This report was prepared by Dave Hebert, Jack Barth, Dave Ullman, and Sandra Fontana. This cruise was sponsored by the National Science Foundation and the National Oceanic and Atmospheric Administration. This work was supported with NSF Grants OCE-9806650 and OCE-9813641.
Cruise Objectives The overall objectives of the cruise were to examine the cross-frontal flow and mixing at the tidal mixing front and shelfbreak front located on the southern flank of Georges Bank. To address these goals, we proposed to:
Cruise Narrative On Monday, 14 June at 0950 EDT (1350 UTC*) we departed Woods Hole to the location of the tidal mixing front (TMF) on the southern flank of Georges Bank (approximately 41deg N, 67deg 45' W).
Throughout the cruise, several underway measurements were continuously recorded. Sea surface temperature and salinity were obtained from the ship's intake water. Standard meteorological measurements were obtained using the IMET system. Figure 1 shows the wind speed that was recorded throughout the cruise. (A time-line of the major events is also shown on that figure.) Throughout the cruise, water velocities were measured with two acoustic Doppler current profilers.
On June 15th at approximately 0600, we arrived at the western end of our survey region. Before deploying the SeaSoar for the first time in this region, a water velocity/depth survey for the northern portion of the radiator pattern [Figure 2] was completed overnight. This mapping would provide us information on any sand waves or other topographic features in the region. The survey was completed at approximately 1400.
At 1400, we started to prepare the SeaSoar for deployment. However, there appeared to be a problem with the transmissometer. This transmissometer was replaced with the one from the R/V Oceanus's CTD package. At 1540, the SeaSoar was deployed at 1N, the northeast corner of the survey grid where lines are numbered from east to west and N or S indicates the northern and southern end of each line [Figure 2]. The radiator pattern was completed at 1926, 16th June. We headed to 40deg 56' N, 67deg 22' W to deploy the surface drifter and COOL float at the tidal mixing front.
After a CTD cast to confirm our location in the tidal mixing front, COOL Float #2 with compressee 2B was deployed at 40deg 55.19' N, 67deg 20.57' W on June 16th at 2159. The COOL float was ballasted for the st = 25.5 kg/m3 surface. It was programmed to surface 30 hours later. At 2207, an ARGOS-tracked surface drifter (Drifter #1, ARGOS ID 27613) was deployed at 40deg 55.15' N, 67deg 20.68' W. This drifter had a 10-m long drogue centered at a depth of 15 m. Self-contained ONSET temperature loggers were placed at nominal depths of 0, 5, 10, 13.3, 16.7 and 20 m.
At 2315, June 16th, the SeaSoar was deployed at 40deg 50.59' N, 67deg 19.69' W. Butterfly patterns were made around the COOL float and surface drifter in order to acoustically track the subsurface float using the ship's 12-kHz transducer and to map the hydrographic fields following a Lagrangian particle. Before recovering the COOL float and drifter, the SeaSoar was brought back on board at 0055, June 19th. The surface drifter was only 2 nm away; it was recovered first. At 0140, the drifter was on board. We headed to the location of the COOL float. At 0447, the COOL float was on the surface (40deg 51.68' N, 67deg 32.60' W). The float was recovered at 0535. At 0804, the SeaSoar was deployed at 1N for another radiator survey of the tidal mixing front [Figure 2]. An additional line (9) was added to the west of the previous radiator pattern. The radiator pattern was completed at 1214, June 20th. We continued to SeaSoar to the start of the shelfbreak front (SBF) radiator pattern (AN, the northeast corner of the grid with lines labelled alphabetically from east to west).
At 1705, June 20th, the shelfbreak front radiator pattern was started [Figure 7]. On line E, from EN to ES, we started losing CTD data frequently at the start of an up cast. We lost the CTD signal completely at 1014, June 21st. The SeaSoar was recovered. However, during the recovery, the level wind on the winch was not working properly and the cable had to be paid out and rewound on the drum.
