JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. C6, PAGES 9611-9628, JUNE 15, 1990

Circulationin the CanaryBasin' A Model/Data Analysis

MICHAEL A. SPALL

National Center .for Atmospheric Research, Boulder, Colorado

The Canary Basin region of an eddy-resolving, primitive equation model of the North Atlantic is studied in terms of its time mean fields, eddy variability, and role in three fundamental oceanic processes.The model calculation was carried out by Bryan and Holland as the first Community Modeling Effort of the World Ocema Circulation Experiment. Each of the major currents in the region is represented in the model fields. Several aspects of the large-scaleclimatological density field which was used for initialization were improved by the dynaxnicM model while other aspects were degraded. The major deficienciesin the model are the representationof the Mediterranean salt tongue and low values of the eddy kinetic energy. The differencein the model salt tongue is believed to be at least partially due to an insufficient treatment of the Mediterranean outflow. Several numerical paxaxneterizationswhich may be partially responsible for the low eddy kinetic energy are suggestedas topics for future study. The role of this region in the ventilation of the thermocline, meridional flux of temperature, and formation of the Mediterranean salt tongue is studied. In general, the model results compare well with the available data and, in some instances,suggest mechanisms for previously observedflow characteristicsand possibly important phenomena which have not yet been observed. Final recommendations are made to both the modeling and observationalcommunities for future activities in this region.

1. INTRODUCTION There have been many recent observationalprograms in the Canary Basin. Someof theseefforts have been designed The Canary Basin is roughly defined as the area extend- to study the general physical oceanographyof the region ing from 40*W to the easterncoast and 10*N to 40*N. This while others have been aimed at specific processesor cur- regionof the North Atlantic, which containsthe easternpor- rents. A generalsurvey of the large-scale(1000 kin) prop- tion of the subtropicalgyre, has traditionally been thought erty distributions in the eastern subtropical gyre is given to be governedprimarily by Sverdrup dynamics. Although by Arrni and Storereel[1983]. Gould[1985] discusses the this is generally reflected in the climatologicaldata base, structure of the Azores Current through a combinedanaly- recent observationalprograms have confirmedthe existence sis of hydrographicdata, float data and seasurface tempera- of mesoscalejets, eddies,and lensesin the region. Currents ture. A summary of what is known about the deep velocity representedin the mean fields include the Azores Current, fields based on current meter data is given by Dickson et the Portugal Current, and the North Equatorial Current. al. [1985]. A seriesof studiesfrom the "Kiel Warmwater- The Azores Current separates from the Gulf Stream near sphereof the Atlantic" project (SFB133) has lookedat the the Grand Banks and flows into the Canary Basin just south currentdistribution and seasonalvariations [Stramma, 1984; of the Azores Islands. The Portugal Current flows to the Strammaand Siedler, 1988] and the roleof the CanaryBasin south along the coast of Portugal and the North Equatorial in the polewardflux of temperature[Stramrna and Iserner, Current flows to the north between 40*W and the coast of 1986; Stramrnaand Isemer, 1988]. Float programsby the Africa. All three of these currents turn to the west and exit Woods Hole Eastern Basin Study [Price et al., 1986; Ze- the Canary Basinbetween 10*N and 30*N. This flowsystem, rnanovicet al., 1988), W. J. Gould (privatecommunication, combined with the presenceof mesoscaleeddies and lenses 1989), and Maillard and Kgise[1989] have been designed to of undiluted Mediterraneanwater, makesthis an interesting investigate the general circulation and variability of the re- region for study. gion. Hydrographicsurveys by KSse et al. [1986], Zenk The Canary Basin is important to the general circula- et al. [1989], and KSse et al. [1985]have helped to de- tion of the North Atlantic for several reasons. The pole- scribe the water mass distribution, temperature variability, ward transport of heat is fundamental to the coupled ocean- and geostrophictransports. The moored arrays of Schmitz atmospheresystem and it is believed that this region con- et al. [1988]and Zenk and MGller [1988]have given direct tributes a significant portion to the total southward heat measurementsof the velocity and temperature fields and an transport in the North Atlantic Basin [Strammaand Ise- estimate of the eddy variability in the region. met, 1986]. This regionis alsocentral to the formationand Previousnumerical modelingstudies in the Canary Basin maintenanceof the Mediterranean salt tongue in the North have been limited to idealized thin jet representationsof a Atlantic. Thirdly, the eastern region of the North Atlantic singlecomponent of the basincurrent structure[Kielmann is believed to be important for the ventilation of the ther- and KSse,1987; Kgiseet al., 1989]. Thesestudies have been mocline[Luyten et al., 1983].Knowledge of this interaction usefulfor the identification of possibleprocesses active in the betweenthe oceanand the atmosphereis important for cou- current under study but they do not relate those currents pled ocean-atmosphere,climate, and biologicalstudies. to the general circulation or observedstatistical properties of the region. There are several reasonsfor the lack of ex- tensive modelingstudies in the Canary Basin. Becausethe Copyright 1990 by the American Geophysical Union. primary forcing mechanismsof the Azores Current are not Paper number 89JCO3680. well known, fundamental process studies on its formation 0148-0227/90/89JC-03680505.00 and maintenanceand its relation to the general circulation

