WOCE Line: AR11
Expocode:  32OC258_3



                    WOODS HOLE OCEANOGRAPHIC INSTITUTION

                     Subduction in the Subtropical Gyre
                        Seasoar Cruises: Data Report 
                            May 1991 - May 1993 
                        Technical Report WHOI-95-13

by 
* Julie S. Pallant 
* Frank B. Bahr 
* Terrence M. Joyce 
* Jerome P. Dean 
* James R. Luyten 

September 1995

Funding sponsored by the Office of Naval Research, grants N00014-91-J-1585,
N00014-91-J-1508 and N00014-91-J-1425. 

Reproduction in whole or in part is permitted for any purpose of the United
States Government.  This report should be cited as Woods Hole Oceanog. Inst.
Tech. Rept.; WHOI-95-13

             Approved for public release; distribution unlimited

                       Approved for Distribution:
                      Philip L. Richardson, Chair
                   Department of Physical Oceanography




                                ABSTRACT:


   The overall objective of the Subduction Accelerated Research
Initiative (ARI) was to bring together several techniques to address
the formation and evolution of newly formed water masses.  The Seasoar
component provided surveys of temperature and salinity to help
determine the spatial variability of the temperature, salinity and
density fields in both the active frontal regions and in the vicinity
of subducting water tagged by bobbers.  Data were collected in the
Eastern North Atlantic Ocean in Spring 1991, Winter 1992, Winter 1993
and Spring 1994.  "Star" patterns were used to study the mesoscale
variability.  Temperature, pressure and thickness for each pattern were
objectively mapped on potential density surfaces of 26.5, 26.7 and 26.9
kg/m^3.  Acoustic Dopler Current Profiles (ADCP) maps were also created
for the two shallower density surfaces. We describe the Seasoar data
collected during the four cruises.  A CD-ROM includes 1 and 3 second
CTD, cruise navigation, ADCP and Seasoar engineering data, as well as
color figures of these data.  This data report can also be viewed using
an internet information browser (i.e. Mosaic, Netscape) using the
provided CD-ROM.



LIST OF FIGURES:

Figure 1: Seasoar Cruise Track:  May 1991 - May 1993.  Heavy lines denote 
          when seasoar was actually being towed. Heavy Circles are location of 
          subduction moorings.

Figure 2: Seasoar vehicle with sensors.

Figure 3a & 3b: Bobber location at the time of Seasoar mesoscale survey 
          determined from ALFOS data.  Figure 3a - Subduction 3.  Figure 3b - 
          Subduction 4.

Figure 4: One hour summary plots of Seasoar track.  In the upper plot, the 
          dotted line represents pressure over a 0-500 db range vs time.  The
          solid line represent sigma-theta over range of 25 - 27.5. 
 
Figure 5: Theta Contour plot of gridded Seasoar data along transect 2 of 
          Cruise 3.  Gray areas denote unavailable data.  The darker lines 
          represent average sigma-theta, during Subduction 1, where the 
          bobbers were deployed. (see Fig. 7).

Figure 6: Example Summary plot of Pressure, Theta, Salinity and Thickness
          vs. Sigma-theta. See Appendix A.

Figure 7: Representative range of bobber movements during Subduction 1.

Figure 8a - 8i: Representative contour maps of theta, pressure and thickness
          for density levels 26.5, 26.7, 26.9.  Each map contains a mean and 
          standard deviation (STD) of the observed property over the 
          entire star pattern .  See Appendix D.

Figure 9: Representative map of ADCP velocities on a star pattern at density
          levels 26.5 (fig 9a) and level 26.7 (fig 9b). See Appendix E.



LIST OF TABLES:

Table 1: Cruise names, dates and Chief Scientists for Subdcution experiment.

Table 2: Mean locations, start and end times of star patterns and sections.

Table 3: Mean and standard deviation of theta, pressure and thickness on
         density surfaces 26.5, 26.7, 26.9 for all star patterns.



