RRS CHARLES DARWIN CRUISE 68
08/05/1992 - 08/06/1992

THE NORTH ATLANTIC TRACER
RELEASE EXPERIMENT (NATRE):
FIRST SAMPLING LEG

Principal Scientist: A. J. Watson
Co-Principal Investigator: J. R. Ledwell

Report prepared by: C. S. Law, J. R. Ledwell, A. J. Watson, M. Beney,
S. Becker, T. Donaghue, C. Fernandez, B. J. Guest, C. Kinkade,
M. I. Liddicoat, K. Lubcke, P. Nightingale, C. Marquette,
R. Oxburgh, D. A. Phillips and S. Watts

Acknowledgement: We thank the personnel of the RRS Charles Darwin under
Captain P. Macdermott. Their hard work and skilful seamanship helped to make
this a successful and enjoyable cruise.

Funding for NATRE is provided by the Natural Environment Research Council
in the UK and the National Science Foundation in the USA.

For a complete set of figures*, tables and appendices see:

RRS Charles Darwin Cruise 68 08/05/1992 - 08/06/1992, The North Atlantic 
Tracer Release Experiment (NATRE): First Sampling Leg, Internal  report, 
Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth.  Date 1992.

1	INTRODUCTION AND CRUISE OBJECTIVES

The North Atlantic Tracer Release Experiment (NATRE) is a study of vertical 
and horizontal mixing processes in the thermocline region of the subtropical 
gyre of the North Atlantic, with the aim of characterising rates of diapycnal 
and isopycnal mixing and the processes which give rise to them. The experiment 
was initiated by the WHOI vessel R/V Oceanus on a cruise concurrent with CD68; 
between 0100 on 05/05/1992 and 0700 on 13/05/1992, Oceanus released a total of 
945 moles of sulphur hexafluoride (SF6) in a series of 9 streaks on an 
isopycnal surface at approximately 300m. The streaks were 5-10km long, and 
were injected into a region of order 20 x 20km centred at 25° 38'N, 28° 15'W. 
Along with the tracer, ten SOFAR "bobber" neutrally buoyant floats were 
released and ten RAFOS floats.

The plan for the experiment as a whole is to follow the dispersal of this 
tracer over a period of a year following release. The spread of the tracer in 
the vertical will give a direct measurement of diapycnal mixing, while the 
horizontal dispersion of the floats and tracer will enable the quantification 
of the isopycnal mixing. The floats serve the additional purpose of enabling 
the location of the tracer. In order to study the processes driving vertical 
mixing, measurements of fine structure have been and will be made during 
cruises by R/V Oceanus and the Canadian vessel CSS Hudson in the area of the 
tracer patch. The overall cruise plan for the experiment is:

	April 1992 - site survey		R/V Oceanus
	May 1992 - injection			R/V Oceanus
	May 1992 - first sampling		RRS Charles Darwin (this cruise)
	October 1992 - 2nd sampling		R/V Oceanus
	March 1993 - microstructure		CSS Hudson
	April 1993 - final sampling		RRS Charles Darwin

In this context, the objectives of CD68 were therefore:

a) To document the "background" concentrations of SF6 in the region of the 
   experiment. Industrial use of SF6 has led to a significant global atmospheric 
   background concentration, presently of order 2 x 10^-12 v/v, and this has in 
   turn generated a concentration in ocean water which decreases with increasing 
   depth. It will be important in the later stages of the experiment to know what 
   background this atmospheric source has generated at the depth of the release, 
   since as the tracer spreads and dilutes, the concentrations will decline 
   sufficiently that the background will begin to become significant in the total 
   after a year.

b) To document the initial distribution of the tracer as soon as practicable 
   after release. The "quality" of the release needed to be verified by obtaining 
   measurements of its initial vertical and horizontal spread, since it is from 
   these values that any future spreading due to mixing processes within the 
   ocean will be measured. An initial vertical spread as narrow as possible was 
   therefore desirable. In practice we hoped ideally to confine the initial 
   vertical spread to within ±5m, with ±10m considered acceptable.

c) To show that the entire tracer patch has been documented, we wished to 
   attempt a "budget" of the patch, that is, a reconstruction to sufficient 
   accuracy to estimate the total amount of SF6 in it and show that this agrees 
   with the amount injected.

To achieve these objectives, two main scientific areas were set up on the 
ship. These were a gas chromatography analysis area in the main lab, and a 
sample handling area in the wet lab.

2	ITINERARY

Figure 1* shows the cruise track:

2145	8 May 1992: The vessel departed from Las Palmas, Gran Canaria and set 
	course for the working area. At Las Palmas we had been delayed about 12 hours 
	awaiting a package emergency-shipped from Woods Hole containing parts for an 
	experimental Richardson Number float to be launched from Oceanus. It was 
	agreed that the time lost from our schedule was worthwhile in order to save 
	this project. During the passage we made frequent stops to test equipment and 
	techniques, described in Section 3 below.

0600	12 May to 1600 14th May: Arriving near 25°N 28°W we documented the 
	background concentrations of SF6 in the working region, in 8 casts. These are 
	described in Section 4 below.

1600	14 May to 1130 31st May: We documented the tracer and float initial 
	distributions resulting from the releases by Oceanus. This was the main part 
	of the work and is described in Section 5.

