REPORT ON C.S.S. Parizeau CRUISE 99-028

23 - 30 September, 1999

Peter C. Smith, Rick Boyce and Liam Petrie

Ocean Sciences Division

Bedford Institute of Oceanography

Dartmouth, Nova Scotia

CANADA

October, 1999

BEDFORD INSTITUTE OF OCEANOGRAPHY CRUISE REPORT Parizeau 99-028

Local Cruise Designation: 99-028

Vessel: C.S.S. Parizeau

Dates: 23-30 September, 1999

Area: Southwest Nova Scotia/Georges Bank

Responsible Agency: Ocean Sciences Division

Maritimes Region, DFO

Ship's Master: Capt. J. Dockrill

Scientific Personnel:

R. Boyce Ocean Sciences

M. Scotney Ocean Sciences

L. Petrie Ocean Sciences

B. Nickerson Ocean Sciences

D. Kellow Ocean Sciences

M. Holtom MEDS

1. PURPOSE

The scientific objectives of this cruise were:

1. Time-series measurements of currents, temperature, and salinity at key locations for the cross-over flow from Browns to Georges Bank in the surface layers,

2. Distribution of temperature, salinity, dissolved oxygen and nutrients in the vicinity of SWNS and eastern Georges Bank,

3. Lagrangian measures of surface drift on Browns and eastern Georges Banks.

The activities planned for the cruise period included:

1. Recover eight moorings at four sites (NECE,NECW,BBO,BBI) on Browns Bank and in Northeast Channel (Figure 1a),

2. Conduct CTD/ADCP survey of SWNS and eastern Georges Bank,

3. Conduct a CTD/ADCP section of the local Gulf Stream/WCR front,

4. Run CTD section on Halifax line

2. NATURE OF DATA GATHERED

Eight moorings were successfully recovered at four different sites (Table 1, Fig.1a). The top two instruments on mooring 1291A had been previously parted from the mooring and the RCM was returned to BIO, but the Microcat was lost. All instruments (18 RCMs, 1 Microcat) appear to have worked, except for moderate amounts of growth and one missing rotor (Table 1)

A total of 67 CTD stations (Fig.1b, Table 2) were occupied at the mooring sites and along the following sections:

1) Section Ia - from the 50 m isobath off Cape Sable to the outer edge of Browns Bank (Fig.3),

2) Section Ib - across Northeast Channel from Browns to Georges Bank on the mooring line, i.e. near the sill (Fig.4),

3) Section II - across the mouth of Northeast Channel (Fig.5),

4) Section III - across the western flank of Browns Bank (Fig.6)

5) Section IV - along the 130-200 m isobath on the eastern side of the Channel, from Georges Basin to the mouth (Fig.7),

6) Section V across the Gulf Stream/WCR front on 65^o 30’W (Fig.8),

7) Section VI across the shelf break off Baccaro Bank (Fig.9), and

8) the Halifax Section (Fig.10).

The quality of the CTD salinity measurements is quite acceptable (Table 2a), especially considering the relatively high variability of the standards used. The YSI dissolved oxygen sensor showed an offset with respect to the Orion probe samples (Table 2a), but stable calibrations were obtained by linear regression of sensor vs. the Orion probe and titrated values (Table 2b, Fig.2a,b). The YSI sensor exhibited occasional noise, spikes, and hysteresis between the up- and downtraces. In addition, roughly 100 oxygen isotope samples were collected throughout the water column at even numbered stations (Fig.1b) for Dr. Robert Houghton of Lamont-Doherty Earth Observatory. Nutrient samples were collected throughout the water column as well at the even-numbered stations.