While repairs were being made to the SeaSoar's termination, we decided to make CTD casts at 5 km spacing to map out the front. At 1239, a CTD cast was made. However, the CTD was not working properly. First, the CTD slip rings were changed between this cast and the previous one. Several of the connections appeared to be noisy. Different conductors were tried until a good set was found. It was discovered that the power cable to the pump had an intermittent break such that the pump was not always on during a cast. This power cable was replaced and the CTD was back in working operation. Cast #1E was made.
At 1400, the level wind for the winch was repaired. We steamed back to EN and deployed the SeaSoar at 1615. We proceeded south to ES to complete the radiator pattern at 2020. Then, we headed to CS and northwards to locate the front, where the st = 25.8 kg/m3 surface was at 50 m depth, for deployment of the COOL float. At approximately 2100, the deployment site was located. At 2339, the SeaSoar was recovered after proceeding farther northwards.
At 0011 on June 22nd, COOL Float #2 (compressee 2A) was deployed at 40deg 48.30' N, 67deg 5.37' W and programmed to surface 30 hours later. The target density was st = 26.15 kg/m3. At 0017, the surface drifter was deployed at 40deg 48.28' N, 67deg 5.37' W. This was followed by a CTD (#4) at 40deg 48.21' N, 67deg 5.30' W.
On June 22th, at 0140, the SeaSoar butterfly pattern was started at 40deg 44.69' N, 66deg 58.52' W. The SeaSoar pattern was completed on June 24th, 0112 at 40deg 32.18' N, 67deg 13.53' W. At 0245, the surface drifter was recovered at 40deg 41.13' N, 67deg 26.84' W followed by the recovery of the COOL float at 0657 (40deg 38.61' N, 67deg 8.67' W). While wiping water off the outside of the float, it was noticed that some water was inside the float. This problem is discussed in more detail in the float section. Fortunately, all of the data was recovered successfully with the assistance of Mark Prater and Jim Fontaine back at URI. We steamed to location CS to deploy the SeaSoar.
At 0803 on June 24th, the SeaSoar was deployed at 40deg 38.70' N, 67deg 57.67' W for the start of a line from CS to 1S, the start of the tidal mixing front radiator pattern. The radiator pattern was started at 1104 and completed (location 8S) at 1350 on June 25th [Figure 2]. The SeaSoar was recovered and underwent maintenance. Meanwhile, a CTD survey was started at location CN and headed southward.
After completion of the SeaSoar maintenance, a line starting at 1822 on June 25th was undertaken to determine the location of the shelfbreak front (i.e., st = 25.8 kg/m3 at 50 m). The location of the front was determined and the SeaSoar recovered at 2154. COOL Float #4 (compressee 4A) was deployed at 40deg 46.95' N, 67deg 4.30' W. Eight minutes later at 2215, the surface drifter was deployed at 40deg 46.97' N, 67deg 4.28' W. For the start of the butterfly patterns, at 2315, the SeaSoar was deployed at 40deg 42.89' N, 66deg 56.83' W. Surveys around the float and drifter were continued until June 28 at 0312 when the SeaSoar was recovered. Due to the heavy fog, the COOL float was recovered first. At 0546, it was onboard. The surface drifter was recovered shortly afterwards at 0704.
We decided to head south of location CS on the shelfbreak radiator pattern so that the cross-frontal structure of the shelfbreak front would be more completely surveyed. At 0848 on June 28th, a SeaSoar transect northwards from 40deg 34.65' N, 66deg 53.25' W to 1S, the start of the tidal mixing front radiator pattern, was started. Another TMF survey started at 1234 and was completed at 1613 on June 29th [Figure 2]. Then, we headed to line 3. From 1838 on June 29th until 1418 on June 30th, we surveyed portion of the line repeatedly in the hope to watch the evolution of solitons interacting with the tidal mixing front.
After recovery of the SeaSoar at 1418 on June 30th, we proceeded towards Woods Hole. At 1255 (0855 EDT) on July 1st, the R/V Oceanus arrived at Woods Hole.
