Cruise Report-- Oceanus 250 Leg 4, High Resolution Profiler Survey for the North Atlantic Tracer Release Experiment: (NATRE) March 25 - April 24, 1992

Transcription

1 REPORT DCCUMENTATION PAGE 1. REPORT NO. WHOI Title ad Subtitle Cruise Report-- Oceaus 250 Leg 4, High Resolutio Profiler Survey for the North Atlatic Tracer Release Experimet: (NATRE) March 25 - April 24, Recipiet's Accessio No. 5. Report Date August Author(s) Elly T. Motgomery ad Raymod W. Schmitt, Jr. 9. Performig Orgaizatio Name ad Address Woods Hole Oceaographic Istitutio Woods Hole, Massachusetts Performig Orgaizatio Rept. No. WHOI Project/Task/Work Uit No. 11. Cotract(C) or Grat(G) No. (C) N00014-I (G) 12. Sposorig Orgaizatio Name ad Address Office of Naval Research 13. Type of Report & Period Covered Techical Report 14. IS. Supplemetary Notes This report should be cited as: Woods Hole Oceaog. Ist. Tech. Rept., WHOI Abstract (Limit: 200 words) This report describes fie- ad microstructure profile data take o R/V Oceaus cruise 250 leg 4, betwee March 25 ad April 24, Durig this cruise, a area of the Caary Basi ear the Subductio Experimet's cetral moorig was surveyed with the High Resolutio Profiler (HRP). The goals of the survey were to describe the hydrographic properties of the water adequately to recommed a locatio for the North Atlatic Tracer Release Experimet (NATRE) tracer ijectio, ad to characterize the microstructure for compariso with the NATRE results. The work performed at sea, istrumetatio, data retur ad processig procedures will be summarized i this report. 17. Documet Aalysis a. Descriptors turbulet mixig Caary Basi iteral tide b. Idetifiers/Ope-Eded Terms c. COSATI Field/Group 18. Availability Statemet Approved for public release; distributio ulimited. 19. Security Class (This Report) UNCLASSIFIED 20. Security Class (This Page) 21. No. of Pages Price (See ANSI-Z39.18) See Istructios o Reverse OPTIONAL FORM 272 (4-77) (Formerly NTIS-35) Departmet of Commerce

2 LIBRARY RESEARCH REPORTS DIVISION NAVAL POSTGft^OUATE SCHOOL mhterey, CA V WHOI Woods Hole ^ Oceaographic Istitutio. <^T\ 1930 /A ^Cruise Regort - Oceaus 250 Leg 4, High Regoiutio ProfTler Survey for the Nqrth Atlatic Tracer Release Experimet: (NATRE) March 25-April 24,1992 by Elly T. Motgomery ad Raymod W. Schmitt, Jr. VAugusLiasa., Techical Report Fudig was provided by the Office of Naval Research through Grat No. N Approved for public release; distributio ulimited.

3 WHOI Cruise Report - Oceaus 250 Leg 4 High Resolutio Profiler Survey for the North Atlatic Tracer Release Experimet: (NATRE) March 25 - April 24,1992 by Elly T. Motgomery ad Raymod W. Schmitt, Jr. Woods Hole Oceaographic Istitutio Woods Hole, Massachusetts August 1993 Techical Report Fudig was provided by the Office of Naval Research through Grat No. N Reproductio i whole or i part is permitted for ay purpose of the Uited States Govermet. This report should be cited as Woods Hole Oceaog. Ist. Tech. Rept., WHOI Approved for public release; distributio ulimited. Approved for Distributio: ypmjo^i Luu. James Luyte, Chair Departmet of Physical Oceaography

4 -1 Table of Cotets Abstract 4 Overview 5 Cruise Log 7 Descriptio of the High Resolutio Profiler 17 Data Processig 20 Summary 28 Ackowledgemets 41 Refereces 41

5 List of Figures 1 Chart of the easter North Atlatic showig the survey area 6 2 Locatios ad dive umbers of profiles for large scale survey. 8 3 Profile locatios ad umbers for the small scale ad iertial surveys Schematic of the High Resolutio Profiler (HRP) 18 5a Plot of Pressure, Temperature, Coductivity, ad A/D groud versus sca umber for dive b Plot accelerometer data for dive c Plot of magetometer ad velocimeter data for dive d Plot Microstructure quality cotrol for dive Plot of computed velocity (east ad orth compoets), temperature ad saliity versus pressure for dive Plots of eergy dissipatio rate (a), dissipatio rate of thermal variace from micro-temperature (b), micro-coductivity (c) sesors for dive Profiles of potetial temperature, saliity, potetial desity ad desity ratio versus pressure for dive Theta-saliity relatioship for dive Cotour plot of the dyamic height field for the large scale survey Cotour plot keep the relative vorticity field for a subset of the statios i the large scale survey Cotour plot showig the desity ratio field at 300 db for the large scale survey Cotour plots of potetial temperature (a), saliity (b), pressure (c), ad desity ratio (d) o the craoo = surface for the large scale survey Same as 14, but small scale survey 36

6 Plot of pressure o the sigma-theta = surface as a fuctio of time (a). Plot of dyamic height at 300 db refereced to 1000 db as a fuctio of statio umber (b) Plots of the umber of temperature iversios (a) ad the average thickess of these iversios (b) for each statio of the large scale survey Plots showig the variatio of eergy dissipatio rate (a) ad thermal diffusivity (b) with pressure for the 100 statios of the large scale survey List of Tables High Resolutio Profiler Survey for NATRE: Profile ad Operatios List.. 12