1130	31 May to 0000 8th June: Passage to Barry, UK.

3	EQUIPMENT TESTS (8th - 11th May)

9 May - Casts 1 and 2: The CTD sled to be used for tracer "tows" (see below) 
was modified and tested in the water towing at speeds up to 1kt close to the 
surface. Its behaviour was monitored and we verified that it showed no 
tendency to rotate or fishtail. We monitored the load on the wire (which did 
not exceed 400kg) and then tested various switches which would form part of 
the main sampling array (see Section 5).

10 May - Casts 3 and 4: We tested the automated winch control system designed 
and built by R. Powell of RVS, by hanging a heavy (650kg) weight on the wire, 
paying out wire and verifying that the winch responded correctly when 
artificial CTD input was fed into the system. We then tested the system for 
real by putting the CTD sled on the wire, lowering to 150m and transferring to 
automatic control. The system showed some tendency to "hunt" but did respond 
basically correctly.

We also stopped to deploy a hydrophone (cast 5) to listen for the "bobber" 
SOFAR floats deployed by Oceanus from 1630 to 1930, but could not hear any of 
them.

11 May - Cast 6: Intercalibration of CTD and autonomous "SEACAT" CTDs. The 
SEACATs were mounted on the CTD cage and lowered to 600m, stopping every 100m 
for 5 minutes to allow time for equilibrium of all sensors.

Cast 7: (nearly) full casts of samplers and flying cage at 300m. We were 
pleased with the automatic winch driving system which on quiet sections was 
able to maintain density surface with a standard deviation of about 0.0012-
0.0015 units of sigma. However, it also hit noisy sections where it was many 
times worse. This cast flew for 4 3/4 hours.

4	BACKGROUND SAMPLING (12th - 14th May)

These casts were made in an octagonal pattern around the central region where 
the tracer had been laid, but at about a distance of 40 nautical miles to 
ensure that the injection did not affect the background values. Initially the 
samples were collected using the canister samplers designed for obtaining 
integrated samples of a streaky tracer. As the analysis proceeded it became 
apparent that the canister samplers were not well suited for very low levels 
of SF6, and we therefore reverted to standard Niskin bottle sampling. Figure 2* 
shows the positions of the stations, details of which are given in Table 1.

The cross-pattern was chosen to make sure that we bracketed the central 
position, and also to allow close approach and VHF contact with the Oceanus in 
the centre.

4.1	Hydrographic Data

We used the SEACAT autonomous CTDs rather than the Neil Brown CTD as this 
avoided having to dismount the CTD or the pylon from the cage. The station 
list gives details of the positions of the SEACATs on the wire. A description 
of the SEACATs is given in Appendix III. The SEACATs are not well configured 
for measuring salinity because of the slow response of the conductivity cell, 
and especially for vertical profiles because the cells are oriented 
horizontally. Therefore, we used temperature as a surrogate for potential 
density for the casts performed only with the SEACATs, the mean relationship 
between temperature and potential density having been determined later in the 
cruise with the EG&G MkIII CTD.

The SEACATs were calibrated for the upper 600m of the water column during cast 
6, when they were deployed together with the MkIII CTD on the sampling sled. A 
multiplier and addend were established for each SEACAT for temperature and 
conductivity, and for pressure for the two SEACATs with strain gauges, namely 
884 and 885 (Table 2).

4.2	SF6 Data

Casts 8-12 showed some obvious contamination and considerable scatter, though 
the precision of duplicates taken from the same sampler were generally good. 
This confirmed a trend seen earlier in the samples analysed from Cast 7, the 
first flying of the winch, in which two bags gave high results. Several 
samples had bubbles in, so for instance a total of 5 samples out of cast 9 had 
to be discarded.

Because of the contamination problem, all interpretation of the background 
concentrations was made with the last four casts only, for which Niskin 
samples were taken. Figure 3* show SF6 concentrations from these casts plotted 
as a function of depth. Three of the profiles agree closely but the last (cast 
15) is consistently high - it is just possible that this is due to generalised 
contamination picked up during the transfer of personnel from the Oceanus 
which took place a few hours before this cast was performed. However, in the 
absence of firm evidence of contamination this cast was treated in the same 
way as the other three.

For each sample from casts 12-15, a potential density was assigned using the 
observed temperature and the cruise-mean temperature vs potential density 
relationship. The four profiles were then linearly interpolated to standard 
densities and averaged. Figure 4* shows the results of this exercise and also 
tabulates the basic cast data. At the "target" isopycnal of 28.05, the men 
background value is 4.23 x 10^-16 moles/l.

5	TRACER SAMPLING (14th - 31st May)

5.1	Method

The difficulties in sampling a tracer when the distribution is streaky are 
well known and have been described in the literature. Grab samples, as 
obtained with Niskin bottles, cannot be relied on to give an accurate estimate 
of the vertical profile because there is no way of knowing whether the sample 
was taken in a streak or out of it, and even small-scale vertical shear in the 
water column will distort the apparent profile. The method we employed to 
circumvent the "streakiness problem" was to use a vertical array of custom-
designed water samplers which fill with water at a slow and reasonably 
constant rate, interspersed with SEACAT CTDs. The array was towed through the 
water for a period of several hours, with its centre kept homed in on the 
isopycnal surface on which the tracer was released. During this time the array 
may cross one or more streaks of tracer, and subsequent analysis revealed a 
vertical cross-section averaged over the tow track. Figure 5* shows the 
position of samplers, SEACATs and central CTD in the array used for casts 16 - 
28. From cast 29 onwards, extra samplers were added at +30m and -30m from the 
centre.