3. PROGRAM SUMMARY

Date From(UTC) To(UTC) Operation

23 Sept 1000 1200 Basin trials for CTD, Lifeboat drill

1200 0230(24) Steam to first CTD site

24 Sept 0234 1755 CTD1–17

1850 2330 Recovery of mooring’s 1294 & 1295

25 Sept 0048 1140 CTD18–24

1214 1445 Recovery of mooring’s 1291 & 1291A

1632 1641 CTD 25

1658 2117 Recovery of mooring’s 1290 & 1290A

2158 2200 Deployed drifter #3012

26 Sept 0024 1141 CTD26–35

1240 1610 Recovery of moorings 1292 & 1293

1800 0100(27) Searching for drifter

27 Sept 0129 1733(29) CTD36–67

29 Sept 2000 Arrive BIO

4. MOORING OPERATIONS

Recovery of 8 current meter moorings and 12 guard buoys at 4 sites (Table 1) proceeded without incident. The Fairey float and current meter (RCM7122)) at BBO mooring 1291A had been previously recovered and returned by a fisher, however the Microcat 0373 was not returned. The rest of the mooring was recovered successfully however it was off position by 2.3 cables to the northwest. Just two guard buoys went adrift during the mooring period with the Coast Guard replacing one. This is not a bad record, considering they had been in the water for roughly 10 months and had survived both winter storms and Hurricane Floyd. Some difficulty was encountered with snarling of the chain at the base of the guard buoy mooring lines, causing it to come up in large lumps. A guard buoy at NECW was instrumented with a Microcat and and Argos transmitter. This system failed during part of the mooring period and was repaired and replaced in Jan/99.

The moorings had moderate growth toward the top with less growth towards the anchor end. Only two conductivity sensors and rotors were fouled. One current meter was recovered with the rotor missing but this may have happened during recovery. The rest of the moorings and instruments were recovered in good shape and appeared to have worked during the mooring period.

Problems/Recommendations:

(1) A solution is needed to prevent the chain at the base of the guard buoy moorings from snarling.

5. HYDROGRAPHIC MEASUREMENTS

Hydrographic and chemical measurements were made at a total of 67 stations (Table 2) using a Seabird 9/11 Plus system (the “WOCE system”), equipped with a SBE 23Y Yellow Springs Instruments (YSI) dissolved oxygen sensor. The data were logged on a 33 Mhz 486 PC and post-processed between stations using SEABIRD's software. Once processed, the data were backed up to a CD-ROM.

Water column sampling was accomplished with a SeaBird Carousel 12-bottle rosette. Duplicate nutrient and single oxygen isotope samples were drawn at roughly standard depths on alternate stations only. Duplicate dissolved oxygen samples were drawn at 50m. In addition, two calibration bottles were tripped at the bottom of each cast for roughly replicate nutrients, salinities and dissolved oxygens. Due to limited numbers of sample bottles, nutrient and oxygen isotope sampling were curtailed after stations 43(save 66) and 23, respectively (see Table 2).

The SeaBird system worked well throughout the cruise. However, sampling was a problem in that there was often not enough water in the 1.7L bottles to fill all of the required sample bottles. Part of this problem was due to the use of 300 ml bottles for the O₂ samples. The accuracy of the salinity samples seemed to suffer most, and the suggestion was made to draw the salinity sample first because of its importance in sorting out bottle misfires and leaking.

5a. Processing

The processing of CTD data was initiated by a single command on the PC at the end of the station. This command, called PROCESS, starts a batch job that sequentially passes the data through a number of programs. Most were from SEABIRD's SEASOFT package. A few were custom written at BIO. The following is a summary of the processing procedure [modifications required for the manual bottle trip procedure are indicated in square brackets]:

(0) [Run MARKSCAN to create .BSR (bottle scan records) file from .MRK file, created by hitting (cntrl-f5) after a bottle is tripped.]

(1) Convert raw frequency data to binary pressure, temperature and conductivity using SEABIRD's DATCNV program. [.ROS file is based on either bottle flags within the data stream (normal), or on information from the .BSR file (manual procedure)]

(2) Split the file into the up and down traces using SEABIRD's SPLIT program.

(3) Check downcast for and mark any 'wild' data points with SEABIRD's WILDEDIT program.

(4) Filter downcast conductivity and temperature using SEABIRD's FILTER program. This is a low pass filter and we used a time constant of 0.045 seconds for conductivity and 0.15 seconds for temperature.

(5) Mark downcast scans where the CTD is moving less than the minimum velocity of 0.10 m/s using SEABIRD's LOOPEDIT program.