7 Abstract This report describes fie- ad microstructure profile data take o R/V Oceaus cruise 250 leg 4, betwee March 25 ad April 24, Durig this cruise, a area of the Caary Basi ear the Subductio Experimet's cetral moorig was surveyed with the High Resolutio Profiler (HRP). The goals of the survey were to describe the hydrographic properties of the water adequately to recommed a locatio for the North Atlatic Tracer Release Experimet (NATRE) tracer ijectio, ad to characterize the microstructure for compariso with the NATRE results. The cruise started i Madeira, ad o the way to the experimetal area, two HRP dives were coducted to test the istrumet's fuctios. The HRP was the deployed 100 times i a 10 by 10 grid with 400 kilometer sides cetered o the cetral subductio moorig. The first 90 profiles were to 2000 meters, ad the last 10, comprisig the eastermost lie of the mai survey, were to 3000 meters. Fially, aother 55 HRP profiles were completed i smaller spatial surveys, time series, ad additioal deep statios. O this cruise the existig record for deep microstructure measuremets was broke by the four profiles to 4000 meters take off the Africa shelf o the way back to port i the Caary Islads. The work performed at sea, istrumetatio, data retur ad processig procedures will be summarized i this report.

8 5- Overview The High Resolutio Profiler (HRP) was used to perform the pre-cruise survey for North Atlatic Tracer Release Experimet (NATRE) because of its uique ability to make both fie- (10 Hz) ad micro- (200 Hz) structure measuremets. The HRP collects ad stores data as it falls freely through the ocea durig a dive. HRP profiles ca be completed at almost the same rate as CTDs to equivalet depth, ad the HRP provides additioal velocity ad microstructure data useful for quatifyig the properties of the area surveyed. The objective of the cruise was to survey the area of the Caary Basi targeted as the potetial site for the NATRE tracer ijectio, ad based o the data collected, recommed a specific locatio for the ijectio. The chart i Figure 1 shows the area surveyed with relatio to North Africa. Because the tracer must be foud at six moth itervals to make the esuig experimet a success, a area with relatively little largescale eddy stirrig was desired. The HRP survey data foud a area of getly slopig dyamic height cotours isolated from large eddy fields ad water property frots that seemed suitable for the ijectio. The HRP surveys were the oly work coducted durig OCEANUS 250, leg 4. Durig the 30 day cruise, 6 days were spet i trasit, ad 155 HRP profiles were completed. Most (111) of the dives were doe to 2000 meters, 25 were doe to 1200 meters, 10 dives were doe to 3000 meters, ad four were doe to betwee 3800 ad 4000 meters. The remaiig 5 dives were to various depths shallower tha 2000 meters. The chroological log of the activities of the cruise is preseted i the ext sectio. Cruise Log. Two modificatios were made to the HRP for this cruise. First, the ecessary chages were made to eable use of the shadowgraph, a optical microstructure istrumet, with the HRP. Secod, i order to avoid spikes i the microstructure data caused by the acoustic trasducer used to track the HRP, a secod trasducer was istalled at the upper ed of the profiler. More iformatio about the HRP ad modificatios is preseted i the sectio etitled High Resolutio Profiler Descriptio. The data processig methods used at sea are described i the ext sectio, ad fially the results are summarized. The sciece party for this cruise o the R/V Oceaus cosisted of employees ad studets of the Woods Hole Oceaographic Istitutio. The participats are listed below. Scietist Affiliatio Ray Schmitt Joh Toole WHOI, Chief Scietist WHOI

9 -6- OCEANUS 250 : NATRE (North Atlatic Tracer Release Experimet) 35 U N Madeira f o P 30 N Su: ~vey Area Caary Isl ads. ^0 25 N / f Afria 20 N 35 W ( 30 W 25 W 20 W 15 W ICT LONGITUDE Figure 1: Chart of the easter North Atlatic showig the survey area.

10 Scietist Affiliatio Dick Koehler WHOI Elly Motgomery WHOI Tom Bolmer WHOI Dave WeUwood WHOI Kurt Polzi MIT/WHOI Joit Program Dave Gloss MIT/WHOI Joit Program Pascal LeGrad MIT/WHOI Joit Program Cruise Log OCEANUS departed Fuchal, Madeira, at 1300 o Wedesday, March 25 ad bega the three day trasit to the iitial poit of the survey grid, ' N, 30 o 43.8' W. This was the orthwest corer of the 10 by 10 grid; the plaed statio spacig was 24 autical miles (44.5 kilometers). I route, two test dives of the High Resolutio Profiler (HRP) were performed, i order to adjust the ballast ad examie sesor performace. The first was to 500 meters, the secod to over 1750 meters. No major problems were discovered o these dives ad all ewly implemeted features fuctioed well. The grid of statios was commeced with HRP dive 3 at 1400 o March 28. This was the first statio of a southward leg at the wester edge of the survey area. By workig eastward i alteratig orth ad south legs, we miimized the time betwee adjacet statios ad progressed upstream ito the expected southwestward mea flow of the regio. The grid of statios for the large survey ad their umbers are show i Figure 2. The goal was to complete six dives per day, with a roughly eve split betwee statio ad steamig time. I practice this worked well. The HRP took about a hour to reach 2000 db, ad 22 miutes to retur to the surface. Recoveries took about 15 miutes, leavig over two hours for the 24 mile steam betwee statios. The OCEANUS typically steamed at kots, so it was possible to achieve six statios per day so log as the watches maitaied efficiecy. Three perso sciece watches were established to work three eight hour shifts. Moderate weather, with kot wids ad commesurate seas, provided a challegig eviromet for startig HRP operatios. After the fourth day, wids dropped to 10 kots or less ad profiler hadlig became relatively routie. Data quality was high ad o major problems were experieced util the eveig of April 3, whe the HRP computer failed at the begiig of dive 39, immediately followig a battery chage. The weights dropped o commad of the backup timig system but the lack of a acoustic sigal from the profiler made for a axious wait durig its retur to the surface. The problem arose durig the battery chage whe a loose coectio o oe computer card required a substitutio of cards ad trasfer of EPROM (erasable