Overall, we had a very successful cruise. We completed four large-scale (30 ×40 km) detailed CTD/ADCP surveys across the the tidal mixing front on the southern flank of Georges Bank [Figures 2 and 3]. As well, a high resolution, small-scale (18 ×10 km) survey was conducted around the subsurface water parcel tagged with a COOL float for several tidal cycles [Figures 5 and 6]. For contrast, a large scale survey was made of the shelfbreak front south of the tidal mixing front [Figure 7]. A COOL float and surface drifter drogued at 15 m were deployed in the shelfbreak front. High resolution surveys were made around these floats. Finally, one of the north-south transect lines was repeatedly surveyed to investigate the evolution of solitons propagating onto the shelf.
SeaSoar Measurements. Jack Barth To measure the hydrographic and bio-optical properties over the southern flank of Georges Bank, we deployed the towed, undulating vehicle SeaSoar. SeaSoar was towed using a 9/16'' 7-conductor hydrographic cable and profiled from the sea surface to 110 m over deep water and to within 10 m of the bottom over the Bank. Cycle time in deep water was about 6.5 minutes resulting in surface points being separated by 1.3 km at the ship's typical 7 knot tow speed - horizontal separation between profiles at mid-depth is half this value. Cycles over the shallow Bank, 0-70 m, took 2 minutes so surface points where separated by 420 m. Bottom avoidance was accomplished by using Oceanus' 3.5 kHz Knudsen echosounder as input to the OSU SeaSoar flight control software.
The SeaSoar was equipped with a Seabird 911+ CTD with pumped, dual T/C sensors pointing forward through the nose of SeaSoar. Fluorescence was measured with a WETLabs FlashPak fluorometer (460-nm excitation, 695-nm emission) mounted on the upper tail fin of SeaSoar. Light transmission was measured with a Seatech 25-cm pathlength transmissometer mounted on top of SeaSoar. In addition, engineering information (pitch, roll, propeller rotation rate) was obtained from sensors aboard the vehicle. All data were sent topside where they were merged with GPS location and time before archiving. The data were averaged over one second from which realtime displays of hydrographic and bio-optical properties were made.
SeaSoar was towed across the southern flank of Georges Bank in several basic patterns. The first was a radiator pattern consisting of 35-km long legs oriented roughly perpendicular to the local isobaths. Cross-bank lines were separated by 5 km for a total alongbank coverage of 35 km. This ``tidal mixing front'' (TMF) radiator pattern was centered on about the 60-m isobath and was repeated four times during the cruise [Figures. 2, 3, and 4]. One cross-bank line paralleled the set of moorings between the 50 and 70-m isobaths (Schlitz et al.) and passed close to the long-term mooring at 40° 57.99' N, 67° 18.92' W (Irish et al.). The 3-m temperature maps [ Figure 2] reveal generally colder water over the bank, but there is much mesoscale variability associated mainly with onbank excursions of warm (salty) water from offbank. The 39-m temperature maps [Figure 3] show the location of the TMF more clearly across the northern end of the sampling region. The two early maps show approximately 16-km peak-to-peak tidal excursions of the TMF. The two later maps reveal excursions with significantly less amplitude presumably due to weakening of the tides due to the spring-neap cycle. The 39-m temperature maps also show the movement of a warm water feature of slope origin across the TMF study region from 19-25 June. The repeated sections along the line just to the SW of the moored array [Figure 4] also show mesoscale variability impinging from offbank. In the first realization (16 June), the TMF is located at about 41.15° N near the 55-m isobath. In the later sections, both the interior and surface water are significantly warmer and the water column is stratified past the northern edge of our study region (45-m isobath).
After defining the local hydrography, bio-optical and velocity fields around the TMF, a subsurface COOL float and a 15-m drogued drifter were released seaward of the TMF. The COOL float was tracked acoustically while towing SeaSoar on a butterfly pattern extending 20 km across the bank and 10 km alongbank. The pattern was repeated in place or shifted to the west depending on the location of the float [Figures 5 and 6]. The butterfly pattern took approximately 6.5-8 hours to complete depending on which phase of the tide it was started on and was repeated about eight times before recovering the drifter and float.
