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Seismic characterization of water masses and mesoscale eddies associated with 1
the subtropical front SE of New Zealand 2
3
4
Andrew R. Gorman, Matthew W. Smillie, Joanna K. Cooper, M. Hamish Bowman, Department of 5
Geology, University of Otago, Dunedin, New Zealand 6
7
Ross Vennell, Department of Marine Science, University of Otago, Dunedin, New Zealand 8
9
Steve Holbrook, Department of Geology and Geophysics, University of Wyoming, Laramie, USA 10
11
Russell Frew, Department of Chemistry, University of Otago, Dunedin, New Zealand 12
13
Corresponding author: Andrew R. Gorman, Department of Geology, University of Otago, PO Box 14
56, Dunedin 9054, New Zealand. ([email protected]) 15
16
Draft Date: 17 April 2014 17
18
19
Key Points 20
• Seismic oceanography images water masses adjacent to the Subtropical Front 21
• Eddies are shown to be mixing water in vicinity of Subtropical Front 22
• Submarine water mass boundaries are more stable than surface expressions 23
Seismic Oceanography of the Subtropical Front SE of New Zealand 2
Abstract 24
The Subtropical Front, a major global boundary separating subtropical and subantarctic waters, is 25
locally diverted to the south by the New Zealand landmass. In this region, large volumes of 26
dissolved or suspended material are intermittently transported across the front; however, the 27
mechanisms of such transport processes are enigmatic. Existing maps and cross-sections of such 28
oceanic boundaries generally depend on measurements collected from stationary points on the 29
ocean surface or seafloor. The details of these datasets, which are critical for understanding how 30
water masses interact and mix at the fine- (< 10 m) to mesoscale (10 to 100 km), are poorly 31
constrained due to resolution considerations. The new method of seismic oceanography, applied to 32
serendipitously located multi-channel seismic data, provides detailed images of reflectivity 33
associated with oceanic water masses. Three nearly coincident profiles have been reprocessed to 34
produce remarkable images of three main water masses, the boundaries between them, and 35
associated thermohaline fine-structure. Interpretations of the data show that the Subtropical Front is 36
a zone that dips landward, with a width that can vary between summer and autumn by as much as 37
20 km. The boundary zone between subantarctic waters and the underlying antarctic intermediate 38
waters is also observed to dip landward. Several isolated lenses have been identified on the three 39
data sets, ranging in size from 9 to 30 km in diameter. These lenses are interpreted as mesoscale 40
eddies that form at depth in the vicinity of the Subtropical Front. 41
42
Index Terms 43
Seismic oceanography, Subtropical Front, mesoscale eddy, Southland Current, subantarctic water 44
45
Seismic Oceanography of the Subtropical Front SE of New Zealand 3
1. Introduction 46
The Subtropical Front (STF) off the SE coast of New Zealand [Heath, 1981; 1985; Rickard et al., 47
2005] is an enigmatic ocean boundary that separates subtropical waters (STW) that are generally 48
warm, low in nutrients, and sustain high levels of phytoplankton growth (primary productivity) 49
from subantarctic waters (SAW) that are generally cold, high in nutrients, and low in productivity 50
(Figure 1). As roughly a third of New Zealand’s offshore waters are associated with the circumpolar 51
subantarctic water mass [Murphy et al., 2001] (comprising about 10% of the world’s oceans), this 52
region is ideally positioned for studying the interaction of this water body with subtropical waters to 53
the north. 54
55
The region of the ocean south of the Chatham Rise regularly exhibits unexpectedly sustained 56
productivity across the STF. This region can be considered a natural laboratory for studying a 57
global phenomena involving the oceanic transport of iron that episodically plays a determining role 58
in increased productivity [Boyd et al., 1999; Boyd et al., 2000; Boyd, 2004; Boyd et al., 2004]. 59
These enhanced productivity events can be stimulated by sporadic iron supply mechanisms [Banse 60
and English, 1997], such as dust storms [DiTullio and Laws, 1991; Husar et al., 2001] and eddy-61
driven transport of iron-rich coastal waters to offshore regions [Whitney and Robert, 2002]. 62
However, iron carried in airborne dust has been ruled out as the mechanism for this productivity 63
[Boyd et al., 2004], which implies that significant ocean mixing must be occurring here to transfer 64
iron across the front. 65
66
To understand the processes involved with mixing ocean water masses, we need to finely sample 67
and model properties such as bathymetry, fluid composition, temperature, and current velocity 68
[Davis, 1991; de Ronde et al., 2005; Robinson and Eakins, 2006; Smith and Sandwell, 1994; 69
Seismic Oceanography of the Subtropical Front SE of New Zealand 4
Vanneste et al., 2006; Vennell, 1994; 1998]. High-resolution observations of these water properties 70
have improved greatly in the last 20 years. However, these measurements generally are made by 71