The purpose of the SEACATs was to provide information on the variation in 
density and density gradient over the region from 30m above to 30m below the 
centre of the array. Without such information it would be necessary to assume 
that these properties remained constant over each sampling tow, whereas 
examination of the data shows that this is frequently far from the truth. Use 
of the SEACATs is described in more detail in Appendix III and Figures 30-34* 
show examples of the kind of data obtained from them.

In order to measure the true tracer distribution with respect to density, it 
was necessary to keep the centre of the array "flying" along a constant-
density surface. Since these surfaces are not at constant depth, a CTD sensor 
at the centre of the array was required which measured the density there, 
along with a method of altering the amount of wire out in response to this 
signal to continuously adjust the sensor to the correct depth. In addition to 
the CTD, samplers, calibration equipment and an experimental "multichamber 
sampler" system were mounted at the centre of the array. To carry this 
equipment, a custom "sled" was built at WHOI. The upper half of the array was 
attached to the CTD wire itself, while the lower part was attached to an 
auxiliary wire hung from the bottom of the sled. Upper and lower arrays were 
triggered by messengers.

The sled, a frame, approx. 1m x 1m x 2m, held the following:

	(a)  The CTD (Neil Brown MkIII).
	(b)  Two or more integrating samplers.
	(c)  A multisampler holding a carousel of 18 sampling syringes filled 
	     sequentially during the tow.
	(d)  A rosette pylon which fired to perform several tasks, in the 
	     following sequence:

	(1)  Release a messenger which tripped the samplers below the sled.
	(2)  Start the samplers on the sled.
	(3)  Start the multisampler.
	(4)  Collect up to seven salinity samples and turn four reversing 
	     thermometers during the course of the tow, for purposes of 
	     calibrating the CTD.

The procedure for the tracer tows was to head due north, that is with the wind 
on the starboard bow, while towing the sample array at 0.5 - 1kt from the 
starboard "A" frame and CTD winch. In order to counteract the westerly drift 
and wind effect, the ship actually headed at about 30° to starboard to do 
this. Deployment of the sled and all the associated samplers took a time of 
order 2 hours initially, but decreased to one hour after a little practice. 
The sled was then lowered to 500m in order to get CTD data through the depth 
of interest. It was then returned to 300m and transferred to automatic winch 
control, and the messenger dropped to start the samplers filling. After a time 
of 3.5 hours all the samplers should be filled, but to be sure they were left 
down for an extra 0.5 hours. Finally another drop to 500m was performed 
followed by recovery to deck, which took about 45min. Turn-around of the 
apparatus was performed during steaming to the next site. The total time of 
the cycle was generally of order 8 hours.

5.2	Float Location

Hydrophone listening stations were occupied as detailed in the station list on 
3 occasions. The vessel steamed a good distance to the NE or SW of the tracer 
patch (typically 20nm) in order to obtain a favourable geometry with respect 
to a Drifting SOFAR receiver deployed by Oceanus, which was about the same 
distance to the NE of the patch. The hydrophone was lowered to listen during 
the period 0630-0930 or 1830-2130 when the SOFAR floats deployed by Oceanus 
were programmed to transmit. We heard respectively 4, 8, and 5 floats out of 
10 released on casts 31A, 37A and 44A.

On three occasions we were able to obtain good fixes on some of the floats by 
using the Simrad PES to listen for the 10kHz pings emitted by them. We used a 
box-search technique whereby as the point of closest approach is passed 
(indicated by a flattening out and then decrease in the pattern of pings on 
the Simrad) a 90 turn to port or starboard is made. We used this technique 
between 2340 on 22 May and 0440 on 23 May to obtain fixes on four floats. The 
Simrad proved an excellent instrument for listening to the signal from the 
floats, with a range of about 3nm.

5.3	Hydrographic Data

Additional calibrations of the SEACATs against the MkIII CTD were made at 
intervals. It was found that, because of a software imperfection, a correction 
must be applied to the time record from the SEACATs. The time since Startup of 
the SEACATs must be multiplied by the drift correction in Table 2 to determine 
the correct time.

The SEACATs were calibrated again at the target density of the sampler tows by 
towing them for 1 hour on the sled during Cast 28 (Table 3). They were checked 
once more in this way during Cast 49 (Table 4), and found to have drifted very 
little.

The MkIII CTD was calibrated for temperature and pressure on shore prior to 
departure. The temperature calibration was checked throughout the cruise with 
four digital reversing thermometers mounted on the Sampling Sled (Appendix 
II). These were the same thermometers used during the injection cruise on 
Oceanus. However, for both cruises the strong temperature gradients and the 
mismatch in time and space between the reversing thermometers and the CTD 
probes resulted in rms noise of around 0.090°C. Therefore the shore-based 
calibrations, which should stand to better than 0.004°C will be relied upon 
for temperature.