9611 9612 S•AtL:CIRo0•Tto• I• • C•Am' BnsI•:A MODe/DaTAA•A•VS•S have been difficult. Eddy-resolvingbasin-scale calculations available from observations,and relevancefor future obser- with realistic geometry, which may form the proper cur- vational programs. rent structure, have previously not been available due to This paper is outlined as follows. In section 2, the dy- the computationalexpense. Limited regionmodels are more namicalmodel and physicalassumptions are presented.Sec- affordablebut the problem of providingthe proper bound- tion 3 describesthe basic velocity and density structure of ary conditionsand coupling with the rest of the basin is a the mean model fields and makes comparisonswith avail- difficult one. able data. The eddy kinetic energy and role of the variabil- An eddy-resolving,primitive equation calculation of the ity in instability processesis investigatedin section 4. In North Atlantic has been carried out by Bryan and Holland section5, the role of the Canary Basin region in three fun- [1989] at the National Center for AtmosphericResearch. damental oceanicprocesses are investigated. Conclusions This calculation is the first experiment in the World Ocean and recommendationsare presentedin section 6. CirculationExperiment (WOCE) CommunityModeling Ef- fort. The model providesa significantincrease in resolution 2. NUMERICAL MODEL and physics compared with previous calculations with real- istic geometry such that the fundamental processesrespon- The numericalintegration which is analyzedin this paper sible for water mass formation and hydrodynamic instabil- wascarried out by Bryan and Holland[1989] at NCAR as ities are both represented. In addition, representationsof the first experiment in the WOCE Community Modeling physical processessuch as an upper mixed layer and the Effort. The model domain extends from 15øS to 65øN and Mediterranean outflow are included. It is intended that the from the west coast to the east coast of the Atlantic Basin. 2 ø 1 o results of this model can be used to address questionson The horizontaldiscretization is g in longitudeand • in the role of the mesoscalein the general circulation of the latitude, giving approximately equal grid spacing of 35 km North Atlantic as well as provide a new benchmark for the at 40øN. The vertical discretization uses 30 levels distributed developmentof basin-scalenumerical models. The analysis over a maximum depth of 5500 m. The vertical spacingis in this paper will concentrateon the Canary Basin regionof nonuniform with a minimum of 35 m at the surface and the model fields. expanding to 250 m at depths below 1000 m. Topography A model/data comparisonin the Canary Basinis poten- which was representedby a single grid point in the model tially very useful. At present, enough data exist to indicate was removed. The Mediterranean Basin is not included in that the region is especially interesting and worthy of de- this calculation; the treatment of the Mediterranean outflow tailed study. The current data base may be used to identify will be discussed later in this section. some of the shortcomingsof the model calculation and, as The numerical model used for the simulation is the three- a result, provide considerationsfor the future designof sim- dimensionalprimitive equation model developedby Bryan ilar numerical calculations and prompt more basic process- and Cox[1967]. The modelwas initialized at rest usingthe orientedmodeling studies. Basedon the model/data com- temperatureand salinity fieldsof Levitus[1982] and inte- parisons,the model fields may be usedto extend estimatesof grated for 25 years. The form of the model equations and relevant processesbeyond that which is presently obtainable parameterizations developed for this experiment are sum- directly from the observations. In addition, the model may marized in Appendix A. As mentioned in the introduction, provide possibleinterpretations for observedflow character- the main focus of this paper is the Canary Basin. Details istics. There are severalobservational programs in the area of the experiment design and implementation, as well as a which are currently under way or being planned for basic discussionof the basin-scalecirculation, can be found in the study in the region; these include studies of the Mediter- work by Bryan and Holland[1989]. ranean salt tongue and Meddy formation, mesoscalevari- In the real ocean there is flow across the latitudes of ability, and ventilation of the thermocline. It is hoped that the northern and southern boundaries of the model. This the results of the model analysispresented in this paper may model does not attempt to treat the open boundary condi- suggestinteresting phenomena in the region and influence tion problem but imposesno flow through the northern and the developmentof these future observationalprograms. southern boundaries. The interaction with the rest of the A conclusivemodel/data comparisonis difficult because world oceansis parameterized through a NewtonJandamp- of the complete four-dimensional nature of the model fields ing of the model temperature and salinity fields back toward and the fact that the observationsare taken from many dis- the monthly mean data set of Levitus. This restoration is connectedprograms which vary in spatial and temporal cov- done over a band of five grid points adjacent to the bound- erage and experimental objectives. Becausethe purposeof aries. The artificial reflectionand generationof wavesat the this paper is both to evaluate the model fields and to learn boundarieswill be suppressedwhile it is hoped that someof about the physical oceanographyof the region, the analy- the effects of interbasin exchangeof water masseswill still sis also follows a somewhat disconnectedpath. One of the be included. problems with this type of investigation is that there are A similar Newtonian damping term is added to the tracer many criteria by which to evaluate and study the fields. The equations in the vicinity of the Mediterranean outflow and analysis presented in this paper falls into two categories: the Labrador Sea. There is no explicit flow through the a direct comparison of dynamic variables and an indirect Strait of Gibraltar but the net flux of temperature and salin- comparison via the study of the meridional flux of temper- ity from this region is parameterizedthrough this noncon- ature, the ventilation of the thermocline, and the formation servative term. Water parcels leaving the damping region of the Mediterranean salt tongue. These quantities have will take on water properties characteristic of the climato- been chosen becauseof the availability of observationsfor logical temperature and salinity field in this region. The comparison,the importance to the general circulation of the damping in the Labrador Sea is required to maintain real- North Atlantic, the potential for additional information not istic sea temperatures over the shelf region where there is SP•: •T•o• • 'rimC•r B•e•: A M•/I•TA Aro•YsIs 9613

extreme cooling to the atmosphere. Sea ice forms in the graphicdata and Conductivity-temperature-depth(CTD) actual ocean but those processesare not included in the surveysis shownin Figurelb, from Stramma[1984], (here- dynamical model. in-after referredto as S84). Note that Figure la. is plotted The surfacewind forcingis taken from the monthly mean in degreeswhile Figure lb is plotted in kilometerssuch that winds derivedby Hellerman and Rosenstein[1983]. The there is approximately25% stretchingin the longitudeof surfaceflux of temperature is derived from a relation for the Figure la. The overall descriptionof the flow field in the surfaceheat flux basedon the analysisof Hah [1984]. The upper thermoclineis that of three distinct currentsentering time scale of the surfaceheat flux dependson the latitude, the region and all flowing toward the west between 10øN longitude, time of year, and sea surfacetemperature but is and 30øN. The AzoresCurrent (AC) entersfrom the west on the order of 50 days. Because the surface flux of salt near 32øN, turns to the south, and flows out to the west (or evaporationminus precipitation) is not well known,it near 26•N. The PortugalCurrent (PC) flowsto the south is parameterized through a rela•xationof the surfacesalinity betweenthe AzoresIslands and the coastof Portugal, turns toward the monthly mean climatology.It appearssimilar in westward off the northwest coast of Africa and flows out of form to the surface heat flux and usesa constant damping the region just south of the AC recirculation. The North time of 50 days. EquatorialCurrent (NEC) entersfrom the south between The mixed layer formulation is similar to the bulk ki- the coast and 40*W, flows northward, and then turns to the netic energymodel of Camp and Elsberry[1978]. The en- west. A more detailed description of each of these currents ergy available for deepeningof the mixed layer from wind follows. work is convertedinto potential energy and distributed over The model produces an AC of 6-7 Sv entering the re- the depth of the mixed layer. This model allowsfor a mixed gion near 32•N. The historicaldata baseshows a somewhat layer which is of arbitrary depth down to a maximum of stronger flow, 11 Sv, entering the region a little further to 800 m. Detrainment of the mixed layer is also included such the north, near 35•N. The modelcurrent is narrowerbut this that if the mixed layer depth shallows,the deeperremnants is probably due to the larger temporal and spatial smooth- of the old mixed layer remain. ing in the historical data set comparedto the model. Other measurementsput the transport of the AC at 10 Sv in the 3. GENEP•L DE$C•PTION upper 1500m [Gould,1985] and 2000 m [Strammaand lse- met, 1988].The transportof the modelfields over the upper In this section, the basic characteristicsof the model ve- 1500 m showsa very similar pattern to that in Figure la but locity and density fields in the Canary Basin will be dis- with an AC transport of 9 Sv, in better agreementwith the cussedand comparedwith observedfields. Similarities, dif- deeper estimates. The westwardrecirculation is very tight ferences,and information not available from observations in the model. The AC entersthe regionsouth of 35øN and will be highlighted.Unless otherwise stated, the modelfields exits the region north of 25•N, spanningonly 10• of lat- are the mean fields averagedover the last two years of in- itude, whereas the recirculation in the climatology covers tegration. The quantities to be analyzed are upper thermo- over 16•. Again, someof this smoothingmay be due to the cline transport stream function, presenceof Mediterranean low-frequencyvariability of the subtropicalgyre whichis not water, deepvelocity field; large-scaletemperature and salin- representedin the 2-year mean model fields. There is only ity structure, and water mass distribution. a weak indication in the model of the three-band structure seenin Figure lb. 3.1. Upper Thermocline Transports The PC is a broad, weak flow enteringthe regionbetween The transport strearnfunction for the upper 800 m of the the Azores Islands and Portugal with a total transport of model fields is shown in Figure la. For comparisonpur- 8 Sv. Transports estimated from observationsalong this poses,the stream function calculated from historic hydro- sectionvary widely. The transport in Figure lb is only about

40N 40N

,,m 50N g 50N

20N

ION ION 40W 30W 20w lOW 20W 40W 3,0W 20W lOW LONGIT UDE LONGITUDE LONGITUDE

Fig. 1. Transportstream function, 0-800 m (contourinterval I sv): (a) primitive equationvelocity (letters A-D indicate the locationsof T-S profile• shownin Figure5), (b) historicaldata base,and (c) geostrophicbalance. 9614 SeA•:C1RCXJLA'nON INarm CANARY BAS•: A ]•EL/I•TA ANALYSIS

3 Sv, while Saunders[1982] calculated a transportof 3-4 (a) Sv in the upper 850 m. These are considerablyless than õON that seen here, but some calculations based on observations placethe transportmuch higher. Sverdrup[1942] estimated zION the total southward transport at 18 Sv while Dietrich et al. [1980]report a southwardtransport of 14 Sv between0 and 1000 m from the International Geophysical Year data.

