Meghan_Rochford_Thesis

Canyons: Conduits of the shelf?
Meghan Rochford
MSc Physical Oceanography
Supervisors:
Dr. Mattias Green (SOS)
Dr. Joanne Hopkins (NOC)
Author contact details:
E-mail: meghanrochford@gmail.com
Address: 97 Grattan Lodge, Balgriffin, Dublin 13, Ireland.
Submitted September 2015

Canyons: Conduits of the Shelf?
i
Declaration
Statement 1
This work has not previously been accepted in substance for any degree and is not being
concurrently submitted in candidature for any degree.
Statement 2
This dissertation is being submitted in partial fulfilment of the requirements for the degree of
M.Sc. in Physical Oceanography.
Statement 3
This dissertation is the result of my own independent work/investigation except where
otherwise stated.
Statement 4
I hereby give consent for my dissertation, if accepted, to be available for photocopying and
for interlibrary loan, and for the title and summary to be made available to outside
organisations.
Signed: (candidate)
Date: 14/09/2015

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Acknowledgements
I would like to thank my supervisors Dr. Mattias Green and Dr. Joanne Hopkins (NOC) for their
continued support and guidance throughout this thesis.
Thank you to all my fellow PO classmates, who were always there to answer questions and
listen to concerns.
I would also like to thank those who carried out the research cruise to gather the data this
thesis is based on: Prof. Mark Inall (SAMS), Dr. Marie Porter (SAMS), Dr. Claire Mahaffey
(University of Liverpool), the scientific team, the captain and crew of the RRS Discovery cruise
to the Celtic Sea shelf break in June 2012.
I am grateful to my parents for their encouragement throughout my academic journey, and
to the EOS girls for encouraging me to leave San Francisco early to undertake this MSc course.
Finally, thank you Edward for your advice and motivation during the final few months of the
course.

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Abstract
Shelf seas are important both physically and biogeochemically. It has long been determined
that topography is the controlling factor of shelf sea-open ocean exchange. Submarine
canyons are therefore very important when determining the cause and effect of physical and
biogeochemical processes along the shelf edge. Flow within a canyon has its own unique
characteristics due to current velocity magnification, causing it to become more energetic,
thus increasing mixing, and therefore enhancing upwelling or downwelling. The three phases
of a typical upwelling/downwelling scenario are the first initial transient phase, secondly an
advection-dominated phase, and finally a third relaxation phase. This thesis aims to
determine the phase in which the Petit Sole Canyon, located along the Celtic Sea shelf break,
was operating during a two week research cruise on board the RRS Discovery in June 2012,
using temperature, salinity and current data. The salinity results show that there was
enhanced upwelling during the period of data collection, while the current velocity results
show that the slope current was flowing in a southerly direction. This reverse in the slope
current direction from the ‘normal’ north flowing current, combined with the enhanced
upwelling show that the canyon was in phase two, the advection-dominated phase, of Allen's
(2004) upwelling/downwelling scenario. The direction of flow through the canyon is contrary
to previous literature, with the upwelled water flowing from the upstream side of the canyon,
and downwelling occurring at the downstream side of the canyon.

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Table of Contents
Declaration...................................................................................................................................i
Acknowledgements......................................................................................................................ii
Abstract...................................................................................................................................... iii
List of Figures.............................................................................................................................. vi
List of Tables............................................................................................................................. viii
Abbreviations and Symbols......................................................................................................... ix
Chapter 1: Introduction................................................................................................................1
1.1 Rationale .......................................................................................................................................1
1.2 The Northwest European Shelf and the Celtic Sea Shelf ..............................................................2
1.3 Canyons.........................................................................................................................................5
1.3.1 Upwelling ...............................................................................................................................7
1.3.2 Dense Water Cascades...........................................................................................................8
1.3.3 Internal Waves.......................................................................................................................9
1.4 Aims and Objectives....................................................................................................................10
1.5 Expected Results.........................................................................................................................11
Chapter 2: Methodology ............................................................................................................ 12
2.1 Observations...............................................................................................................................12
2.2 Data Analysis...............................................................................................................................14
2.2.1 Meteorological Data ............................................................................................................14
2.2.2 Temperature and Salinity.....................................................................................................14
2.2.3 Acoustic Doppler Current Profilers ......................................................................................14
2.2.4. Scanfish ...............................................................................................................................15
2.2.5 Gliders..................................................................................................................................15
Chapter 3: Results...................................................................................................................... 16
3.1 Meteorological and Tidal Data....................................................................................................16
3.2 Gliders.........................................................................................................................................19
3.3 Slope Current & Direction...........................................................................................................22
3.4 CTD & ADCP.................................................................................................................................25
3.4.1 Undulating CTD ....................................................................................................................25
3.4.2 LADCP...................................................................................................................................27
3.4.3 Spatial Survey.......................................................................................................................28
3.5 Scanfish.......................................................................................................................................30
3.5.1 Repeat Canyon.....................................................................................................................30
3.5.2 Zigzag Canyon ......................................................................................................................33

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Chapter 4: Discussion................................................................................................................. 37
4.1 Principle relationships shown by results.....................................................................................37
4.1.1 Current Velocity and Direction ............................................................................................37
4.1.2 Salinity and Temperature Profiles........................................................................................40
4.1.3. Nutrient Fluxes....................................................................................................................41
4.2 Comparison with previous literature..........................................................................................42
Chapter 5: Conclusion ................................................................................................................ 44
References ........................................................................................................................................45
Appendices........................................................................................................................................48

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List of Figures
Figure Description Page
1.1 Schematic showing the different physical processes which occur in a
shelf sea and at the shelf break.
2
1.2 Map of the Northwest European Shelf. 3
1.3 Schematic showing the flow of water through a submarine canyon. 7
1.4 Schematic showing flow at different depths through a canyon. 8
1.5 Map showing the locations of all instrument deployments, and the
location of the site along the Celtic Sea shelf break.
11
3.1 Absolute wind speed, direction and stress from the 12th June to 1st July
2012.
17
3.2 Barotropic tidal cycle at LT1 from 23rd June to 3rd July 2012. 18
3.3 Barotropic tidal cycle at ST4 from 17th to 28th June 2012. 18
3.4 Salinity profiles of the Glider data along the Celtic Sea shelf break. 20
3.5 Temperature profiles of the Glider data along the Celtic Sea shelf break. 21
3.6 Time and depth averaged north-south current velocities from LT1. 23
3.7 Time and depth averaged east-west current velocities from LT1. 23
3.8 Along- and across-slope currents from the 30th June 2012 to the 7th
March 2013, recorded at LT1.
24
3.9 Salinity profile of the undulating CTD from 29th to 30th June 2012. 26
3.10 Temperature profile of the undulating CTD. 26
3.11 Averaged across- and along-slope velocities from each of the 12 LADCP
casts.
28
3.12 Nitrate plots from the four CTD casts (38-41). 29
3.13 Salinity, temperature and density plots for the four CTD casts (38-41). 29
3.14 Salinity profiles of the nine repeat Scanfish transects, from 23rd to 24th
June 2012.
31
3.15 Temperature profiles of the nine repeat Scanfish transects, from 23rd
to 24th June 2012.
32

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3.16 Temperature and salinity profiles of T8 from the zigzag Scanfish canyon
survey, from 25th to 26th June 2012.
34
3.17 Salinity profiles of the seven zigzag Scanfish transects. 35
3.18 Temperature profiles of the seven zigzag Scanfish transects. 36
4.1 Across- and along-shelf current velocities recorded by the LACDP for a
13 hour period from the 29th to 30th June 2012.
39
4.2 Schematic showing the different flow regimes for the Astoria and
Barkley Canyons and the Petit Sole Canyon.
43

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List of Tables
Table Description Page
2.1 List of instruments, their locations, and the start and finish time of
deployment.
13