Salinity calibrations were also performed throughout the cruise, with samples 
from the 5-litre Niskin bottles during the background casts and from 1.2-litre 
Niskin bottles mounted horizontally on the Sampling Sled during the tows 
(Appendix II). They showed a consistent salinity error of -0.022psu for the 
MkIII system for the part of the water column sampled. This correction was 
applied in processing the CTD data. The processed CTD data are considered as 
accurate as possible; no post-cruise adjustment of the data is anticipated.

Each sampler tow was preceeded by a downcast to 500m at about 25m/min, and was 
followed by an upcast from 500m at roughly the same speed. Since the ship was 
generally moving at about 1kt (30m/min) the descent and ascent angles are 
about 45° from the vertical. Also the CTD probes, while located in an open 
area just a few cms from the leading edge of the sled, may see thermal 
contamination from the sled. For these two reasons the finestructure at scales 
of 1m or so are not to be taken as representing the vertical hydrographic 
structure. Averages of the data over many casts and the individual profiles 
over scales of 10m or more should be accurate, however.

Table 1 lists the times and positions at the start and end of the tows, and 
the tow tracks are plotted in Figure 6*. These can be taken as applying to the 
downcasts and upcasts, respectively. The data stream from the MkIII Deck Unit 
to the SUN-based processing system was not always turned on at the start of 
the cast and was sometimes turned off early. Also, although the wire out was 
always brought to 500m or more at the bottom of each cast, the pressure did 
not always reach 500db. Tables 5 to 8 list the properties caught at the top 
and bottom of each descent and ascent to show what data are available from the 
processed CTD files.

Representative profiles from the MkIII are shown in Figures 7 to 11*. These are 
the raw data, with only the spikes near the rosette trips removed. There are 
still obvious spikes of noise in the data. These also appear in the plots of 
potential temperature versus salinity shown in Figures 12 to 16*. These spikes 
are not severe enough to seriously affect the means discussed next.

The data from the CTD descents and ascents were interpolated to standard 
levels every metre from 10m to 500m, where data are available. These were then 
averaged at the 1m intervals to produce the mean profiles of Figures 17* and 
18, and the plots of mean potential temperature versus salinity in Figures 19* 
and 20. Profiles are allowed to drop out of these averages where there are no 
data. This can lead to discontinuities in the mean profiles, but does not in 
this case because the number of profiles remaining is so large. A listing of 
the mean of the descent data every 10m is given in Table 9.

Considerable CTD data near the target density surface of sigma-p = 28.05 were also 
obtained with the MkIII and SEACAT CTDs. Figures 21 to 29* show samples of data 
from the MkIII for several selected casts, including the ones where problems 
following the target density were encountered. These figures show about 4 
hours of data, and sometimes include the approach and leaving of the target 
surface at the ends of the records. However, if the winch control system is 
operating properly, simga-p should remain close to a constant value for the 
remainder of the record. Figures 27 to 29* show the CTD data for the tows for 
which this was not the case.

The winch control system failed during Casts 35, 49, and 54. Fortunately, no 
SF6 was found during Cast 35, Cast 49 was a calibration tow without samplers, 
and only small amounts of SF6 were found from Cast 54. It is possible that the 
anomalous shape of the SF6 profile from Cast 54 is due to the poor flight 
control shown in Figure 29*.

The 4 SEACAT CTDs were hung on the wire above and below the MkIII on the 
sampling sled to give data on the vertical temperature distribution along the 
sampling track. The SEACATs were hung just below the integrating samplers 
nominally at 16m, 6m, -9m, and -20m above the CTD. The SEACAT probes were 
located between 40 and 60cm below the inlet of the sampler above. The 
positions of these inlets are listed in Table 10. The approximate heights of 
the SEACAT probes are then: 15.5m, 5.1m, -9.8m, and -20.8m. The heights for 
SEACATs 885 and 884, at 15.5m and -20.8m, inferred from the pressure records 
average 15.8m and -20.2m, in fair agreement. The differences are not 
understood completely but may be due in part to the 0.3db digital resolution 
of the SEACAT sensors and a slight hysteresis in the MkIII gauge.

The discrepancies cannot be explained by wire angle. The angle at the sheave 
was typically less than 10°, and the 500lb weight at the bottom of the array 
will reduce this angle to nearly 0° in the vicinity of the array. Furthermore, 
the pressure offset at 15.5m is in the wrong sense to be explained by wire 
angle. Therefore, no corrections need be attempted for wire angle in the 
sampler heights.

Examples of the temperature records from the 4 SEACAT CTDs and the MkIII are 
shown in Figures 30 to 34*. It is clear from these figures how variable the 
spacing of isothermal and isopycnal surfaces is. A correction for the effect 
of this finestructure on the vertical distribution of the tracer as it is used 
to infer the diapycnal distribution will be made in post-cruise analysis.

The data from the 5 CTDs obtained during the 43 tows of the sampling sled give 
an excellent record of the hydrographic properties in the vicinity of the 
target density surface. Table 11 summarises the mean and rms temperature, 
salinity and density at the tow level of the sled. The target density was not 
always 28.05 because of errors made in correcting for the calibrated offsets 
earlier in the cruise. A history of nominal target densities is given in Table 
12, along with the actual target density, accounting for the errors made, and 
the actual mean density of the tow as measured by the MkIII CTD.