A slightly larger radiator pattern (44-km cross-bank, 7.5-km separation between legs for a total alongbank coverage of 45 km) was centered near the 95-m isobath and covered a region just offbank of the TMF radiator. This pattern sampled the ``shelfbreak front'' (SBF) and was occupied once during 20-21 June [Figure 7]. Hydrographic properties at 39 m reveal the strong SBF located along the 95-100 m isobaths. The warm, salty anomaly seen at the offbank end of the TMF radiator [Figure 3] is evident in the NW corner of the SBF survey grid [Figure 7]. While some small scale (i.e., with a size less than the between-track spacing) variability is likely an artifact of the first-cut gridding routine used to produce these results (e.g., near 40.95° N, 67.05° W), other features are real. For example, the warm, salty, light anomaly at 40.8° N, 67.2° W is the manifestation of an internal soliton propagating onbank. The feature is 3-4 km wide and depresses the property clines by about 40 m [Figure 8].
Two float and drifter deployments took place in the SBF and SeaSoar was towed in a butterfly pattern on 22-24 [Figures 9 and 10] and 25-28 June while tracking the float acoustically.
The final activity was five repeated occupations of an approximately 50-km long cross-bank line during 29-30 June which parallel the moored array just to its NE.
ADCP Measurements. Sandra Fontana Both the 150-kHz narrowband and 300-kHz broadband RDI acoustic Doppler current profilers (ADCPs) were in operation throughout the cruise. Real-time running colour raster plots of the velocities from the narrowband ADCP were displayed continuously. The narrowband was initially configured to run using Charlie Flagg's autoadcp mode, in which the configuration files change automatically based on specific geographic regions. Upon entering the survey region, the autoadcp mode was abandoned in favor of a fixed configuration file (beginning on June 14th at 23:30). The ensemble length was set to 150 seconds (from 300 seconds). The pulse length and bin length were both set to 8 meters, and bottom track data were collected during the entire time. The configuration was changed back to autoadcp mode at 14:53 on June 30th, shortly after the start of the return transit to Woods Hole.
The Ashtech 3DF attitudinal GPS data were recorded every 1/2 second, until a reset at 14:14 on June 19 resulted in data every 1.0 second. The data were then averaged over the ensemble length by the user exit program ue4, with the Ashtech-gyro difference stored in a user buffer (in the pingdata files). Positions from the P-Code navigation data were extracted at the beginning and end of each ensemble and also stored in the user buffer by ue4. The Ashtech data will be used in post-processing to correct for gyrocompass variations to determine the transducer offset and more accurate final velocities. On June 19th at approximately 01:25, the bridge officer on watch switched gyrocompasses, a factor to be considered in the post-processing.
The broadband data (300 kHz) were collected with RDI's TRANSECT software (version 2.80). Both raw and averaged data were recorded. The data were averaged over 150 seconds. Gyro and navigation data were recorded throughout the cruise. Cell length was set to 4 meters for deeper water and 2 meters for shallower water. Bottom track data were also collected.
Figures 11 and 12 show the velocity vectors every 5 minutes from the narrowband ADCP for the 4 TMF surveys at 30 m and one SBF survey at 35 m, respectively.
COOL Float/ Drifter Measurements. Dave Hebert A major aspect of this cruise was to map the change in the hydrographic structure of the front from a Lagrangian point of view. Thus, the plan was to survey around the position of a subsurface isopycnal float. The dominant current advecting the float would be the M2 tide. The subsurface float would be tracked acoustically from the ship. Given the success on the previous cruise (OC340), we decided to deploy a GPS/ARGOS surface drifter drogued to 15 m at the same time as the subsurface float.
After completing our first radiator pattern of the tidal mixing front, a CTD cast was made to confirm our location in the tidal mixing front. The subsurface isopycnal COastal Ocean Lagrangian (COOL) float #2 with compressee 2B (Hebert et al., 1997) was deployed at 40deg 55.19' N, 67deg 20.57' W. It was programmed to surface 30 hours later. The COOL float was ballasted for the st = 25.5 kg/m3 surface. The COOL float contains a compass and is equipped with vanes angled at 15deg to horizontal. Thus, a diapycnal velocity past the float will make the float rotate and this rotation rate is measured using a compass. Compass angle, pressure and temperature were recorded every 64 s [Figure 13]. The COOL float has a 12-kHz pinger which sends an 8-ms ping every 8 s. The float is tracked acoustically from the ship [Figures 5 and 6].