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Abbreviations and Symbols
DW Dense Water
CTD Conductivity, Temperature, and Depth
LADCP Lowered Acoustic Doppler Current Profiler
FASTNEt Fluxes Across the Sloping Topography of the North East Atlantic
SAMS Scottish Association for Marine Science
NOC National Oceanographic Centre
BODC British Oceanographic Data Centre
UCTD Undulating CTD
VMADCP Vehicle Mounted ADCP
τ Wind stress
ρ Density of air
cd Drag coefficient
u10 Absolute wind speed
x Across-slope current
y Along-slope current
u East-west velocity
v North-south velocity
θ Angle of canyon relative to north
NADW North Atlantic Deep Water
MIW Mediterranean Intermediate Water

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Chapter 1: Introduction
1.1 Rationale
The coastal and open ocean have their own diverse physical and biological processes. The
region where they meet, the shelf edge, is therefore a dynamic and unique environment.
Exchange between the open ocean and shelf seas has important consequences for shelf sea
currents, flushing and the supply of nutrients from the deep ocean to surface waters
(Huthnance et al. 2009), which in turn, have consequences for phytoplankton production
(Rees et al. 1999). This exchange, and subsequent shelf sea processes, are believed to have
some effect on open ocean circulation patterns in ocean basins and mixing over slopes. This,
in turn, effects the overall density structure of the water column (Munk & Wunsch 1998). It
has long been understood that topography is the dominant factor steering flow along the
continental slope. It constrains geostrophic flow to flow along isobaths, rather than across
them. This forces along-slope flow, not across-slope flow (Huthnance et al. 2009). However,
at the equator this constraint is less severe, and more relaxed (Huthnance 1995). This allows
for an increase in exchange between deep ocean waters and surface waters. Carbon and
nutrient exchange between shelf seas and the open ocean is important for both the carbon
and nutrient cycles, however it has not been sufficiently quantified (Huthnance et al. 2009).
Figure 1.1 shows the main environmental physical processes which effect shelf sea-open
ocean exchange; these are wind stress, tidal currents, stratification, and shelf geometry.

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Figure 1.1. Schematic showing the different physical processes which occur in a shelf sea and at the
shelf break (Huthnance et al. 2009).
1.2 The Northwest European Shelf and the Celtic Sea Shelf
The Northwest European Shelf encompasses the Hebrides and Malin shelves, the English
Channel, the Irish Sea, the Celtic Sea, and Irish Shelf (Figure 1.2). It is approximately 2000 km
long, located at the eastern side of the North Atlantic Ocean, from the Amorican Shelf in the
south, up to the North Sea in the north (46-60°N). Due to the presence of the British Isles, the
region experiences a range of complex topography.
The region experiences strong tidal forcing at the ocean boundary (Simpson 1998; Egbert et
al. 2010). The English Channel, Irish Sea and Bristol Channel are characterised by strong tidal
responses, with the largest ranges occurring on the eastern side of the basins. The largest
tidal ranges (>8 m at M2 tides) have been recorded near the port of St. Malo, at the head of
the Bay of Seine, and Avonmouth, Bristol (Simpson 1998).
The M2 tide enduces frictional stresses at the seabed, over much of the region, with a
maximum stress of 0.25 Pa, which is the equivalent of a sea surface wind stress of 13 m/s. In
the Irish Sea, extreme stresses have been recorded at 4 Pa, which is on the scale of hurricane

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force wind stresses. Due to this, the European Shelf is believed to cause approzimately 12.5%
of the global tidal energy loss (Simpson & Bowers 1981).
Figure 1.2. Map of the
Northwest European Shelf,
with the locations of the Celtic
Sea, Irish Sea, Irish Shelf,
English Channel, the Hebrides
and Malin Shelf (200m depth
contours, red lines separating
shelves) (Huthnance et al.
2009).
The area is subject to strong seasonal forcing, with surface heating and cooling changing the
structure of the water column. In regions with strong tidal flows (the English Channel and
eastern Irish Sea) the water column is continually mixed, while regions such as the Celtic Sea
and Hebridean Shelf experiences strong seasonal stratification. In the North Channel,
between the Irish Sea and the Malin Shelf, there is complete vertical homogeneity to a depth
of 200 m, due to a strong tidal current of 1.5 m/s at spring tides (Simpson 1998).
The Celtic Sea is a 500 km wide (approximately), 100-200 m deep shelf sea with a highly
dynamical environment (Figure 1.2; Huthnance et al. 2009; Green et al. 2008). It has a large
tidal energy input originating from the Atlantic Ocean, and is characterised by strong tidal
currents, which are known to be the dominant source of mechanical energy (Simpson 1998).
The area is subject to strong seasonal variations in surface heating and cooling. Freshwater
supply to the area is limited, meaning stratification is dominated by temperature (Huthnance
et al. 2009; Heathershaw et al. 1987). This stratification becomes established over summer,
where buoyancy input out-competes stirring by wind and tidal stirring.

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At the shelf edge, internal tides generate internal waves due to the forcing of the barotropic
tide. It forces the water column up and down the steep slope, generating waves (Sharples et
al. 2007). This also produces a baroclinic energy flux (Green et al. 2008). These internal waves
cause mixing and diffusion across the thermocline, bringing cooler, deeper water to the near
surface. This water is then exposed by wind mixing (Figure 1.1). These processes lead to a
large ocean-shelf exchange of 3 m2/s (Huthnance et al. 2009).
North Atlantic water forms a poleward slope current, that is warm and saline, flowing along
the continental slope from Portugal, past Ireland to Norway (Cooper 1952). This barotropic
current is centred at approximately 500 m on the slope (Cooper 1949; Pingree & Le Cann
1989; Huthnance et al. 2009). The depth of the slope current is suggested to be forced by the
dynamic height of warmer subtropical waters (Huthnance 1984). Below this current is the
bottom Ekman layer, where the current is reduced to zero, due to friction. Off-shore Ekman
transport in the region is believed to be of the order of 1 m2/s (Huthnance 1995).
In the Celtic Sea, low-frequency circulation is generally weak, except at the upper slope, and
when channelled through topographic features (e.g. canyons). This localised exchange is
equal to the slope current transport (of order of 1 Sv.) (Huthnance et al. 2009). The
discontinuous coastline allows wind-driven flow eastward through the English Channel (0.1
Sv.: Prandle et al. (1996)) and northward through the Irish Sea (0.1 Sv.: Knight & Howarth
(1999)). At the shelf edge, tidal currents exceed 0.5 m/s, with tidally reflected flow reaching
0.1 m/s (Huthnance et al. 2009). Internal tides are particularly strong in the region of the shelf
edge. At spring tide they have wavelengths >50 m, and propagate as decaying waves both off-
and on-slope. This means that off-shore exchange occurs in wave-form of approximately 1.3
m2/s (Huthnance et al. 2001).
The enhanced mixing that is associated with internal waves, causes a flux of nutrients and
chlorophyll over the thermocline, associated with a cooler band of water (Sharples et al. 2007;
Green et al. 2008; Huthnance et al. 2009). This flux of nutrients allows the subsurface
chlorophyll maximum to be sustained; primary production is therefore localised. It has also
been documented that there is an increase in chlorophyll near seamounts and banks, possibly
due to intensified mixing (Green et al. 2008).