The mean and rms potential temperature, salinity, and sigma-p at the MkIII are 
listed for the tows in Table 11. This table also includes an estimate of the 
gradient dT/d sigma-p from the SEACAT and MkIII data and the ratio of rms 
temperature to this gradient. This last ratio gives an estimate of the rms 
sigma-p which is generally lower than that made directly from sigma-p, 
presumably because the latter is strongly affected by noise in the salinity 
record.

The array of SEACATs and MkIII CTDs has been used to estimate pressure, 
potential temperature, and salinity at the sigma-p = 28.05 surface by 
interpolation between the MkIII and the appropriate neighbouring SEACAT, 
usually SEACAT 882, located 9.8m below the MkIII. Estimates of the gradients 
dT/dz, dS/dz, and d sigma-p/dz, as well as the Density Ratio (R_) and the 
Buoyancy Frequency (N) are listed in Table 13. These gradients are estimated 
using the differences between the mean temperature, conductivity and pressure 
at the two extreme SEACATs, 885 and 884, separated by 36.3m. Many of these 
quantities are shown in contour maps below.

The gradients calculated from the MkIII and the neighbouring SEACAT have also 
been used to estimate the mean height of the MkIII above the sigma-p = 28.05 
surface for the tows. These are listed in Table 14, and have been used to 
estimate the heights of the sampler inlets above the 28.05 surface in reducing 
the concentration data.

5.4	Lateral Motion of the Patch

The data from the "bobber" floats are described in detail in Appendix III. 
These data have been used to infer the motion of the tracer patch during the 
sampling survey. These estimates have been used in choosing locations for the 
survey, and are used here for plotting and reducing the data. Two estimated 
for the motion of the patch have been made. The first is based on the 
positions of 5 of the floats, namely, 55, 56, 57, 58, and 59.

The positions of the floats on the days between actual fixes were estimated by 
linear interpolation. The longitudes for the days from the last fix to the end 
of the survey on 31 May were estimated by linear interpolation, while the 
latitudes, being more variable, were held at the last known position. The 
velocities of the floats for each day were estimated from simple differences 
of the positions. The velocity of a water parcel at any position and day was 
estimated from a weighted mean of the float velocities, the weighting function 
taken as inversely proportional to the distance from the float. Displacements 
of water parcels over a period of several days were then estimated using a day 
by day stepping procedure of position and velocity. Thus, the sampling tracks 
could be transformed from their original positions to positions on 23 May, the 
approximate central time of the survey (see Figure 35*).

The translation inferred in this way from the 5 floats mentioned above is 
considered conservative, in that 3 of the floats, 55, 57, and 58 are 
relatively stationary and remain close to one another. Thus they are 
redundant, and their low velocities are weighted too heavily.

A more liberal estimate of the translation was made by using only data from 
floats 56, 59, and 64, the last representing the slow velocities in the 
eastern area of the patch (see Figure 36*). Some data have been reduced using 3 
sets of positions, namely the original positions of the tracks, those 
translated with the conservative estimate, and those translated to 23 May with 
the liberal estimate. Most of the data, however, is presented using the 
conservatively translated positions.

5.5	Lateral Hydrographic Patterns at sigma-p = 28.05

Contour maps of some of the hydrographic properties listed for the sampler 
tows in Table 1 are shown in the contour maps. The map of pressure, Map 1, 
simply illustrates the mean and variation of the pressure on the surface. The 
pattern is probably a badly aliased picture of internal wave displacements. 
The potential temperature map (Map 2) shows that there are systematic 
variations of potential temperature and salinity at constant sigma-p, even over the 
small area of our survey. The salinity map (Map 3) reflects the potential 
temperature map, and confirms this conclusion.

Density ratio has proven to serve well as a water mass tracer. It is contoured 
in Map 4. It appears that its value in the western part of the patch, where 
the tracer was found, is about 1.80, which is characteristic of the region at 
this level on the larger scale. Since density ratio is the best water mass 
tracer, it is also mapped using the original positions (Map 5) and in the 
liberally translated positions (Map 6). The unrealistically fine structure 
seen in the original positions suggests that the translation was justified, 
while there is little basis in these figures to choose between the liberal and 
the conservative translation.

Map 7 contours the density gradient, which is proportional to the local 
absolute potential vorticity. Like the pressure, this is mostly useful for 
evaluating the mean and the variance, since the dominant features are probably 
aliased internal tides and waves.

5.6	Distribution of SF6 within the Patch

Forty two tows were made to document the distribution of the released tracer. 
Of these, 13 had no measurable SF6 (these were casts 19-21, 35, 38-42, 45, 56, 
57, and 59). Each of the remainder is summarised in figures* and tables 15 to 
43. In these summaries the concentrations are plotted at the nominal heights 
of the samplers above or below the sled, except for casts 16-18 and 23 for 
which the sled was flying substantially off the true target density surface 
and the profiles have accordingly been offset.

Figure 37* summarises the contribution of the individual profiles to the mean 
profile. The profiles are arranged in chronological order. The largest 
contribution comes from profile number 31, with profiles 16, 17, and 18 close 
behind. Figure 38 shows the same data with the vertical scale percentage 
contribution of each cast at the given distance from the target surface. It is 
notable from this diagram that the earlier profiles tend to be more narrow and 
higher in the water column than the later profiles. A few profiles (i.e. casts 
30, 37, and 55) tend to dominate in the wings of the distribution.