Shortly after deploying the COOL float, an ARGOS-tracked surface drifter (Drifter #1, ARGOS ID 27613) was deployed at 40deg 55.15' N, 67deg 20.68' W. This drifter (Brightwaters Instrument Corporation Model 115) had a 10-m long drogue centered at a depth of 15 m. This drifter obtains its GPS location every 30 minutes and transmits the last 7 positions via ARGOS every 90 s. On the ship, a GONIO ARGOS receiver was used to intercept the ARGOS messages when we were within range. A computer attached to the receiver displayed the drifter positions obtained. ARGOS messages from the drifter were transmitted also to the ship via normal twice daily e-mail transfers. The surface drifter was equipped with a night-time flasher. Self-contained ONSET temperature loggers were placed at nominal depths of 0, 5, 10, 13.3, 16.7 and 20 m.
Due to a combination of unfortunate events, it appears that the COOL float was resting on the bottom for parts of its mission after 10 hours. During the previous cruise, it was realized that there were problems with the ballasting of the COOL floats using the recently moved Graduate School of Oceanography Equipment Development Laboratory's pressure vessel. This resulted in the floats being ballasted approximately 0.3 kg/m3 denser than expected during the previous cruise (OC340). We had reballasted the floats and believed to have corrected this problem. It appears that the floats were ballasted heavier than expected again. The original target density surface for the first deployment was st = 25.3 kg/m3. As well, the Oceanus' CTD was not working properly (see cruise narrative for details). Finally, the tidal mixing front is very sharp. There was a short delay in launching the COOL float after the CTD cast and the front could have been advected past the ship by the tidal currents.
The second and third deployments of the COOL float and surface drifter were made at the shelfbreak front which is broader than the TMF. After determining the location of the front, COOL Float #2 (compressee 2A) was deployed at 40deg 48.30' N, 67deg 5.37' W and programmed to surface 30 hours later [Figure 14]. The target density was st = 26.15 kg/m3. Shortly later, the surface drifter was deployed at 40deg 48.28' N, 67deg 5.37' W [Figure 15]. When the COOL float was recovered, it was rinsed with fresh water on the deck and brought into the lab, placed horizontally and dried. Normally, the optical interface is connected to the float and the onboard CPU accessed in order to turn off the flasher and pinger. While drying the outside of the float, it was noticed that water had gotten into the float. Suddenly, the flasher stopped working followed shortly by the failure of the pinger. We believe that some of the saltwater that was at the bottom of the float when it was vertical in the water shorted some of the electrical components higher in the float when the float was placed horizontally. The release plug was quickly opened and the float oriented vertically. About 1 cup of seawater was removed from the float. At this time, we could not communicate with the float through the optical interface. Mark Prater and Jim Fontaine were contacted and detailed instructions on how to open the float, test to see whether the float was still active, and to access the hard reset for the CPU were sent to us. With these instructions, the data from the float was successfully recovered. We believe that the leak was a low pressure one that occurred at the end of the float's mission while it was on the surface waiting recovery. The amount of water contained in the float was enough to make it rest on the bottom, which it did not do [Figure 14], if the compressee was still attached to the float.
The COOL float upwelled across the front [Figure 14] and moved offbank as it was advected alongbank [Figures 9 and 10]. Interestingly, the surface drifter moved onshore [Figure 15]. There also appears to be a change in the stratification of the upper ocean on a semi-diurnal timescale. A second deployment of a COOL float and surface drifter in the shelfbreak front was made several days later (not shown). The COOL float did not move far from its initial deployment location. The surface drifter moved in a cyclonic loop. The shelfbreak front structure was much more convoluted at this time.
References Hebert, D., M. Prater, J. Fontaine and T. Rossby. 1997: Results from the test deployments of the COastal Ocean Lagrangian (COOL) float, GSO Technical Report, 97-2, University of Rhode Island, 27p.