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1.3 Canyons
Submarine canyons are abundant features along continental shelves to deep ocean basins.
Shepard & Dill (1966) mapped just 96 major submarine canyons globally. Using modern
satellite bathymetry data, more than 660 submarine canyons have been mapped globally (De
Leo et al. 2010). These are areas of enhanced cross-shelf exchange, primarily because they
are regions of large Rossby numbers. In such regions, the effects of planetary rotation are
secondary to the effects of advection of momentum. Because canyons are much smaller than
the surrounding slope, the Rossby number gets increasingly larger (Wåhlin 2002; Allen 2004).
Canyons are features with complex topography, hydrography, flow, and sediment transport
and accumulation. Because of these complexities, they are known for their distinctive
characteristics, such as accelerated currents, enhanced mixing, and dense water cascades.
These can be forced by topographic or climatic forcings (Klinck 1996; Mulder et al. 2012).
Submarine canyons are important features for physical and biogeochemical reasons. They
have been observed to be hotspots for enhanced upwelling and downwelling, allowing for an
increased exchange between shelf waters and the open ocean (Allen & Durrieu de Madron
2009; Allen & Hickey 2010).
Geostrophic flows cannot cross isobaths, restricting cross-shelf flows and forcing along-slope
continental slope flows. Due to geostrophy, this causes an across-slope pressure gradient
(Allen & Durrieu de Madron 2009). In a submarine canyon, flow cannot be along-slope due to
the restrictions of the canyon walls. This means that the Coriolis force cannot balance the
pressure gradient force, allowing for flow through the canyon (Freeland & Denman 1982).
Therefore, flow is dominated by the pressure gradient at the canyon rim. There is no direct
impact on the near surface flow. Canyon flow can be broken down into two types (Allen &
Durrieu de Madron 2009):
1. A wind-driven shelf-break or slope current, with the strongest effect felt at the canyon
rim.
2. On-shelf deep water formation with an equally strong cross-slope pressure gradient.
However, this water cascades deep into the canyon, making it independent of the
wind-driven flows.

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A typical upwelling or downwelling event can be divided into three main phases (Allen &
Durrieu de Madron 2009):
1. An initial transient phase;
2. A near steady advection-dominated phase;
3. A relaxation phase.
The first phase (the initial transient phase) is a time-dependent response, as the shelf-break
flow increases. It is generally quite strong and occurs quickly, normally within an inertial
period (Allen & Durrieu de Madron 2009). If there is a steady wind, causing the along-current
to continue, density advection within the canyon reduces the time dependent upwelling after
about five days (She & Klinck 2000). It is essentially linear, with similar responses for both
positive and negative flows (see Figure 1.3).
The second phase (the advection-dominated phase) is not linear, and therefore more
complicated. It is dependent on the canyon topography and flow strength. This phase occurs
when the shelf-break flow is reasonably steady. In this phase, upwelling is generally stronger
than downwelling (She & Klinck 2000). Upwelling is driven by negative flows (Figure 1.3), thus
opposing the shelf waves and arresting them, leading to strong across isobath flow.
Downwelling is driven by positive flows, moving in the same direction as wave propagation,
allowing along-isobath flows to be established around the canyon and onto the shelf (Allen &
Durrieu de Madron 2009).
Oscillatory flows over the canyon have been suggested to create mean flow over the canyon,
due to the asymmetry of upwelling and downwelling. In the positive phase, the flow leaves
the canyon via the downstream wall, having diverged from the upstream wall (Figure 1.3). In
the negative phase, flow follows the upstream wall into the canyon, and leaves via the
downstream wall (Allen & Durrieu de Madron 2009).
In the final phase, the relaxation phase, shelf-break flow reduces (Allen & Durrieu de Madron
2009). Hickey (1997) suggested that upwelled water leaves the canyon laterally in this phase,
rather than horizontally.

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Figure 1.3. Schematic showing the
flow of water through a submarine
canyon. The negative phase
(upwelling conditions) is flow in the
opposite direction of Kelvin wave
propagation, while the positive
phase (downwelling conditions) is in
the same direction as wave
propagation. The hashed line on the
right represents the coastline, and
the black line in the centre presents
an isobath, highlighting the shape of
the canyon. Edited from Allen &
Durrieu de Madron (2009).
1.3.1 Upwelling
Submarine canyons are important mechanisms for coastal upwelling, with a high
concentration of zooplankton seen around them (Allen et al. 2001). However, there is a
difference in upwelling between short canyons and long canyons. Short canyons are those
that the head of the canyon reaches the continental slope, long before it reaches the
coastline, for example, Astoria (off the west coast of the USA) and Barkley canyons (west of
Vancouver Island; Hickey 1997; Allen 2000). In a long canyon, the head of the canyon does
not reach the continental slope before the coastline, but rather it extends far into the coastal
region, usually into an estuary (Hickey 1995; Allen 2000). Examples of long canyons are Juan
de Fuca, Mackenzie and Monterey canyons (Waterhouse et al. 2009). Flow in short canyons
has been studied and documented along the west coast of North America, while long canyons
remain largely unstudied.
In short canyons, as a geostrophic flow passes over the canyon, water is driven up the canyon.
This occurs due to a pressure gradient imbalance caused by restrictions in the topography
(Freeland & Denman 1982). This imbalance is what causes enhanced mixing and upwelling
(Hickey 1995). Water columns, originating upstream of the canyon, flow over the top,
becoming stretched. This is due to an increase in bottom depth downstream of the canyon
rim. This stretching creates a cyclonic vorticity in the flow (Hickey 1997; Allen 2004). This has
been linked to flow separation at the mouth of the canyon, which is then advected into the

8
canyon. The flow then turns towards the canyon head and is advected onto the shelf. Due to
vortex stretching, a cyclonic eddy is formed from the shelf break down to a depth in the
canyon mouth (She & Klinck 2000). Flow above the canyon (<100 m) does not feel the effects
of the canyon, except for a possible elevation of isopycnals (see Figure 1.4 for all flow
scenarios; Hickey 1997; Allen 2004; Waterhouse et al. 2009).
Figure 1.4. Schematic showing the different depth-dependent flows through a canyon: 1) the surface
flow; 2) flow just above the canyon rim; 3) flow at the canyon-rim depth; and 4) deep water flow (Allen
2004).
1.3.2 Dense Water Cascades
Dense Water (DW) cascades form on the shelf, where water is cooled and cascades down the
slope. Submarine canyons have been documented to be conduits for this process (Allen &
Durrieu de Madron 2009). DW cascading contributes to the ventilation of intermediate and
deep water of the open ocean, which has a substantial impact on biogeochemical cycles. The
effect of canyons on DW cascades varies with the length, width (Wåhlin 2004) and orientation
(Chapman 2000) of the canyon, as well as the topographic features present (Wåhlin et al.
2008). In a uniformly sloping shelf with a canyon cutting into it, a portion of DW will cascade
into the canyon, forming a plume flowing offshore along the right side of the canyon axis. The

9
formation of eddies, due to a density front, have been documented to slump into the canyon,
disrupting this DW plume (Chapman & Gawarkiewicz 1995). Dependent on the magnitude of
the along-slope current, DW has been observed to cascade through the canyon, with an
accumulation of this DW in the canyon (Wåhlin 2002).
The length and width of canyons was studied to determine its influence on DW cascading
(Wåhlin 2004). It was determined that the transport capacity of deep channels was larger
than that of shallow channels. When gently sloping topography was used in the model, there
was a maximum downward flow through a wide canyon (>10 km), however, steeper regions
were the most active, when the canyon was a few km wide. The influence of the overall shape
of the canyon (v-shaped) with respect to different flow regimes and topographic features was
also studied (Wåhlin et al. 2008). They determined that small scale topography has a bigger
influence on mixing than large scale topography.
Three different canyon orientations were studied to determine any influence on DW cascades
(Chapman 2000): normal, diagonal and parallel to the shelf. It was found that little DW enters
the normal and diagonal canyons. This is due to the fact that along-slope flow follows
isobaths, and cannot flow down into the canyon. In regards to the parallel canyon, there was
a higher portion of the flow in this canyon. The amount of DW in the canyon was dependent
on the rate of flow over the canyon. A slower along-slope flow meant that there was more
DW in the canyon, due to the reduced speed and therefore increased meandering. It was also
found that DW cascading has shown to induce localised upwelling of deep water onto the shelf
(Kämpf 2005).
1.3.3 Internal Waves
Upwelling and DW cascading are both processes of advection, which cause the movement of
water through the canyon. Another process of transport is the mixing of deep water, due to
tides. Submarine canyons act as conduits for deep ocean water further onto the shelf then if
it were of uniform length. This means that canyon heads of canyons is are areas of enhanced
tidal mixing (Allen & Durrieu de Madron 2009).
Long canyons are particularly strong areas of enhanced tidal mixing. This is due to them
stretching from the shelf break, along the slope and up to large estuaries. Deep ocean water
circulates in these estuaries, and with the axis and head of the canyon being subjected to

