The source of the Leeuwin Current seasonality

The seasonal circulation around the southwestern boundary of Australia is documented using sea level anomalies from satellite altimetry. Results extrapolated to the coast agree closely with tide gauge observations indicating that seasonal altimeter fields are realistic. Monthly sea level maps identify an annual propagating wave along a waveguide extending along the shelf edge, from the Gulf of Carpentaria to southern Tasmania. The annual sea level pulse does not originate from the Pacific Ocean, as annual Pacific sea level variations are completely out of phase with signals south of the Indonesian archipelago. The presence of a phase discontinuity is demonstrated in annual sea level, temperature, and salinity observations. The origin of the Leeuwin Current seasonality is in the Gulf of Carpentaria where monsoonal winds drive a massive buildup of sea level from November to December. During December–February, a sea level “pulse” emerges from the region, and rapidly propagates poleward along the western and southern Australian boundary. In the broad shelf region centered at 19°S, an independent process forms a high sea level feature when a positive heat flux anomaly induces an annual increase in sea surface temperature which is rapidly mixed through the water column by the strong regional tides. In March, the winds relax and switch to a downwelling favorable alongshore component. In this period, the sea level pulse is essentially in a quasi-static equilibrium with the annual propagating wind systems. The change in cross-shelf sea level gradient along the 8000 km path length at the western and southern boundaries, drives the seasonal changes in the Leeuwin Current flow.

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

[2]  P. Holloway,et al.  Leeuwin current and wind distributions on the southern part of the Australian North West Shelf between January 1982 and July 1983 , 1985 .

[3]  P. Oke,et al.  Eddy-resolving ocean circulation in the Asian Australian region inferred from an ocean reanalysis effort , 2008 .

[4]  J. Church,et al.  Circulation in the Gulf of Carpentaria. II. Residual currents and mean sea level , 1983 .

[5]  E. Wolanski,et al.  The net water circulation through Torres strait , 2013 .

[6]  P. Ridd,et al.  Currents through Torres Strait , 1988 .

[7]  P. Holloway.  Leeuwin current observations on the Australian North West Shelf, May–June 1993 , 1995 .

[8]  M. Cirano,et al.  Aspects of the Mean Wintertime Circulation along Australia's Southern Shelves: Numerical Studies , 2004 .

[9]  H. Hellmer,et al.  Tidal Mixing in the Southern Weddell Sea: Results from a Three-Dimensional Model , 2002 .

[10]  James T. Potemra,et al.  Contribution of equatorial Pacific winds to southern tropical Indian Ocean Rossby waves , 2001 .

[11]  K. Badcock,et al.  Tidal mixing near the Kimberley coast of NW Australia , 2000 .

[12]  R. Morrow,et al.  Variability in the southeast Indian Ocean from altimetry: Forcing mechanisms for the Leeuwin Current , 1998 .

[13]  M. Batteen,et al.  Modeling Studies of the Leeuwin Current off Western and Southern Australia , 1998 .

[14]  A. J. Clarke.  On the reflection and transmission of low-frequency energy at the irregular western Pacific Ocean boundary , 1991 .

[15]  Yukio Masumoto,et al.  Forced Rossby waves in the southern tropical Indian Ocean , 1998 .

[16]  G. Meyers.  On the Annual Rossby Wave in the Tropical North Pacific Ocean , 1979 .

[17]  E. Wolanski.  Water circulation in the Gulf of Carpentaria , 1993 .

[18]  J. Wilkin,et al.  Ocean Interpolation by Four-Dimensional Weighted Least Squares—Application to the Waters around Australasia , 2002 .

[19]  N. Bindoff,et al.  Dynamics of the Leeuwin Current: Part 1. Coastal flows in an inviscid, variable-density, layer model , 2013 .

[20]  K. Brink,et al.  Alongshore currents and mesoscale variability near the shelf edge off northwestern Australia , 2007 .