Subduction in the Subtropical Gyre:  Seasoar Cruises Data Report

Julie S. Pallant
Frank B. Bahr
Terrence M. Joyce
Jerome P. Dean
James R. Luyten

1. INTRODUCTION

Seasoar CTD data were collected during the Subduction Experiment in the
Eastern North Atlantic during the period of May 1991 - May 1993.  The
Seasoar work is part of the Subduction Accelerated Research
Initiative (ARI) supported by the Office of Naval Research.  The
overall objective was to bring together several techniques to address
the formation, evolution and subduction of newly formed water masses
over a two year period.  Other activities include synoptic mesoscale
sampling of tracers, including potential vorticity in the upper
pycnocline, and direct tagging of water parcels with bobber floats, as
well as independent Eulerian velocity and meteorological measurements
from surface moorings.  The Seasoar provides well-resolved surveys to
help determine the spatial variability of the temperature, salinity and
density fields in both the active frontal regions and in the vicinity
of subducting water tagged by the bobbers.

The Seasoar, manufactured by Chelsea Instruments, Ltd., is a towed
vehicle equipped with impeller-forced wings that can be adjusted to
undulate in the upper ocean.  The wings are controlled by signals
from the ship, and moved by an hydraulic unit.  The Seasoar undulates
between 0-450 dbars while being towed at about eight knots, cycling to
the surface approximately every 12 minutes.  The Seasoar group
participated in four cruises during the experiment (Table 1).  On the
initial cruise in May 1991, 18 bobbers were deployed, three mesoscale
surveys ("star patterns") and a frontal survey were completed (Luyten,
1991).  In Feb 1992, a second frontal survey was completed
(Rudnick, 1992).  The following November, six star patterns near
bobbers and two long transects were executed (Joyce, 1992).  The final
cruise, May 1993, surveyed near four bobbers and towed along three long
transects (Luyten, 1993); (Fig 1, Table 2).  In conjunction with four
star patterns (two on the first and two on the last cruise), a series
of closely spaced CTD stations, using a profiling CTD, were made
overlaying the star pattern in an L-shaped pattern for the tracer
studies. We used the CTD data from these stations to augment the
Seasoar dataset.  All cruises were on the R/V Oceanus.




Table 1:  Dates corresponding to the cruises during the Subduction Experiment. 
                                                                
Subduction 1 (OC240 Leg 2)-  5 May 1991 - 3 June 1991            - J. Luyten
Subduction 2 (OC250 Leg 3)-  2 March 1992 - 20 March 1992        - D. Rudnick
Subduction 3 (OC254 Leg 4)-  24 November 1992 - 16 December 1992 - T. Joyce
Subduction 4 (OC258 Leg 3)-  18 May 1993 - 16 June 1993          - J. Luyten




Table 2:  This table includes the mean locations, start and end times for all the 
        star patterns surveyed on the Subduction cruises.  In addition, it 
        includes the start and end locations and times for the sections 
        presented in this data report.
        


Cruise  Star   Start      End      Latitude   Longitude 
                time      time    
   1     1    05/16/91   05/20/91  22.8655 N  -27.0472 E
   1     2    05/23/91   05/27/91  29.0139 N  -23.5313 E
   1     3    05/31/91   06/02/91  29.9845 N  -21.6587 E
   3     1    11/28/92   11/30/92  20.3393 N  -29.6893 E
   3     2    12/01/92   12/02/92  22.8685 N  -27.0472 E
   3     3    12/02/92   12/04/92  22.7953 N  -28.6941 E
   3     4    12/04/92   12/06/92  23.2972 N  -29.4279 E
   3     5    12/08/92   12/11/92  25.1120 N  -24.4478 E
   3     6    12/11/92   12/12/92  26.9678 N  -24.9349 E
   4     1    05/24/93   05/27/93  18.9862 N  -31.9748 E
   4     2    05/30/93   05/31/93  24.2855 N  -37.4544 E
   4     3    06/02/93   06/03/93  23.5120 N  -31.6550 E
   4     4    06/05/93   06/08/93  28.1707 N  -26.3902 E



Cruise  Sect   Start      End           Start                     End 
                time      time     Latitude   Longitude    Latitude   Longitude 
   3     1    12/06/92   12/08/92  23.2020 N  -29.0249 E   24.6882 N  -24.6651 E
   3     2    12/12/92   12/14/92  26.8574 N  -24.8574 E   32.6342 N  -24.7439 E
   4     1    05/27/93   05/30/93  19.0518 N  -32.0232 E   24.6200 N  -24.6200 E
   4     2    06/04/93   06/05/93  26.3285 N  -28.2048 E   27.8200 N  -26.1847 E
   4     3    06/08/93   06/09/93  28.9257 N  -25.8696 E   28.8426 N  -23.0890 E
   4     4    06/09/93   06/10/93  28.8509 N  -23.0497 E   30.4189 N  -23.8573 E
   4     5    06/10/93   06/11/93  30.4394 N  -23.8296 E   32.9475 N  -21.5498 E