11 I I I I I 1 I I I I I I I I I I I I I I I I I I 1 I I I I I I 1 I I I I 1 I o D ^ I I I I I I I I I I I I I I 1 I I I I I I 1 I I I I I I I I I I I I I I LONGITUDE (DEG W) Figure 2: Chart showig the dive umbers ad positios of the profiles comprisig the large scale survey.

12 -9- programmable memory) chips. The trasfer apparetly caused a failure i oe of the EPROMs; reprogrammig a ew set of EPROMs cured the problem. The survey the progressed without major icidet. The weather worseed a bit, with heavy swell from the orth, kot wids, ad overcast skies, for most of the rest of the survey. Eve with the heavy weather, the efficiecy of HRP operatios improved with practice. A secod battery replacemet was effected without icidet o April 11, after dive 83. The HRP requires a ew battery every dives (depedig o dive duratio). For the sake of more tha doublig the world total of deep microstructure profiles, the last 10 statios of the large survey (the eastermost lie) were performed to a depth of 3000 meters. This lie was completed with dive 102 o the afteroo of April 14. Followig the hudred dive large survey, a small scale survey was started i the ortheast quadrat of the origial survey box. Dives 103 to 107 comprised a east-west lie south of statio 66 ad orth of statio 67 of the large grid; these cofirmed the dyamic height field suggested by the large survey. To detail this field, a grid of statios, 4 dives ad 25 kilometers o a side, was completed with 16 statios, umbers 108 to 123. This box was cetered o / N, ' W ad executed betwee April 15 ad 17. The positios of profiles 103 to 123 are show i Figure 3. A third battery chage was made without problems after statio 122. The weather had fially become calm, so o April 17 a profile with the shadowgraph mouted o the HRP was attempted. The coectio of the two istrumets is delicate ad complicated, but was accomplished with little difficulty. The first shadowgraph/hrp dive to 500 meters was the commeced. Recovery of the shadowgraph/hrp was more difficult tha aticipated. The calm weather ad experieced watchstaders made the damage-free recoveries possible. The video did ot record as expected o this dive, so a modificatio was made to the shadowgraph, ad aother profile attempted. This time the profile was to 1200 meters, ad the images of the microstructure were recorded successfully. The HRP ad shadowgraph were the ucoupled i preparatio for more profilig. A time series of repeated HRP dives to 1200 meters, over two iertial periods i a L shaped patter (to obtai some measure of horizotal gradiets ad coherece), was doe from April 18 to 20. This time series cosisted of dives 126 to 150, ad was performed at 26 0 N, 28 0 W, overlayig statio 121 i the southeast sector of the small survey. O completio of the time series, a eutrally buoyat pop-up float to mark the parcel of water thought to be optimal for the tracer ijectio was deployed i a regio of relatively weak horizotal gradiets at 300 m. The float was deployed at 26 10' N, / W o April 20 at 1137 hours (GMT). Sice the weather remaied calm, aother shadowgraph/hrp dive was performed before commecig the steam to port.

13 10- M I I I I I I I I I I I I I I I I I I I! I (, 26, cu o D l^l I I I I I I I I I I I I I I I I I I Logitude (W) Figure 3: Chart showig the dive umbers ad positios of the profiles comprisig the small scale ad iertial surveys.

14 -11 - Fially, four very deep HRP dives were performed o the trasit to the Caaries, reachig uprecedeted depths (for microstructure profilig) of 4000 meters. The last three were ear the cotietal rise, but outside Africa's 200 mile limit. The profiles desceded to withi 150 meters of the bottom allowig the possibility of iertia! wave reflectio off weakly slopig topography to be explored. Dive 155 was completed the eveig of April 22, expedig the last available profiler weights ad ship time. A list of statio locatios, deploymet time ad depth for all the HRP dives is give i Table 1. The OCEANUS arrived i Las Palmas, Caary Islads o April 24, after two days of steamig. I port, discussios with Jim Ledwell ad the other scietists o the ext cruise were held to pass o the iformatio gaied from the surveys.

