Transcript The shelf edge - Ocean and Earth Science

Processes at the shelf edge.
At the shelf edge:
The current speed can vary with depth, but the direction of the current is
always along the isobaths.
This assumes a linear, steady, frictionless flow; i.e. a geostrophic flow cannot
move across the shelf slope or shelf edge.
Fundamental result:
Shelf waters have different characteristics to the open ocean, and the two are
often separated from the ocean by a shelf break front.
Shelf water dynamics controlled
by local tides, heating,
freshwater, mixing, etc.
low salinity
high salinity
Pingree et al. 1999. Continental
Shelf Research, 19, 929-975.
Souza et al. 2001. Oceanologica
Acta, 24S, S63-S76.
Along-slope flow
constrained to follow
isobaths
1. Shelf seas tend to be isolated from the open ocean.
2. Cross-shelf edge transports will be weak, and require the
breakdown of geostrophy.
Section of salinity south of Cape Cod
(NE USA).
After: Beardsley et al., J. Phys. Oceanogr.,
15, 713-748, 1985.
Useful general reference:
Sharples, J., & J. H. Simpson. 2001. Shelf
Sea and Shelf Slope Fronts. In:
Encyclopedia of Ocean Sciences,
Academic Press Inc., (eds. J. H. Steele, K.
K. Turekian, & S. A. Thorpe), 2760-2768.
Cross-shelf exchange of water is inhibited by the bathymetry.
But, weak cross-shelf exchange is vitally important:
•
Main supply of new nutrients to shelf, fuelling new
primary production.
•
Cross-shelf edge transfer of carbon important in the
global carbon cycle.
•
Recent evidence that shelf edge is used as a migration
highway for fish - slope current can change swimming
behaviour.
References:
Reid, et al.1997. Cross-shelf processes north of Scotland in relation to the
southerly migration of Western mackeral. ICES J. Mar. Sci., 54, 168-178.
Wollast, R.. 2003. In: Ocean Margin Systems (eds. G. Wefer, et al.). SpringerVerlag, Berlin-Heidelberg-New York, 15-31.
Ways to drive cross-shelf edge exchange:
1. Topographic irregularity - flow does not have time to adjust to a sharp change
in bathymetry if its transit time past the change is quicker that the inertial period.
Quantified in terms of the Rossby number of the flow:
V
Ro 
Lf
V = flow speed (m s-1)
L = lengthscale of irregularity (m)
f = Coriolis parameter (s-1)
i.e. R0 > ~ 0.1  non-linear terms are
large enough to allow breakdown of
geostrophy.
Example: Kuroshio
v ~ 1 m s-1; f = 7.2x10-5 s-1; L ~ 50 km
 Ro=0.3
Hsueh, Y., et al, 1996. Journal of Geophysical Research, 101 (C2), 3851-3857.
2. Upwelling by an along-shelf edge wind.
[Northern hemisphere]
wind
coast
sea surface
Ekman transport
thermocline
Rl
R
•
Along-shelf wind stress drives water offshore (to the right, N hemisphere).
•
Bottom water forced by pressure gradient to move onshore.
•
Thermocline pushed upwards.
Useful refs:
Mann & Lazier, chapter 5.
Huyer, J. Mar. Res., 34, 531-546, 1976. [Peru upwelling system].
Sharples, J., & M.J.N. Greig. 1998. New Zealand Journal of Marine and Freshwater Research,
32(2), 215-231. [Episodic upwelling]
3. Downwelling by an along-shelf edge wind.
wind
coast
sea surface
Ekman transport
thermocline
Rl
R
•
Along-shelf wind stress drives water onshore (to the right, N hemisphere).
•
Bottom water forced by pressure gradient to move offshore.
•
Thermocline pushed downward.
Remember: deeper water at the shelf edge tends to have
higher nutrient concentrations. Upwelling/downwelling events
often have associated biological and chemical responses.
4. Upwelling or downwelling by an along-shelf slope current.
At the upper shelf slope, the
nearbed flow “feels” the seabed 
geostrophy breaks down.
Nearbed current has small onshore
(upwelling) component due to bottom
friction
Off shelf, the slope-driven flow is
geostrophic (i.e. slope is balanced
by Coriolis)
Thus we have a possible upwelling mechanism in western boundary current
regions.
Reverse the current along the slope, and the system is favourable to
downwelling.
See: Blackburn & Cresswell, 1993.
Australian Journal of Marine &
Freshwater Research, 44, 253-260.
See also:
Condie, 1995. J. Geophysical
Research, 100, 24,81124,818.
Geyer, 1993. Journal of
Geophys. Research, 98(C1),
955-966.
5. Dense water cascades off the shelf.
Heat loss
Remember:
Upwelling is important as a mechanism for
transferring nutrients onto the shallow shelf,
driving primary production.
Any process that drives water off the shelf
is important as a mechanism for transferring
material (i.e. carbon) off the shelf (and
become either buried in the slope
sediments, or mixed into deeper oceanic
water).
Dense water cascades: a result of shelf
water cooling and becoming denser than
the adjacent oceanic water in winter.
Gravity-driven dense
water flow
Density contrast across shelf edge
~0.01 kg m-3
Cascading water has higher turbidity
(I.e. it contains material from the
shelf)
Clear evidence if cascade in
existence of “healthy” chlorophyll at
500 metres depth.
Hill et al., 1998. Journal of Marine
Research, 56, 87-106.
Yool & Fasham, Global Biogeochem.
Chem. Cycles, 15, 831-844, 2001.
6. The Internal Tide at the shelf edge.
Stratification can support waves within the ocean. These internal waves are
often set up by the interaction between flow and topography, an important
example being the interaction between the barotropic tidal currents and the
shelf edge.
In a simple two-layer system:
Ebb tide
Flow of ebb tide off shelf drags thermocline down at the shelf edge.
As tidal flow decreases, thermocline depression propogates on-shelf (and offshelf) as an internal wave.
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Example:
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Sharples et al., J. Geophys. Res., 106,
14,069-14,081, 2001.
0
20
Cape Brett
36
m
IW1
Latitude S
35.5
IW2
174
178
IW4
IW5
Mokohinau
Islands
36
36.5
174.5
175
175.5
Longitude E
SAR image of internal tidal
wavefronts and associated
solitons.
Temperature (oC)
18
17
16
15
14
336
336.5
337
Time (yeardays 1998)
337.5
338
Example temperature time series influenced by the passage on
internal tidal waves.
Sample period = 1 minute.
Such a mixing source within the water column will have important consequences
for the supply of nutrients to surface waters at the shelf edge. [See Holligan et al.,
1985, Nature, 314, 348-350; Pingree et al., 1982. Cont. Shelf Res., 1(1), 99-116].
Cool band of water at the shelf
edge (elevated nutrients).
Response of primary producers
at the shelf edge.
Shelf edge
The open shelf sea
ROFIs