2. METHODS


A: TEMPERATURE, CONDUCTIVITY AND PRESSURE

The Seasoar CTD is a Sea-Bird model 911 with redundant sensors for
conductivity and temperature and a single oxygen sensor.  Data were
telemetered to the ship at 24 Hz.  The CTD sensors are openly mounted
on the top cover of the vehicle, the temperature sensors are located
behind and slightly above the conductivity cells (Fig, 2).  Peak flow
rates past the sensors typically reached 5 m/s, with occasional
extremes of 7 m/s.  Flow rate exceeded the capabilities of the standard
pump on Sea-Bird CTD's and therefore no pumping of sea water through the
sensors was done. Seasoar sensors were exchanged with those on a pumped
profiling CTD, also a Seabird 911, for calibration purposes where they
could be compared with rosette samples directly.  The 24 Hz data were
logged and displayed on a personal computer (PC) or a Sun Computer.


B: LOCATION OF BOBBERS 

In May 1991, 18 bobbers were launched during Oceanus Cruise 240, Leg
2.  The bobbers are sound fixing and ranging (SOFAR) floats which
control their buoyancy to cycle every other day between prescribed
isotherms. (J. Price, personal communication) Bobbers transmit a swept 250
hertz signal for 80 seconds, precisely 12 hours apart on a preset
schedule.  The range to the float can be derived from the travel time
and the speed of sound in the water.  While at sea, onboard tracking
was done using shipboard listening stations and SOFAR receivers.
Either special hydrophone arrays or a Sonobouy float was used at these
stations to listen for the bobbers. In addition, drifting SOFAR
receivers (DSRs) and ALFOS floats, which were deployed from the ship
earlier, relayed the times of arrival (TOAs) of bobber signals to WHOI
via ARGOS satellite.  The TOAs and receiver position were then
transmitted to the ship via INMARSAT FAX where the range from the
drifting receivers to the bobber was calculated.  Range information
from two or three receivers was combined to locate the bobbers by
triangulation.  On the final cruise, the moored Autonomous listening
stations (ALS), which had been recording TOAs from the bobbers since
May 1991, were recovered.  The ALS data were decoded and actual
positions were determined for the bobbers for the times of the Seasoar
surveys (Fig. 3a - Subduction 3, Fig. 3b - Subduction 4).


C: DATA PROCESSING

The CTD temperature and conductivity sensors were calibrated for each
cruise using a combination of lab calibrations (done by Sea-Bird at the
North-West Calibration Center) and comparisons with water samples
collected on a profiling CTD.  All sensors were calibrated before the
initial cruise and following all subsequent cruises.

The temperature sensors were corrected for drift based on the lab
calibrations alone, by assuming a linear change in time between two
calibrations. Corrections to the lab calibrations were +/- 0 to 2 mK
(offset) and  1 +/- 0 to 0.15 mK/K (slope).

Using the corrected temperatures, water sample salinities from
approximately seven deep stations per cruise were converted to
conductivity for comparison with the conductivity sensors.  The
calibration for conductivity in shallow water where the vertical
gradients are large and spatially variable is particularly difficult.
The profiling CTD maintained one sensor pair (primary) throughout a
cruise, the secondary sensors were swapped with the Seasoar's for
calibrations.  Thus, the primary sensor pair had the greatest number of
water samples to use for calibration.  For cruises one and three, we
determined the Seasoar sensor calibration by performing a water sample
calibration for the primary CTD sensors, and then fit the secondary
sensors to the primaries using data from the complete cast. The bottle
data for the secondary sensors then served as a consistency check for
the obtained calibration values.  For Subduction 4, however, this
approach generated a correction to the pre-cruise lab calibration that
largely exceeded the post-cruise lab calibration.  This can not occur
if the sensors drift essentially linearly in time. We therefore relied
only on the direct bottle comparison to calibrate the Seasoar
conductivity sensors.  Additionally, the vertical conductivity gradient
during this cruise was at times so strong that the vertical separation
of 1.5 meters between bottles and sensors introduced an error large
enough to affect the calibration.  To correct for the spatial
difference, a polynomial fit of the conductivity gradient was
determined for each station, and an offset was applied based on the
polynomial and the distance between bottles and sensors. The
conductivity gradients from the other cruises were not large enough to
require this correction.  Conductivity corrections ranged from -1.7 to
+0.7 mS (offset) and 1 +/- 0 to 0.6 mS/S (slope).  The remaining
differences between calibrated CTD conductivity and bottle
conductivities were of the order of 0.2 mS/m (deep samples) to 0.5 mS/m
(shallow samples), corresponding to salinity differences of 0.002 to
0.005 psu.