10
large tidal currents, mixing is enhanced. Long canyons are therefore areas of strong nitrate
concentrations (Allen & Durrieu de Madron 2009).
In long canyons that do not reach into large estuaries, tidal currents do not penetrate into the
canyon, but rather across the canyon, parallel to the shelf-break (Allen & Durrieu de Madron
2009). It has been documented that many of these canyons are areas of very high internal
tidal energy, such as Hydrography Canyon (Wunsch & Webb 1979) and Monterey Canyon
(Kunze et al. 2002). They are areas of enhanced tidal and internal wave energy due to focusing
(Wunsch & Webb 1979) or due to them being large regions of critical slope (Hotchkiss &
Wunsch 1982). It has also been suggested that small scale roughness causes enhanced mixing
(Wåhlin et al. 2008) and internal tidal energy (Kunze et al. 2002). Diffusivity values in such
canyons are very large at the canyon axis (0.05 m2/s), with the canyon rim have values just a
factor of ten smaller, recorded in the Monterey Canyon (Kunze et al. 2002).
1.4 Aims and Objectives
The Petit Sole Canyon is located along the Celtic Sea shelf, northwest of France (Figure 1.5).
It is approximately 5-10 km wide, and 15-20 km long. It is therefore characterised as a short
submarine canyon, similar to the Astoria and Barkley Canyons on the west coast of North
America.
The aim of this thesis is to determine the mechanism(s) which enhance upwelling and
downwelling within the Petit Sole Canyon, and determine what phase the canyon is in, during
data collection, in relation to Allen's (2004) three phases. The phase of the canyon has
important consequences for the nutrient cycle; enhanced upwelling brings nutrient rich water
to the surface, allowing for a ‘bloom’ in phytoplankton, while a downwelling scenario would
cause nutrient limited water to be replenished in the open ocean.
To achieve these aims, data collected, during a two week cruise, were processed and
analysed. This data included both spatial and temporal Conductivity, Temperature and Depth
(CTD) measurements within the canyon, attached with a Lowered Acoustic Doppler Current
Profiler (LADCP), two Scanfish surveys through the canyon and onto the shelf, long and short
term moorings, and a six transect Glider survey (Figure 1.5).

11
Figure 1.5. Map showing the locations of all instrument deployment, and the location of the site
along the Celtic Sea shelf break.
1.5 Expected Results
According to Allen & Durrieu de Madron (2009) there are three phases of an
upwelling/downwelling event, and since the general trend of the Celtic Sea slope current is
northerly, it is expected that the results show one of the three phases, in a negative direction,
i.e. downwelling is the more prominent process occurring. The likelihood is that the
combination of the canyon and a northerly flowing slope current are forcing enhanced
downwelling. However, if the current is flowing in the opposite direction (i.e. southerly) this
would alter the regime and cause enhanced upwelling through the canyon, according to Allen
et al. (2001) and Allen & Durrieu de Madron (2009). The exact phase in which the canyon is
acting in, during the cruise, cannot be commented on, without looking at data collected.

12
Chapter 2: Methodology
2.1 Observations
All data were collected during cruise 376 of the Royal Research Ship Discovery, from 11th June
to 1st July 2012. This cruise was part of the NERC funded FASTNEt (Fluxes Across the Sloping
Topography of the North East Atlantic) consortium led by The Scottish Association for Marine
Science (SAMS). Researchers from SAMS, Bangor University, the National Oceanographic
Centre (NOC), University of Liverpool, and the British Oceanographic Data Centre (BODC)
were responsible for operating the instruments used.
The aim of the cruise was to investigate the internal tide and its contribution to cross-shelf
exchange. However, a submarine canyon was found during the start of the cruise (Figure 1.5).
Data were collected within this canyon towards the end of the cruise; this data, along with
some longer term cruise data will be presented in this thesis. The location, timing and
duration of instrument deployment is displayed in Table 2.1. Meteorological data were
collected using the ships own central logging system, for the duration of the cruise. CTD
profiles were made at four locations through the canyon (Figure 1.5). An undulating CTD
(UCTD) and LADCP were used to gather temporal (15 hours) data mid-way through the
canyon. A vehicle mounted ADCP (VMADCP) was used to recorded current measurements for
the duration of the cruise. Two Scanfish transects were carried out. The repeat survey was
located towards the head of the canyon; and the zigzag survey extended from the canyon
onto the continental shelf (Figure 1.5). The data from two moorings were used for this thesis:
LT1 was used to look at current velocities on the continental slope, while ST4 was used to look
at current velocities on the continental shelf. Glider data were collected at after the cruise,
on the continental shelf, above the heads of the Petit Sole and Grand Sole Canyons.

13
Table 2.1. List of instruments, their locations, and the start and finish time of deployment.
Data Source Location Start Finish Julian Day
Meteorological
Data
Ship Track 12/06/12 01/07/12 163-182
ST4 48°39.904’N
009°06.358’W
15:03
16/06/12
15:35
28/06/12
166-179
LT1 48°04.500’N
009°44.400’W
18:08
22/06/12
07/03/13 173-66
Scanfish Repeat Ship track
(Figure 1.5)
10:17
23/06/12
01:27
24/06/12
174-175
Scanfish Zigzag Ship track
(Figure 1.5)
11:13
25/06/12
04:38
26/06/12
176-177
CTD 38 48˚ 26.226’N
009˚ 26.303’W
21:03
24/06/12
21:37
24/06/12
175
CTD 39 48˚ 24.386’N
009˚ 32.366’W
22:22
24/06/12
23:30
24/06/12
175
CTD 40 48˚ 22.155’N
009˚ 36.650’W
00:38
25/06/12
01:53
25/06/12
176
CTD 41 48˚ 20.530’N
009˚ 42.910’W
03:14
25/06/12
04:43
25/06/12
176
LADCP/UCTD 48°09.19’N
009°37.46’W
13:01
29/06/12
04:41
30/06/12
180-181
Gliders Six Transects
(Figure 1.5)
12:15
28/07/12
14:07
14/08/12
210-227

14
2.2 Data Analysis
2.2.1 Meteorological Data
The absolute wind speed and direction (AbsWindSpd, AbsWindDir) were output every ten
seconds (as opposed to every one second). This is due to the routine pro_wind, which
calculates the absolute wind, relying on output from the programme bestnav, which is only
available every 10 seconds. This data were used to calculate wind stress during the cruise
(Matlab script Met.m). The wind equation used was:
𝜏 = 𝜌 ∗ 𝑐𝑑 ∗ 𝑢102
Equation 2.1
Where ρ is the density of air (1.25 kg/m3), cd is the drag coefficient, and u10 is the absolute
wind speed.
2.2.2 Temperature and Salinity
The CTD was calibrated at the beginning of the cruise, and during the middle of the cruise.
Following the cruise the raw data were processed. This processed data has been used in this
thesis. The Matlab script CTD_Code.m was used to further process and analysis the
temperature and salinity data from the UCTD.
There was also a spatial survey completed through the canyon to determine changes in
density, salinity and temperature. There were also sampling bottles attached to the CTD, used
to look at nitrate levels in the water. Processing and analysis of this data were done in Matlab
script Nutrients.m.
2.2.3 Acoustic Doppler Current Profilers (ADCP)
A VMADCP was fitted to the RRS Discovery, with 75 kHz and 150 kHz frequency Ocean
Surveyor ADCPs. These measured the relative velocities, and where able, bottom tracking was
enabled. The bin sizes of both ADCPs was different: OS75’s bin size was 8 m, while OS150’s
bin size was 4 m. For the sake of this thesis the VMADCP data was not used as it was too noisy.
The LADCP was attached to the CTD frame and data were obtained from every cast. The
LADCP used was a 300 kHz RDI ‘Workhorse’ LADCP. The data collected from the UCTD allowed