tools that are either (a) dropped from stationary boats or (b) attached to floating or seabed moorings. 72
These measurements require synthesis and interpolation to produce laterally-continuous 73
representations. Measurements of chemical parameters are even sparser because they usually rely 74
on the shipboard collection of samples that are processed after a cruise. In our target area, these 75
observations are incapable of answering basic questions such as, ‘How much water is involved in a 76
mixing event?’ or ‘Does the water mix as the result of an eddy or an upwelling tongue?’ 77
78
The emergence of the field of seismic oceanography [Holbrook et al., 2003; Holbrook et al., 2006; 79
Nandi et al., 2004; Páramo and Holbrook, 2005; Tsuji et al., 2005; Wood et al., 2008] has enabled 80
the visualization of oceanic water masses in the same way and with the same data that sedimentary 81
basins are imaged in the sea floor. Investigations of the geology of continental margins by the 82
petroleum industry over the last few decades contain a wealth of unexploited information on ocean 83
water masses above the seafloor. Fine-scale variations in temperature and, perhaps, salinity 84
manifest themselves as sound-seed contrasts; conventional seismic reflection methods record such 85
variability with a vertical resolution of ~10 m and a sensitivity to abrupt temperature contrasts as 86
small as 0.05°C [Holbrook et al., 2006]. Simple processing techniques readily produce clear images 87
of ocean structures that were below the visible threshold in previous versions of the seismic data 88
processed to show sub-seafloor geology. A range of oceanographic features has been investigated 89
including the evolution and dispersal of large eddies (e.g., Meddies at the mouth of the 90
Mediterranean [Biescas et al., 2008]), the position and variability of ocean boundary currents (e.g., 91
the Agulhas Current off the Cape of Good Hope [Uenzelmann-Neben et al., 2008]), and dynamics 92
of internal waves occurring on boundaries between ocean layers with different densities, 93
Seismic Oceanography of the Subtropical Front SE of New Zealand 5
temperatures and salinities (e.g., interaction of internal waves with continental shelf sediments at 94
the Rockall Trough off Ireland [Jones et al., 2010]). Existing oceanographic methods are incapable 95
of observing the the fine detail of such processes due to their sporadic and spatially–constrained 96
nature. 97
98
In this paper, we present detailed cross-sectional seismic images of the water masses spanning a 99
section of the Subtropical Front – as they mix – using multi-channel seismic data collected by the 100
petroleum industry for deeper geological targets. We capitalise on recent petroleum exploration 101
efforts SE of New Zealand to analyze data showing the temporal and spatial characteristics of water 102
masses on either side of the Subtropical Front and indications of dynamic mixing processes 103
underway as the result of mesoscale eddies. 104
105
2. Seismic data acquisition and processing 106
Recent hydrocarbon exploration efforts SE of New Zealand have led to the acquisition of several 107
extensive seismic surveys in the region of the Subtropical Front (Fig.1). The DUN-06 2D seismic 108
survey was undertaken by the New Zealand government to encourage petroleum exploration in the 109
Great South Basin prior to an exploration block offer made in 2007. This then led to the acquisition 110
of the OMV-08 2D seismic survey (and additional on-going exploration efforts). Both surveys 111
contain seismic lines suitable for imaging oceanographic features across the Subtropical Front. A 112
summary of the acquisition parameters of the DUN-06 and OMV-08 surveys is provided in Table 1. 113
114
Three parallel seismic lines from the two surveys (lines DUN-06-13, OMV-08-45, and OMV-08-115
42) are presented here to highlight the oceanographic features visible in these data and to 116
investigate the spatial and temporal variability of such features. The lines were chosen from these 117
Seismic Oceanography of the Subtropical Front SE of New Zealand 6
datasets with optimal orientations for examining the water mass extending from the shallow shelf to 118
moderately deep waters of the Campbell Plateau at the south end of New Zealand’s South Island 119
(Fig. 1). DUN-06-13 lies parallel to and halfway between OMV-08-42 and OMV- 08-45. The main 120
difference in the survey designs is the airgun arrays used during the acquisition (a 26-gun array for 121
DUN-06 compared to a 48-gun array for OMV-08), but the energy of the airguns was similar (just 122
over 65 L for both surveys). 123
124
Processing of these data has focused on the water column, rather than the sub-seafloor geology for 125
which the data were originally collected [Smillie, 2012]. These datasets span the continental shelf, 126
slope and rise in water depths from a few metres to more than 1500 m; they are optimally located 127
for investigations of water variability caused by interactions between shoreline processes and deep-128