The calibrated 24 Hz data were then screened for anomalous points using
a 9-point median filter.  To determine the proper relationship between
temperature and conductivity sensors influenced by their physical
separation and sensor response times, salinity was calculated for
various lags of temperature and pressure relative to conductivity. A
lag of 4 scans (1/6 second) was found to minimize salinity spiking
across sharp gradients.  This lag was consistent over the course of the
experiment.  The data were edited further by excluding data shallower
than 1 dbar. This excludes salinity spikes due to air in the
conductivity cell when the Seasoar breaks the surface.  Summary Figures
for quality control were produced (Fig. 4).  The data were then binned
into 1 and 3 sec datasets (available in ASCII and matlab format on
CDROM) of time, pressure, potential temperature, salinity, and
potential density.  Salinity and potential density were calculated
after binning.

The 3 second averaged data were interpolated onto a uniform grid in
depth/distance along the Seasoar track using a second order exponential
filter with vertical and horizontal scales of 5 dbar and 4km,
respectively. Grid points for which the sum of weights were less than
or equal to 0.1 were flagged (Fig.  5).  Data were then mapped onto
density surfaces at intervals of 0.05 sigma-theta (Table 3, Fig. 6).
Where appropriate, CTD data from the L-shaped tracer surveys were
combined with the Seasoar data and input into the objective mapping
programs. We chose to focus on sigma-theta levels of 26.5, 26.7 and
26.9.  The levels correlate with the isotherm boundaries and
corresponding average densities of the bobbers when they were initially
deployed (Fig. 7). The thickness of each density surface is
based on the density gradient centered on the density surface of
interest with a fixed density difference of 0.05 Sigma-theta.  The
mapping technique used a spatial correlation scale of 10 km, and a
signal to noise ratio of 50 percent was assumed.  Areas with errors
exceeding 95 percent were not contoured.  Data was objectively mapped
for all Seasoar surveys on the above-mentioned density surfaces for
potential temperature, salinity, pressure and thickness (Figs. 8a - i).

Despite the variety of shipboard location tools for determination of
bobber position, the actual location of bobbers during the experiment
was problematic. In some instances, insufficient fixes were available
to locate bobbers or two Seasoar surveys were carried out because of
possible ambiguities in bobber location. Why post-experiment bobber
tracks (using the moored ALS data) seem to be 'offset' from at-sea
locations has not been resolved.  Thus, the Seasoar maps around bobbers
should be considered only to reflect the general characteristics of the
water masses at that particular time.


Table 3: Mean and standard deviation of theta, pressure and thickness on
        density surfaces 26.5, 26.7, 26.9 for all star patterns surveyed.
	Not enough data were available for density level 26.9 for surveys
	3 and 4 during Subduction 4.


 SUBDUCTION 1 - STAR PATTERN 1:
*************************************************************************
DATES:  05/16/91 10:30 - 05/20/91 07:30
BOBBER #: 24,29,22,5,8,21,16,17,18,23,25
MEAN LAT: 31.4729 N
MEAN LONG: -22.4486 E
*************************************************************************
Sigma-Theta	26.5		|	26.7		|	26.9
Avg. Theta	18.7702		|	16.2514		|	13.9505
Std. Theta	0.0201		|	0.0680		|	0.0206
Avg. Press	60.7186		|	240.5842	|	364.9566
Std. Press	10.9056		|	10.0753		|	4.1349
Avg. Thick	78.4115		|	31.4357		|	36.6801
Std. Thick	5.6687		|	1.5292		|	1.3638
# grid pts	86		|	469		|	223
*************************************************************************


 SUBDUCTION 1 - STAR PATTERN 2:
*************************************************************************
DATES: 05/23/91 06:00 - 05/27/91 20:00
BOBBER #: 19,14,11,12,26,20,15
MEAN LAT: 29.0139 N
MEAN LONG: -23.5313 E
*************************************************************************
Sigma-Theta	26.5		|	26.7		|	26.9
Avg. Theta	18.8185		|	16.1601		|	13.9314
Std. Theta	0.1699		|	0.0929		|	0.0249
Avg. Press	124.4070	|	241.9765	|	366.1262
Std. Press	6.6596		|	6.3833		|	4.9197
Avg. Thick	41.0442		|	30.1820		|	35.9329
Std. Thick	8.8076		|	1.6768		|	1.3890
# grid pts	473		|	473		|	346
*************************************************************************