15
for the creation of a velocity time series. The Matlab function residual.m was used to remove
the barotropic tidal signal, and the Matlab script LADCP_2.m was used to further process and
analyse the velocity data. Within this code, the direction of flow was manipulated from east-
west and north-south, to along-/across-slope, using Equations 2.2 and 2.3.
𝑥 = 𝑢𝑐𝑜𝑠𝜃 + 𝑣𝑠𝑖𝑛𝜃 Equation 2.2
𝑦 = −𝑢𝑠𝑖𝑛𝜃 + 𝑣𝑐𝑜𝑠𝜃 Equation 2.3
Where u is the east-west velocity, v is the north-south velocity and θ is the angle of the canyon
relative to north.
The moorings LT1 and ST4 also had an ADCP attached to each of them. The purpose of this
was to determine the slope current velocity over a long period (LT1) and to determine
velocities on the continental shelf (ST4). During processing, both stations had the barotropic
tidal signal removed. Further processing was done in Matlab scripts LT1.m and ST4.m.
2.2.4. Scanfish (Repeat and Zigzag)
The Scanfish data were obtained from the FSI CTD unit, Haardt Fluorometer and AANDERAA
CTD sensors. The CTD data collected was then combined with the ships own navigation data
to accurately map the vertical distribution of the water parameters, using the ship as a
reference. The first survey, in this thesis, was the repeat survey. This was positioned towards
the head of the canyon. The Matlab script M_Sc_ Scanfish_canyongrid.m was used to further
process and analyse this data. The final Scanfish survey was the zigzag survey, which extended
from the mouth of the canyon, through the canyon, and onto the continental shelf. This was
processed and analysed in the Matlab script Scanfish_zigzag.m.
2.2.5 Gliders
Glider data (temperature, salinity and density) was collected after the cruise, along the
continental shelf break, just above the head of both the Grand Sole Canyon and the Petit Sole
Canyon. Data processing and analysis were done in the Matlab script Gliders_2.m.

16
Chapter 3: Results
3.1 Meteorological and Tidal Data
Meteorological data was gathered from the 12th June to 1st July 2012 (Figure 1.5, Table 2.1),
using the ships own central logging system. Although the cruise was during summer, two
storms occurred during this time. The first, from day 166 to 168 (14th-16th June), was the more
severe of the two. Winds reached speeds up to 24 m/s, with wind stresses reaching as high
as 1.5 N/m2 (Figure 3.1). The second storm occurred from day 175 to 176 (24th-25th June)
closer to the collection of canyon data. Wind speeds reached up to 20 m/s, while wind
stresses only reached 1 N/m2 during this storm. However, in the case of both storms, wind
direction was predominantly around 200° (south-southwest).
Tidal currents were recorded at LT1, and show that during a ten day period of the cruise, the
tidal cycle moved from a spring tide at the beginning, to that of a neap tide towards the end
(Figure 3.2). Current velocities changed from -0.05 m/s to 0.05 m/s during the spring cycle to
-0.04 m/s to 0.04 m/s during the neap cycle, in the east-west direction. The opposite occurred
for the north-south tidal currents, in that they were of weaker strength (-0.03 m/s to 0.03
m/s) at the beginning of the cruise and became strong (-0.05 m/s to 0.05 m/s) towards the
end. Figure 3.3 shows the tidal currents recorded at station ST4, which is located on-shelf.
Here it can be seen that the tidal currents are an order of magnitude larger than those of LT1.
It can also be noted that the north-south currents are more dominant, while at LT1 the east-
west currents are more dominant. This is due to the tidal currents moving on-shore, across
the shelf-break at LT1. At ST4 the tidal currents are not largely constrained by topography,
and therefore have more freedom to move north-south.

17
Figure 3.1. Top: absolute wind speed for the period of the cruise (12th
June to 1st
July 2012). Middle: measure of the absolute wind direction (direction in
which the wind is blowing from). Bottom: wind stress from the same period.

18
Figure 3.2. Barotropic tidal cycle for a ten day period from the 23rd
June to the 3rd
July 2012 at LT1.
Figure 3.3. Barotropic tidal cycle for an 11 and a half day period from the 17th
to the 28th
June 2012 at
ST4.

19
3.2 Gliders
Gliders were deployed from the 28th of July 2012 until the 14th of August 2012 (Table 2.1).
They were deployed parallel to the slope, cutting across two canyons (Figure 1.5). Salinity
measurements were recorded along these transects to a depth of 180 m (Figure 3.4). The first
three transects show higher salinity pockets (35.65) below 75 m, while the last three transects
do not show these pockets, instead of an almost uniform salinity of 35.6. Apart from these
high salinity pockets the structure of the water column, with a graded low to high salinity
structure from top to bottom, was expected.
Temperature plots (Figure 3.5) were contorted to show smaller temperature values, to give a
better idea as to temperature changes above both the Petit Sole Canyon and the Grand Sole
Canyon. Temperature changes were not as significant as that of salinity during the 15 days of
data collection. There is evidence of an ever so slight decrease in depth of the surface warm
layer between T1 and T2, and an increase in depth of the same layer between T2 and T3. In
T3 to T6 the temperature at the head of both canyons varies. In T3 and T5 there is a lower
temperature seen above the head of the Petit Sole Canyon, while in T4 and T6 lower
temperature are seen above the Grand Sole Canyon.
The potential density contours in both the salinity plots and the temperature plots show
evidence that the density structure is controlled by temperature, rather than salinity. The
salinity plots in Figure 3.4 are somewhat chaotic and do not follow much of a pattern, while
the temperature plots in Figure 3.5 are almost uniform and are similar to the density
structure.

20
Figure 3.4. Salinity transects from 28/07 to 14/08, parallel to the slope, to a depth of 182 m, along the Celtic Sea shelf-break. Potential density contours are
shown in each transect.

21
Figure 3.5. Temperature transects from 28/07 to 14/08, parallel to the slope, to a depth of 182 m, along the Celtic Sea shelf-break. Density contours are
shown in each transect.

22
3.3 Slope Current & Direction
The slope current direction and velocity were calculated from LT1 (Figure 1.5), the long term
moored ADCP (22nd June to 3rd July, 2012) (Table 2.1). Using residual.m the barotropic tide
was removed from the data, and the residual currents were used to calculate an average
overall current direction of 173.5°. This means that for the period in which data was collected,
the slope current was flowing southeast, contrary to a northwest flowing slope current from
previous literature. Figure 3.6 shows both the time averaged velocities and depth averaged
velocities of the north-south flow. The direction of the flow is predominantly south (negative
values) until c. day 184 when it reverts back to flowing north. The depth averaged flow also
shows that throughout the water column the flow was predominantly south flowing for this
period.
The east-west velocities (Figure 3.7) show that the current was flowing to the east, therefore
it was an on-shelf flow. This again occurred from day 175 until c. 184 when it changed to west
flowing. This is mirrored in the depth averaged flow which shows a positive flow (east flowing)
throughout the cruise period. Since this is depth averaged, it can be assumed that any nutrient
rich bottom water, that may have been upwelled, would flow onto the shelf.
Along the continental shelf, the slope current is constantly changing from north flowing to
south flowing. This has been made evident from data collected at LT1 from 30th June 2012 to
the 7th Match 2013. The current, however, does, in general, move in a northerly direction, as
can be seen by the increased velocities between mid-August and the start of November 2012
(Figure 3.8).