(6) Align downcast pressure, temperature and conductivity using SEABIRD's ALIGNCTD program by advancing the conductivity signal by 0.01 sec.

(7) Apply the thermal mass correction for the conductivity cell using SEABIRD's CELLTM program.

(8) Compute dissolved oxygen in ml/l using SEABIRD's DERIVE program.

(9) Create WOCE 2-dbar dataset using OSD program PRO-WOCE.

(10) Bin average downcast data to 1.0-dbar intervals using SEABIRD's BINAVG program.

(11) Compute downcast salinity, potential density(s_q), potential temperature(q), and depth using SEABIRD's DERIVE program.

(12) Convert the down cast from binary to ASCII using SEABIRD's TRANS program.

(13) Convert downcast to ODF format using PCS program SEAODF.

(14) Create IGOSS message using PCS program ODF_IGOS.

(15) Prepare batch and command files to transfer the data to the VAX and create the input for SEABIRD's ROSSUM program using our customized MAKEFILE program.

(16) Check for bottles, then use ROSSUM to create the rosette summary file.

(17) Convert the resulting .BTL file to a format suitable for ingestion into Quattro PRO (.QAT file) using our customized QPROBTL program.

(18) Create the calibration file of merged up- and down-cast data using OSD program CALIB.

(19) Copy Quattro, downcast, and ODF files to appropriate directories and clean up.

Plots and status info displayed by the SEASAVE program during the acquisition are discarded when the program terminates. The post-processing plotting was not included in the batch job because SEABIRD's SEAPLOT program requires interactive operator attention. Plots produced after each station include T, S, O₂, and s_q vs. pressure and T vs. S. Attempts to produce section plots with Igor Yakashev’s contour package, modified to accept .ODF files, were frustrated by improper header information in the ODF files. “Station” and “Cast” numbers were confused, causing problems to propagate into the processed files. The only foolproof solution appears to be reprocessing of the entire data set, which is presently underway.

5b. Calibration

At the base of each CTD cast two rosette bottles were tripped and a single salinity sample was drawn from each to be analyzed onboard with an Guildline AutoSal salinometer. Assuming that the temperature offset for the WOCE system is negligible, the comparison of the salinity standards against the SeaBird CTD (Table 2a below) shows that the mean offset is small and not significantly different from zero. However, the standard deviation about this mean is unacceptably large (order of magnitude higher), even after accounting for outliers based primarily on replicate sampling. Some of this variability may be due to real time dependence in the properties at the base of each cast, but part may be due to sampling techniques which need to be re-examined.

The performance of the YSI O₂ sensor was similar to that on previous cruises. The surface values on the downtrace appeared to be fully equilibrated, thanks to a 3 min. waiting period (as suggested by the manufacturer), but there was usually a large hysteresis between the down- and uptraces in the vicinity of the pycnocline, and there were occasional spikes from the electronics. The hysteresis probably results from a mismatch of the temperature and O₂ sensors in the probe and might be improved by applying filters with appropriate phase lags. The dissolved oxygen samples collected from 300-ml calibration bottles were analyzed on board with an Orion 853 oxygen probe, borrowed from Marine Chemistry. The mean offset of the YSI over the entire cruise was significantly different from zero (Table 2a) and the standard deviation about the mean was substantial (0.3 ml/l). A linear regression (Table 2b) of the Orion standard on the YSI values from the CTD provides a calibration for the YSI with a standard error of ±0.3 ml/l (Figure 2a). To assess the quality of the Orion standard, “replicate” O₂ samples were obtained from the 50-m rosette bottle, and “near-replicate” samples were drawn from the two bottles at the base of the cast. Overall, these “replicates” agreed to within a mean difference of 0.02 ml/l, but with a standard deviation of 0.11 ml/l (or better discounting outliers; Table 2a). As a further check on the Orion sensor, standard Winkler analysis was performed on 8 samples drawn from a late CTD cast (CTD65) and “pickeled” before their return to BIO 12 hours later. Although the Winkler replicates showed substantial variation (Table 2a), their average was quite consistent with the Orion results (Table 2b: Figure 2b)

Problems/Recommendations:

(1) Efforts should be made to remove the hysteresis between the up and down traces from the YSI sensor by application of filters with various lags.