 SUBDUCTION 1 - STAR PATTERN 3:
*************************************************************************
DATES:  05/31/91 06:30 - 06/02/91 01:30
BOBBER #:  
MEAN LAT:  29.9845 N
MEAN LONG: -21.6587 E
*************************************************************************
Sigma-Theta	26.5		|	26.7		|	26.9
Avg. Theta	18.7825		|	16.4236		|	13.9782
Std. Theta	0.2247		|	0.1287		|	0.0264
Avg. Press	51.0511		|	216.5273	|	340.1730
Std. Press	4.2587		|	15.9862		|	18.1612
Avg. Thick	45.1072		|	27.1940		|	36.2476
Std. Thick	5.5786		|	4.7615		|	1.0980
# grid pts	418		|	420		|	360
*************************************************************************



 SUBDUCTION 3 - STAR PATTERN 1:
*************************************************************************
DATES: 11/28/92 22:00 - 11/30/92 14:00
BOBBER #: 26
MEAN LAT: 20.3393 N
MEAN LONG: -29.6893 E
*************************************************************************
Sigma-Theta	26.5		|	26.7		|	26.9
Avg. Theta	17.6773		|	15.6800		|	13.5893
Std. Theta	0.1868		|	0.1361		|	0.0823
Avg. Press	163.4997	|	234.0999	|	342.2342
Std. Press	10.8143		|	11.4590		|	10.1150
Avg. Thick	13.0774		|	22.1654		|	30.2678
Std. Thick	0.8380		|	1.6846		|	1.5926
# grid pts	308		|	308		|	304
*************************************************************************



 SUBDUCTION 3 - STAR PATTERN 2:
*************************************************************************
DATES: 12/01/92 01:00 - 12/02/91 18:30
BOBBER #: 15,25
MEAN LAT: 22.8685 N 
MEAN LONG: -27.0472 E
*************************************************************************
Sigma-Theta	26.5		|	26.7		|	26.9
Avg. Theta	18.2682		|	16.0629		|	13.8362
Std. Theta	0.0742		|	0.0673		|	0.0426
Avg. Press	180.6118	|	262.3036	|	373.9740
Std. Press	5.6818		|	5.9471		|	5.1209
Avg. Thick	14.5908		|	24.6175		|	28.2718
Std. Thick	0.6068		|	1.1954		|	1.4681
# grid pts	308		|	308		|	308
*************************************************************************



 SUBDUCTION 3 - STAR PATTERN 3:
*************************************************************************
DATES: 12/02/92 19:00 - 12/04/92 19:30
BOBBER #: 19
MEAN LAT: 22.7953 N
MEAN LONG: -28.6941 E
*************************************************************************
Sigma-Theta	26.5		|	26.7		|	26.9
Avg. Theta	18.3048		|	16.0337		|	13.8438
Std. Theta	0.0858		|	0.0732		|	0.0374
Avg. Press	191.6362	|	271.8771	|	376.0974
Std. Press	6.4222		|	5.5544		|	5.1748
Avg. Thick	14.3554		|	23.5976		|	24.8414
Std. Thick	0.7220		|	0.8780		|	1.5926
# grid pts	312		|	312		|	303
*********************************************************************



 SUBDUCTION 3 - STAR PATTERN 4:
*************************************************************************
DATES: 12/04/92 20:00 - 12/06/92 01:00
BOBBER #: 15
MEAN LAT: 23.2972 N
MEAN LONG: -29.4279 E
*************************************************************************
Sigma-Theta	26.5		|	26.7		|	26.9
Avg. Theta	18.1389		|	15.8712		|	13.7206
Std. Theta	0.2898		|	0.2891		|	0.1830
Avg. Press	187.0571	|	268.6239	|	374.4001
Std. Press	5.9160		|	8.1597		|	9.1982
Avg. Thick	14.3723		|	24.3347		|	25.0590
Std. Thick	0.8294		|	1.0564		|	1.4868
# grid pts	301		|	301		|	298
*************************************************************************