_ _ _ There is a large-scale wave pattern superimposedon the _ southward flow of the PC in the model which is not observed 20N :• in the climatologicaldata base. The presenceof theselarge- 60W 40W 20W scale meanders is due to low-frequency waves which have remained in the 2 year average. A similar low-frequency (b) zonal variability has been observedeast of the Azores Islands 50N ' ' • • - -'--- in the float data of gernanovicet al. [1988]. The NEC enters at 10*N between 40*W and the coast of Africa. The model transports 7 Sv in the upper 800 m.

After entering the region, the water meets with the west- __

ward recirculationof the Azores Current and the Portugal _ Current and exits to the west. Figure lb showsa weak in- dication of a NEC but there is not a coherent circulation. 20N • - • 60W 40W 20W A seasonalanalysis by Strarnrnaand Iserner [1988] indicates a northward transport of between 0 and 7 Sv in the upper (c) 1500m. Hellerman[1980] used wind stress data to calculate 50N •, ,•-•. _• .... •• the Sverdrup transports between 3 and 10 Sv at 230W and 9 and 23 Sv at 38'W. The model transport compareswell with these other estimates, being in the low range of the calculationsbased on the wind stressand at the high end of those based on the historical data base. The geostrophictransport calculatedfrom the model tem- _ perature and salinity fields is shown in Figure lc. Rather 20N • " • than use a level of no motion, the geostrophic velocity 60W 40W 20W was calculated relative to the primitive equation velocity LONGITUDE at 1500 m. In this way, the differencesare due only to ageostrophiceffects in the upper 1500 m and not dependent Fig. 2. Salinityat 1125 m (contourinterval 0.1 ppt): (a) Levi- on assumptionsabout a level of no motion. The geostrophic tus data, (b) meanmodel salinity years 24-25, and (c) difference transport is similar to the full primitive equation transport between Levitus and model data. in the region of the Azores and Portugal currents. The most striking differencebetween the two fields is in the NEC re- gion. Where the full primitive equation fields show a slow but clear flow enteringfrom the south and turning west, the Mooney,1975]. In thissection, the salinityfield at 1125m in geostrophicbalance indicates only a weakflow enteringfrom the model will be comparedwith the Levitus salinity which the south and flowing into the coast. The implication that was used to initialize the model. A more detailed discus- the NEC transport is mostly ageostrophicis consistentwith sion of the mechanismsof the salt tongue formation in the the conclusionsof Maillard and K&e [1989]based on surface numerical model will be given in section5.3. drifter data. The lack of a strong NEC in the analysisof S84 The salinity at 1125 m averagedover the last 2 years of may be because the primary transport in the NEC is due model integration is shownin Figure 2b. The salt tongue to the Ekman flux. This also suggeststhat hydrographic can be seen extending westward between 25'N and 35'N. surveyswould not be a very good method to measurethe There is a rapid decreasein salinity away from the eastern total NEC transport. boundary, reduced from 36.0 ppt at the coast to approxi- mately 35.4 ppt at 20*W. This is muchless than the 35.6 ppt 3.œ. Mediterranean Water, 11œ5 rn seenin Figure2a at 20*W. The modelsalt tonguealso has a highly variablestructure in the meridionaldirection near the One of the most prominent features of the North At- coast. There are two long filaments of higher-salinity water lantic thermohaline field is the presence of the Mediter- extendinginto the interior of the tonguepast 20*W. Further ranean salt tongue. Shown in Figure 2a. is the salinity at to the west, the salt tongue is smoother and has a salinity 1125m from the climatologicaldata baseof Levitus[1982]. almost 0.2 ppt greater than climatology. The model tongue This is a warm, salty water mass which is strongest be- has nearly constant salinity over most of the interior with tween 1000 and 1200 m and extends westward from the sharp fronts marking its northern and southernextent. By Mediterranean outflow. The mechanisms of its formation contrast, the climatologicaltongue has nearly uniform gra- and maintenance are not well known but are believed to be dients throughout its extent in the interior, possiblydue to a mixed advection/diffusionprocess with isolatedlenses of the extensivesmoothing in the mapping procedureof Levi- undilutedMediterranean water (Meddies)acting as moving tus. Due to its large spatial extent, synoptic representations point sources[Arrni and Haidvogel,1982; Richardsonand of the entire salt tongue are not presently available. ANALYSIS 961$

As was mentioned in section 2, the model was initialized flow from the Madeira Abyssal Plain into the Iberia Abyssal with temperature and salinity fieldstaken from Levitus. The Plain. The magnitudes of these mean currents are 1-2 cm differencein salinity between the last 2 years of the model s-1 but the coverageis inadequate for any meaningful esti- calculation and the climatology which was used for initial- mateof the transport.The float data of Gould[1989] show a ization is shownin Figure 2c. In the vicinity of the damping similar pattern near 3000 m with anticyclonic circulation in region the differenceis nearly zero; however,the low model the regionof 20øW to 26øW and 32øN to 36øN. These data salinity over most of the east and high model salinity in the are in general agreement with the magnitude and senseof west are deafly seen. Most of this restructuring of the salt the deep circulation found over the Madeira Abyssal Plain in tongue occurs over the first 20 years of model integration, the model calculation. No estimate of the deep flow pattern this difference field remains nearly constant over the last 5 over the Canary Abyssal Plain can be made from present years. This indicates that the model is approachinga steady observations. state which is significantly different from climatology. $.4. Meridional Sections 3.3. Deep Velocity Field In this section, meridional sectionsof the model temper- A map of the average model velocity field at 2875 m is ature and salinity fields are used to get a picture of the shown in Figure 3; fields averagedover the last 5 years of large-scalewater massdistributions in the region. Observa- integration are qualitatively the same. The general senseof tional programsin the Canary Basin haveindicated that this the flow field at this depth is that of two weakly connected is a region of complex water massdistributions with North anticyclonic circulations. The larger one is over the Canary Atlantic Central Water (NACW), South Atlantic Central AbyssalPlain and extendsfrom 15*N to 30*N. A smallercir- Water (SACW), Antarctic Intermediate Water (AAIW), culation pattern also exists over the Madeira Abyssal Plain MediterraneanWater (MW), and North Atlantic DeepWa- between30*N and 37'N. The maximum time-averagedve- ter (NADW) all present.Although the modelwas initialized locitiesin both circulationsare on the order of 1 cm/s with with a coarse representation of these water mass distribu- most velocities much smaller. There is some flow from the tions, it is of interest to see to what extent the model main- Madeira AbyssalPlain over the Azores-PortugalRidge (at tains and refines the observed characteristics of the water 38'N) and into the Iberia AbyssalPlain. This transportis properties. barotropic down to the top of the ridge, so that the total Shown in Figure 4a. is the meridional section of the transport into the Iberia Abyssal Plain is estimated to be model temperature field along 20*W. In the upper ocean 0.25 to 0.5 Sv. (_• 600 m) there is a strong front located at 21*N. This There have been some deep current meter and float mea- boundary between the cooler SACW to the south and the surementstaken in this region but the data are sparse. Cur- warmer N ACW to the north has been observed in CTD sec- rent meter measurementsnear 3000 m from several pro- tions and was called the Central Water Boundary(CWB) grams are found to have mean horizontal velocitieson the by Zenket al. [1989].Another prominent feature of the up- orderof 0.1 cm s-a to 1.0 cm s-a Dicksonet al. [1985]. per thermocline is the presenceof a layer of nearly isother- Although speculative, this data coverage in the Madeira mal water near 17'C between27'N and 34'N, similar layers Abyssal Plain indicates that the general circulation pattern of water have been found in this region by several hydro- is that of northward flow along 25'W with a weak south- graphicsurveys ISlediet et al., 1987; Gould 1985]. Sledlet ward flow in the east. There are also several current meters et al. [1987]hypothesize that this water is formedlocally, along the Azores-PortugalRidge which indicate that there is near the Madeira Islands, and have called it Madeira Mode Water. The spreadingof the 11*C to 12'C isothermsat the northern end of the section is similar to the spreadingdue to Mediterranean water seen in the sections of K•se et al. 40N [1986]but it occursat a shallowerdepth in the model. The deeper temperature structure showsa gradual deep- ening of the isothermsfrom the south to the north between 10*N and 30*N. This is a signature of the bowl shape of LI.I the thermocline in the subtropical gyre. The warmer MW c• SON is deepeningthe isothermsbetween 22'N and 36'N. The eddy-like feature seen in the MW near 33'N is the inter- leaving of more pure MW with NACW of the interior seen in Figure 2b. The deep isothermsare nearly horizontal and 20N lessthan 3'C, indicative of NADW. It shouldbe noted that the model integration has not been carried out long enough for equilibration of the deep model fields. Shownin Figure 4b is the meridional sectionof the model salinity along the longitude 20*W. Many of the featuresob-