(2) Salinity samples should be drawn first from small (1.7L) rossette bottles.

(3) Smaller O₂ sample bottles should be used

(4) The new CTD computers require WINZIP and OFFICE 97 software.

5c. Sections

CTD sections Ia,b, II, III, IV, V, VI and VII (Figs. 3-10) depict hydrographic conditions, respectively, 1) along the eastern boundary of the Gulf of Maine, 2) across the sill in Northeast Channel, 3) down the western flank of Browns Bank, 4) from Georges Bank across the mouth of Northeast Channel, 5) along the 200 m isobath on the eastern side of the Channel, 6) across the front of a Gulf Stream warm-core ring, 7) across the shelf break off Baccaro Bank, and 10) along the Halifax Section, spanning the entire Scotian Shelf off Halifax. Section Ia (Fig.3) shows two surface layer pockets of fresh water lying between Cape Sable and Browns Bank and over the outer edge of Browns. Conditions in these two zones are stratified by both temperature and salinity, while the inshore region off Cape Sable and the cap of Browns are vertically well mixed. A weak cold intermediate layer is evident over the outer Bank (CTD10), and a similar hint appears at CTD4.

Section Ib (Fig.4) shows the presence of Warm Slope Water or Gulf Stream water, centered at 100m, in the region of strong inflow currents near the NECE mooring (CTD13). A strong oxygen minimum (O₂<3.5 ml/l) at 120-160m is associated with this warm salty tongue on the eastern side of the Channel. The warm, fresh (T>15^oC; S<33.5) surface layer thickens markedly over the western side of the Channel. Upward-sloping isopycnals to the east suggests deep inflow and surface outflow.

Section II (Fig.5) again reveals a hint of the cold intermediate layer in Georges Basin (e.g. CTD37), with minimum temperatures at 100m dropping below 10^oC. There is also a vestige of the O₂ minimum at ~150m, and salinities below that depth uniformly exceed 35. The surface layer is again warm and fresh and there is some evidence of vertical mixing on the upper part of the slope, where the isopycnals tend to splay.

Section III (Fig.6) also shows the warm, saline water extending almost entirely across the mouth of Northeast Channel, with maximum salinities >35.5 and temperatures >14^oC at depths of 100-150m isobath.. The surface layer on the eastern side of the Channel is both fresher and warmer than on the western side, where the density stratification is also weaker. The core of a thick oxygen minimum layer lies roughly between 150 and 250 m.

Conditions on Section IV (Fig.7), from the mouth of Northeast Channel to Georges Basin along the eastern 200m isobath show the gradual erosion and mixing of the incoming layer of Warm Slope/Gulf Stream Water. The S=35 isohaline sinks along the Channel from <100m to around 130m, and the temperatures do not exceed 13^oC in the Channel itself. The surface layer is also cooler and fresher inside the Channel. The oxygen minimum layer extends into the Channel at depths of 150-200m.

Section V (Fig.8) depicts the sharp transition in properties at a Gulf Stream/Warm Core Ring front which lay off the mouth of Northeast Channel near 41^o 10’N on 65^o 30’W. Surface layer temperatures and salinities south of the front were in excess of 20^oC and 36, respectively, with a salinity contrast of 3.0 across the front itself. Beneath the Gulf Stream water, isopycnals slope sharply upward to the north, indicating the strong vertical shear in the eastward flow. Similarly, on the coastal side of the front, the near surface isopycnals slope weakly upward to the north. The core of the oxygen minimum layer rises up from deep under the Gulf Stream water to lie between 200-250m to the north.

The offshore end of Section VI (Fig.9) also appears to pass through a bolus of Warm Slope Water at depths of 50-200m, with salinities in excess of 35.5 and temperatures of 13-18^oC. Over the shelf, the surface layers are well stratified by both temperature and salinity, but the deep water properties on the shelf show little influence of the offshore waters. The cold intermediate layer is most pronounced at CTD57 near the shelf break.