15 -12- Table 1: HRP Survey for NATRE: Profile ad Operatios List March 24 - April Cast Date Deploymet Max. # Mo. Day Time Latitude Logitude Pres. Commets 1992 (GMT) Deg. N Deg. W (db) test cast - all sesors test to survey depth start survey: NW corer **started South o lie cotiued S o lie » » « » » o » last cast o lie **begi lie 2 orthboud cotiue N o lie » V » » )) » last cast o lie **begi lie 3 southboud cotiue S o lie ) " « » » last cast o lie **begi lie 4 orthboud cotiue N o lie «

16 -13- Table 1: (Cotiued) Cast Date De ploymet Max. # Mo. Day Time Latitude Logitude Pres (GMT) Deg. N Deg. W (db) Commets CHANGE HRP BATTERY cotiue N o lie 4 last cast o lie 4 **begi lie 5 southboud cotiue S o lie 5 at Subductio Surface Moorig cotiue S o lie 5 last cast o lie 5 **begi lie 6 Northboud cotiue N o lie 6» last cast o lie 6 **begi lie 7 Southboud cotiue S o lie 7 i «11 ii ii ed lie 7 ew atea **begi hue 8 Northboud cotiue N o lie 8 Micro temperature sesor TN died 675 db

17 14 Table 1: (Cotiued) Cast Date Deploymet Max. # Mo. Day Time Latitude Logitude Pres. Commets 1992 (GMT) Deg. N Deg. W (db) replaced TN sesor cotiue N o lie » ed lie 8 Northboud **begm lie 9 Southboud CHANGE HRP BATTERY cotiue S o lie " rt » ed of lie 9 southboud **begi lie 10 Northboud cotiue N o lie V r> " » ed of lie 10 Northboud ** END LARGE SURVEY ** Trasit to E-W lie site **begi small E-W survey cotiue W « » ed E-W survey ** begi 4x4 survey **

18 15- Table 1: (Cotiued) Cast Date Deploymet Max. # Mo. Day Time Latitude Logitude Pres. Commets 1992 (GMT) Deg. N Deg, W (db) cotiue top lie east » eastermost o lie *start W o lie cotiue westward ) westermost o lie * start east o lie cotiue eastward ?! eastermost o lie * start west o last lie cotiue westward CHANGE HRP BATTERY ** ed small 4x4 survey mout & test SHADOWGRAPH * shadowgraph * * shadowgraph * remove SHADOWGRAPH ** BEGIN iertial survey cotiue iertial survey « » )) « "

19 -16 Table 1: (Cotiued) Cast Date Deploymet Max. # Mo. Day Time Latitude Logitude Pres. Commets 1992 (GMT) Deg. N Deg. W (db) » » » » » « * END iertial survey deploy ARGOS beaco HRP with SHADOWGRAPH * start deep drops at base of Africa cotietal slope * ** last HRP cast **

20 -17- Descriptio of the High Resolutio Profiler The High Resolutio Profiler (HRP) is a vertically profilig free vehicle. Beig a free vehicle, the measuremets take by the HRP are ot subject to cable-iduced oise. Each deploymet of the HRP, ad the data collected durig that deploymet, is referred to as a statio, profile, or dive. The HRP was desiged ad developed at WHO! to make high quality fie- ad microstructure measuremets usig the iterface bus computer (IBC). The IBC is the HRP's cotroller, hadlig everythig from software setup to data acquisitio ad storage. A suite of sesors iterfaced to the IBC provides data o the physical properties of the water sampled as the HRP desceds. All the data collected are stored iterally i a 16 Mb RAM (radom access memory) mass storage area. A schematic of the HRP ad its compoets is show i Figure 4. For additioal iformatio o the developmet of the HRP ad IBC, see the papers by Schmitt et al. (1988) ad Melliger et al. (1986). The HRP has two data streams: "fie" ad "micro". The begiig ad ed of each deploymet is triggered by the measured pressure reachig user-defied threshold values. The fie-scale data cosists of iputs from the o-board CTD (coductivity, temperature, depth sesor), ad a suite of aalog devices is iterfaced with the aalog to digital (A/D) coverter chaels. Durig a profile, micro-structure measuremets of turbulet velocity, as well as temperature ad coductivity fluctuatios, are acquired simultaeously with the fie-scale data. The samplig i both modes is drive by the 200 Hz iterrupt, with micro-scale data acquired every cycle, ad fie-scale data acquired every twetieth cycle, for a rate of 10 Hz. The sesor cofiguratio used for the NATRE cruise o the R/V Oceaus is show below. (Pressure, temperature ad coductivity do ot have A/D chaels assiged to them because they are acquired by the oboard CTD, which has its ow A/D coverter.) fie sesors (10 Hz samplig) A/D chael pressure temperature coductivity accelerometer top 0 accelerometer top Y 1 accelerometer bottom 2

21 18 Liftig Rig Drag Whiskers Float Release Foam Buoyacy Accelerometers Navigatio Navigatio Battery Coectig Cable Aalog Circuits e i E O CPU Mass Storage System Battery CTD Weight Release Microstructure Electroics Ballast Weight Accelerometers Trasducer CTD Sesors Acoustic Velocimeter Figure 4: Schematic of the High Resolutio Profiler (HRP)