23
Figure 3.6. Left: Time averaged residual north-south velocities with tides removed from LT1 from day
175 to 185, 2012. Right: Depth averaged residual north-south velocities from LT1 from the same
period.
Figure 3.7. Left: Time averaged residual east-west velocities with tides removed from LT1 from day
175 to 185, 2012. Right: Depth averaged residual east-west velocities from LT1 from the same period.

24
Figure 3.8. Along- and across-slope currents from the 30th
June 2012 to the 7th
March 2013, recorded at LT1.

25
3.4 CTD & ADCP
3.4.1 Undulating CTD
The UCTD was deployed from day 181.5 to 182.2, to collect temperature, salinity and oxygen
concentration data within the Petit Sole Canyon (Figure 1.5 & Table 2.1). During this period
the tidal cycle changed from the ebb tide to high tide slack water (Figure 3.2). There is a very
subtle change in the overall structure of the water column, almost a slight wave-like
appearance, which indicates that the water column is somewhat influenced by the tide.
However, there are no significant changes in the overall vertical structure of either the salinity
profile (Figure 3.9) or the temperature profile (Figure 3.10), indicating that the tide does not
play a major role in the structure of the water column, at this point in the canyon.
The salinity profile for the UCTD (Figure 3.9) shows what was to be expected at the shelf
break. The bottom waters are almost 0.15 units less saline than that of the top water. This is
due to the presence of the North Atlantic Deep Water (NADW), which has a general salinity
of 35. There is also an indication of the slope current, which appears to be located at
approximately 350-700 m depth driven by the Mediterranean Intermediate Water (MIW),
known for its high salinity. The temperature profile for the UCTD (Figure 3.10) shows an
almost uniform stratification throughout the time period that data were collected, with the
exception of 600-700 m when there is some mixing. The structure of the temperature profile
is a textbook example of seasonal stratification.

26
Figure 3.9. Salinity profile of UCTD, from the 29th
June to the 30th
June 2012.
Figure 3.10. Temperature profile of UCTD, from the 29th
to 30th
June, 2012.

27
3.4.2 LADCP
The LADCP was attached to the UCTD frame, so measurements of current velocities were
taken within the canyon, over a 13 hour period, between the 29th and 30th June 2012 (Figure
1.5 & Table 2.1). The velocities recorded show that throughout the water column, and
throughout the time period, the velocity was ever changing. The across-slope current was
expected to have a much higher velocity magnitude than the along-slope current. This was
expected because, within the canyon, upwelling and downwelling occur, and therefore in an
across-slope direction, rather than along-slope. This does not seem to be the case as both
velocities seem to be similar in magnitude and direction, apart from Cast 59 when the across-
slope current is much faster than the along-slope current.
The along- and across-slope currents follow the same directional pattern during the
deployment. In Figure 3.11, it was observed that the current was flowing in a south-west
direction from 14:13 (Cast 47) to 17:47 (Cast 50), until it turned north-east from 18:53 (Cast
51) to 00:02 (Cast 56). After that it was south-west flowing for the remainder of the
deployment. This current pattern is indicative of possible upwelling and downwelling patterns
during the period, possibly influenced by the barotropic tide. The average current velocity
for the across-slope current, from the entire period was -0.363 m/s, while the average velocity
for the along-slope current was -0.0111 m/s. This indicates that for the 13 hour period the
LADCP was deployed the overall direction of flow through the canyon was south-west.

28
Figure 3.11. Averaged across- (top) and along-slope (bottom) current velocities (blue line) from each
cast, with the total deployment average velocity (red line) plotted through them.
3.4.3 Spatial Survey
Four separate CTD casts were obtained from the head of the Petit Sole Canyon (Cast 38) to
the fan (Cast 41)(see Table 2.1 & Figure 1.5). These were obtained to determine possible
nutrient migration through the canyon, either onto the shelf or off the shelf, a.k.a. nutrient
supply or dump. All the casts show that there is up to 10 µM of nitrate in the upper 150 m of
the water column (Figure 3.12). There is a general trend of increasing nitrate concentration
with depth, meaning that bottom waters, should they be upwelled, would cause
phytoplankton blooms. However, the high concentrations in the upper water column would
still have caused an increase in phytoplankton growth.
Temperature and density results (Figure 3.13) for the four CTD casts were as expected, a
decrease in temperature and an increase in density, with depth. However, the salinity results
show an increase in salinity in the top 250 m, then a sharp decrease between 250 and 750 m,
followed by a rapid increase. This figure shows that there is evidence of the slope current
located between 300 and 700 m depth.

29
Figure 3.12. Nitrate concentrations with depth from four CTD casts taken from the head of the Petit
Sole Canyon to the fan.
Figure 3.13. Salinty (top left), temperature (top right) and density (bottom left) plots of all four CTD
casts.

30
3.5 Scanfish
3.5.1 Repeat Canyon
The repeat canyon survey was undertaken from the 23rd June to the 24th June 2012, for
approximately 15 hours (1.25 cycles of the M2 tidal cycle). It was undertaken in the upper part
of the Petit Sole Canyon (Figure 1.5), where the SE side of the canyon extended to a depth of
180 m, the central trench extended to 900 m and the NW side of the canyon extended to 400
m. The Scanfish reached a depth of 120 m, limiting the results. The NW side of the canyon
experienced higher salinities (>35.45) than that of the SE side (<35.44; Figure 3.14). These
high salinities were consistent for the entire 15 hour period of LADCP data collection, and
indicates upwelling along the NW side of the canyon, and possibly downwelling along the SE
side of the canyon. At the beginning of data collection, there is evidence in T1 to T3 for very
low salinity water on the SE side of the canyon, possibly indicating a period of enhanced
downwelling, reversing to enhanced upwelling by T4.
The period the Scanfish data was collected was during summer, meaning that the water
column was stratified due to an increase in atmospheric temperature, causing the surface
water to heat while bottom water remained cool. Figure 3.15 shows the propagation of an
internal wave, through the water column. This is interpreted from the increase in warmer
surface water in T3 and T4. After this, from T5 the water column becomes more stable and
stratification becomes much clearer. T4 represents the possibly point of maximum
wavelength of the internal wave. This internal wave was observed to cause an increase in
mixing with depth in the water column.

31
Figure 3.14. Salinity plots for the repeat canyon transects using a Scanfish. Data collection from 23rd
to 24th
June 2012. The plots are orientated southeast on
the left, and northwest on the right. The location of the common centre point (0 km) is 48.3726 N 9.5526 W.

32
Figure 3.15. Scanfish plots of temperature from 23rd
to the 24th
June 2012. Left: SE side of the canyon, right: NW side of the canyon. The location of the
common centre point (0 km) is 48.3726 N 9.5526 W.

33
3.5.2 Zigzag Canyon
The zigzag canyon grid was a series of eight transects in a zigzag from the head of the canyon,
onshelf towards ST4 and ST5, and back through the canyon again (Figure 1.5), carried out
from 25th to 26th June 2012. The final transect cut through T1 to T4, and the repeat canyon,
heading southwest (Figure 3.16). High saline (>34.45) pockets of water are evident in T1, T2
and T3, on the NW side of the canyon (Figure 3.17). The further from T1 these pockets get,
the less concentrated the pockets become. The salinity plot from Figure 3.16 shows a very
high saline body of water just south of the canyon. The high saline pockets of water are
believed to have migrated from this body of water, dispersing the further onshelf the water
travels. This is evident by T5, with the water column becoming much less saline (<34.4), and
highly stratified.
Temperature profiles (Figure 3.18) of the same transects show that there is increased mixing
in T1, T2 and T3, causing a less stable and contorted water column, while from T5 and onwards
the water column is much more stable and stratified. The minimum temperatures reached in
T1 to T3 barely go below 12°C, whereas in T5 to T7 temperatures reach a minimum of 11°C.
The difference in temperature from the surface waters to the bottom of the profiles is much
larger in the last three transects (4°C), while in the first four transects this difference is only
2°C. The temperature plot from T8 shows an undulating thermocline, getting shallower
towards the south, where the high saline pocket is. This indicates a high saline, low
temperature water body, possibly from deep open water.