The Halifax Section data (Fig.10) also reveal the presence of Gulf Stream water lying just off the shelf at depths of 50-200m. A subsurface front near the shelf break separates the war, saline water from the cooler, fresher waters of the shelf. Over Emerald Basin, a very fresh surface layer extends from the coast to Emerald Bank, a cold intermediate layer is pressed against the coast at depths of 50-100m, and relatively warm (T=8-10^oC), saline (S>35) slope water resides in the deepest parts of the section.

Acknowledgements:

We are greatly indebted to the officers and crew of the C.S.S. Hudson for their skilled assistance and friendly cooperation, which was vital to the success of this mission.

TABLE 1. Moorings Recovered During Parizeau 99-028, 23-30 September 1999

Mooring Site N. Lat. Placement Instrument Comments

No. (Depth,m) W. Long. Time(Z),Date (Depth,m)

1290 BBI 42^o20.53’ 2035, Sept.25 RCM7592(30) Growth in cond. cell

(120) 65^o44.13’ RCM4355(50)

RCM5574(100)

1290A BBIA 42^o20.77’ 1838, Sept.25 RCM7525(16)

(122) 65^o43.95’

1291 BBO 42^o09.75’ 1420, Sept.25 RCM6403(30) Growth on rotor

(120) 65^o34.31’ RCM3196(50)

RCM4195(100)

1291A BBOA 42^o09.99’ 1350, Sept.25 RCM7122(14) Previously recovered

(118) 65^o34.31’ by a fisher in July/99

Microcat 0373(13) Missing

1292 NECEA 42^o17.75’ 1405, Sept.26 Microcat 0286(23) Growth on cell

(216) 65^o50.41’ RCM7013(24) Growth on rotor and

cell

1293 NECE 42^o17.69’ 1530, Sept.26 RCM4342(35)

(215) 65^o50.66’ RCM9355(55)

RCM4602(105)

RCM5577(155)

1294 NECWA 42^o07.79’ 2125, Sept. 24 RCM5359(23)

(216) 66^o00.91’

1295 NECW 42^o07.61’ 2210, Sept. 24 RCM7131(33)

(213) 66^o00.84’ RCM9607(53)

RCM2664(103) Rotor missing

RCM6411(153)

TABLE 2. CTD Stations During Parizeau 99-028, 23-30 September, 1999

TABLE 2a. Temperature and Salinity Calibration Results for Parizeau 99-028

QUANTITY NO. SAMPLES MEAN DIFF. STD. DEV.

±STD. ERR.

CTD vs. Standard

Salinity:

CTD-AutoSal. (0-66) 62 0.0039±0.0042 0.0329

“ (no outliers) 53 0.0012±0.0020 0.0145

Dissolved Oxygen:

YSI-Orion(0-66) 273 -0.081±0.018 0.299

Standard vs. Standard

Salinity:

Btl.1-Btl.2 (0-66) 29 -0.0060 0.0404

“ (no outliers) 20 -0.0009 0.0037

Dissolved Oxygen:

Btl.1-Btl.2 (0-66) 48 0.018 0.109

“ (no outliers) 37 0.013 0.060

Winkler1-Winkler2(same btl.;65) 8 0.076 0.060

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TABLE 2b. Dissolved Oxygen Regression Results for Parizeau 99-028

Y = aX+b (Y=Orion Standard, X=sensor)

SENSOR(CTD) NO. SAMPLES a±da b±db(ml/l) ±dY(ml/l) r²

YSI(0-67) 273 0.976±0.020 0.194±0.098 ±0.299 .894

Winkler (65) 8 1.036±0.014 -0.064±0.058 ±0.045 .999

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FIGURE CAPTIONS:

Figure 1 a) Mooring sites, and b) CTD and biological stations for C.S.S. Parizeau

Cruise 99-028, 23-30 September 1999

Figure 2 a) Linear calibration curve for YSI O₂ sensor, based on Orion 853 standard (see Tables 2a,b); and b) intercalibration of Orion and Winkler O₂ standards.

Figure 3 Hydrographic section Ia (CTD1-11) from Cape Sable to the offshore edge of

Browns Bank.

(a) temperature,

(b) salinity,

(d) dissolved oxygen