22 19 accelerometer bottom Y 3 acoustic curret meter - velocity 4 acoustic curret meter - Y velocity 5 compass 6 Y compass 7 groud 14 micro sesors (200 Hz samplig) A/D chael micro coductivity 10 micro temperature 11 shear 12 shear Y 13 Modificatios to the HRP for this cruise icluded the followig: addig a secod trasducer ad modifyig the software to support output of pressure data to the shadowgraph to be logged with the video images. O previous cruises, the pulse geerated by the trasducer mouted i the ose of the HRP caused spikes i the microstructure data because of its proximity to the turbulece sesors. To solve the problem, a secod trasducer was mouted at the top of the istrumet (5 meters above the sesors) to pig o the descet, while data is beig collected. The origial trasducer was used durig the ascet, so the trasducer i use would be submerged, eve at the surface. The HRP cotrol program was modified to cotrol two trasducers. This cruise was the first time the shadowgraph was used simultaeously with the HRP. The iitial (ad oly other) deploymets, i 1985 durig the C-SALT (Carribea Sheets ad Layers Trasects) experimet, were doe idepedetly. Iformatio o the developmet of the shadowgraph is available i the paper by Coverse et al. (1988). I order to use the shadowgraph i this experimet, umerous issues had to be resolved. Dave Wellwood worked out the details of the mechaical coectio to the HRP ad made plas for istrumet couplig ad hadlig o deck. The mechaical coectio to the HRP was desiged with a shear pi release, so that if the shadowgraph ad HRP were to desced beyod a preset pressure, the shear pi would break, releasig the shadowgraph so the HRP could be recovered. Paul Fucile revamped the electroics ad cotrol program ruig the shadowgraph, ad added a pressure display to the video record. He also desiged a serial coectio to the HRP employig a optical lik where a beam of light trasmits the data sigal betwee cable termiatios. This allowed the cable to separate clealy at the optical lik ad ot cause the HRP to be dragged to the bottom by the

23 -20- data cable if the shadowgraph was released. Fortuately, the fail-safe mechaisms were uecessary, ad both the HRP ad shadowgraph retured after each deploymet. Elly Motgomery implemeted software to cotrol the serial output of HRP pressure data to the shadowgraph. Durig the cruise three profiles were completed with the shadowgraph coected to the HRP. Very deep casts were also completed o this cruise. Some of the dive cotrol variables were stored as itegers, meaig the largest umber possible was ( meters). This was ot large eough to allow 4000 meter dives, so the parameters storig the dive termiatio pressures were chaged to usiged itegers allowig values of up to ( meters) ad data acquisitio for deep profiles proceeded smoothly. With the above improvemets to the HRP, o trasducer-iduced spikes occurred i the microstructure data, the shadowgraph worked as we hoped, ad dives to 4000 meters were made successfully. Data Processig The HRP collects ad stores fiestructure, microstructure, ad avigatio data. Each of the three types is treated separately after it is trasferred from the HRP. The followig sectio describes the data processig carried out routiely oboard the ship durig a cruise. The HRP is programmed to store oly oe profile at a time i its memory. Cosequetly, after each deploymet, the data must be offloaded to aother computer for permaet storage. Serial data trasfer at 34 kilobaud is used to move the data from HRP memory to the hard disk of a type PC for itermediate storage. The trasfer is first eabled at the HRP, the software o the PC cotrols the trasfer. The data trasfer rate allows the microstructure data for a 1000 meter profile to be set to the PC i about 20 miutes. Oce the data is trasferred to the PC, the HRP ca be programmed to start aother dive. After several profiles of raw data have accumulated o the hard disk they are archived to a optical disk. Additioal iformatio o the data trasfer is provided i the report by Motgomery (1991). Oce the data is o the PC, it ca be trasferred to the post processig computer via etheret, usig File Trasfer Protocol (FTP). The bulk of the data processig is accomplished o Digital Equipmet Corporatio VA VMS computers usig Fortra programs developed at WHOI. The first step is to covert the data ito egieerig uits, ad the store it i a biary format to coserve storage space (Millard ad Galbraith 1982). Quality cotrol time series plots are the geerated for each fiestructure ad microstructure data chael. Sample quality cotrol plots from profile 78 are show i Figures 5a through 5d.

24 -21- o i Ld oo >- cr GO UJCM Q_ < LxJtvJ O 0 Q UJ._ 0( co. _ o. _ 4500, SCAN NUMBER Figure 5a: Plot of Pressure, Temperature, Coductivity, ad A/D groud versus sca umber for dive 78.

25 22 o >- 0 <i. CM o i o 10 I o CVJ o i._ <.i._ CM o m CM.. o _ o 10 r-. _ , SCAN NUMBER Figure 5b: Plots of accelerometer data for dive 78. The upper plot shows the y compoet of the two accelerometers ad the lower plot shows the x compoets of the two accelerometers.

26 -23- o CD O _ o o. _ CO x. x 0 - o _ o o o iu_ , SCAN NUMBER CD CO o Q o lo C\J o ZDi. _ SCAN NUMBER Figure 5c: Plot of x ad y compoets of the magetometer (top) ad x ad y compoets of the velocimeter (bottom) data for dive 78.

