WHP Ref. No.: AR10 Last updated: 6 November 1995 A. Cruise Narrative A.1 Highlights A.1.a WOCE designation AR10 A.1.b EXPOCODE 18HU92053/1 A.1.c Chief Scientist Neil S. Oakey Bedford Institute of Oceanography Box 1006 Dartmouth N.S. B2Y-4A2 Canada Fax 902 426 3147 Internet neil@oakey.bio.dfo.ca A.1.d Ship C.S.S. Hudson A.1.e Ports of call Halifax, N.S. Canada; Las Palmos, Canary Islands; Halifax, N.S. Canada A.1.f Cruise dates 5 April, 1993 to 14 May, 1993 A.2 Cruise Summary Information A.2.a Geographic boundaries A.2.b Stations occupied At the 46 Fast CTD tracer stations a Seabird CTD was used to obtain temperature, salinity and dissolved oxygen profiles to a depth of about 350 meters. Two rosette bottles were fired at each of these positions for analyses of salinity, dissolved oxygen, nutrients and total carbonate. At the 65 Full CTD tracer stations a Seabird CTD was used to obtain temperature, salinity and dissolved oxygen profiles to a depth of about 500 to 700 meters. Twenty rosette bottles were fired at each of these positions for analyses of salinity, dissolved oxygen, nutrients and total carbonate. 45 Biology casts, both shallow and deep casts were performed. 66 ESPONDE profiler stations were occupied. Nearly 1000 profiles were obtained with the vertical microstructure profiler. 3 Cartesian diver deployments A.2.c Floats and drifters deployed 2 ALFOS floats were deployed. Floats #76 and #72 were deployed. A.2.d Moorings deploued or recovered A.3 List of Principal Investigators ------------------------------------------------------------------------------- Name Responsibility Affiliation Neil S. Oakey Chief Scientist, Microstructure BIO neil@oakey.bio.dfo.ca Studies James Ledwell Tracer Sampling WHOI ledwell@tracer.whoi.edu Tim Duda Cartesian Diver Profiling WHOI timd@salsa.whoi.edu Barry Ruddick Microstructure Studies Dal U. barry@phys.ocean.dal.ca Rolf Lueck TAMI U. Vic rolf@george.seaor.uvic.ca Glen Harrison Carbon, Nitrogen Uptake BIO g_harrison@bionet.bio.dfo.ca Kinetics Edward Horne Optical Measurements, Salinity BIO user@bodvax.bio.dfo.ca Paul Kepkay Dissolved Organic Carbon BIO p_kepkay@bionet.bio.dfo.ca Brian Irwin Total Carbon Dioxide BIO b_irwin@bionet.bio.dfo.ca Oxygen, Nutrients ------------------------------------------------------------------------------- Institute Abbreviations and Addresses BIO Bedford Institute of Oceanography Box 1006 Dartmouth, N.S., B2Y4A2, Canada WHOI Woods Hole Oceanographic Institute Woods Hole, MA, 02543 USA Dal.U. Dalhousie University Halifax, N.S., Canada U. Vic University of Victoria Victoria, B.C., Canada A.4 Scientific Programme and Methods The North Atlantic Tracer Release Experiment, NATRE, is a contribution to the core 3 WOCE study. The Hudson voyage to the eastern Atlantic Basin had as one primary purpose the measurement of vertical mixing processes from the study of the vertical spread of a purposeful tracer by J. Ledwell of WHOI. The second primary purpose was the physical estimates of mixing made using the vertical profiling instrument EPSONDE and other instruments including a free floating profiler called the Cartesian diver and a microstructure mooring. Secondary to the NATRE studies biological studies of carbon and nitrogen uptake, dissolved organic carbon, primary production, total CO2, and optical studies were carried out by scientists of Biological Sciences Division who continued their studies in a follow on experiment. It should be noted that the current Hudson voyage was a continuation of the study started on the Woods Hole Vessel, the R/V Oceanus from 26 October to 19 November, 1992 on which Oakey was chief Scientist. Summary of Mission Along with Comparison with a Previous Cruise The North Atlantic Tracer Release Experiment, NATRE, is a study of the rate of mixing in the eastern Atlantic carried out by a group of scientists from the United States, the United Kingdom and Canada as a part of the international WOCE Core Project 3 study. Diapycnal mixing is integral to the dynamics of ocean circulation; the temperature and salinity of water masses are altered by diapycnal mixing, and this affects the pole ward transport of heat by the circulation. Knowing the magnitude and the mechanisms causing mixing is important to developing better models of ocean circulation. NATRE was planned as part of WOCE Core Project 3 to be the first direct measurement of the diapycnal mixing rate in the main thermocline of any ocean basin. The essence of this experiment was to release a chemically inert, easily measurable tracer on a target isopycnal surface, and to measure the subsequent tracer dispersion over the following year. Observations of turbulence and microstructure were part of the tracer experiment in order to understand the mixing processes that occur. This understanding is needed so that the results for the tracer can be applied appropriately and confidently to heat, salt, and density, and extrapolated from the experimental sites to the global ocean. NATRE provided an outstanding and unique opportunity to test the concepts, models and methodologies presently used to study mixing, and to refine them further, by comparison with direct measurements of diapycnal spreading rates of a tracer. The NATRE experiment started in the spring of 1992 when Dr. Jim Ledwell and his group from WHOI on the R/V Oceanus injected 139 kg of sulpher hexafluoride at a depth of approximately 300 meters in an area of about 20x20 km in the Canary Basin. This patch of tracer diffused vertically and moved horizontally over the months to follow and various surveys explored this evolution, allowing us to deduce the rate of mixing or the vertical diffusivity in this region of