water currents. Processing of the seismic data followed a fairly routine flow that focuses on water 129
column reflectivity [cf. Nandi et al., 2004; Pinheiro et al., 2010] using petroleum industry software 130
[Ravens, 2001], including pre-stack deconvolution, manual trace editing, removal of the direct 131
arrival, detailed velocity analysis, seafloor muting, and NMO correction; post-stack flow included a 132
simple band-pass filter and finite difference migration. Amplitude gain was addressed by applying a 133
spherical divergence operator based on the final velocity model. The processed sections are shown 134
in Fig. 2. 135
136
3. Detailed seismic observations 137
3.1 Reflection variability 138
In seismic oceanography reflection sections, laterally coherent reflections occur as a result of 139
relatively abrupt changes in acoustic impendence (the product of sound speed and water density). 140
These changes predominantly arise from thermohaline contrasts, which advect laterally along 141
Seismic Oceanography of the Subtropical Front SE of New Zealand 7
isopycnals. Isopycnals can be assumed to have very shallow spatial gradients [Cox, 1987] and 142
therefore, reflections are typically observed to be sub-horizontal. Coherent reflections can extend 143
horizontally for >10 km as can be seen in the reflective zones associated with regions of increased 144
temperature variability. Such reflections (e.g., the Southland Current (SC) and transition zone (TZ) 145
regions of Fig. 2) likely correspond to waters lying between more stable water masses. Reflections 146
also can be short (<100 m), like the isolated reflections seen in some of the highlighted boxes 147
showing specific reflective features in the data (Fig. 2). In addition to the length of reflections, three 148
characteristic arrangements of reflections are identified in the seismic data as: (a) undulating 149
reflections, (b) vertical stacks of reflections, and (c) individual reflections with high apparent dip. 150
151
Undulating reflections are widespread in the data. The undulations have amplitudes of around 15 m 152
and horizontal wavelengths that vary on the order of hundreds of meters to kilometers. The 153
displacement is seen to be in phase with multiple reflections vertically separated by hundreds of 154
meters. Holbrook and Fer [2005] attributed such features to the displacement of isopycnals by 155
internal wave motion. Their findings are consistent with the Garrett and Munk internal wave model 156
[Munk, 1981]. 157
158
At numerous locations in the data, short moderately sloping stacks of reflections are observed to 159
cover a vertical distance as great as 500 m (e.g., highlighted boxes in Fig. 2). Similar structures 160
associated with eddies have been reported previously [e.g., Biescas et al., 2008; Pinheiro et al., 161
2010]. The sub-horizontal reflections within the features are thought to represent thermohaline 162
intrusions, the inter-fingering of adjacent water masses along isopycnals [Ruddick, 2003]. 163
Thermohaline intrusions are driven by a double diffusion process that occurs at thermohaline fronts 164
between waters with differing temperature-salinity profiles [Ruddick, 1992]. The interleaving of 165
Seismic Oceanography of the Subtropical Front SE of New Zealand 8
water between the eddy core and its surroundings give rise to narrow (<1 km) regions of 166
temperature and salinity variability that can be detected by seismic methods. 167
168
There are several locations where reflections appear too steep to be explained by thermohaline 169
intrusions [cf. Pinheiro et al., 2010; Ruddick, 1992]. An example of this is seen in the highlighted 170
body from line OMV-08-42 (Fig. 2a), which shows the upper NW boundary of the lens feature 171
sloping at around 5°. The set of reflections appears to cross isopycnals, the orientations of which are 172
assumed to be subhorizontal in accordance with nearby reflections. There are currently two schools 173
of thought surrounding the presence of steeply dipping features in seismic oceanography profiles. 174
One hypothesis involves the tilting of thermohaline intrusions by sheared isopycnal advection 175
[Pinheiro et al., 2010]. Others dismiss the steep dips as artifacts resulting from the insufficient 176
resolving power of low-frequency seismic data in a region of fine-scale vertical variations in the 177
physical properties of the ocean [Géli et al., 2009; Hobbs et al., 2009]. However, the two 178
hypotheses do require further investigation. 179
180
3.2. Water mass characterization 181
The configuration of water masses in the vicinity of the STF southeast of New Zealand has 182
previously been established using expendable bathy-thermograph (XBT) and conductivity-183
temperature-depth (CTD) instrument deployments [Sutton, 2003]. Such data can be used to produce 184
a cross-section of the water properties along the trajectory of coincident seismic profiles (Fig. 3). 185
Temperature and salinity data can then be used to identify subtropical and subantarctic waters in 186