Heights from the target isopycnal were subject to two further minor 
adjustments: these take account of changes in the positions of the samplers on 
the wire as detailed in Table 44, and a further adjustment based on the 
observed mean density at which the sled was flying during each cast and the 
temperature gradient between the MkIII and the SEACAT which bracketed the true 
target surface. These adjustments are small, typically <<1m. The observed 
profiles (except for cast 51 which was discarded for subsequent analysis 
because the samplers below the sled did not trigger) were then linearly 
interpolated to an evenly spaced grid. The results of this interpolation are 
shown in Table 45. Finally, a mean profile was obtained by averaging the 
interpolated profiles together. The mean profile is shown in Table 46 along 
with the summary statistics. The mean profile has a nearly symmetric shape, an 
rms width of 6.8m and a displacement below the target surface of 0.84m. Such a 
profile is very satisfactory for the start of the mixing experiment, and 
should enable accurate measurements of vertical mixing as even if the Kz is as 
low as 0.01-0.02cm2/s.

The profiles from each cast can be column-integrated to obtain an amount of 
SF6 in moles per unit area for each tow track, as well as a displacement from 
the target surface, rms width and second moment about the target surface. The 
spatial distribution of these data around the patch area are contoured in Maps 
8 - 13. The data for column integrated amounts are the best, being constrained 
not only by the profiles which did have SF6 in them, but also by those which 
did not.

Map 8 shows the column-integrated SF6 in the float-guided, conservatively-
translated co-ordinates - those which we believe are the closest to a 
lagrangian co-ordinate system. According to this and the other maps, the 
tracer has strained out mostly in an east-west direction as might be expected 
from the observed direction of drift. It remains however relatively simple in 
shape, and contours well. In absolute co-ordinates (Map 9) the distribution is 
much more difficult to contour. In "liberally translated" co-ordinates (Map 
10), the patch shows a more nearly circular shape. Integration of the 
distributions over the maps yields our best estimates of the total amount of 
tracer in the measured patch. These values are:

		1128 moles (conservative co-ordinates)
		1111 moles (liberal co-ordinates)
		1586 moles (absolute co-ordinates)

Of these, the "absolute" value is clearly suspect because we have plenty of 
evidence that the patch did move substantially during the course of the 
cruise. Of the three, we favour the "conservative" figure. The number is about 
20% greater than the best estimate of the amount actually released, but this 
represents a much closer agreement than we had expected to be able to achieve 
and clearly shows that no major areas of the patch were undocumented. The 
distribution of zero and near-zero values also indicates that we succeeded in 
closing off the patch on all sides.

The first moment of the vertical distribution, i.e. the displacement of the 
centre of mass from the target surface, is mapped in conservative co-ordinates 
in Map 11. There is a clear indication of high values on the north-west edge 
and low values to the south. Quite substantial offsets are shown in an area to 
the east, but comparison with Map 8 shows that there are only very low 
concentrations of tracer here. The gradient in height across the patch can be 
interpreted as evidence for shear, tending to move the upper portion of the 
distribution more rapidly to the north-west than the lower part. The second 
moment of the patch, that is the rms widths relative to the centre of mass and 
to the target surface, are shown in Maps 12 and 13. Once again, the centre of 
the patch shows widths of 6-8m, with more extreme values on the edges of the 
distribution where the concentrations are low.


TABLE 1: STATION LIST

Cast no. 1
	09/05/1992, 1530: 27°17.9'N, 18°19.7'W.
	Test CTD sled at surface.
Cast no. 2
	09/05/1992, 1830: 27°15.0'N, 18°43.7'W.
	Test CTD sled and samplers at 100m.
Cast no. 3
	10/05/1992, 1330: 26°32.7'N, 22°42.7'W.
	Test CTD wire with 1350lb weight using automatic winch controller.
Cast no. 4
	10/05/1992, 1600: 26°32.3'N, 22°49.7'W.
	Test CTD sled with weight on lower wire. Test fire samplers. Test 
	automatic winch controller.
Cast no. 5
	10/05/1992, 1745: 26°35.1'N, 22°50.0'W.
	Hydrophone cast to listen for SOFAR floats.
Cast no. 6
	11/05/1992, 1100: 26°15.0'N, 26°42.8'W.
	Intercalibration of RVS CTD and SEACAT autonomous CTDs.
Cast no. 7
	11/05/1992, 1700: 26°14.7'N, 26°43.3'W.
	Test complete sampler array and automatic control system for CTD winch.

Casts 8-15: 600m casts to determine background SF6 concentrations, using CTD 
wire, SEACAT CTDs and 14 water samplers. The arrangement of samplers and 
SEACATs on the wire was as follows:

Depth
10m		sampler
50m		sampler
100m		sampler, SEACAT
150m		sampler
200m		sampler
250m		sampler, pressure SEACAT
300m		2 samplers
350m		sampler
400m		sampler, SEACAT
450m		sampler
500m		sampler
550m		sampler
600m		sampler, pressure SEACAT, pinger, weight.