34
Figure 3.16. Salinity (top) and temperature (bottom) plots of transect 8 during the zigzag survey of the
canyon.

35
Figure 3.17. Zigzag Scanfish salinity transects 1 through 7, from the period 25th
to 26th
June.

36
Figure 3.18. Temperature plots from the Zigzag Scanfish transects 1 to 7.

37
Chapter 4: Discussion
The aim of this thesis was to determine the processes which induce upwelling/downwelling
through the Petit Sole submarine canyon, and to determine the extent at which upwelled
water travels onto the shelf. To achieve this aim, CTD, ADCP, Scanfish, and Glider data were
used to monitor current velocities, and temperature and salinity profiles throughout the
canyon (Figure 1.5).
4.1 Principle relationships shown by results
4.1.1 Current Velocity and Direction (Figures 3.6, 3.7, 3.8, 3.11, and 4.1)
The direction of the slope current is of significant importance when determining the processes
effecting upwelling and downwelling through a canyon. Chen & Allen (1996) determined that
an equatorward slope current produces enhanced upwelling, while a poleward slope current
reduces the strength of upwelling and allows for enhanced downwelling. Allen (2000) & Allen
et al. (2001) showed that an equatorward slope current forced upwelling through both the
Astoria and Barkley canyons, on the west coast of Canada. This hypothesis led to the study of
currents within the Petit Sole Canyon. The LT1 mooring has shown that the slope current is
constantly changing direction, neither at a seasonal scale or a tidal scale. During the cruise,
the current was flowing in a south-easterly (equatorward) direction. There was also an
increase in easterly flow, indicating an on-shelf flux of water. Towards the end of the cruise
the current then reversed back to its ‘normal’ north-westerly flow direction (Figure 3.8).
The velocity profile of a point in a canyon would be expected to have a minimal along-slope
current and a maximum across-slope current. Within the canyon, flow along-slope is
restricted due to the presence of the canyon walls (Allen et al. 2001). Figure 4.1 shows
averaged current velocities at depths of 100 m, 500 m and 700 m. These depths were chosen
because they were thought to represent the different flow regimes through the canyon. At
100 m flow should not be affected by the canyon. At 500 m flow is advected into the canyon

38
(during the second phase: advection-dominated) causing enhanced upwelling or
downwelling. At a depth of 700 m flow should not be upwelled through the canyon, rather it
should be stretched and form cyclonic vortices. Therefore, the LADCP data shows that the
across-shelf flow is generally larger than along-shelf, and both flows follow a similar pattern
for the 100 m and 500 m (Figure 4.1). However, at a depth of 700 m the velocity of the along-
slope current changes direction, compared with the across-slope current, between casts 52
and 56. The change in direction of the along-shelf flow is possibly indicative of the water
column stretching. This causes vorticity at the bottom of the water column, and not reaching
the surface waters, seen by Allen et al. (2001) in the Barkley Canyon.
The velocity within the canyon is an order of magnitude larger than that of the residual
currents recorded at LT1. This is due to the change in magnitude of the canyon, compared
with the continental shelf. As the currents enter the canyon, they become stretched and
magnified, causing increased velocities, and therefore increased mixing.

39
Figure 4.1. Across- and along-shelf current velocities recorded by the LACDP for a 13 hour period from the 29th
to 30th
June 2012.

40
4.1.2 Salinity and Temperature Profiles (Figures 3.4, 3.5, 3.9, 3.10, 3.13 – 3.18)
The structure of the UCTD salinity profile shows that there is a low saline (<35.55) body of
water at the bottom of the water column, while temperature profile shows that it is also cold
water (<10°C). This possibly indicates the presence of North Atlantic Deep Water (NADW).
There is also evidence of a ‘sandwich’-like structure further up the water column. Figure 3.9
shows high saline water (>35.62) from 800m and above, however the structure is variable.
There is a lower saline (c. 35.6) body of water sandwiched between two higher saline (>35.62)
water bodies, possibly indicating a slope current driving by Mediterranean Intermediate
Water (MIW). The structure seen in UCTD is also present in the spatial survey through the
canyon.
The salinity profiles of all four CTD casts show that there is a low saline body of water between
two higher saline water masses, centred around 500 m. This illustrates an ever-present lower
saline water mass extending through the canyon from the head to the fan. Cast 41 shows a
higher salinity for the surface and bottom water masses, possibly indicating a source off-shelf,
migrating through the canyon onto the shelf. Both Cast 40 and Cast 41 show a rapid decrease
in salinity towards the very bottom of the water column, possibly indicating NADW.
Although canyons remain largely under-studied, they are believed to be hugely important in
the transport of water both on- and off-shelf. The zigzag canyon survey shows that the Petit
Sole canyon can act as a conduit for high saline waters to move on-shelf. The salinity profile
of T8 in the zigzag survey shows a very high saline water mass south of the canyon, in the
open ocean. T1 to T3 in the zigzag salinity survey show the migration of this high salinity water
mass through the canyon, onto the shelf, where it dissipates before reaching T5.
The temperature profiles in in the repeat Scanfish survey show an increase in mixing through
the water column, in the first three transects (T1-T3), indicating that the head of the canyon
is an area of increased mixing, possibly due to internal tides. According to Hickey (1997) and
Allen & Durrieu de Madron (2009) water above 100m is not affected by the canyon and flows
over it. However, from the above results it can be said that water is affected to a certain
degree, in that mixing is enhanced, causing a change in the temperature gradient.
Allen & Durrieu de Madron (2009) determined that, with a southerly slope current flow,
upwelling will occur at the downstream wall of the canyon, while downwelling will occur along

41
the upstream flow of the canyon. The results presented in Figure 3.14 show that this is not
the case in the Petit Cole canyon. During the repeat Scanfish survey there were higher
salinities recorded on the NW (upstream) side of the canyon, compared with that of the SE
(downstream) side of the canyon. The canyon also appears to be in the second phase of a
typical upwelling scenario, in that upwelling is much stronger than downwelling. This
enhanced upwelling is possibly due to an increase in wind stress. During the second half of
the repeat Scanfish survey there was a storm, this continued into the zigzag survey, where it
reached a peak around day 175.5. This increase in wind stress potentially caused increased
mixing through the water column, enhanced upwelling.
There is a clear change from upwelling to downwelling seen in the salinity profiles of the glider
data, caused by a change in the physical dynamics of the shelf break. During the time of data
collection it has been established that the slope current was flowing a south-easterly
direction, opposite to its ‘normal’ direction. During July the slope current was constantly
changing direction, until the 5th August 2012, when it reverted back to this ‘normal’ north-
westerly flowing direction. This will have brought changes to upwelling and downwelling.
4.1.3. Nutrient Fluxes (Figure 3.13)
The nutrient data collected from Cast 38 to 41 suggests that there is a higher level of nitrate
within the first 100m at the head of the canyon compared to the mouth of the canyon.
However, at the mouth of the canyon there is an extremely high concentration (>15 μM) of
nitrate at the bottom of the water column. This high concentrations is formed through the
decomposition of organic matter and can lead to ‘dead zones’. However, if this water is
upwelled onto the continental shelf, it is consumed by phytoplankton to form algal blooms,
possibly leading to toxic events.