27 24-4 I I I I h I I I h- :.*_ H ' :: O i 1 ' i I ' ' I ' ' I ' ' I ' I ' ' I ' ' I ' ' i I ' ' i I ' ' "r 3.0x10* 3.6x10" 4.2x10* 4.8x10* 5.4x10* 6.0x1 -I : 4.8x10' 5.4x10* 6.0x10* '.'l.< \ I I I I I I I I \ I I I^H I I I I I I I \- -I h 1.6x10* 1.2x10* 1.8x10* 2.4x1, 10, 3.0x10* 3.6x10* 4.2x10* 4.8x10* 5.4x10* 6.0x1 0" 6.6 -L i i f H i i I i v v Y \ i i i i i i i \ i i-.6x10' 1.2x10* 1.8x10* 2.4x10* 3.0x10* 3.8x10* 4.2x10* 4.8x10' 5.4x10' 6.0x10* 78 NATRE,,, MICRO DATA CYCLES... ^ 'T 1-1^ 1 1 f l (~ PRESSURE DB Figure 5d: Example of a microstructure data quality cotrol plot usig dive 78. Micro-temperature, micro-coductivity, shear x ad shear y are plotted agaist sca umber.

28 -25- As the above plots are geerated, a routie is ru that computes the fiescale velocity, potetial temperature-saliity profiles ad bis the data i a uiformly icremeted pressure series (typically 0.5 db). The velocity computatio employed is described by Schmitt et al. (1988) ad uses the acceleratio ad magetometer data to correct the raw acoustic curret meter data for istrumet motio. Laboratory-derived calibratio data are used to covert raw pressure ad temperature data to scietific uits. A laboratory-derived relatioship is also utilized for the iitial estimate of the coductivity cell calibratio. Adjustmets of this scale are subsequetly derived to obtai cosistet deep water potetial temperature-saliity relatioships. The output is stored i aother biary file from which a plot of temperature, saliity, east ad orth velocities versus pressure is created. A example of this type of plot, usig profile 78, is show i Figure 6. Microstructure data processig is started cocurretly with the fie, but takes much loger to complete due to more desely sampled data ad more computatios performed. The scheme used follows procedures developed by Neil Oakey (Bedford Istitute of Oceaography). A report by Polzi ad Motgomery (i prep.) describes the microstructure data processig, so oly a brief summary is icluded here. The processig utilizes laboratory-derived calibratio coefficiets for the shear probes (micro-scale velocity sesors), while i-situ calibratio data for the microscale temperature ad coductivity sesors are obtaied by referece to the fiescale temperature ad coductivity from the HRP. The microstructure data are bied i time blocks aliged with the uiformly icremetig pressure series of the reduced fiescale data. Gradiet variaces are estimated i the frequecy domai after fast Fourier trasformig by itegratig spectra out to a local miimum i eergy desity. Spectral correctios are the applied for the fiite resposes of the sesors. After automated edit ad cosistecy checkig, scalig to scietific uits yields estimates of the kietic eergy dissipatio rate (e, epsilo), ad two measures of the dissipatio rate of thermal variace (from the microscale temperature ad coductivity sesors: Chi-T ad Chi-C respectively). Profile plots (i "stick diagram" form) of the dissipatio rates are the produced, examples of which (agai usig profile 78) are show i Figures 7a-c. Navigatio data is acquired oly whe a et of acoustic traspoders is deployed o the seafloor. It is used to determie the absolute velocity profile. There were o profiles o this cruise for which a traspoder et was deployed, so all velocity profiles are relative to a depth average of zero i each compoet.

29 i r - EAST VELOCITY i 1 1 r NORTH VELOCITY f h TEMPERATURE -r r T T P 24. SALINITY (\3 O o PROS;CPRO.OC250D036.VELOCITYDFINEOTS.VEL Figure 6: Plot of computed velocity (east ad orth compoets), temperature ad saliity versus pressure for dive 78.

30 27 a. EPSiLON CHI-T, ' CHI-C 10"' 10- lo"* 10"' lo -7 10"' 10"" lo-'" 10'' 10"' lo" 7 lo - ' lo"" IO- 0 io-' 10-' ID" 7 lo" 6 ^ IIIIHI o 500 looo K3 UJ a JOOO Figure 7: Plots of eergy dissipatio rate, epsilo (a), dissipatio rate of thermal variace from micro-temperature (b), micro-coductivity (c) sesors for dive 78.

31 -28- Summary The large scale survey is the first of such magitude to be doe with a free profiler; typically oly 10 to 20 dives per cruise have bee made with free microstructure vehicles i the past. Ideed, it is relatively uusual to perform as may as 150 CTD casts i a cruise. A total of 300 km of vertical profile data was obtaied o this cruise (over a gigabyte of raw data). The surveys ad time series provide a accurate descriptio of mesoscale variability for the subsequet tracer release experimet ad a firm basis for characterizig mixig i the regio. The regio surveyed met most of the expectatios for NATRE; there was a mai thermoclie stratificatio favorable for salt figers ad little itrusive activity. Figure 8 shows vertical profiles of potetial temperature, saliity, potetial desity ad desity ratio from statio 78 of the survey. The mixed layer is deep, ofte 125 m or more. Temperature ad saliity decrease mootoically through the thermoclie from maxima i the mixed layer ad display a "irregular steppiess," possibly related to salt figerig. Desity ratios are low ad ear two for most of the thermoclie; the lowest values ted to be i the upper thermoclie. The shadowgraph dives revealed weak horizotal lamiae, like those i C-SALT, i the mai thermoclie ad i itrusios at depth. Itrusios are foud below 700 m, where the lateral mixig of warm, salty Mediterraea water ad cold, fresh Labrador Sea water begis to appear. The temperature-saliity structvue has a classic cetral water structure, a tight, smooth arc from the mixed layer dowward, reflectig the relatively costat desity ratio of the thermoclie (Figure 9). Itrusios are foud at lower temperatures, as the large scale saliity miima ad maxima are ecoutered. Some small T/S iversios are foud i the upper waters i some casts. Mesoscale eddy variability is a primary cocer for NATRE. Accordigly, the dyamic height at 300 db relative to 1500 db is mapped for the large scale survey (Figure 10). Three hudred db is the target depth for the tracer deploymet. The map reveals two high pressure regios i the west ad east cetral areas, separated by a low which is particularly strog i the south cetral area. The low is cetered o statio 48, which is at the site of the cetral subductio moorig. Typical relief of these features is 2-4 dyamic cetimeters; characteristic geostrophic velocities would be order 5-15 cm/s. A strog easterly flow is idicated i the ortheast corer; the velocity profiles from HRP also showed this flow. Some reservatios must be expressed with regard to the ifluece of the iteral tide o such maps, a issue discussed later, but we geerally see good coherece of features from sectio to sectio. (The orth-south sectios took typically a day ad a half each.)

