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The Leeuwin current originates off the northwestern coast of Australia
and flows
poleward along the shelf edge down, beyond Perth, to the southwestern
corner of
Australia. It is relatively well characterized off the west coast of
Australia (Smith et al,
1991), but off the northwestern coast, it has rarely been observed,
and is apparently
sometimes masked by the energetic tidal currents in this region (Holloway,
1995). It is
typically found as a narrow, energetic jet just offshore of the shelf
edge, and it appears to
be driven from offshore as the return flow of a basin- scale current
system. One anomaly
of the Leeuwin Current system is that, although winds are often upwelling
favorable off
the west coast of Australia, upwelling is generally not observed and
the region is not
nearly as biologically productive as the seemingly analogous Peru-Chile
system.
Thompson (1987) provided a dynamical explanation for this anomaly,
but his results
need to be revisited in light of current ideas about buoyancy shutdown
in bottom
boundary layers.
We seek to examine the shelf and slope waters off the northwestern coast
of Australia.
This region is characterized oceanographically by three pronounced
features:
1) Extremely strong tidal currents,
2) The formation region of the Leeuwin Current near the shelf break,
and
3) Strong evaporation associated with dry air flowing off the Australian
continent.
Our goal is to improve our understanding of the system by:
1) Characterizing the strength, structure and variability (including
associated eddies)
of the Leeuwin current near and offshore of the shelf edge, and
2) Investigating how shelf waters respond to evaporative forcing.
The general strategy of the June 18-July 17, 2003 R/V Melville cruise
(Port Hedland to
Darwin) is to start with broad surveys and to refine to sampling schemes
that resolve the
energetic modes of variability. To that end, sampling proceeded as
follows:
1) June 18: An ADCP mooring was deployed near 117 53.1 E, 18 49.8
S. This
will allow us to quantify the tides and so develop means to remove
them from
shipboard ADCP records. The mooring was recovered on July 13, 2003.
2) June 18-June 24, 2003: Three cross shelf CTD sections were completed
with 10
km station spacing. Temperature, pressure, conductivity, chlorophyll
fluorescence, beam transmission and other properties were measured
at each cast.
These gave us a preliminary sampling of scales, the overall structure
(both of the
Leeuwin current and of shelf salinity field), and allowed a preliminary
identification of the current itself. These lines spanned the region
between about
117 E and 121 E.
3) June 24- June 30: Seven long SeaSoar transects were completed over
the same
general region. The SeaSoar was equipped to measure temperature, conductivity,
pressure, chlorophyll fluorescence, beam transmission, bioluminescence
and
yellow-wavelength (CDOM) fluorescence. This gives us a far more resolved
view
of phenomenology and, along with the CTD sections, allows some
characterization of water mass changes and current location as a function
of
alongshore distance.
4) June 30 -July 9: Repeated mesoscale SeaSoar surveys were carried
out near the
western edge of our study area in order to resolve mesoscale variability
(meanders
and eddies) associated with the Leeuwin current core, and to sample
the
interaction of saline shelf waters with the less salty slope waters.
5) July 9- July 13: Minibat surveys (measuring temperature, conductivity,
pressure, light transmission and chlorophyll fluorescence) were conducted over the
broad shelf near Port Hedland (centered near 118 30 E, 19 45 S)
in order to study inner shelf effects associated with evaproative forcing.
The Captain and crew of the R/V Melville were most gracious and helpful in the
accomplishment of this work. They are much appreciated. This research is sponsored
by the Office of Naval Research, Physical Oceanography program.
References:
Holloway, P.E., 1995: Leeuwin Current observations on the Australian
north west shelf,
May-June 1993. Deep Sea Research I, 42, 285-305.
Smith, R.L., A. Huyer, J.S. Godrey, and J.A. Church, 1991: The Leeuwin
Current off
Western Australia, 1986-1987. Journal of Physical Oceanography, 21,
322-345.
Thompson, R.O.R.Y., 1987: Continental shelf-scale model of the Leeuwin
Current.
Journal of Marine Research, 45, 813-827.
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