 SUBDUCTION 3 - STAR PATTERN 5:
*************************************************************************
DATES: 12/08/92 09:30  - 12/11/92 13:30
BOBBER #: 21
MEAN LAT: 25.1120 N
MEAN LONG: -24.4478 E
*************************************************************************
Sigma-Theta	26.5		|	26.7		|	26.9
Avg. Theta	18.5389		|	16.2373		|	13.9424
Std. Theta	0.0331		|	0.0267		|	0.0070
Avg. Press	176.1963	|	268.6849	|	387.1770
Std. Press	4.8472		|	4.9632		|	6.2923
Avg. Thick	18.6155		|	28.0117		|	30.1917
Std. Thick	1.3784		|	0.8609		|	1.1335
# grid pts	272		|	272		|	272
*************************************************************************



 SUBDUCTION 3 - STAR PATTERN 6:
*************************************************************************
DATES: 12/11/92 13:30 - 12/12/92 18:30
BOBBER #: 20
MEAN LAT: 26.9678 N
MEAN LONG: -24.9349 E
*************************************************************************
Sigma-Theta	26.5		|	26.7		|	26.9
Avg. Theta	18.2821		|	16.1338		|	13.9678
Std. Theta	0.0512		|	0.0342		|	0.0164
Avg. Press	164.3603	|	256.8055	|	362.1923
Std. Press	3.9648		|	4.1351		|	3.9743
Avg. Thick	19.5107		|	28.0923		|	33.5705
Std. Thick	0.7727		|	1.1078		|	0.8828
# grid pts	265		|	265		|	265
*************************************************************************



 SUBDUCTION 4 - STAR PATTERN 1:
*************************************************************************
DATES: 05/24/93 07:30 - 05/27/93 12:00
BOBBER #: 19
MEAN LAT: 18.9862 N
MEAN LONG: -31.9748 E
*************************************************************************
Sigma-Theta	26.5		|	26.7		|	26.9
Avg. Theta	17.8358		|	15.6538		|	13.5999
Std. Theta	0.1302		|	0.0988		|	0.1453
Avg. Press	199.7486	|	261.7677	|	361.3810
Std. Press	10.6113		|	11.6073		|	8.3183
Avg. Thick	11.1412		|	20.6743		|	28.2127
Std. Thick	0.8561		|	0.9830		|	2.1628
# grid pts	306		|	306		|	279
*************************************************************************



 SUBDUCTION 4 - STAR PATTERN 2:
*************************************************************************
DATES: 05/30/93 02:00 - 05.31.93 13:30
BOBBER #: 26
MEAN LAT: 24.2855 N
MEAN LONG: -37.4544 E
*************************************************************************
Sigma-Theta	26.5		|	26.7		|	26.9
Avg. Theta	18.0407		|	16.0338		|	13.9275
Std. Theta	0.0810		|	0.0271		|	0.0122
Avg. Press	224.9581	|	318.8388	|	389.9046
Std. Press	8.3322		|	9.2240		|	4.6894
Avg. Thick	18.3139		|	27.8029		|	30.8733
Std. Thick	1.4372		|	1.4772		|	1.5640
# grid pts	335		|	335		|	132
*************************************************************************



 SUBDUCTION 4 - STAR PATTERN 3:
*************************************************************************
DATES: 06/02/93 01:30 - 06/03/93 14:00
BOBBER #: 15
MEAN LAT: 23.5120 N
MEAN LONG: -31.6550 E
*************************************************************************
Sigma-Theta	26.5		|	26.7
Avg. Theta	18.1924		|	16.0873
Std. Theta	0.0251		|	0.0184
Avg. Press	192.4304	|	281.1632
Std. Press	5.9920		|	5.0778
Avg. Thick	17.1146		|	26.1395
Std. Thick	1.3808		|	1.8345
# grid pts	306		|	306
*************************************************************************



 SUBDUCTION 4 - STAR PATTERN 4:
*************************************************************************
DATES: 06/05/93 15:00 - 06/08/93 17:30
BOBBER #: 21
MEAN LAT: 28.1707 N
MEAN LONG: -26.3902 E
*************************************************************************
Sigma-Theta	26.5		|	26.7
Avg. Theta	18.4098		|	16.0449
Std. Theta	0.0979		|	0.0424
Avg. Press	183.8366	|	292.0272
Std. Press	11.0004		|	10.4090
Avg. Thick	31.0742		|	28.0224
Std. Thick	3.0547		|	1.4874
# grid pts	366		|	362
*************************************************************************




D: UNDERWAY CURRENTS - ADCP

Shipboard Acoustic Dopler Current Profiler (ADCP) data were collected
during all four Subduction cruises using a standard 150KHz RD Instruments
transducer. The setup used 8 meter vertical bins with 8 or 16 meter
pulse lengths averaged over 5 minutes.  Bottom tracking data were
collected over the continental shelf leaving Woods Hole, and for very
short periods over the slopes of the Azores, Madeira and Gran Canaria.
One-second navigation data were provided by a Magnavox MX4200 Global
Positioning System (GPS) receiver.

