscale biophysical coupling and the mesoscale eddies. A anticyclonic eddies that spin up from the current number of modeling papers (Domingues et al., and propagate westward away from the coast 2007; Rennie et al., 2007; Mueleners et al., 2007; (Pearce and Griffiths, 1991; Morrow et al., 2004). Batteen et al., 2007) document various mechanistic Interannual variation in LC strength influences the aspects of the LC including its sources, structure, local marine ecosystem, and is positively correlated and eddy generation. Hanson et al. (2007a) then with settlement of western rock lobster, P. cygnus, examine the magnitude and significance of the to juvenile habitat (Pearce and Phillips, 1994; ubiquitous deep chlorophyll maximum layer found Caputi et al., 2003), though the mechanism is within LC and offshore waters, while Hanson et al. unknown. (2007b) and Twomey and co-workers (2007) in- Globally, mesoscale eddies (50–200 km diameter) vestigate the nitrogen uptake dynamics of phyto- have been seen to generate large productivity pulses plankton in broad geographic regions north and in many nutrient-poor regions of the world ocean south of the Abrolhos islands from field efforts in ARTICLE IN PRESS A.M. Waite et al. / Deep-Sea Research II 54 (2007) 789–796 791 2000 and 2003, respectively. Moore et al. (2007) report the first measurements of chlorophyll a in the very large WC eddy of 2000. The rest of the papers in this volume form part of the Waite et al. study of two counter-rotating LC eddies in 2003 (Eddies 2003): Feng and co-workers (2007) detail the physical structure of the eddies studied. These are followed by a discussion of vertical mixing within the warm WC eddy in particular (Thompson et al., 2007) and fluxes of nitrogen into the eddy (Greenwood et al., 2007). Phytoplankton productivity and nutrient uptake are discussed by Waite and co-workers (2007a) with a special effort devoted to nutrient dynamics and nitrogen fixation (Holl et al., 2007). Microzoo- plankton grazing and picoplankton abundances are investigated by Paterson et al. (2007a, b), followed by enumeration of the mesozooplankton by Strze- lecki et al. (2007). Larval fish abundances and distributions are discussed in detail by Muhling et al. (2007). Waite et al. (2007b) investigate food web structure within the eddies as elucidated by the N and C isotope signature of the organisms, in combination with all other data. And finally, Gaughan (2007) discusses implications of mesoscale eddy formation for fisheries management of coastal species. In the present paper, we provide a brief context for, and introduction to, the 20 papers contained in this volume. 2. Results and discussion 2.1. Physical oceanographic modelling New model results using a Lagrangian approach Fig. 1. Simplified schematic of surface currents off Western have indicated that the warm, low-salinity tropical Australia emphasizing the connection between the Leeuwin water transported by the LC along the coast sources Current (LC) and Leeuwin Undercurrent (LUC) as a curved vertical arrow.
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