ION "' servedin the temperature sectionare apparent in the salinity 40W sow 2ow lOW section as well. The separation of the fresh SACW from the saltier NACW by the CWB is visible at 21'N. There is also LONGITUDE 2 cm/s evidence of the poleward transport of fresh water near the surfacein this region by the Ekman transport. The Madeira Fig. 3. Mean velocity field at 2875 m. Mode water is visible as a halostad near 36.3 ppt between 9616 Se•: ••o•/n,/•sm C,•AmCI3•: A ]•DD•L/tDATAANALYSIS

{0) TEMPERATURE {øC) (b) SALINITY (ppt) 0

I000 •-----•••_ ' -'"'-•--•- - •_•-•_•• I000 - I • • --••••

_

-• 2000 2000 -

_

a. :5000 3000

4000 4000

5OOO 5OOO

ION 20N 5ON 40N ION 20N 50N 40N

(C) TEMPERATURE (øC) (d) SALINITY (ppt) o

IOOO

2000 . 2000

5000 .5000

4OOO 4000

5OO0 5OOO

ION 20N 5ON 40N ION 20N 30N 40N LATITUDE LATITUDE

Fig. 4. Meridional sectionat 20øW: (a) model temperature(b) model salinity (c) Levitus temperatureand (d) Levitus salinity.

28øN and 34øN. The separation of AAIW and MW at mid- contours associated with the intrusion of the Mediterranean depths is shown more clearly in the salinity field than it was salt tongue in the Levitus sectionsis significantly reduced in the temperature field. The freshet AAIW can be seen by the model. This is consistentwith the weak representa- extending northward at 900 m overlyingthe MW extending tion of the salt tongue in the model at 1125 m discussedin to the south between 1200 m and 1400 m. A similar profile section 3.2. The model has also reducedthe strength of the was found in a CTD section taken in the area by Zenk et AAIW extending into the region from the south at 900 m. al. [1989].To the north, a weakincrease in salinityis found Because the source of AAIW is south of the southern model due to the core of the Mediterranean salt tongue. The in- boundary, its reduction may be a result of the Newtonian terleavingof the MW with the NACW is seenin the salinity damping used to approximate the interbasin exchangesat section as well. The deep salinity fields are nearly constant the boundary. The Madeira Mode water which is seen in at near 34.9 ppt. the model fields was not present in the initial conditions but The extent to which the model has modified the climatol- was formed by the model during the courseof integration. ogy usedfor initialization can be seenby comparingthe tem- The width of the CWB has been reduced from over 500 km perature and salinity sectionsin Figures4a. and 4b with the in the climatology to 200 km in the model. This is in better same sectionstaken through the Levitus data in Figures 4c agreementwith estimates basedon hydrographicdata. and d. In general, the Levitus data have the same tenden- 3.5. T-S Curves cies as were discussed for the model sections but the features are much smoother due to the analysis procedure of Levi- SelectedT-S profiles from the averagedmodel fields are tus. The model has modified the fields in several ways, some shown in Figure 5. The locations of the profiles are indi- are in better agreementwith small-scalestructure observed cated in Figure l a for profiles A-D. The bold lines in Fig- in the region and others are in disagreementwith what is ure 5 indicate the regimesof NACW and SACW as defined known about the water massdistribution. The broadening by Tomczak[1981]. Also indicatedin the figure are MW, of the 9ø to 11øC isotherms and the 35.5 to 35.6 ppt salinity AAIW, and NADW for reference. SPALL:(•TION IN• CANAKY•31: A i•D•/,/DATAANALYSIS 9617

0 i , i , ! , ! , i , i , ! , ! , i , ! , i , ! , 4. EDDY VARIABILITY

Aspects of the three-dimensional structure of the eddy kinetic energy are describedin section 4.1. The role of the mesoscalevariability in the instability processesis presented in section 4.2. In section 4.3, these results are related to the 20- mode] discretization and parameterizations.

J.1. Eddy Kinetic Energy 15 The eddy kineticenergy (K•r) at 1125m is shownin Fig- ure 6a. This depth has been chosenbecause of the availabil- ity of data and relevance to the formation of the Mediter- I0 A ranean salt tongue. The pattern at 1125 m is representative of the eddy variability in the upper thermocline, although the magnitudesare smaller at this depth. The K•r is largest 5 AA between 15øN and 33øN and east to 20øW to 25øW. This is IADW the region bounded to the north by the Azores Current and to the east and south by the Central Water Boundary. There I I • I I I • :35 :36 57 are two other locationsof significantK•r, one concentrated right along the coast of Africa and the other to the east of SALINITY the AzoresIslands (at 38øN, 15øW to 25øW). In the vicinity of the AC, KE is generally2-3 cm2/sz with valuesas high Fig. 5. T-S profries;locations indicated in Figure la. as4 cmz/s z. KE is largestwhere the AC turnsto the south and in the westward flowing recirculation region. Also indi- catedon the figureare the locationsof moorings831 (circle) Profile A is located at 10øN, 20øW. SACW is clearly and K276 (triangle). The measuredkinetic energiesare 5.9 presentbetween 10øC and 15øC. Belowthe SACW is AAIW cmZ/sz and9.7 cmZ/sz for 831and K276, respectively. The between 8øC and 5øC. The coldest water is the NADW. The actual location of the mooringsplaces them just to the north presenceof SACW over AAIW and NADW was discussed of the AC in the model fields. When compared to the his- in the previoussection. Profile B (25øN, 35øW) is com- torical position of the AC, they are located in the eastern posed almost completely of NACW and NADW. In profile part of the AC just as it begins to turn to the south, see C (37øN,15øW) the mixingof MW with NACW is seenbe- Figure lb. The analogouslocation in the model would be tween 6øC and 10øC. Overlyingthis MW is NACW, this is near28øN, 29øW, where the modelK•r are2-3 cmZ/sz, low consistent with the northern end of the meridionM section by a factor of 2-5. discussedearlier. The final profile is located very close to It is also of interest to study the vertical distribution of the Mediterranean outflow at 37øN, 10øW. Undi]uted MW K•r in the Azores Current. The K•r as a function of depth is found between 5øC and 10øC and NACW exists at tem- is shownin Figure 6b for the model and three setsof current peratureswarmer than 10øC. meter moorings. The model profile has been chosento be