Times and positions of the casts were:

Cast 8	0600, 12/05/1992 Position A: 26°08.0'N, 27°38.0'W.
Cast 9	1530, 12/05/1992 Position B: 25°11.0'N, 28°42.0'W.
		(Done in two hoists, Messenger dropped too soon).
Cast 10	2300, 12/05/1992 Position C: 25°11'N, 27°38'W.
Cast 11	0830, 13/05/1992 Position D: 26°08'N, 28°42'W.
Cast 12	1300, 13/05/1992 Position E: 25°39'N, 28°55'W.
Cast 13	2000, 13/05/1992 Position F: 24°58'N, 28°10'W.
Cast 14	0300, 14/05/1992 Position G: 25°39'N, 27°24.5'W.
Cast 15	1450, 14/05/1992 Station H: 26°22'N, 28°10'W.

Casts 16-59: Tracer sampling tows and hydrophone casts. See text for 
descriptions of these. The following table gives times and positions of 
deployment, triggering of the sampler array and end of the sampling tow.

Cast no.  Date / Time	Posn		Time		Posn		Time		Posn
	    begun	begun		triggered	triggered	ended		ended
16	14/5, 2035	24 35.0N,	2240		24 37.0N,	0342		25 42.0N, 
			28 18.0W			28 18.0W			28 18.0W
17	15/5, 0915	25 40.1N,	1110		25 41.5N,	1521		25 45.9N, 
			28 18.0W			28 18.7W			28 18.0W
18	16/5, 1915	25 34.8N,	2113		25 36.7N,	0142		25 41.4N, 
			28 23.5W			23.5W				28 23.3W
19	15/5, 0443	25 29.5N,	0620		25 06.0N,	1100		25 36.7N, 
			28 23.5W			28 23.5W			28 23.5W
20	16/5, 1521	25 28.1N,	1652		25 28.8N,	2130		25 32.9N, 
			28 18.1W			28 18.2W			28 18.0W
21	16/5, 2324	25 28.1N,	0050		25 21.1N,	0442		25 33.0N, 
			28 12.3W			28 12.3W			28 12.3W
22	17/5, 0638	25 36.5N,	0753		25 37.4N,	1200		25 41.8N,
			28 12.5W			28 12.5W			28 12.9W
23	17/5, 1510	25 40.0N,	1630		25 41.1N,	2050		25 45.6N, 
			28 23.7W			28 23.4W			28 23.7W
24	17/5, 2316	25 44.7N,	0040		25 44.6N,	0415		25 49.0N, 
			28 23.4W			28 23.3W			23.5W
25	18/5, 0705	25 47.9N,	0828		25 48.9N,	1236		25 53.2N, 
			28 23.5W			28 23.4W			28 23.6W
26	18/5, 1532	25 44.1N,	1701		25 45.2N,	2112		25 49.1N, 
			28 32.9W			28 33.1W			28 32.9W
27	18/5, 2354	25 42.0N,	0145		25 43.5N,	0548		25 47.4N, 
			28 29.1W			28 30.2W			28 30.1W
28	19/5, 0841	25 43.7N,	SEACAT				1102		25 45.4N, 
			28 30.6W	CALIB.						28 30.0W
29	19/5, 1244	25 46.7N,	1442		25 47.6N,	1843		25 51.4N, 
			28 30.2W			28 30.0W			28 30.0W
30	19/5, 2335	25 38.5N, 	0109		25 40.0N,	0519		25 44.5N, 
			28 30.0W			28 30.1W			28 30.0W
31	20/5, 0815	25 38.9N, 	0912		25 39.7N, 	1344		25 43.7N, 
			28 26.7W			28 26.5W			28 26.7W
31A	20/5, 1740	26 05.7N, 	H'PHONE 			2152		26 05.4N, 
			28 41.1W	CAST						28 41.3W
32	21/5, 0051	25 34.8N, 	0220		25 35.9N, 	0620		25 40.1N, 
			28 26.6W			28 26.4W			28 26.5W
33	21/5, 0836	25 36.0N, 	0946		25 36.8N, 	1354		25 40.9N, 
			28 20.7W			28 20.7W			28 20.7W
34	21/5, 1650	25 32.2N, 	1810		25 32.8N, 	2216		25 36.3N, 
			28 20.6W			28 20.8W			28 20.7W
35	22/5, 0106	25 31.3N, 	0215		25 31.9N, 	0620		25 36.0N, 
			28 27.3W			28 27.3W			28 27.2W
36	22/5, 0900	25 38.9N, 	1008		25 39.7N, 	1410		25 44.3N, 
			28 24.1W			28 24.0W			28 24.0W
37	22/5, 1612	25 37.8N, 	1736		25 38.5N, 	2136		25 41.0N, 
			28 28.6W			28 28.4W			28 28.6W
37A	23/5, 0615	28 55.8N, 	H'PHONE 			1000		25 54.6N, 
			28 30.7W	CAST						28 31.1W
38	23/5, 1150	25 40.3N, 	1303		25 41.5N, 	1722		25 46.7N, 
			28 21.0W			28 21.0W			28 21.2W
39	23/5, 1913	25 40.5N, 	2025		25 41.5N, 	0045		25 46.0N, 
			28 17.9W			28 18.0W			28 18.0W
40	24/5, 0230	25 37.8N, 	0400		25 38.8N, 	0824		25 43.5N, 
			28 13.1W			28 13.0W			28 13.1W
41	24/5, 1039	25 36.2N, 	1148		25 37.2N, 	1552		25 42.6N, 
			28 17.0W			28 18.0W			28 17.9W
42	24/5, 1837	25 30.8N, 	1955		25 32.2N, 	0000		25 37.2N, 
			28 17.4W			28 18.0W			28 18.0W
43	25/5, 0226	25 43.3N, 	0342		25 43.9N, 	0707		25 47.2N, 
			28 26.8W			28 26.6W			28 26.7W
44	25/5, 1024	25 40.0N, 	1128		25 40.8N, 	1453		25 43.6N, 
			28 33.0W			28 33.0W			28 33.0W
44A	25/5, 1754	25 52.1N, 	H'PHONE 			2154		25 50.5N, 
			28 49.3W	CAST						38 50.3W
45	26/5, 0146	25 50.0N, 	0302		25 50.6N, 	0627		25 53.4N, 
			28 58.0W			28 52.0W			28 52.1W
46	26/5, 0915	25 46.3N, 	1037		25 47.0N, 	1402		25 49.7N, 
			28 47.9W			28 48.0W			28 48.1W
47	26/5, 1700	25 43.3N, 	1823		25 44.0N, 	2148		25 46.8N, 
			28 43.4W			28 43.3W			28 43.3W
48	27/5, 0043	25 41.5N, 	0202		25 42.5N, 	0527		25 46.4N, 
			28 38.2W			28 38.1W			28 38.2W
49 SAMP. 27/5, 0850	25 41.9N, 	0948		25 42.3N, 	1313		25 44.5N, 
TEST			28 38.1W			28 38.3W			28 38.2W
50	27/5, 1631	25 38.3N, 	1823		25 39.5N, 	2148		25 42.9N, 
			28 43.6W			28 43.5W			28 43.6W
51	28/5, 0124	25 47.8N, 	0232		25 48.2N, 	0557		25 52.4N, 
			28 45.8W			28 43.5W			28 43.5W
52	28/5, 1906	25 50.0N, 	2040		25 50.7N, 	0115		25 53.7N, 
			28 33.0W			28 33.0W			28 33.0W
53	29/5, 0447	25 48.6N, 	0603		25 49.9N, 	1015		25 55.1N, 
			28 38.3W			28 38.2W			28 38.2W
54	29/5, 1306	25 38.6N, 	1419		25 39.4N, 	1826		25 43.2N, 
			28 49.9W			28 49.8W			28 49.2W
55	29/5, 2032	25 38.4N, 	2139		25 39.5N, 	0215		25 44.3N, 
			28 39.6W			28 39.6W			28 39.7W
56	30/5, 0421	25 34.3N, 	0527		25 35.0N, 	0939		25 38.9N, 
			28 39.7W			28 39.6W			28 39.6W
57	30/5, 1115	25 34.9N, 	1222		25 35.7N, 	1650		25 39.8N, 
			28 45.5W			28 45.5W			28 45.5W
58	30/5, 1945	25 44.2N, 	2056		25 45.1N, 	0120		25 48.3N, 
			28 26.0W			28 25.9W			28 26.0W
59	31/5, 0248	25 48.6N, 	0458		25 53.6N, 	0910		25 57.0N, 
			28 26.5W			28 31.5W			28 31.5W