cross-section that correspond to surface expressions of the water masses (Fig. 1). However, 187
conventional physical oceanography data do not have the capability of examining large cross-188
Seismic Oceanography of the Subtropical Front SE of New Zealand 9
sectional regions of the ocean with high lateral resolution. This leaves the fine-scale nature of such 189
water masses largely unknown. 190
191
Used in conjunction with physical oceanography data, the seismic oceanography method can 192
provide a more detailed view of ocean structures and boundaries. In the seismic data presented here, 193
four distinct regions can be identified by their reflectivity characteristics. From top to bottom (and 194
northwest to southeast) these are: highly reflective waters associated with the Southland Current 195
(SC) at depth, relatively non-reflective Subantarctic Waters (SAW), a highly reflective transitional 196
zone (TZ), and the basal non-reflective Antarctic Intermediate Waters. The high lateral resolution of 197
the data also enables detailed observation of the fine structure associated with the boundaries 198
between these water masses. 199
200
Note that the position of the STF from sea surface temperature (SST) data (Fig. 1) is inboard of 201
much of the SC reflectivity seen in the seismic data (Fig. 2). There are two likely reasons for this. 202
First, the largest share of water moving in the SC is likely to be from the SAW side of the STF. 203
Second, the seismic data do not image near-surface conditions, whereas SST data do. The SST 204
images show that the position of the greatest temperature change occurs just NW of the seismic 205
sections. Even so, the surface water at the NW end of the profiles is still much warmer, ~11°C, than 206
the observed SAW, <9.5°C. The high-amplitude SC reflections are assumed to be the result of this 207
temperature change. This interpretation is also supported by CTD data from May 1994 (Fig. 3) 208
[Sutton, 2003]. 209
210
Seismic Oceanography of the Subtropical Front SE of New Zealand 10
3.3. Mesoscale eddies 211
Mesoscale eddies, with horizontal spatial scales of roughly 10 to 100 km, evolve over time scales of 212
weeks to months and are an important part of oceanic circulation systems. They range in size from 213
large eddies (>100 km in diameter) involved with net transport through constricted waterways such 214
as the Mozambique Channel [e.g., Biastoch and Krauss, 1999] to baroclinic eddies (perhaps 10 km 215
in diameter) that are linked to the horizontal and vertical transport of water masses in association 216
with strong currents [Stern, 1967]. 217
218
In the seismic data presented here, each line contains several features that resemble eddy lenses 219
from previously published work. Four of these features are highlighted (Fig. 4). These lenses have 220
lateral dimensions between 8 and 15 km, so could be classified as baroclinic or perhaps even sub-221
mesoscale. This contrasts with most previously reported lenses, which are mostly greater than 222
30 km across. Another point of difference is the depth at which the cores of the eddies are centered. 223
The centers of the eddies seen in this project are all shallower than 500 m, whereas nearly all of the 224
lenses from other data are deeper, around 1000 m. The depth of a particular lens is dependent on its 225
average density relative to its surroundings. For example, at the Mediterranean outflow, dense salty 226
water flows through the Straits of Gibraltar, with a maximum depth of 400 - 500 m to meet the 227
North Atlantic Current Water (NACW). The Mediterranean water has a density anomaly of around 228
0.7 kg/m3, causing it to sink about 500 m to a depth of around 1000 m until the difference in density 229
can be compensated for, i.e., where the Mediterranean water reaches neutral buoyancy [Buffett et 230
al., 2009; Price et al., 1993; Richardson, 1993]. 231
232
Three types of reflectivity features are presented here to highlight the unique characteristics of the 233
eddies observed at the STF off New Zealand. 234
Seismic Oceanography of the Subtropical Front SE of New Zealand 11
3.3.1. Eddies with bowl-‐shaped reflections 235
Lens A (Figs. 2c and 4a) is situated both within and below the interpreted Southland Current, close 236
to the location where eddies may be forming. This lens does not exhibit the typical elliptical shape 237
of a meddy, which is thought to exist due to a stable density relationship with its surroundings. 238
Instead, Lens A has a shape tending more towards that of a bowl, similar to eddies seen in contact 239
with the surface (e.g., a warm core ring eddy). If this were the case, however, the overlying 240
reflections would be expected to follow a similar bowl-like curvature [Yamashita et al., 2011]. 241
Another explanation for the shape of Lens A is that it may be gravitationally unstable, i.e., it might 242
be sinking. As mentioned earlier, the make-up of the lens may be influenced by the upwelling of 243
cold, dense SAW. This would cause a relatively abrupt density contrast leading to the downward 244
