Pilot investigation of the origins and pathways of marine debris found in the
northern Australian marine environment
Draft for comment - 14/3/2008
David Griffin
Centre for Australian Weather and Climate Research:
A partnership between CSIRO and the Bureau of Meteorology
Contents
Summary
Not written yet.
Introduction
Needed?
Methods
Scope of study restricted to large, heavily sunken items of debris (like ghost nets) whose
movement should be well-approximated by the drifters, and our Bluelink model (0-10m
vertical average of current). This means that the direct effect of the wind on the the
movement of debris is assumed to be negligible in comparison to the indirect effect via
the wind’s influence on the movement of the water. We model the movement of items,
but do not attempt to simulate the stranding process.
Info about drifters. (http://www.aoml.noaa.gov/phod/dac/gdp_drifter.html#design)
Model: run of hydrodynamic model took many months of super-computer time. The
resulting archive is several TBytes. For this study, we merely use the stored near-surface
(0-10m average) velocity fields to track ‘model drifters’ by stepping their positions
forward a few hours at a time. For the simulations described here, the model is seeded
with particles from a relatively small number of locations, 12 in the ‘near field’ and 12 in
the ‘far field’, every second day for a year at a time. These particles are then all tracked
for a second year during which no more are released from the starting points. This
‘particle tracking’ model has been run several times for all years between 1992 and 2006.
Here we present results for a single 4-year run of the model. Inter-annual variability of
results is surprisingly low.
For further information on the hydrodynamic model see Oke et al. (2008) and Schiller et
al. (2008). The particle tracking model has also recently been used by Bruce et al. (2007)
for a study of the dispersal of the larvae of Southern Rock Lobster.
Model weaknesses
Flow through Torres Strait is too fast by about a factor of two. This is because the model
does not have tides. Friction scales approximately with the square of the instantaneous
velocity, so omitting the tides artificially reduces the effect of friction. We have
demonstrated this effect experimentally with our model, and are working on a way to
make a compensatory change. In the meantime, however, we have no option but to use
the information available from our present archive of model output.
The model is on a 10km by 10km horizontal grid, and a 10m vertical grid. For numerical
reasons, no part of the model can be less than two grid cells wide, or two grid cells deep.
This means some narrow passages have been closed, and some widened. Some shallow
areas have been filled in, some have been made deeper.
Results
Global Lagrangian Drifters
Since January 1990, 920 satellite-tracked drifters have entered the region 19S-15N,
110E-156E. Not a single one of those entered the Gulf of Carpentaria. Five were tracked
in the GoC, but it appears that they were deployed there, rather than drifting there. Three
drifters traversed the Arafura Sea immediately north of the GoC, all in a westward
direction, having passed through Torres Strait in June 1998 or July 2007. Figure 2.1
shows the tracks of all drifters during the SE trades season. Tracks for the individual
months are at http://www.marine.csiro.au/~griffin/debris/drifters/monthly/index.html .
Two drifters entered the Arafura Sea from the west, but their tracks end at 135E, in the
northern Arafura Sea. Both did this in February 1994 (Fig 2.2), and came from the
northern Indian Ocean, one via Lombok Strait and one via the Sawu Sea (north of
Timor). These directions of movement are, as one might expect, in the downwind
direction for the respective time of year.
These drifter tracks suggest that there is very little surface-layer inflow to the Arafura Sea
/ GoC region. However, it must be remembered that the drifters have a 20m-long seaanchor that will result in them snagging in deeper water than many forms of marine
debris, thereby reducing their chance of traversing the archipelagos and reef systems
bordering the GoC.
Bluelink model
The best way to visualize the results of the modeling is to view animations of the
movement of ‘model drifters’. For clarity these show, separately, model drifters seeded
in the
 far field: http://www.marine.csiro.au/~griffin/debris/traj/e007/
 near field: http://www.marine.csiro.au/~griffin/debris/traj/e006/
Trajectories of model drifters are qualitatively similar to the trajectories of real drifters in
many regards. The southward Mindanao current feeding into Makassar Strait and
clockwise Halmahera Eddy are clear
(http://www.marine.csiro.au/~griffin/debris/traj/e007c/monthly/ ), as are many other well
known features of the regional oceanography. (could expand this, with the general theme
of ‘model validation’).
Model drifters of Far Field origin
In April – October (mainly April – June), several model drifters go westward through
Torres Strait to the Arnhem Land and Groote Island coasts (Fig 3.1). In November-March
(Fig 3.2), a few of these move east to the Cape York side of the GoC, and there is northeastward movement of a few model drifters in the Arafura Sea towards Irian Jaya.
Model drifters of Near Field origin
Even when the model is seeded with drifters in the Indonesian Archipelago, few of these
enter the GoC (eg:
http://www.marine.csiro.au/~griffin/debris/traj/e006/2001tracks/reg_07h_d0723.html )
Discussion
Both real and modeled drifters arriving in the GoC or along the Arnhem land coast are of
South Pacific origin, having come through Torres Strait. There is no evidence from either
the existing archive of satellite tracked drifters, or from our model simulation, that large,
heavily sunken items of marine debris reaches Australia’s northern shores from either the
west (ie, the Indian Ocean) or north (through Indonesia).
Is this a plausible result? The main argument in favour is that the SE Trades blow
stronger and for longer than the NW Monsoon. The strongest argument against it is that
many more drifters, both real and modeled, wash up on the Great Barrier Reef than on the
shores of northern Australia. Why do we not see many ghost nets on the GBR?
Further Work
Still to write
iTracker
In the meantime, before an explanation is found for the existence of debris north of
Australia, we still have the problem of dealing with the debris. To assist with the task of
intercepting items that have been spotted by aircraft, we have implemented a
demonstration version of a system called, for the time being, iTracker. This uses the same
basic methodology as the present study, except that the velocity fields are those produced
twice-weekly by OceanMAPS, the operational version of the Bluelink model used here.
To use iTracker you only need a web browser, but to make better use of its output you
can import the results to Google Earth. See:
http://www.marine.csiro.au/bluelink/exproducts/itracker/
Bibliography
Oke, P. R., G.B. Brassington, D.A. Griffin and A. Schiller (2008). The Bluelink Ocean
Data Assimilation System (BODAS), Ocean Modelling, 21, 46-70,
doi:10.1016/j.ocemod.2007.11.002. \
Saint-Cast, F. and S. Condie (2006). Circulation Modelling in Torres Strait. Geoscience
Australia, Record 2006/18, 82 pp.
Schiller, A., P. R. Oke, G. B. Brassington, M. Entel, R. Fiedler, D. A. Griffin, and J.
Mansbridge, 2008: Eddy-resolving ocean circulation in the Asian-Australian region
inferred from an ocean reanalysis effort. Progress in Oceanography, in press.
Barry D. Bruce, David A. Griffin and Russell W. Bradford (2007). Larval transport and
recruitment processes of southern rock lobster. Final report to FRDC.
http://www.marine.csiro.au/~griffin/FRDC2002-007/