a 40N b 0

u.i .SON :::) E -moo0

-J 20N • -2000

ION . 40W 50W 20W lOW

LONGITUDE -:5000 o IO 20 $o 40 EDDY KINETIC ENERGY

Fig. 6. Eddykinetic energy (contour interval 0.5 cmzs -z): (a) eddykinetic energy at 1125m and(b) verticalprofiles in the Azores Currentfor modeland observations(A, model;B 831 first year;C 831 secondyear, D K276). 9618 SPA•:••o•/I/q 'nmC•Am' t•I•: A MDD•/,/DATAANALYSIS representative of the model KE in the eastern extent of the wardflux of 0.001*Ccm s -•. The positivetemperature flux AC and the mooringsare K276 and 831, where mooring 831 is an indication that the mean potential energy field is a has been broken up into its first and second years. The sourceof energy for the fluctuationsbecause the isotherms model KE is significantlylower than that measuredby the of the recirculationgyre shallowto the north in this region. current meters, and the curvature of the model Kr is much The eddy temperature flux is to the south acrossthe Central too low in the upper ocean. Above 500 m the model K• is Water Boundary, between 14øN and 23•N, but there are no between a factor of 3 and 6 lower than the measurements. estimates available from observations. At 1000 m, the model values are a factor of 2-5 less than The total conversionof mean potential energy to eddy the observations,and the deep values are low by a factor of energy is written in terms of the temperature as 5. It is evident from these figures that the Kr in the model pogav'T'T u pogau'T'Tx AC is much lower than that measured by moored instru- S•,•= - T, - T, (1) ments. The upper thermocline distribution of Kr is similar where contributions due to both the zonal and meridional to the map shown for 1125 m; the largest values are found near the Azores Current and Central Water Boundary, but mean temperaturegradients are included.In (1), p0 is the they are still much less than estimates from observations. mean density of seawater, g is the gravitational acceleration, The low KE reflected in the vertical profiles near the AC is a is the coefficient of thermal expansion, T is the mean tem- found throughout the Canary Basin. Becausethe variabil- perature field, and T • and u•, v• are the eddy temperature ity of the model fields is much lessthan the observedvalues, and velocity components,respectively. a detailed analysisof the kinetic energy budget would have The baroclinic conversionrate averagedin the zonal and questionable relevanceto the real ocean and will not be done vertical directions is shown in Figure 7b as a function of here. Instead, the analysis in the remainder of this section latitude. The contribution due to meridional eddy fluxes is will concentrateon determining the reasonsfor this low Kr. labeled as curve A, the zonal contribution is curve B, and the total is curve C. Negative values indicate that the net transfer of energy is from the mean to the eddy field. The 4.œ. Energy Conversion by the Mesoscale Variability Canary Basin falls into three-regions:26•N to 36•N is the The baroclinic and barotropic energy conversionbetween AC and its associatedrecirculation; 23øN to 26øN is the the mean and variable fields are calculated in the upper quiet water within the subtropicalgyre; and 14ONto 23•N ocean. Comparisons are made with the limited estimates is the westward flow of the Central Water Boundary. available from observationsin terms of the magnitude, di- In the vicinity of the AC there are two distinct peaks rection, location, and form of the energy conversion.While in S•,r. The first is located at 30ON and is primarily due it has been stated that a detailed analysis of the eddy kinetic to the meridiona] component. This is associatedwith per- energy budget will not be done, the conversionterms are of turbations of the eastwardflowing AC and the north-south interest because direct comparisonswith previous observa- gradient of the mean temperature field. The secondpeak is tional and modeling analysisare possible.If the conversion located at 33•N and is due to the zonal component.Analysis rates are lower than these other estimates, this may be at of the three dimensional fields indicates that this conversion least partially responsiblefor the low Kr found in the model sourceis locatedin the deep(300-500 m) westernregion at fields. this latitude. The sourceof the potential energyis the zonal One potential sourceof energy for the time variable fields variation of the mean temperature field due to the reduction is the potential energy of the time mean fields. This energy in size of the bowl shapedstructure of the gyre recirculation conversionis accomplishedby the down density gradient with increasingdepth. ttux of the perturbation density by the perturbation velocity The region south of the AC recirculation and north of the field. This density flux has contributions due to both the CWB is a region of low baroclinic instability. This is not temperature and salinity fields. In the following analysis, surprisingbecause it is within the recirculationgyre where the discussionis presented in terms of the temperature ttux the horizonta• velocities and mean horizontal temperature only. The contributions due to the ttux of salinity have been gradients are small. calculated and are in generalagreement with the conclusions The temperature gradient alongthe CWB is anotherlarge drawn from the temperature analysis. sourceof eddy energy. The major contribution to the insta- The poleward eddy temperature ttux has been calculated bility is due to the meridional componentof the eddy heat for the upper 500 m. The value of v•T • averagedover the flux. There are two peaks in energy conversions,located at upper 500 m and between 40øW and 25øW is shown as a 21øN and 17øN. Theseare the latitudesof the locallylarge function of latitude in Figure 7a. The largest fluxes are to KE seenin Figure 6a along the CWB. This eddy variability the north alongthe AC (30ON)and to the southalong the is seenin the velocity field at 460 m on October 1 of the last CWB. In the vicinity of the AC, the average value is ap- year of model integration, shownin Figure 8. There are two proximately0.0018øC cm s-•. Alsoindicated in the figure wavelikepatterns a•ongthe CWB at the latitudesof largest are estimates of the eddy heat flux based on measurements baroclinicconversion. These eddies are frequentlyseen along acrossthe AC by Kdse et al. [1985],(hereinafter referred the CWB in the model, have baroclinicsignatures from the to as KZSH). Their measurementsshow a similar average surface to below 1500 m, and persist for time scales of 1 value(0.002øC cm s-•) with somewhatlarger variability. year. During this year, the wave packetspropagate to the The variability is not surprisingsince the model fluxesrepre- west along the CWB front where they interact and merge sent an averageover two years of integration. The numerical with the westwardrecirculation. Strong interleaving of the modelingstudy by Kielmannand Kdse [1987] of an idealized salinity front is often seenin hydrographicdata off the coast thin jet representation of the AC produced a similar north- of Africa near 20oW [Zenket al., 1989]and a similarpat- S•Ar•:••ON n•35m C•arur BAs•: A ]VJDDFI,/DATAANALYSIS 9619

(a) EDDY HEAT FLUX (b) BAROCLINIC CONVERSION , i , ! , I ' 40 - '"----- T .... T ..... T ---• ! ' I 40

36 A $2 uJ 28 w 28 i- 24 I- 24

._1 20 .

