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.

low salinity

high salinity

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.

Along-slope flow constrained to follow isobaths

Shelf water dynamics controlled by local tides, heating, freshwater, mixing, etc.

Pingree et al. 1999. Continental Shelf Research, 19, 929-975.

Souza et al. 2001. Oceanologica Acta, 24S, S63-S76.

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.

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.

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.). Springer-Verlag, 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:

Lf

VRo

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.

R

Rl

wind

Ekman transport

coast

thermocline

sea surface

[Northern hemisphere]

• 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.

R

Rl

thermocline

wind

sea surfaceEkman transport

coast

• 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.

Off shelf, the slope-driven flow is geostrophic (i.e. slope is balanced by Coriolis)

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

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, 24,811-24,818.

Geyer, 1993. Journal of Geophys. Research, 98(C1), 955-966.

5. Dense water cascades off the shelf.

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.

Heat loss

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 off-shelf) as an internal wave.

174.5 175 175.5Long itude E

36.5

36

35.5

35

La

titu

de

S

36

40

174 178

200 m

IW 5IW 4

IW 2

IW 1

M okohinau Is lands

C ap e B re tt

SAR image of internal tidal wavefronts and associated solitons.

Example:

Sharples et al., J. Geophys. Res., 106, 14,069-14,081, 2001.

336 336.5 337 337.5 338T im e (yeardays 1998)

14

15

16

17

18

Te

mp

era

ture

(o C

)

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.

ROFIsThe open shelf seaShelf edge