[21]  O. Alves,et al.  Propagation characteristics of coastally trapped waves on the Australian Continental Shelf , 2013 .

[22]  Gentio Harsono,et al.  Halmahera Eddy Features Observed from Multisensor Satellite Oceanography , 2014 .

[23]  K. Brink,et al.  Evaporative dense water formation and cross-shelf exchange over the northwest Australian inner shelf , 2010 .

[24]  Richard Brinkman,et al.  Seasonal circulation and temperature variability near the North West Cape of Australia , 2012 .

[25]  K. Katsumata.  Tidal stirring and mixing on the Australian North West Shelf , 2006 .

[26]  W. Kessler,et al.  The Annual Wind-driven Rossby Wave in the Subthermocline Equatorial Pacific , 1993 .

[27]  J. McCreary,et al.  Thermohaline forcing of eastern boundary currents : With application to the circulation off the west coast of Australia , 1986 .

[28]  M. Maltrud,et al.  El Niño Effects and Upwelling off South Australia , 2007 .

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

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

[31]  J. S. Godfrey,et al.  Ekman transports, tidal mixing, and the control of temperature structure in Australia's northwest waters , 2000 .

[32]  J. McCreary,et al.  On the Dynamics of the Throughflow from the Pacific into the Indian Ocean , 1986 .

[33]  D. Quadfasel,et al.  Circulation in the Timor Sea , 1993 .

[34]  Ocean circulation on the North Australian Shelf , 2011 .

[35]  R. Morrow,et al.  Source of the baroclinic waves in the southeast Indian Ocean , 2001 .

[36]  K. Lambeck,et al.  GRACE estimates of sea surface height anomalies in the Gulf of Carpentaria, Australia , 2008 .

[37]  Scott A. Condie,et al.  Modeling seasonal circulation, upwelling and tidal mixing in the Arafura and Timor Seas , 2011 .

[38]  A. J. Clarke,et al.  El Niño/La Niña shelf edge flow and Australian western rock lobsters , 2004 .

[39]  J. Hunter,et al.  Fronts in the Irish Sea , 1974, Nature.

[40]  J. S. Godfrey,et al.  The Large-Scale Environment of the Poleward-Flowing Leeuwin Current, Western Australia: Longshore Steric Height Gradients, Wind Stresses and Geostrophic Flow , 1985 .

[41]  P. Holloway.  Tides on the Australian north-west shelf , 1983 .

[42]  G. Meyers,et al.  An Intersection of Oceanic Waveguides: Variability in the Indonesian Throughflow Region , 2004 .

[43]  D. Nof,et al.  The Mindanao and Halmahera eddies: Twin eddies induced by nonlinearities , 2003 .

[44]  A. J. Clarke,et al.  Coastline Direction, Interannual Flow, and the Strong El Niño Currents along Australia's Nearly Zonal Southern Coast , 2004 .

[45]  G. Meyers.  Variation of Indonesian throughflow and the El Niño‐Southern Oscillation , 1996 .

[46]  R. Weisberg,et al.  Tides on the West Florida Shelf , 2002 .

[47]  G. Meyers,et al.  Transmission of ENSO signal to the Indian Ocean , 2005 .

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

[49]  G. Cresswell,et al.  The Leeuwin Current south of Western Australia , 1993 .

[50]  Scott A. Condie,et al.  The 5500‐km‐long boundary flow off western and southern Australia , 2004 .

[51]  Peter R. Oke,et al.  Evaluation of a near-global eddy-resolving ocean model , 2012 .

[52]  Lukijanto,et al.  Observed features of the Halmahera and Mindanao Eddies , 2013 .

[53]  J. Bye,et al.  Long-period variations of sea-level in Australasia , 1986 .

[54]  J. S. Godfrey,et al.  Observations of the Shelf-Edge Current South of Australia, Winter 1982 , 1986 .

[55]  J. S. Godfrey,et al.  The Leeuwin Current off Western Australia, 1986–1987 , 1991 .