Appendix 1. Personnel List
David Hebert University of Rhode Island Chief Scientist John A. Barth Oregon State University co-Chief Scientist David Ullman University of Rhode Island Scientist Anatoli Erofeev Oregon State University Scientist J. Marcus Willis Oregon State University Marine Techn. Linda Fayler Oregon State University Marine Techn. Sandra A. Fontana University of Rhode Island Technician Robert T. O'Malley Oregon State University Technician Jennifer Simeon Oregon State University Grad. Student Che Sun University of Rhode Island Grad. Student LaQuieta Huey Grambling State University Undergrad. Student Laura Goepfert Woods Hole Oceanographic Inst SSSG Techn.
Lawrence T. Bearse Master Courtenay Barber III Ch. Mate Emily Sheasley 2nd Mate Jeffrey M. Stolp Bos'n Colin Walcott OS James R. Ryder AB Peter J. Liarikos AB Glen Loomis Chief Engineer J. Kevin Kay Jr. Engineer Alberto Collasius, Jr. Jr. Engineer Torii Corbett Steward Raul E. Martinez Mess Attendant
Appendix 2. Figures
Figure 1. Wind speed measured throughout the cruise. Above each panel, shaded boxes represent the time periods for major events conducted throughout the cruise.
Figure 2. Temperature at 3-m depth for the radiator patterns of the large-scale CTD/ADCP surveys at the tidal mixing front (TMF). Hydrographic properties obtained by averaging SeaSoar data over four minutes (0.85 km horizontally) and by 2-db vertically (averaged data locations indicated by white dots).
Figure 3. Temperature at 39-m depth for the radiator patterns of the large-scale CTD/ADCP surveys at the tidal mixing front (TMF). Hydrographic properties obtained by averaging SeaSoar data over four minutes (0.85 km horizontally) and by 2-db vertically (averaged data locations indicated by white dots). Mooring locations indicated by black plus signs.
Figure 4. Temperature cross-section along the radiator line just to the southwest of the mooring array.
Figure 5. Horizontal maps of the depth of COOL float target density of st = 25.5 kg/m3 for each of the butterfly patterns at the tidal mixing front. COOL float locations for each survey are shown by the circles. The white circle is the first location fix for that survey.
Figure 6. Horizontal maps of the temperature on COOL float target density of st = 25.5 kg/m3 for each of the butterfly patterns at the tidal mixing front. COOL float locations for each survey are shown by the circles. The white circle is the first location fix for that survey.
Figure 7. Temperature, salinity and st at 39-m depth for the radiator pattern of the large scale CTD/ADCP survey at the shelfbreak front (SBF). Hydrographic properties obtained by averaging SeaSoar data over four minutes (0.85 km horizontally) and by 2-db vertically (averaged data locations indicated by white dots). Mooring locations indicated by black plus signs.
Figure 8. Cross-section maps of temperature, salinity and st along line D showing a soliton observed.
Figure 9. Horizontal maps of the depth of COOL float target density of st = 26.15 kg/m3 for each of the butterfly patterns at the shelfbreak front. COOL float locations for each survey are shown by the circles. The white circle is the first location fix for that survey.
Figure 10. Horizontal maps of the temperature on COOL float target density of st = 26.15 kg/m3 for each of the butterfly patterns at the shelfbreak front. COOL float locations for each survey are shown by the circles. The white circle is the first location fix for that survey.
Figure 11. Horizontal map of the velocity at a depth of 30 m for each of the tidal mixing front radiator pattern surveys.
Figure 12. Horizontal map of the velocity at a depth of 35 m for the shelfbreak front radiator pattern survey.
Figure 13. Time series of compass angle, pressure and temperature for the deployment of COOL Float 2 in the tidal mixing front.
Figure 14. Time series of compass angle, pressure and temperature for the deployment of COOL Float 2 in the shelfbreak front.
Figure 15. Trajectory of the surface drifter (upper panel). Temperature recorded by the ONSET temperature loggers attached to the surface drifter and drogue.
* For the remainder of this report, all times will be reported as UTC.