16 16

12 12

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(C) BAROTROPICCONVERSION 4O

36 ....•.... I.... I.... I.... •.... 1''''•••'''• ....•.... •...._ . A $2

tu 28

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12 •_taa_taa.'LIii it l,, ii i,,, il 1111t -.:SO -.20 -.10 0 .10 .20 .$0

Fig.7. Energyconversion byeddy variability: (a)meridional fluxof temperature byeddy field (øC cm s-1 ), (b)baroclinic conversion ofenergy (10 -6 J m--3 8 --1 ), and(c) barotropic conversion ofenergy (10 -6 J m--3 8 --1 ). tern has been observed in several float trajectories at 1000 that KZSH did not considerthe contribution from the zonal m near this samelocation by Price et al. [1986]and Ze- variationin the mean temperaturegradient. It is important rnanovic•t al. [1988].Although it is not clearwhat a float to notethat, whilethe AzoresCurrent is primarilyzonal, it trajectoryin oneof thesemodel wave patterns might look mayinteract with zonal variations of thelarge-scale density like,the modelfields do suggestthat the variabilityfound structureof the subtropicalgyre circulation.Other conver- in the realfloats and hydrographic data maybe dueto local sion rates of 1.2 x 10-s J m-a s-• have been calculatedin baroclinicinstability processes along the temperaturegradi- theAntarctic Polar Front by Brglden [1979] and 1.5 x 10-4 J ent of the Central Water Boundary. If this is the case,the m -a s-• in the Iceland-ScotlandFrontal Zone by Willebrand meanderingof the CWB not onlymay be importantto the andMeincke [1980]. This indicates that baroclinicinstabil- localdynamics of the region,but may alsoact as a source itiesin the modelare a significantsource of eddyenergy in of eddyvariability to the westernNorth Atlantic. the CanaryBasin and are comparableto thoserates found Thelargest value of theenergy conversion rate in theAC in similar fronts elsewhere in the ocean. is 1.1 x 10-s J m-a s-1. The maximum found along the The meankinetic energy field is alsoa potentialsource of CWBis slightlylower, 0.9 x 10-s J m-a s-•, butis active energyfor the eddyfield. The total conversionfrom mean overa largerrange of latitudes.These values compare with kineticenergy to eddyenergy is written as an estimateof 0.4 x 10-s J m-a s-• from observationsin the AC by KZSH.Part of the differencemay be dueto thefact (2) 9620 SPALL:Qaar•noN m • C•,•,• •: A Moi)•./I•T,• fi.NALYSIS

25N south to 25 km in the north. Over much of the region the horizontal model resolution is the same or less than the ra- dius of deformation. While the scale of most flow features is 20N larger than this, energy exchangeprocesses for geostrophic turbulence take place on scales of the Rossby radius. As such, adequately resolving this scale is likely to be impor- tant for an accuraterepresentation of the flow of energyin 15N the model. Although it is desirable to investigate the re- lationship between horizontal resolution and eddy energy, this is not yet computationally feasible on the scaleof this ION numerical calculation. Possible alternatives include models 40W 30W 20W with simplified geometry,nested and expandablegrid mod- els, and regional models. LONGITUDE IOcm/s The horizontal subgrid-scaleparameterization is a bihar- monic operator on the momentum and tracer variables. The Fig. 8. Velocity at 460 m on October 1, model year 24. biharmonic operator is more scale selectivethan the tradi- tional Laplacian operator. The coefficientof horizontal dif- fusionused in the calculationwas 2.5 )<10 •9 cm4 s-•, this is high for the horizontal resolution of the model and was The barotropicconversion rate averagedin the zonal and required to maintain numerical stability. Analysisof a high vertical directions is shown in Figure 7c as a function of resolutionsynoptic survey of the Azores Current and nearby latitude. In general, the magnitude of the barotropic con- version term is much smaller than the baroclinic term. At recirculationby Kdseet al. [1985]indicates that there are two dominant wavelengths,one at 570 km associatedwith all latitudes, the meridional component is dominant over the the meanderingof the AC and the other at 200 km associ- zonal component. There are two distinct peaks,one over the ated with a mesoscaleeddy field superimposedon the me- southernportion of the AC and into the recirculationregion anderingjet Using the smaller of these two wavelengths,the (28øN to 30øN) and the other overthe northernportion of estimated time scale of the horizontal friction term is 50 the AC (30•N to 33•N). In the southernpeak, the energy days. This is much shorter than time scalesof dissipation transfer is from mean kinetic energy to eddy energy. Be- generallybelieved to exist in the real ocean. Meddies,which tween 30•N and 33•N the energy transfer is in the opposite are believed to be formed near the Mediterranean outflow, direction; the eddy field is giving up its energyto the mean are only 100 km in diameter and have been tracked for up flow. Estimates of the barotropic conversionin the idealized to 2 years, indicative of much weaker dissipation than the numericalmodeling study of Kielmann and estimate for this model. conversionfrom eddy to meankinetic energy of 5.0)<10 -6 The coefficient of vertical viscosity in the momentum J m-z s-•, muchlarger than that foundhere. The lower equationsis 30 cm2/s2 andconstant over the basin. This is conversion rate found in this study may be due to lower 3-10 timeslarger than that whichis typically usedfor primi- horizontalresolution (0.33 ø versus0.1 •) or the interaction tive equationEGCM's. This large valuewas chosen to damp of the Azores Current with the recirculationgyre. It is pos- gravity wave generationwhich occurredon a very shallow sible that the barotropic conversionin the AC is too small shelf along the coast of Brazil, far from this region under in this model but it is not believed that this is a major study. The time scaleof the vertical friction term in the up- contributing factor to the low model per thermocline is approximately 100 days. This is longer than, but comparableto, the horizontal friction term, and againit is muchless than estimatesfor the real ocean.This roughscale analysis is consistentwith the findingthat, away 4.3. Model Parameterizationsand Low KE from the NEC, the horizontal friction accountsfor approx- In this section, several aspects of the model parameter- imately 65% of the total Kz sink and the vertical friction izations which may influence the development and growth accountsfor the other 35%; bottom friction is very small. It of eddies are discussed. It is impossible to say with any is believedthat reducingthe horizontal and vertical dissipa- certainty if these numerical considerationsare the causeof tion usedin this calculation will increasethe eddy variability low Kz. The following analysisis intended only to point to be in better agreementwith observationalestimates. out likely areas which deservefurther investigation. In the The surface boundary condition on wind stress may in- designof a numerical model for a given oceanicapplica- fluence the developmentof eddies in the ocean. The wind tion there of course must be trade-offs. Many of the model forcing has been filtered in both spaceand time. The time parameters used in this first Community Modeling Effort filtering eliminates all frequenciesless than 2 months. The were constrainedby considerationsother than the eddy ki- grid representationof the wind stress field has been sub- netic energy structure. To name a few, computationalre- ject to spatial smoothingthrough the analysisand mapping quirements, suppressionof gravity waves, and the progres- procedureof Hellermanand Rosenstein[1983]. In the real sion from simple to more complex models all influencedthe ocean, variability in the surface wind stress may directly model design. force small-scalevariability in the upper ocean, where it is The horizontal resolution of the model is approximately subject to vertical diffusion and advection, into the ocean 35 km and has been chosento resolvegeographical features interior. The effect of high-frequencywind forcing on the such as the Florida Straits while remaining affordable in generation of eddy variability in the ocean is an important terms of computational requirements.In the Canary Basin, questionfor the future developmentof numerical modelsand the Rossby radius of deformation varies from 60 km in the should be studied. S•'A•:••noN I•/• C.•A•y l•,sI•: A I•'aODF/,/]•A'I'AANALYSIS 9621