movement of the lens. Both of these ideas are highly speculative and require further examination of 245
similar lenses associated with the SF to better constrain the processes here. 246
3.3.2. Eddies with strong and irregular lateral boundaries 247
Several lenses exhibit strong lateral boundaries. Generally, these boundaries have an overall convex 248
outward shape (e.g., Lens D in Figs. 2a and 4d). However, in some cases these boundaries have 249
irregular configurations. For example, the boundaries of Lens C have an inflection that occurs near 250
the top of the lens at a water depth of about 200 m (Figs. 2b and 4). The stacks of reflections on the 251
boundaries of Lens C generally narrow from their base and then at 200 m they begin to open up 252
again. The inflection is more notable on the SE boundary, but its cause is unknown. One possibility 253
is that it may be related to interactions with the sea surface. An alternative hypothesis is that the 254
inflection may represent a second overlying eddy core; however, if this were the case, a reflection 255
separating the cores would be expected [Biescas et al., 2008]. 256
Seismic Oceanography of the Subtropical Front SE of New Zealand 12
3.3.3. Eddies with strong internal reflections 257
Several eddies contain strong internal laterally continuous reflections (e.g., Lenses B and C in Figs. 258
2, 4b and 4c). Such reflections suggest that there is a somewhat regular stratification of water layers 259
within the eddy. Most of these internal reflection are sub-horizontal as seen in Lens B. However, in 260
some cases, the reflections are inclined, like the low-amplitude concave-down reflection in Lens C 261
that separates the lower NW section from the rest of the feature. Measurements taken from the 262
steepest section of this reflection indicate an apparent dip of 5° SE. This dip contrasts greatly with 263
the subhorizontal reflections that make up the lateral boundaries of the lens. Based on observations 264
made by Géli et al. [2009] and Hobbs et al. [2009], however, it is assumed that the ‘dipping’ 265
structure is likely to be made up of irresolvable short flat reflections. Such a structure could indicate 266
a second core in the eddy, but confirmation of such an interpretation would require direct physical 267
measurements of its thermohaline properties. 268
269
3.4. Confirmation of lens interpretations with satellite data 270
Support for the interpretation of eddies at depth could come from Sea Surface Temperature (SST) 271
and Sea Level Anomaly (SLA) satellite data [e.g., Pinheiro et al., 2010]. SST images will only 272
provide information on the near surface temperature characteristics of eddies, but SLA data will be 273
affected by density contrasts throughout the water column and therefore low or high density eddy 274
lenses should be manifested by a corresponding drop or rise in sea level. Seismic images show that 275
much of the detail of the lenses appears to be below the surface, which means that the overlying 276
water will mask the thermal and density anomalies of the eddies. Also, the identification of smaller 277
mesoscale eddies may be difficult with both SST and SLA data due to their limited lateral extent 278
[cf. Pinheiro et al., 2010]. 279
280
Seismic Oceanography of the Subtropical Front SE of New Zealand 13
Both SST and SLA data sets can be used to corroborate seismic oceanography interpretations. SLA 281
satellite data are limited by their spatial resolution (data tracks are separated by up to 270 km) and 282
can therefore only accurately resolve large structures [Pinheiro et al., 2010]. Although SST data are 283
more accurate (resolution of 1 – 4 km), they are highly limited by cloud cover. Cloud-free SST 284
satellite images coincident with previously acquired seismic lines are not common. The best SST 285
satellite data found to supplement the findings in this project consist of eight-day composites of the 286
area with a resolution of 4 km (e.g., Fig. 1). Such images help to constrain of the position of the 287
STF, but are not helpful for identifying specific eddies because they would travel too far during an 288
eight-day period. However, in the satellite image, there are signs of eddy-like patterns to the SE of 289
the STF that support variability and mixing of surface waters in this region. 290
291
5. Discussion 292
Several prior seismic oceanography studies have characterized water-mass boundaries, the first 293
being an investigation of the front separating the Labrador Current from adjacent North Atlantic 294
Current (NAC) off of the east coast of North America [Holbrook et al., 2003]. This specific front is 295
identified as a ‘slab’ of sloping reflections beneath a relatively transparent zone (the warm NAC). 296
Such an observation is consistent with observations of the STF/SC in the data presented here, 297
particularly in Line OMV-08-42 (Fig. 2a). Similar observations are reported on the other side of the 298
Atlantic Ocean, where a sloping collections of reflections separate the warmer Norwegian Atlantic 299
Current from the underlying Norwegian Sea Deep Water [Nandi et al., 2004], and off of the east 300
