20
BAMOS
Mar 2019
Article
What is Shelf-Break Upwelling and
How Does It Work?
Nathan Teder
Flinders University
The continental shelf is one of the more biologically productive
regions in the ocean. This is due to the shelf being able to
support an influx of nutrients (nitrates, phosphates, and
silicates) from runoff, or from the deeper parts of the ocean
(upwelling), as well as being shallow enough for those nutrients
to get into the euphotic zone and trigger photosynthesis. One
method by which deeper water can be upwelled on to the shelf
is through an ambient current interaction with a canyon that
has incised into the continental shelf. This interaction has been
shown to be important to Australia’s ecological make-up, due
to canyon based upwelling being a source of fish, whales, krill
and other marine life in both the Great Australian Bight through
the Kangaroo Island pool (Kämpf, 2010), and the Perth canyon
off the Western Australia coastline (Rennie et al., 2009) in the
summer months. This essay looks at breaking down how the
interaction between the shelf-break canyon and the ambient
conditions works, and what can occur when an upwelling event
happens.
For upwelling to occur in the canyon, a few conditions must be
met first. It must be cut into the shelf in such a manner that it is
a deep, steep and narrow formation. The depth of the canyon
influences the amount of nutrients being upwelled when an
event happens, as the deeper the water is located, the more
nutrients are present. Steepness and narrowness both play
a role in the intensity of an event. The steeper and narrower
a canyon is, the more upwards force will be present as the
ambient current passes over the canyon, influencing both the
cyclonic eddy formation and the amount of force the upwelling
current has. To give an idea as to what is looked for, typically
canyons that cut into the shelf break tend to be around 100–
500 m deep, with canyons capable of upwelling being typically
greater than 300 m depth , 10–50 km wide and can have a slope
of 45 degrees as a guideline (Kämpf, 2007).
The other factor which controls if upwelling occurs is the
direction of the ambient current, how deep it reaches and how
uniform the current is. In the southern hemisphere, the ambient
flow required to trigger an upwelling event in this manner
requires the coastline to be on the right of it (and left in the
northern hemisphere). Flows with this characteristic are in the
opposite direction to the topographic steering flow. The depth
that the current reaches is also important. For any upwelling
to occur, a current must exist at a depth that is ideally below
the rim of the canyon. It is worth noting that the presence of
an opposite direction subsurface current can prevent upwelling
despite the surface having conducive conditions (Hickey, 1997).
Equally uniformity of the current matters as it is required that
a flow lasts in an upwelling conducive direction for a couple of
days at minimum for an event to occur.
When an upwelling flow is present around a viable canyon, the
current flow tends to vary depending on the depth it occurs
(Figure 1). Near surface flow tends to only be weakly impacted
by the canyon as it passes over the canyon and has no real
influence in an upwelling event. The flow just above the rim of
the canyon will stretch the water column as it crosses the rim,
by flowing downwards alongside its initial horizontal flow. This
flow will begin to correct itself and rise in the water column as
it flows across the canyon, however it will compress itself once
it flows up the downstream rim. This stretching of the water
can form a cyclonic eddy at the surface of the canyon. Finally,
the flow slightly below the rim will enter the canyon, and is
upwelled over the downstream rim of the canyon. This flow will
carry the deepest water which is advected on the shelf (Allen
and Hickey, 2010).