42
4.2 Comparison with previous literature
Allen & Durrieu de Madron (2009) have previous determined that within canyons on the west
coast of North America, during the advection-dominated phase of an upwelling/downwelling
event water will flow through the canyon in different directions. They determined that during
the negative phase (upwelling enhanced) that upwelled water would flow out of the canyon
via the downstream wall, while during the positive phase (downwelling enhanced) water
would flow from the upstream wall of the canyon. However, the results presented above
clearly show that there is upwelling along the upstream wall of the canyon (NW side) during
a south-flowing slope current. This could indicate that the canyon, being in the advection-
dominated phase, shows that the Petit Sole canyon, and therefore potentially the Celtic Sea,
do not follow the hypothesis of the western North American coastline. The main difference
between previous studies and this thesis, is that the slope current in previous studies has
been to the right of the canyon, while in this study the slope current is to the left of the canyon
(Figure 4.2). This, potentially, is the controlling factor in whether the region experiences
enhanced upwelling or downwelling with different slope current flow directions.
The other explanation, is that the canyon is in a positive advection-dominated phase, meaning
that although downwelling is the main process which occurs within all three phases, during
the second phase, upwelling is stronger than that of downwelling. This was then enhanced
further by the storm event, starting on day 173 and continuing until day 176. During this storm
winds reached up to 20 m/s, with wind stress reaching over 1 N/m2. This upwelling is what is
being seen in the CTD, Scanfish and Glider data. This hypothesis would also contradict
numerous authors (Chen & Allen 1996; Allen 2000) who state that enhanced upwelling is
caused by a south-flowing slope current, since the main process in this scenario is
downwelling.
The actual depth of upwelled water varies considerably through time, and within certain
canyon types. Upwelled water presented above in Figure 3.4 is consistent with the structure
presented in Figure 3.9. However, Figures 3.14 and 3.16 (the Scanfish data) show lower values
for salinity. Again, the Scanfish data was collected only in the first 120 m of water, while both
the Gliders and UCTD reached much further. According to Allen et al. (2001) and Allen (2004)
water is upwelled through a canyon until it reaches a maximum depth, where it becomes less
effected, becoming stretched, forming cyclonic vorticities. Data presented above indicates

43
that this depth is somewhere between 700 and 800 m. This is shown in Figure 3.9 by the
smoothing of the salinity gradient. This means that water above this depth is being upwelled
through the canyon, to the surface layer. And since this water has the highest salinity in the
water column, it brings much more saline water, and therefore nutrients, to the surface of
the water. Allen (2004) also showed that water flowing above the canyon would not be
affected, and would flow directly over it. This is evident in the Scanfish, CTD and Glider data
presented above, with the continual stratification of surface waters.
Figure 4.2. a) shows the flow regime associated with a southerly slope current flow in the Astoria and
Barkley Canyons according to Allen & Durrieu de Madron (2009); b) flow regime associated with a
southerly slope current flow in the Petit Sole Canyon. The hashed line on the right represents the
coastline, and the black line in the centre presents an isobath, highlighting the shape of the canyon.

44
Chapter 5: Conclusion
Submarine canyons are important physical and biogeochemical topographic structures, along
the shelf break. The importance of these structures has yet to be fully studied, meaning that
our current understanding of the physical processes which occur within them is limited.
However, it has been determined by many different authors (Chen & Allen 1996; Allen 2000;
Allen et al. 2001; Allen 2004;) that they act as conduits in which water can be upwelled or
downwelled through them.
This thesis has shown that, in the case of the Petit Sole Canyon, there are two scenarios
possible:
1. The south flowing slope current caused an increase in upwelling through the canyon,
which appears to be most prominent on the northwest side (upstream) of the canyon.
This is contrary to the findings of Allen & Durrieu de Madron (2009), who determined that
upwelling would occur along the downstream side of the canyon, and downwelling would
occur on the upstream side of the canyon.
2. The canyon is in the second phase of a typical downwelling scenario (according to Allen's
(2004) three scenarios). During this phase upwelling is stronger than downwelling, made
stronger by storm events, observed in the results above. This hypothesis also contradicts
literature, in that during a south flowing current it has been found that upwelling is the
dominant process, and during a north flowing current downwelling is dominant. This
thesis shows the opposite of this.
Both of these hypotheses show that the Petit Sole Canyon differs from canyons along the
west coast of North America. Both hypotheses are possible, and only further research into
this area could identify the answer. The location of the slope current to the canyon is an
important factor. Does a slope current to the right of the canyon enhanced downwelling? And
vice versa for upwelling? These are the important questions which need to be answered with
further research.

45
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48
Appendices
A – Description of Matlab scripts
The Matlab scripts and functions written and amended for the use of this thesis are enclosed
on the CD attached. A brief description of the scripts is given below. The headings indicate
the file location.
MSc Project/Cruise Data/Scripts/CTD/CTD_Code.m
This file imports all the CTD files, grids the temperature, salinity and oxygen data, and plots
them using pcolor (Figures 3.9 & 3.10).
MSc Project/Cruise Data/Scripts/Gliders/Gliders_2.m
This file loads the Glider data and produces plots for temperature and salinity (Figures 3.4 &
3.5).
MSc Project/Cruise Data/Scripts/LADCP/LADCP_2.m
This file loads the LADCP data, removes the barotropic tidal signal using the function
residual.m and manipulates the data to show the direction of flow, using the function
polar2met.m. The data was then manipulated to show along- and across-slope flows
(Equations 2.2 & 2.3) and then plotted (Figures 3.11 & 4.1).
MSc Project/Cruise Data/Scripts/Met/Met.m
The meteorological data from the RRS Discovery’s own central logging system was loaded in
this file. The absolute wind speed was used to calculate the wind stress (Equation 2.1) for the
duration of the cruise, and plotted (Figure 3.1).

49
MSc Project/Cruise Data/Scripts/Moorings/LT1.m
The current velocities recorded at LT1 were loaded in this file, the barotropic tide was
removed using residual.m, and the direction of flow was calculated and manipulated using
the function polar2met.m. They were re-orientated to show along- and across-slope flow
through the canyon (Equations 2.2 & 2.3) and all data was plotted (Figures 3.2, 3.6, 3.7 & 3.8).
MSc Project/Cruise Data/Scripts/Moorings/ST4.m
Current data from ST4 was loaded, the barotropic tidal signal was removed using the function
residual.m, and the residual currents were plotted (Figure 3.3).
MSc Project/Cruise Data/Scripts/Nutrients/Nutrients.m
Nutrient data was imported and plotted in this script (Figure 3.12). The temperature, salinity
and density data of CTD Casts 38 to 41 were also uploaded to this script, and plotted (Figure
3.13).
MSc Project/Cruise Data/Scripts/Scanfish/M_Sc_Scanfish_canyongrid.m
This code loads and manipulates the repeat canyon Scanfish survey, using the functions
sw_dist.m and cumsum.m. This data was then sub plotted in temperature and salinity plots
(Figures 3.14 & 3.15).
MSc Project/Cruise Data/Scripts/Scanfish/Scanfish_zigzag.m
This code loads and manipulates the zigzag Scanfish survey, using the functions sw_dist.m
and cumsum.m. This data was then sub plotted in temperature and salinity plots (Figures 3.17
& 3.18).The final transect (T8) was sub plotted in its own figure (Figure 3.16).

50
MSc Project/Cruise Data/Scripts/Functions/sw_dist.m
This function calculates the distance between a two points, using latitude and longitude
coordinates.
MSc Project/Cruise Data/Scripts/Functions/importfile.m
This function was simply used to import data.
MSc Project/Cruise Data/Miss/polar2met.m
This function allowed for radian direction to be changed to degree direction.
MSc Project/Cruise Data/Miss/residual.m
This was a function written, using the function t_tide.m, to remove the barotropic tidal signal,
and produce the residual currents recorded.

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