The ADCP data were processed with the Common Oceanographic Data Access
System (CODAS) software developed by Eric Firing from the University of
Hawaii (Bahr et al., 1990).  After the data were loaded into a
database, the individual profiles were edited for anomalous points
based on editing criteria such as large second vertical derivatives of
eastward (u) and northward (v) velocity components, large vertical (w)
and error velocities, and subsurface maxima of backscatter amplitude.
Aside from the usual amplitude warnings triggered by either bottom
interference or biological scattering layers, we found occasional
interference from the hydrowire when the CTD package had drifted into
one of the ADCP beams.  Next the GPS fixes were screened for outliers
based on number of satellites used and Horizontal Dilution Of Precision
(HDOP) values, and then merged with the ADCP data to provide absolute
velocities. This step involved the intermediate calculation of the
absolute velocity of a reference layer (e.g., Kosro,1985, see Table 4
for layer range).  The velocity of the reference layer is the
difference between the velocity of the ship over the ground, determined
by the fixes, and the velocity of the ship relative to the reference
layer, calculated from the ADCP profiles.  This initial estimate of the
reference layer velocity, which is constant between fixes, was then
smoothed by convolution with a Blackman window function (Blackman and
Tukey, 1959).  The choice of filter width generally depends on the
quality of the fixes. For Subduction 1, which occurred shortly after
the Gulf war Desert Storm, selective availability (SA) was not in
effect, and the fix quality was accordingly good. SA was in effect,
however, for Subduction 3 and 4, and the filter needed to be
correspondingly larger (Table 4).

Bottom track calibration was performed using mostly the Woods Hole
continental shelf data, since the island bottom tracking was often too
short. Underway calibrations were done on cruises with many CTD
stations.  In this type of calibration, velocity differences measured
by the ADCP (e.g., when departing from station) are compared with those
measured by the satellite navigation. This method has a large
uncertainty associated with each individual calibration point and a
large number of points need to be taken.  Calibration values were
computed for each cruise from a combination of bottom track and water
track information (Table 4 ).


          Table 4: ADCP processing parameter settings


                     Subduction 1     Subduction 3     Subduction 4

reference             bins 5-20        bins 5-17        bins 5-20
layer range           (50-170m)        (50-145m)        (50-170m)

smoothing filter      20 minutes       30 minutes       30 minutes
half width

calibration phase    -1.32 degrees     -1.5 degrees     -1.7 degrees
  and amplitude         1.007             1.005            1.005



In order to produce maps of velocity on density surfaces, temperature
and salinity profiles were generated from 15-minute averages of the
Seasoar data. Using this database, the ADCP data were vertically
regridded on density, and 30-minute averaged vectors over the two
shallower density intervals were calculated (Figs. 9a-b). In addition,
30-minute averages of ADCP velocity along the original depth bins were
produced in ASCII format (available on CD-ROM).



4. DISCUSSION


The initial deployment cruise for the bobbers, in May 1991, came just
as the water column began to stratify. The remnant mixed layer was deep
and reflected the characteristics of late winter conditions. The
density modes for the first two star patterns indicated that the
initial winter mixed layer depth was between 100 and 150 meters (See
Appendix B:  Figs. Sub 1, Star 1 - Section SE-NW and Sub 1, Star 2,
Section SE - NW). The Subduction bobber cruises were distributed in
time in such a way as to cover a two year lifespan.  However, due to
the concentration in the northern region near the Azores Front on the
second cruise (February 1992), no Seasoar data were collected near any
of the bobbers. Thus, the temporal sampling between the bobber cruises
was uneven, with intervals of 18 and 6 months.

