Abyssal Canyons and Mixing by Low-Frequency Flow
K. G. Speer and A. M. Thurnherr
Department of Oceanography, Florida State University, Tallahassee, FL, USA
Abstract. The role canyons play in abyssal mixing is discussed, with a
focus on the issue of low-frequency flow over sills and associated overturning
instability as opposed to tidally generated mixing. Flow through several deep
canyons on the mid-ocean ridge is reviewed and some global consequences are
discussed.
Introduction
Most deep mixing, when defined as diapycnal transport, is associated with a meridional overturning cell of
bottom water and deep water. The downwelling part
is entrainment into dense plumes from marginal seas
and shelves, and the upwelling part is the conversion
of the bottom water to deep water, thought to happen predominantly over topography. Mid-ocean ridges
bring the effect of near bottom mixing up to mid-depth,
hence through several thousand meters of abyssal stratification, and one can distinguish stronger diapycnal upwelling in abyssal layers below the ridge crest in the
meridional overturning circulation determined by box
inverse models (Fig. 1). In this example (Lumpkin and
Speer, 2003), the overflow has been included explicitly
in the inversion, and the meridional streamfunction illustrates the dominant diapycnal fluxes due to three effects: surface forcing, overflow, and abyssal upwelling.
What box models do not provide is a mechanism for
this mixing, though regional calculation of upwelling
rates can help to isolate effects, for instance important
passages or sills, or areas of upwelling over ridge flanks.
Below the basin scale, topography includes numerous
structures determined by geodynamic processes, erosion and sedimentation. Since bottom water follows
the deepest path as it fills basins, the morphology of
the mid-ocean ridge is expected to have some impact
on the conversion of this dense water to deep water,
hence on the ocean’s deep thermohaline cell.
Fracture zones and other stress-generated cracks are
common in Earth’s crust; the creation of Earth’s crust
in the spreading process forms axial valleys that are
deep (1000 m or more) on slow spreading ridges like
the Mid-Atlantic Ridge and shallow (10’s of meters) on
fast spreading ridges like the East Pacific Rise. To identify canyon-like structures that might be important for
bottom water flow and mixing we searched bathymetry
data (Smith and Sandwell, 1997) for local zonal and
meridional depth maxima exceeding 250 m (for details
Figure 1. A box inverse model of the N. Atlantic circulation, zonally averaged to display the meridional overturning
streamfunction (black lines and color-coded strength (Sv)).
Bottom Water and Thermocline Water enter the North Atlantic from the south; Deep Water leaves at intermediate
density. The vertical coordinate is neutral density so contours illustrate where diapycnal upwelling or downwelling
takes place. A shallow cell is forced by surface air-sea
fluxes (above the thick white line), a deep cell is driven by
dense overflows and incoming Antarctic Bottom Water below the ridge crest (thick gray line). From Lumpkin and
Speer (2003).
see Thurnherr and Speer, 2003). This reveals the organization of the Mid-Atlantic Ridge flanks into long,
nearly zonal canyons and fracture zones (Fig. 2). Some
are well known fracture zones that form deep gaps in
the ridge, but others are box canyons that do not connect ocean basins. In addition to the Mid-Atlantic
Ridge the flanks of the mid-ocean ridges in the Southern Ocean and Indian Oceans are also characterized by
a large number of canyons. The meridional depth maxima shown in the figure emphasize quasi-zonal canyons;
in the case of zonally trending ridges the ridge-flank
canyons are primarily apparent in the distributions of
17
SPEER AND THURNHERR
18
Figure 2. Zonal-canyon search algorithm results for the
tropical and subtropical South Atlantic Ocean (adapted
from Thurnherr and Speer, 2003); tectonic-plate boundaries
are indicated by solid lines.
Figure 4. Axial bathymetry in the Romanche Fracture
Zone, from Mercier et al.,, 1994.
1800
2000
Depth [m]
2200
2400
I
2600
2800
R
3000
0.05m/s
3200
35.6
35.7
35.8
35.9
36.0 36.1 36.2
Latitude [˚]
36.3
36.4
36.5
36.6
Figure 5. Rift valley bathymetry, hydrography (shaded
contours), flow (arrows), and mixing (“bubbles”); see
Thurnherr et al., 2002 for details.
Figure 3.
Zonal-canyon search algorithm results for
the tropical and subtropical eastern South Pacific Ocean;
tectonic-plate boundaries are indicated by solid lines.
zonal depth maxima.