the ocean. In the month after the injection Ledwell and Dr. Andy Watson from the UK explored the initial distribution from the UK ship the RRS Darwin. The patch had increased to about 50 km across and had thickened vertically consistent with a vertical diffusivity of order 10-5 m2/s. In October through November, Ledwell and Watson explored the evolution of the dye patch during two cruises on the R/V Oceanus. They found about 35% of the original tracer in a narrow (10 to 20 km wide) band about 350 km long. Their measurements in the vertical yielded a vertical diffusivity of 1.1x10-5 m2/s. During the second of these cruises Oakey and his group joined the R/V Oceanus and surveyed the area using EPSONDE, a vertically profiling microstructure instrument. A report of this experiment is included at the end of this report on the Hudson 92053 voyage. The final surveys of the NATRE study were done in the spring of 1993 using the CSS Hudson (cruise 92053) and the RRS Darwin. Along with its other studies, the CSS Hudson gathered water samples using a CTD with a rosette sampler for analysis by Ledwell and his group from WHOI. About 115 CTD tracer stations were done to delineate the extent of the tracers horizontal dispersion including about 60 full profiles to examine the vertical diffusion of the tracer. The RRS Darwin, devoted only to the tracer studies obtained over 160 full tracer profiles. It is estimated that all of the tracer released in the spring of 1992 has been accounted in the survey a year later and that the tracer can be found spread over an area of about 500 km by 700 km and has increased in thickness due to vertical mixing from a few meters to about 30 meters. These measurements are consistent with the above estimates of vertical mixing but indicate that the rates over the winter were higher than during the summer, about 1.8x10-5 m2/s. The largest program carried out on the April-May Hudson 92053 survey was an extensive set of microstructure and turbulence measurements. Nearly 1000 profiles to greater than 360 meters depth were obtained with the vertical microstructure profiler, EPSONDE. These measurements will be used along with the 825 profiles last November on the R/V Oceanus to estimate vertical mixing rates and explore the mechanisms which are important in the vertical dispersion of dye. These mixing studies are part of the WOCE NSERC collaboration between Oakey and Dr. B. Ruddick of Dalhousie. This is the largest set of data collected with this instrument and represents nearly 700 km of sampling of the fluctuations of temperature and velocity fluctuations in the ocean sampled at about 3 mm intervals, about 3 gigabytes of data! The challenge is whether we get the correct answer which has been obtained from the tracer studies. Preliminary results indicate that our results are consistent. Other studies were done on Hudson 92053 as well. A group from the University of Victoria led by Dr. R. Lueck deployed and recovered a mooring designed to measure the intensity of mixing at a fixed site over a period of several weeks using a variety of microscale and larger scale sensors. Several deployments of an instrument called a Cartesian Diver were also carried out successfully. This instrument floated freely measuring vertical profiles of velocity and microstructure from which mixing processes can be explored. To take advantage of the transit to the eastern Atlantic scientists from Biological Oceanography, BSB participated in the survey with extensive biological studies. They continued their experiment in the following experiment. The ship left Halifax on Monday, 5 April, 1993 en route to the position 32o16'N, 34o08'W where a surface mooring buoy which had broken from a Scripps/WHOI mooring was recovered. Tracer surveys were started south from here to approximately 25o30'N, 34o08'W then east to near the site of a mooring central to the experiment at 25o30'N, 29oW. The ship track is shown in Figure 1. A mooring was placed in this region by the group from University of Victoria headed by Lueck, the first deployment of the Cartesian diver was done and coordinated tracer and EPSONDE surveys started. These continued until 18 April when the program was interrupted by a medical emergency that required taking the Chief Mate to Las Palmos. We returned to the site of the experiment on 23 April and continued sampling until the end of the study on 12 May allowing time for the transit to Las Palmos where the expedition terminated on May 14, 1993. A.5 Major Problems and Goals not Achieved A.6 Other Incidents of Note A.7 List of Cruise Participants Name Responsibility Affiliation ------------------------------------------------------------------------------- Neil S. Oakey Chief Scientist, Microstructure BIO Studies James Ledwell PI, Tracer Sampling WHOI Gary Stanbrough Technician, Tracers WHOI Brian Guest Technician, Tracers WHOI Tim Duda Cartesian Diver Profiling WHOI Barry Ruddick Microstructure Studies Dal U. Dave Walsh Post Doc, Microstructure Dal U. James Burke Student, Microstructure Dal U. Nauzer Kalyaniwalla Student, Microstructure Dal U. Rolf Lueck PI, TAMI U. Vic Rick Hudson Engineer, TAMI U. Vic Don Newman Engineer, TAMI U. Vic Robert Ryan Technician, Microstructure BIO Jennifer Hackett Technician, Microstructure BIO Liam Petrie Technician, Microstructure BIO Edward Verge Technician, Microstructure BIO Glen Harrison PI, Carbon/Nitrogen Uptake Kinetics BIO Edward Horne PI, Optical Measurements BIO Brian Fraser Technician, Optical Measurements BIO Paul Kepkay PI, Dissolved Organic Carbon BIO Brian Irwin PI, Total Carbon Dioxide BIO -------------------------------------------------------------------------------