------------------------------------------------------------------------------
APPENDIX 1: SCIENTIFIC PERSONNEL

Embarked at Las Palmas:

Dr Andrew Watson	(Plymouth Marine Laboratory)	Principal Scientist
Ms Susan Becker		(Woods Hole Oceanographic Institution)
Mr Terry Donaghue	(Woods Hole Oceanographic Institution)
Ms Cecelia Fernandez	(Woods Hole Oceanographic Institution)
Dr Clifford Law		(Plymouth Marine Laboratory)
Mr Malcolm Liddicoat	(Plymouth Marine Laboratory)
Mr Kay Lubcke		(Plymouth Marine Laboratory)
Mr Craig Marquette	(Woods Hole Oceanographic Institution))
Dr Phillip Nightingale	(University of East Anglia)
Ms Rachel Oxburgh	(Lamont-Doherty Geological Observatory)
Mr Martin Beney		(RVS)
Mr Darrell Phillips	(RVS)
Mr Simon Watts		(RVS)
Mr Chris Rymer		(RVS)
Mr David Dunster	(RVS)

Transferred at sea from R/V Oceanus:

Dr James Ledwell	(Woods Hole Oceanographic Institution)
Dr Brian Guest		(Woods Hole Oceanographic Institution)
Mr Chris Kinkade	(Woods Hole Oceanographic Institution)


------------------------------------------------------------------------------
APPENDIX 2: CTD CALIBRATION

Temperature and pressure channels of the RVS Neil Brown MkIII instrument (s/n 
01-1195) were both calibrated at RVS against known standards. However, the 
conductivity (salinity) channel had to be calibrated at sea using samples 
drawn from Niskin bottles on the sled.

In total 168 samples were taken from 59 casts and then analysed on a Guildline 
autosal (s/n 52395) which was calibrated against Standard Seawater ampoules 
from batch P118 (see Table A2.1). The results from the comparison of (autosal-
CTD) data showed an average offset of +0.022ppm with a standard deviation of 
0.006 (see Table A2.2).

Also to ensure that there was no offset between the salinity measurements 
taken on Oceanus and Charles Darwin an inter-calibration exercise was carried 
out. This involved measuring 28 duplicate samples from Oceanus.

* All figures shown in PDF file.