32 29- POTENTIAL DENSITY (REF.-O) Ze.jlO DENSITY RATIO (20 DB' P 3<&. 5 0 f POTENTIAL TEMPERATURE 12. T fi 35, 50 SALINITY f' Figure 8: Profiles of potetial temperature, saliity, potetial desity ad desity ratio versus pressure (computed from fiescale data variables) are show for dive 78.

33 30 CM CM. UJ a:. I " < a:. t-ot <. Mo UJ -I I i SALINITY Figure 9: The theta-saliity relatioship is show for dive 78.

34 \ \ 1 1 \ Logitude (W) Figure 10: Cotour plot showig the dyamic height field at 300 db refereced to 1500 db for the large scale survey.

35 -32- If we accept the dyamic topography as a reasoable sap-shot of the structure, we ca examie the higher order dyamics to ifer evolutio of the eddies. Figure 11 is a map of the relative vorticity, scaled with f, calculated from the Laplacia of the dyamic height field (by Joh Toole). It suggests that the strogest eddy feature is a dipole pair i the ceter of the box. Oe would ifer orthwestward propagatio of such a pair from their mutual iteractio. The plaed NATRE deploymet site is i the ceter of the ortheast sector of the box, so it might ot be adversely affected by this dipole pair, though it does lie withi the ortheast sector of the aticycloic eddy. The value of the desity ratio is of particular iterest, i order to determie the potetial for salt figerig. This is mapped at 300 db usig a 20 db least square fit to the vertical temperature ad saliity gradiets i Figure 12. At 300 db, there is a strikig bad of low desity ratio tredig from the southwest to the ortheast across the souther portio of the box. The iteded TRE deploymet site ear 26 0 N, 28 W, falls withi this bad. We have selected the (7300 = surface to represet the lateral thermohalie structure. I this area a 300 = correspods to sigma-theta = This surface lies close to 300 db i the regio of the small scale survey. Theta-300, saliity, pressure ad desity ratio o this surface are show i Figures 13a-d. For the most part, the topography of these surfaces mirrors that of the dyamic height field. Horizotal T/S gradiets are geerally modest, with mesoscale variability domiatig ay large scale mea structure. Gradiets i the tracer release area appear to be small. Further detail o structure i the release area is revealed by the small scale survey (statios show i Figure 3). Figures 14a-d show the potetial temperature, saliity, pressure ad desity ratio for the small survey. Gradiets of temperature ad saliity of order 0.01 o C/km ad o / oo /km are see. However, of more cocer is the strog relief i the depth of this surface. Statios 114 ad 118 have more tha 24 db differece i the depth of = However, oly about 10 hours separate these two profiles, so we suspect aliasig by the iteral tide. The times series of statios take i the viciity of 26 0 N, 28 0 W is valuable for examiig this questio. Figure 15a shows the pressure of the sigma-theta =26.75 surface as a fuctio of time, vertical exclusios up to 40 db ca be see, with a 24 hour period. The effect of the iteral tide o the dyamic height for these statios is show i Figure 15b, the amplitude is of order 2 dyamic cetimeters for the 300 db refereced to 1000 db surface. This variatio raises some ucertaity about the represetativeess of the large scale maps; some of the features there may be due to tidal aliasig. Also, such large vertical exclusios of desity surfaces will be of cocer durig the TRE ijectio ad samplig programs, though the wich systems should certaily hadle such variatios. The exis-

36 CD ) Logitude (W) Figure 11: Cotour plot showig the relative vorticity field at 300 db refereced to 1500 db for a subset of the statios i the large scale survey.

37 = o T Logitude (W) Figure 12: Cotour plot showig the desity ratio field at 300 db for the large scale survey.

38 -35- a. b J L J I L '<* \\^' ) D C. i I I r I I I 1 I l I I r J I I I I L I I I I \ I I I I I I Logitude (W) Figure 13: Cotour plots of potetial temperature (a), saliity (b), pressure (c), ad desity ratio (d) o the a 300 = surface for the large scale survey.