The surface boundary conditions on temperature and tional approachhas been to estimatethe time sincea parcel salinity may also influence the developmentof eddies. The of water hasinteracted with the surfacethrough the behav- salt flux at the surfaceis parameterizedthrough a Newto- ior of passivetracers such as oxygen and tritium. Several nian dampingterm which relaxesthe surfacelayer salinity studiesbased on such data have indicated the presenceof back toward climatology. The climatologicalsalinity field unventilatedwater in the easternNorth Atlantic [Jenkins, is smoothedin both spaceand time and will tend to sup- 1987; Sarmiento,1983]. This is in generalagreement with pressthe mesoscaleactivity at the surface.The strengthof the ventilatedthermocline theory of Luyten et al. [1983], this damping will be proportional to the time scaleof the (hereinafterreferred to as LPS83). In this section,the mag- NewtonJanterm. A similar argument appliesto the tem- nitude and mechanismsof ventilation in the Canary Basin perature field, although it is more complicatedbecause the will be calculated and compared with estimates based on relaxation is calculatedby a variable heat flux at the surface observed tracer distributions. rather than a constantdamping coefficient. The damping The means by which ventilation is estimated in the nu- time scalefor the salinityfield is 50 days;this is alsotypical mericalmodel is througha passiveage tracer r. The ageof of the time constantderived through the heat flux relation a water parcel is the time since that parcel has been in con- for the temperature field. tact with the surfaceof the ocean. The age tracer equation The time rate of changeof the eddy availablepotential is exactly like that of the active tracersexcept that an addi- energy(A') may be written in termsof the time rate of tionalterm is addedwhich accumulates time (seeAppendix changeof the perturbation temperature field as A). The behaviorof this passivetracer is quite similar to that of the age derived from the known behavior of tritium , g•T'T( usedby Jenkins[1987], hereinafter referred to as J87. At 15 years of integration the age tracer was started in the model (3) calculation, hence the oldest value of r in the model will be 10 years at the end of the 25 year integration. At each time step, the value of the tracer at the surface is set to zero. c,zxz (4) The age at 91 m is shownin Figure 9a. There is a clear separation between the ventilated water to the north and the where cr = 10-4C -•, • is the vertical derivativeof the unventilatedwater to the south. This southeasternregion is temperature field, TR is the referencevalue of the temper- the shadowzone describedby LPS83. Conservationof po- ature for the surface heat flux, T * is the deviation of the tential vorticity requiresthat water parcels,once subducted, surfacetemperature from TR, Cv is the specificheat, p is leave the coast and are no longer in direct contact with the the densityof seawater,and AZ is the depth of the surface wind forcing. There is some evidenceof an influenceof the layer of water. The valueschosen for the parameterswere Equatorial Current near 10øN but this figure is in general 7/GvpAZ = 1/50 days,T'= løC, and• = 0.03øCm -•. agreementwith basic ventilated thermocline theory. Using thesevalues, the lossof eddy availablepotential en- The agefield at 370 m givesa muchdifferent picture (Fig- ergy throughthe surfaceboundary condition is 4.0 x 10-5 J ure 9b). The front extendingfrom the coastof Africa at m-36 s-• actingover the upper 50 m of water.This is com- 25øN is the shadowzone predictedby LPS83. As would be pared to the generation of eddy energy by the conversion expected, the shadowzone has shifted to the north deeper of meanavailable potential energy of 1.0 x 10-5 J m-3 s-• in the water column. In the northwest corner of the do- acting over the upper 500 m. This indicatesthat the lossof main the water is ventilated due to strong interaction with eddy energythrough the upper boundarycondition may be the winter mixed layer. Between these two regions, there a significantportion of the generationin the upper 500 m are three subregionswhich differ from the smoothgradient by internal instability mechanisms. It is not known if this which would be expected from Sverdrup flow. There is a is a realisticrepresentation of the actual oceanenergy ex- tongueof ventilated water extendinginto the area from the changeprocess but indicatesthat further study in this area west along 30øN, this is due to the AC. There is a second is needed. region which is nearly homogeneousfurther north and to the east,near 38øN.This is in the regionof the meandering 5. OC•A• P•oc•ss•s PC discussedin section3. Finally, alongthe coast,there is In this section, the role of the current structure described ventilatedwater near the coastof Portugal. This is consis- in section 3 will be studied for three fundamental oceanic tent with the southwardextension of the winter mixed layer processes.In subsections5.1, 5.2, and 5.3 the ventilation of near the coast[Levitus, 1982]. the thermocline, meridional temperature flux, and forma- An analysisof tritium fieldswas done in the Beta Triangle tion of the Mediterranean salt tongue will be discussed. regioncentered at 27øN,32.5øW (indicated in the figure)by J87. He found a generalincrease in age from the north to 5.1. Ventilation of the Thermocline the south as is found here. The age varied from 3.5 at the northernapex of the triangleto 7.5 at the southernedge The ventilationof the thermoclineis an important oceanic on the densitysurface •r = 26.85 (approximately390 m). process. Accurate knowledgeof the interaction of subsur- Those estimatescompare quite well with thosefound in the face water with the atmosphere is important for ocean- model calculation.The age was alsofound to increasefrom atmosphere exchange of heat and CO2. Ventilation of 10 yearsat 20øNto 30 yearsat 10øNat 370 m along38øW. the oceanis also intricately tied to the generalcirculation This is in generalagreement with the shadowzone found in through the conservationof potential vorticity [Luyten et the modelfields but quantitativecomparison is not possible al., 1983;Pedlosky et al., 1984]. Direct observationsof ther- becausethe model age is limited to 10 years. mocline ventilation are difficult to measure because it is be- From Figure 9b, it appears that the ventilation of the lieved to occur intermittently in spaceand time. The tradi- main thermoclineis achievedby a mixture of direct Ekman 9622 Seaua•:•'nora n,/TagC.•Amc Brisk: A ]Vr•ODlq'•/DATAANALYSIS

a 40N •L I t I l• a 40

LI.I c3 50N

-• 20N

500

ION IO 40 3Jo io 40W sow 2ow lOW LONGITUDE LONGITUDE (øW)

b 40N

b -200 LI.I c3 50N

-;500 'J 20N

-400 b) ION 40W 50W 20W lOW -500 LONGITUDE

Fig. 9. Age tracer at (a) 91 m and (b) 370 m; contourinterval 0.5 years. SUBDUCTION VELOCIT Y pumpingand horizontaladvection of water renewedby win- Fig. 10. Subductionvelocity: (a) at 260 m (contourinterval tertime convection. This is consistent with the results of 25, contoursscaled by 1.0 x 10-sm s-1 and (b) verticalpro- J87 as he concludedthat the recirculationis effectivelyven- file (1.0 x 10-6 m s-•; A, modelin BetaTriangle; B, modelin tilated "probablyby passagethrough areas of deep winter Azore• Current; C, model in Portugal C•t: D, J87 in Beta Triangle;and E, Ekman pumping. convection.'Although J87 consideredconvection regions only to the north, it appears here that the winter convec- tion in the westernNorth Atlantic is alsoa significantsource of new water via the eastwardflowing AC. A subductionve- hasbeen limited to 5.0 x 10-6 m s-• for presentation.The locity may be estimatedas Wo = fo/fTrz, wheref0 is regionsouth of the shadowzone is not valid for this a•alysis the Coriolisparameter at the latitude of outcrop,fT is the sinceall water in this regionis essentiMlyunventilated over Coriolis parameter at the location of a•alysis, and rz is the the time of model integration. The subductionvelocity is vertical derivativeof the age tracer (J87). This givesan very large in the northwestbecause it is stronglyinfluenced estimate of the total subduction due to both the direct Ek- by wintertime convection. The effects of winter convection ma• forcingand shoalingof the mixed layer. A lessaccurate are also visible along the coast of Portugal. It is the area estimatewhich makes no correctionfor conservationof po- between the northern and south fronts that is of interest. tentialvorticity is writtenas Wo = r• '• . Bothmeasures will Thevalue of Wais 1.26x 10-6 m s-• overmost of the region, be used to study the relative contributions to thermocline however,there are two largedeviations from this basevalue. ventilation. The first placeis alongthe AC, wherevalues are as large The nonpotentialvorticity conserving subduction velocity as 3.0 x 10-6 m s-•. The secondis in the regionof the is shownin Figure10a at a depthof 260 m. The contouring meanderingPC, along36øN, where values are also as large as S•t•: •a•ON •Na• C•i•tY BtmN:A ]•DF•./DATAANALYSIS 9623