coast of Japan between the boundary of the warm Kuroshio and the cold Oyashio water-masses 301
[Nakamura et al., 2006]. 302
303
Seismic Oceanography of the Subtropical Front SE of New Zealand 14
The greatest factor limiting interpretations of seismic oceanography data is the general lack of 304
coincident physical property measurements of the water. Ocean masses, and therefore their 305
boundaries, are typically defined by a collection of physical parameters such as temperature, 306
salinity, density and, in some cases, oxygen level. Shaw and Vennell [2001], for example, define the 307
SAW (based on previous observations) as water having temperature and salinity ranges of 7 – 12°C 308
and 34.3 – 34.5 psu, respectively. As seismic reflections are the result of relative changes in 309
acoustic impendence, absolute physical properties cannot be directly measured or calculated [cf. 310
Papenberg et al., 2010]. The precise location of the water mass boundaries, therefore, can only be 311
inferred. However, interpretations are still possible, for example, by comparing such properties as 312
sound speed derived from the seismic data with similar properties derived from CTD data (Fig. 3c). 313
314
In contrast to the Mediterranean outflow, the density of the local STW off New Zealand is only 315
slightly less dense than the adjacent SAW [Heath, 1972; Hopkins et al., 2010]. This suggests that if 316
the eddies contained solely STW they would be forced to the surface. The data shown here suggest 317
that this is not the case, with Lens C (Fig. 4c), for example, having core depths of around 400 m and 318
Lens D (Fig. 4d) appearing completely submerged. To account for this, the lenses need to have a 319
higher density than the water at the surface of the SAW. A mechanism that may account for this, is 320
the upwelling of deep cold SAW filaments in the region of the front [Hopkins et al., 2010]. Colder 321
and therefore denser SAW may become entrained during lens formation increasing the lens density. 322
Such upwelling is consistent with the reflectivity patterns seen in these data. 323
324
Variations over seasonal or annual time scales include such things as the position, dimensions, and 325
internal characteristics of major currents or stratified water masses. A comparison of the three 326
profiles presented here (Fig. 2), recorded in close geographic proximity to one another, shows that 327
Seismic Oceanography of the Subtropical Front SE of New Zealand 15
some variability occurs in the position of the main water masses over time. Lines OMV-08-42 and 328
OMV-08-45 (Fig. 2a and b) were recorded about 3 weeks apart, with line DUN-06-13 (Fig. 2c) 329
recorded about 22 months earlier. Optimal weather conditions for seismic operations in this part of 330
the ocean occur during the austral summer, so there is no data available on seasonal variations. 331
However, the positions and thicknesses of water masses over monthly to annual time scales are 332
observed to change considerably. For example, the thickness of the transition zone between the 333
SAW and AAIW water masses changes by as much as 300 m, and the position at which the 334
transition zone intersects the seafloor changes laterally by about 10 km near a change in slope at a 335
water depth of ~620 m. The details of internal reflectivity of the various water masses are also 336
observed to change over time. However, the overall reflectivity is similar and enables the various 337
water masses to be interpreted based on their reflectivity characteristics. 338
339
The stability of water mass boundaries in time and space is known to be variable. For example, off 340
the southeast coast of New Zealand, numerous methods show that the surface position of the STF 341
annually moves nearer or farther from shore by several km. Furthermore, subsurface measurement, 342
including the three seismic sections presented here, show that the variability in the position of the 343
STF boundary in the subsurface is potentially greater that the variability observed on the surface. 344
Given how such ocean boundaries are observed to differ around the world [e.g., Graham and De 345
Boer, 2013], the seismic oceanographic method provides a means to evaluate the details of such 346
change. 347
348
The Mediterranean Outflow into the North Atlantic has been the focus of much ground-breaking 349
research in seismic oceanography [e.g., Biescas et al., 2008; Biescas et al., 2010; Buffett et al., 350
2009; Buffett et al., 2010; Pinheiro et al., 2010]. As a result, Mediterranean outflow eddies (referred 351
Seismic Oceanography of the Subtropical Front SE of New Zealand 16
to as meddies), a well-established mesoscale eddy phenomenon, have been identified in seismic 352
oceanography data as anomalous lens-shaped bodies [e.g., Biescas et al., 2008]. There are only a 353
few other widely varying observations of eddies outside of this region made with seismic 354
oceanography. For example, Yamashita et al. [2011] display a Warm Core Ring, or current eddy, 355