By contrast, the East Pacific Rise does not show the
same raked appearance and is not the center of high
canyon density (Fig. 3). Presumably this difference
between the ridges is related to spreading rates; the
slow-spreading Mid-Atlantic Ridge is associated with
stronger relief than the fast-spreading East-Pacific Rise.
In regions where there is strong relief on the East-Pacific
Rise it appears more random than that on the MidAtlantic Ridge and long quasi-linear canyons are much
less prevalent. This relationship between spreading rate
and canyon incidence appears to hold over most of the
global mid-ocean ridge system.
Why should canyons be more important than random topographic roughness for bottom water mixing?
What are the key parameters of when canyons are involved? It may be of some use to compare abyssal
canyons to coastal canyons (e.g., Hickey, 1995), which
have been studied more extensively. Abyssal canyons
are typically 10× longer (O(1000 km)), have slopes 10–
100× weaker (O(10−3 )), and show substantial alongaxis depth variations. The latter give rise to multi-
ple sills and sub-basins along the length of the canyon.
Coastal canyons span the full water column and alongcanyon excursions are therefore more strongly limited
by stratification. Abyssal canyons may support flow
along their entire length. We proceed under the assumption that density-driven currents dominate the advection of density along the bottom (c.f. Thurnherr and
Speer, 2003), and where these currents encounter sills,
mixing occurs as stratified water spills over them. This
effect is thought to be magnified in canyons which constrict and amplify flow, raising the Froude number. Because of the complexity of bottom water pathways, however, we are unable to offer more than a rough prescription for predicting which canyons ought to have a more
important role. Essentially, canyons with weak average
bottom slope and more sills will lead to greater mixing
and upwelling.
Processes
As water moves through canyons it spills over sills,
even as it rises up the flank. This process was documented in the Romanche Fracture Zone (Fig. 4; from
Mercier et al., 1994) and the Rift Valley on the MidAtlantic Ridge (Fig. 5; from Thurnherr et al., 2002),
which show a series of basins and sills surrounding the
main (shallowmost) sill. The primary mixing mecha-
ABYSSAL CANYONS AND MIXING BY LOW-FREQUENCY FLOW
19
presumably because there are fewer localized diapycnal sources (as upwelling over topography would be) to
these layers.
Acknowledgments. KGS appreciated the opportunity to confront large-scale results with small-scale physics
at the Aha workshop and the many interesting discussions
with the participants.
References
Figure 6. Axial bathymetry in the Chain Fracture Zone,
from Mercier et al., 1994.
nism in both cases is shear-induced overturning with
the shear generated by accelerating flow across the
sills. The flow may be modulated by oscillations at
various time scales (eddies, basin modes, wind-driven
barotropic, tidal), but we distinguish this case from a
system with tidal forcing alone, driving relatively small
(e.g. compared to size of a sill) excursions across the
topography. We hypothesize that this “low-frequency”
mechanism is generic to long abyssal canyons and important to bottom water transformation in the Atlantic
Ocean and elsewhere.
The Chain Fracture Zone is an interesting case because it combines a role as a major conduit with more
of a box-canyon form (Fig. 6). Mean flow is strongest
500 m off the bottom, more or less at the exit sill depth
near 4050 m (Mercier and Speer, 1998). Although the
mean flow decreases with depth into the canyon, both
high and low passed ( 48 hr) variance increases, consistent with observations of flow in other canyons and
the notion of enhanced internal wave activity focussed
by canyon walls.
Conclusions
The evidence supports the view that mixing in major and minor abyssal canyons is fundamental to bottom water upwelling and the dense thermohaline circulation cell. The question remains of the partition
between diapycnal upwelling driven by tidally forced
mixing and lower frequency components. This partition likely varies from one density horizon to another,
depending on the kind of topography a given layer intersects, and the distribution of sills around a basin. One
of the most telling indications of a large-scale difference
between the Pacific, where canyon density is lower, and
the Atlantic, where it is higher, may be the distribution
of large-scale potential vorticity in the near-bottom water (O’Dwyer and Williams, 1997). They find much
more horizontally uniform distribution in the Pacific,
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K.G. Speer and A.M. Thurnherr, Department of
Oceanography, Florida State Univ., Tallahassee, FL,
32306 (e-mail: [email protected];
[email protected])
This preprint was prepared with AGU’s LATEX macros v4,
with the extension package ‘AGU++ ’ by P. W. Daly, version 1.6b
from 1999/08/19.