39 ) o Logitude (W) Figure 14: Cotour plots of potetial temperature (a) saliity (b) pressure (c) ad desity ratio (d) o the (7300 = surface for the small scale survey

40 ^330 i,340 0) 350 w CD April dive umber 145 Figure 15: Plot of pressure o the sigma-theta = surface as a fuctio of time (a). Plot of dyamic height at 300 db refereced to 1000 db as a fuctio of statio umber (b).

41 -38- tece of such a large iteral tide at a ope ocea site with relatively flat topography is uexpected. Also of cocer for NATRE is the potetial for additioal complexity to the vertical ad horizotal mixig due to the presece of lateral itrusios. At the depth of iterest (300 m), itrusios are relatively ifrequet ad weak. For the desity rage of sigma-theta = 26.5 to 26.85, the umber of temperature iversios i our 1/2 db pressure-sorted data set is displayed for the large survey i Figure 16a. There are relatively few iversios i the cetral part of the survey. The average thickess of the iversios is posted i Figure 16b. As they are geerally a meter or less ad some may represet overturs due to vertical mixig, itrusios should ot be major problem for NATRE. Fially, we offer a prelimiary estimate of the vertical diffusivity. We saw o dramatic variatios i iteral wave eergy levels durig the survey, ad believe that a stable estimate of diffusivity ca be expected. We were able to borrow a faster computer for this cruise (a DEC workstatio) which eabled us to keep up with the massive amouts of data obtaied o each dive. Kurt Polzi has calculated average vertical profiles of diffusivity from the turbulet ad thermal dissipatios for the 100 HRP dives of the large survey. These are show i Figures 17a ad 17b. The first is derived from the stadard Osbor formula, the secod from the Cox umber. These diffusivities were estimated with backgroud gradiets calculated from pressure averaged profiles of temperature ad saliity. The turbulet diffusivity (17a) shows little variatio with depth, ad has a value of about 0.1 cm 2 /s. The thermal diffusivity has comparable values i the upper ocea ad gets larger i the itrusio regio at mid-depth. The icrease at depth is primarily associated with the average-based gradiets beig urepresetative of the actual backgroud agaist which the turbulece is straiig. If salt figers cotribute to the turbulet dissipatio, the the salt diffusivity may be ehaced above 0.1 by a possibly large factor (Hamilto et al, 1989). However, careful aalysis will be required to discrimiate properly betwee the salt figer ad turbulet cotributios to the measured dissipatios. To coclude, a very successful cruise was executed utilizig a free profiler i survey mode for the first time. Much credit goes to the members of the scietific party ad ships crew, who all persevered with operatios through a fair bit of adverse weather. The cruise was log ad tirig for all participats ad we trust that the results were valuable to the executio of the tracer ijectio portio of NATRE.

42 I I I I I I I I I I I x a x 0 x > I I I I i I I I I I I I I I I I I I I I I I I I I M :< o.a x * x A x I I I I I I I I [ I I I! I I I I I I I I Logitude (W) Figure 16: Cotour plots showig the umber of temperature iversios betwee 26.5 ad (a) ad the average thickess of these iversios (b) for each statio of the large scale survey.

43 -40 a. KRHO (m 2 /s) KT (m 2 /s) 0.0 x x D 1.0 x CO V) 03 L. _ 1.5 x xlo 3 Figure 17: Plots showig the variatio of eergy dissipatio rate (a) ad thermal diffusivity (b) with pressure for the 100 statios of the large scale survey.

44 -41 - Ackowledgemets We would like to thak V. Gree for preparig this documet, ad B. Gaffro for editig it. The crew ad officers of the R/V Oceaus did their usual exemplary job i hadlig the ship durig HRP operatios. Eve i adverse weather, they were able to place the ship for safe, dry ad quick deploymets ad recoveries. We appreciate all their assistace i port ad at sea. Fially, we wish to ackowledge the support of the Office of Naval Research ad the Natioal Sciece Foudatio, who joitly sposored this project through ONR grat umber N Refereces Coverse, C. H., A. J. Williams, 3rd, P. D. Fucile, ad R. W. Schmitt, A Free Vehicle to measure Optical Microstructure. I: Curret Practices ad New Techology i Ocea Egieerig - OED-, 11, T. McGuiess ad H. H. Smith, eds Hamilto, J. M., M. R. Lewis ad B. R. Ruddick, Vertical fluxes of itrate associated with salt figers i the worlds oceas. Joural of Geophysical Research, 94(C2), Melliger, E. C., K. E. Prada, R. L. Koehler ad K. W. Doherty, Istrumet Bus: A Electroic System Architecture for Oceaographic Istrumetatio. Woods Hole Oceaographic Istitutio Techical Report, WHOI-86-30, 86 pp. Millard, R. C., ad N. Galbraith, WHOI Processed CTD Data Orgaizatio. Woods Hole Oceaographic Istitutio Techical Report, WHOI-82-37, 42 pp. Motgomery, E. T., The High Resolutio Profiler (HRP) User's Guide ad Software Modificatio Documetatio. Woods Hole Oceaographic Istitutio Techical Report, WHOI-91-01, 32 pp. Polzi, K., ad E. T. Motgomery, (i prep) High Resolutio Profiler: Microstructure data processig methods. Woods Hole Oceaographic Istitutio Techical Report. Schmitt, R.W., J.M. Toole, R.L. Koehler, ad K.W. Doherty, The Developmet of a Fie- ad Micro-structure Profiler. Joural of Atmospheric ad Oceaic Techology, 5,

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