approximately 250 km across and in contact with the surface off Japan. In contrast, the eddies 356
observed in the seismic data presented here are generally smaller in scale. In order to compare these 357
eddy structures to those that have been previously published, a tabulation of previously published 358
eddies is shown in Table 2. 359
360
The detailed interpretations of the eddies in these data can play a significant role in understanding 361
the chemical and physical mixing mechanisms that result in the episodic periods of elevated levels 362
of phytoplankton production across the STF [Boyd et al., 1999; Boyd et al., 2000; Boyd et al., 2004; 363
Butler et al., 1992]. In the three seismic lines presented here, at least ten possible eddies at various 364
stages of formation or breakup are imaged (Fig. 2). One of these may be an evolving eddy located 365
within the STF (Lens a, Fig. 4a). Others may be more fully formed eddies lying within the SAW 366
(e.g., Lenses b to d, Fig. 4b-d). The implications are that lenses containing a significant amount of 367
warm, relatively iron-rich, and macro-nutrient-poor STW form within the STF region and then, 368
driven by the flow of the Southland Current, break to the south into the cold, iron and silicate poor, 369
but macro-nutrient rich SAW, thereby enabling an increased rate of productivity. The regularity of 370
such events can only be the subject of speculation at this point, although the common appearance of 371
eddy features in the three profiles presented here suggests that they are regular. They potentially 372
may be the main mechanism for transporting nutrients across the front. However, numerous details 373
remain to be investigated, such as whether or not eddies at depth as well as those within the photic 374
zone can effectively contribute to nutrient transport. 375
Seismic Oceanography of the Subtropical Front SE of New Zealand 17
376
6. Conclusions 377
This project has reprocessed three closely spaced 2-D multi-channel seismic lines, originally 378
collected for the petroleum industry, with the intention of examining the thermohaline finestructure 379
within the water column. Seismic reprocessing has produced three remarkable images of the local 380
oceanographic setting that are the first of their type in this part of the world. Most significantly, the 381
data are interpreted to show the internal structure of a series of eddies that are expected to drive 382
mixing between subtropical and subantarctic water masses? The dense lateral spacing of the seismic 383
data enables an investigation of the finer details of mesoscale ocean features at depth. This work 384
greatly improves the lateral resolution of conventional oceanographic techniques that typically use 385
largely spaced probes (XBTs and CTDs) to measure the physical properties. 386
387
This dataset builds on the existing understanding of the local oceanography and also contributes to 388
the general understanding of eddies from a seismic oceanography perspective. The thermohaline 389
fine-structure of the STF can be identified in the seismic images with never-before-seen detail. Data 390
from this project show that the width of the STF at depth can vary between the summer and autumn 391
by as much as 20 km. These images also identify the local boundary zone between the SAW and the 392
underlying AAIW. 393
394
Several isolated lenses have been identified on the three data sets, ranging in size from 9 to 30 km 395
in diameter. These lenses have been interpreted as mesoscale eddies that appear to form at depth 396
within the region of the STF, in association with the flow of the SC, before spinning off into the 397
SAW. The eddies in these data have been compared to previously published seismic observations of 398
eddies, which on average tend to be larger. The significance of their size may be a result of their 399
Seismic Oceanography of the Subtropical Front SE of New Zealand 18
formation at the STF at a location south of New Zealand where the front is relatively constricted 400
[Smith et al., 2013]. This remains to be tested by observations of similar features elsewhere in the 401
world’s oceans. 402
403
Without further investigation, the amount of water carried by these eddies can only be speculative; 404
however, these observations may provide a means for understanding mixing mechanisms across the 405
STF that episodically give rise to the increased rates of productivity observed SE of New Zealand. 406
407
Acknowledgments 408
The seismic data for this project were graciously provided by the New Zealand Government (line 409
DUN-06-13) and a consortium of petroleum exploration companies: OMV New Zealand, Shell, 410
Mitsui, and PTTEP (lines OMV-08-42 and OMV-08-45). In particular, we personally thank Tim 411
Allan at OMV and Roland Spuij at Shell for their on-going interest and support for our work. 412
Seismic processing was undertaken with an academic licence for Globe Claritas. Phil Sutton and 413
NIWA provided access to archived CTD data from our area of interest. This work was supported by 414
the New Zealand Marsden Fund, contract UOO0920. 415