The 'star' patterns were carried out in order to map the variability
around the bobber floats. During the initial cruise, the star patterns
each consumed about 45 hours of shiptime. The long legs of the patterns
were approximately 110 km in length. An analysis of temperature,
pressure, and thickness variations on the individual legs indicated
that the de-correlation scale was 8-10 km. Error maps made from the
objective mapping of the data showed that the star pattern was too
large:  large areas within the pattern were poorly mapped. In
subsequent cruises, the scale of the pattern was reduced so that the
long legs of the pattern were approximately 80 km.  Not only did this
better 'map' the variability, it took less shiptime (27 hrs/survey)!


5. ACKNOWLEDGMENTS:

The Subduction ARI experiment was sponsored by the Office of Naval
Research, grants N00014-91-J-1585 - Mesoscale Variablility of
Subduction Waters (T. Joyce) and N00014-91-J-1508 - Seasoar Operations
in Subduction and N00014-91-J-1425 - Subduction in The Subtropical Gyre
(J. Luyten).  We wish to thank the captain and crew of the R/V
Oceanus.  Bobber locations were received from James Price and Christine
Wooding.


6. REFERENCES:

Bahr, F., E. Firing, and S.N. Jiang, 1990.  Acoustic Doppler current 
    profiling in the western Pacific during the US_PRC TOGA cruises 5 and 6.
    Data report No. 007 from the Joint Institute for Marine and Atmospheric
    Research, University of Hawaii.  161 pp.

Blackman, R.B. and J.W. Tukey, 1959.  The measurement of power spectra.
    Dover, New York, 190 pp.

Joyce, T.M., 1992. Cruise Report OC254/4: Subduction 3.  Woods Hole 
    Oceanographic Institution, Woods Hole MA.  Unpublished manuscript. 21 pp.

Kosro, P.M. 1985.  Shipboard acoustic current profiling during the Coastal
    Ocean Dynamics Experiment.  SIO reference 86-8, 119 pp.

Luyten, J.R., 1991.  The Subduction Experiment: Cruise Report OC240/2.  Woods
    Hole Oceanographic Institution, Woods Hole MA. Unpublished manuscript.  20 pp.

Luyten, J.R., 1993.  The Subduction Experiment: Cruise Report OC258/3.  Woods
    Hole Oceanographic Institution, Woods Hole MA. Unpublished Manuscript. 20 pp.

Price, J.R., In Progress.  Subduction Bobber Data Report

Rudnick, D.L. 1992.  Cruise Report OC250/3: Subduction experiment.  University
    of California, San Diego.  Unpublished Manuscript. 13 pp.
 

7. APPENDICES


A: Star Pattern - Data Summaries

Seasoar data were summarized for each star pattern surveyed during the
subduction experiment. The gridded data were mapped onto density
surfaces of 0.05 sigma-theta.  Plots of pressure, potential
temperature, salinity and thickness vs potential density for each
survey are presented in figures A-1 - A-13.  Location and time of the
survey is described in Figure 1 and Table 2.


B: Section Contour Plots for star patterns:  

Contour plots of gridded Seasoar data along selected sections of the
radiator pattern are shown in figures B-1 - B-15.  Each figure consists
of a sigma-theta, theta and salinity contour plot for the specified
section.  Location of the section on the star pattern is highlighted on
the star pattern shown in figure B-1.  Position and time of the
individual star pattern is described in Figure 1 and Table 2.  Gray
areas denote unavailable data.  The darker lines represent the average
theta and sigma-theta where the bobbers were deployed during Subduction
1. (see Fig.  7).


C: Long Sections Surveyed between star patterns

Contour plots of gridded Seasoar data along several long transects
during the Subduction 3 and 4 cruises proceed figures C-1 - C-21.
Position and time of the transects can be located on Figure 1 and Table
2.  Gray shading denote areas of unavailable data.  The darker lines
represent the average theta and sigma-theta where the bobbers were
deployed during Subduction 1. (see Fig.  7).  Color versions of these
maps are available on the accompanying CDROM.


D: Star Pattern Objective Maps

Figures D-1 - D-110 present objectively mapped plots of ocean
properties on potential density surfaces of 26.5, 26.7, 26.9. Theta,
pressure and thickness are individually plotted on the selected
surfaces.  The triangles on the plots denotes 15 minute averages along
the  cruise track.  Color versions of these maps are available on the
accompanying CDROM.


E: ADCP maps for star patterns 

Figures E-1 - E-15 show ADCP velocity maps for each star pattern on
potential density surfaces of 26.5 and 26.7.  ADCP vectors were
averaged in density space over 0.05 sigma theta.


F: Contents of accompanying CD-ROM.