416
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568
569
Seismic Oceanography of the Subtropical Front SE of New Zealand 27
Figure Captions 570
571
Figure 1. The locations of the collected multi-channel seismic profiles used in this study with 572
distance labelled as a reference. Also shown are the locations of CTD casts collected by XXX on 573
XXX used in Fig. 3. Also shown are the approximate positions of the Southland Current and the 574
Sub-Tropical Front. The location of the front is based on the sea-surface temperature image shown 575
here (insert relative satellite information here). The inset shows the regional bathymetry (NIWA?). 576
577
Figure 2. Migrated sections of lines (a) OMV-08-42, (b) OMV-08-45, and (c) DUN-06-13 578
processed to reveal oceanographic features. The sections have been divided into water-mass regions 579
based on reflectivity. The Southland Current (SC) and transition zone (TZ) are areas of higher 580
reflectivity and are interpreted to represent areas of increased temperature variability. The 581
continuous high amplitude reflection seen at the surface in lines OMV-08-13 and -42 has been 582
identified as a surface layer (SL). SAW – Subantarctic Water, AAIW – Antarctic Intermediate 583
Water. Note outlined sections labelled (a) – (d) are enlarged in Figure 4. 584
585
Figure 3. Temperature (a), salinity (b), and sound speed (c) sections of CTD data from May 1994 586
(source: NIWA). These data are located near the seismic profiles presented in this study and 587
they provide the basis for the water mass interpretations made on the seismic data. 588
589
Figure 4. Enlarged MCS profiles from Figure 3. (a) – (d) are displayed at the same horizontal and 590
vertical scales. The labelled distances correspond to the distance along the MCS profile. 591
592
593
Seismic Oceanography of the Subtropical Front SE of New Zealand 28
Table 1. Seismic data acquisition parameters 594
DUN06 Survey OMV08 Survey
Client New Zealand Ministry of Economic Development
OMV and partners
Acquisition date January – March 2006 Dec. 2007 to Feb. 2008 Total source capacity: 4140 in³ (67,842 cm³) 3980 in3 (65,220 cm3) Total Number of guns: 26 active 48 (40 active, 8 inactive)
Vessel: Multiwave Geophysical: Pacific Titan Wavefield Inseis: Discoverer II
Streamer length: 6000 m 6000 m Number of channels: 480 480 + 27 aux Record length: 8000 ms 8000 ms Sample interval: 2 ms 2 ms Firing delay from time zero:
50 ms 50 ms
Shot interval: 25 m 25 m Group length: 12.5 m 12.5 m Group interval: 12.5 m 12.5 m Streamer depth: 7 +/- 1 m 10 – 12 m 595
Seismic Oceanography of the Subtropical Front SE of New Zealand 29
Table 2. Summary and comparison of seismic characters of mesoscale and sub-mesoscale eddies 596
previously reported and presented in this paper. 597
Location Publication Profile Name
Core depth (m)
Apparent thickness (m)
Apparent width (km)
Internal reflections
Further comments
Mediterranean Outflow
[Biescas et al., 2008]
IAM-3 1000 900 50 Yes, concentric habit
IAM-4 1000 1200 80 Yes, ~2 quazi-horizontal moderate-low amplitude
Double core?
IAMGB1 1000 1500 65 Negligible [Buffett et al.,
2009] IAM-11 1000 1100 30 Negligible
[Buffett et al., 2009; Pinheiro et al., 2010]
IAM-5 (1) 1200 1000 30 Yes, one seemingly continuous ‘<’ shaped reflection
IAM-5 (2) 1200 1000 40 Yes, concentric habit
[Quentel et al., 2010]
GO-LR-01 and GO-LR-13
1000 700 25-35 Negligible
[Buffett et al., 2010]
GO-LR-05 1100 800 80 Yes, reflections mimic upper surface
Possible inner core
East of Falkland Islands
[Sheen et al., 2009]
Un-named 750 500 10 Negligible Sub-mesoscale
Un-named 1000 1000 40 Negligible Very well defined boundaries, asymmetric in shape
NW coast of Ireland
[Jones et al., 2010] PAD-95-11 (1)
750 500 10 Negligible Sub-mesoscale
PAD-95-11 (2)
Surface >1000 80 Negligible In contact with surface
SE of Hokkaido, Japan
[Yamashita et al., 2011]
A2OBS and A2MCS
Surface >1500 250 Yes, multiple groups of concave (up) reflections
Mesoscale warm-core ring
SE of New Zealand
This work DUN-06-13 (Lens 1)
350 300 9 Yes, moderate amplitude undulating reflections
Sub-mesoscale
DUN-06-13 (Lens 2)
200 950 15.5 Yes, concentric Possible inner core
OMV-08-45 (Lens 3)
400 500 30 Yes, chaotic style in center
OMV-08-45 (Lens 4)
400 500 12.5 Yes, ne low amplitude curving (concave-down)
Lateral reflections inflect toward the surface
OMV-08-42 (Lens 5)
350 300 9 Negligible Sub-mesoscale
598
0
300
600
1500
900
1200
40 60 80 100 120 140 160
Dep
th (m
)
c)
AAIW
SAW
Line DUN-06-13: 23 March 2006NW SE
TZ
SCa)
b)
Seafloor
0 20 40 60 80 100 120 140 160 180 2000
300
600
1500
900
1200
Dep
th (m
)
Distance (km)a)
AAIW
Line OMV-08-42: 12 January 2008NW SE
SAW
TZ
SC
d)
Seafloor
0 20 40 60 80 100 120 140 160 180 2000
300
600
1500
900
1200
Dep
th (m
)
b)
AAIW
Line OMV-08-45: 25 - 26 December 2007NW SE
SAW
TZ
SC
c)
Seafloor
Gorman et al. - Figure 2
0
1.0
0.5
1.5
0
1.0
0.5
1.5
0
1.0
0.5
1.5
Dep
th (k
m)
(a) Temperature (°C)
(b) Salinity (psu)
(c) Sound Speed (m/s)
170.0° 170.4° 170.8°Longitude
10
8
6
4
2
34.5
34.4
34.3
34.2
1500
1490
1480
1470
Gorman et al., Figure 3