3.0x 10-6 m s-• . Thesecompare with an Ekmansubduction its magnitude, meridionaland vertical distributions,source, rateof approximately0.85 x 10-6 s-• for theoutcropping of and degreeof ageostrophy. the • = 26.62 density surface, or at approximately 258 m. The model fields are used to calculate the meridional tem- At this depth, the Ekman pumpingaccounts for 67% of the perature flux as a function of latitude and depth using the total subductionover most of the region but in the areas of direct method. The sectionstaken in this subregionof the large mesoscalecurrents the shoaling of the mixed layer is basin have a net mass transport; as such, the flux is referred dominant. to as a temperature flux instead of a heat flux. Zonal and J87 calculatedr• -• = 1.25 x 10-6 m s-• at the center vertical integrals will be taken in order to distill the large of the Beta Triangle at approximately 225 m. Although he amount of information contained in the three-dimensional did not look into the vertical derivatives over the rest of the fields into a manageableresult. The extent of the zonal in- triangle, there is evidencein his data of increasedsubduction tegration is from 35•W to the coast, chosento match the at the northernmostapex of the Beta Triangle. The value regionof integrationused in previousanalysis of the histor- of rz-• calculatedat 32.5øN,30øW from the data of J87 ical data baseby Strammaand lsemer[1986], (hereinafter indicatesa subductionvelocity of approximately3.25 x 10-6 referredto as SI). The verticalintegration is doneover the m s-• at 225 m. Thesevalues compare quite well with upper 1000 m, again to match the previousanalysis and to the estimates from the age tracer in the model, both in the contain the major components of the meridional tempera- center of the Beta Triangle and at the northern apex. Based ture flux. on the model analysis, the increasedsubduction to the north The three-dimensional velocity and temperature fields in the data of J87 may be evidence of ventilation by the from the time-averagedmodel fields are used to calculate Azores Current. the meridional temperature flux, H, using the direct rela- The vertical profile of the potential vorticity conserving tion subduction velocity is shown in Figure 10b for three loca- tionsin the model(curves A, B, and C), estimatesfrom J87 (curveD), and Ekman pumpingalone (curve E). CurveA H= / / p(7p•vdzdz(5) is placed at the center of the Beta Triangle, at the same where the integration is taken over depth and longitude, p location as the estimates of J87, curve B is in the AC, and is the densityof seawater,C'p is the specificheat capacityof curve C is in the region of zonal jets of the PC. seawater(taken to be constanthere), t• is the potentialtem- perature, and v is the meridional componentof velocity. The Near the surfacein the Beta Triangle, the Ekman pump- eddy componentof the temperature flux has been calculated ing is a significantportion of the total subductionrate in both the model and J87 profiles. The contribution due to (section4.2) and found to be negligiblewhen compared to the mean flux. the Ekman pumping decreaseswith increasingdepth. This The meridional temperature flux is shown in Figure 11a is consistentwith the reduction of Ekman pumping as one as a function of latitude for several flow representations. moves further north. The subduction velocity in the model Negative fluxes are to the south. The curve marked A is Beta Triangle increaseswith increasingdepth such that it the meridional flux of temperature using the full primitive is over 3.0 x 10 -6 m s-• at 500 m. This trend is consistent equation yelpdry field from the model calculation. Curve B with the increasein the depth and horizontal gradient of the is the flux calculated using the geostrophicvelocity derived mixed layer as one movesnorth. The estimatesby J87 also from the model density field, relative to the primitive equa- show a tendency to increase with depth so that it gives the tion velocity at 1500 m. Curve C is the differencebetween same subduction rate as the model at 500 m. The subduc- curvesA and B. Shown in Figure lib are the temperature tion profilein the AC (curveB) is muchdifferent from that fluxes for the same region calculated by SI. In their calcu- in the Beta Triangle. The near-surfacesubduction velocity lations, they used a geostrophicvelocity basedon historical is mostly due to shoalingof the mixed layer from the west, hydrographicdata and an Ekman transport calculatedfrom although the Ekman pumpingis not negligible.The relative the average wind stress field. Triangles denote the total contribution due to shoaling remains almost constant with transport, geostrophicplus Ekman, and circlesand crosses increasingdepth; however,the overall ventilation decreases are the geostrophicand Ekman components,respectively. due to the reduction in Ekman pumping. The AC ventila- tion is much larger than that in the Beta Triangle in the upper 300 m. The AC ventilation does not increase with 4O depth becausethe mixed layer does not go as deep in the western North Atlantic as it does to the north of this region. Curve C, in the PC, has the largest ventilation over the up- 5O per 500 m. Near the surfaceit is comparable to that in the AC, and at 500 m it is comparable to that in the Beta Trian- gle. Although alwaysdue to shoalingof the mixed layer, the 20 upper ventilation is large becauseof the strong horizontal velocity field, and the deeper ventilation is large becauseof the steepeningof the mixed layer depth further to the north. I0 -I.2 -0.8 -0.4 0 0.4 -0.8 -0.4 0 0.4 0.8 5.•. Meridional Temperature Flux Fig. 11. Meridionaltemperature flux (times1015W), 0-1000 m: The importance of the poleward heat transport by the (a) from model(A, total; B, geostrophic;and C, &geostrophic}and atmosphereand oceansis well known. The purpose of an- (b) estimatesfrom Stratamet a,•d Isemer [1986](triangles total; alyzing the heat flux in the model fields is to determine circles,geostrophic; and crosses,Ekman). 9624 SPAI•:•TION IN•241g CANAllY ]•q: A MODEL/DATAANALYSIS

In general, there is reasonable agreement between the An interesting feature is the northward temperature flux at model fluxes and those calculated by SI. At 38øN the to- 32øN between 700 m and 1000 m. This is due to a branch tal modelflux (curveA) and the total flux estimatedfrom of the Azores Current which splits and recirculates to the observations(triangles) are to the south. The modelflux is north near the base of the thermocline. South of the Azores, considerablylarger, -0.65 PW comparedto -0.23 PW, due the flux becomes surface intensified as the Azores and Por- to the larger transport in the PortugM Current producedby tugal Current recirculations turn to the west. The influx of the model than was calculated by S84. The southward flux SACW by the northward flowing NEC is seen down to 350 increases between 34øN and 30øN as the Azores Current en- m between 10øN and 15øN. ters the region from the west and turns to the south. The The geostrophictemperature flux is shownin Figure 12b. model producesa large southward flux of almost 1.1 PW The temperature flux due to the Ekman layer is not present compared to 0.7 PW from SI. The peak produced by the in the geostrophicfields; the transport by the upper ther- model at 29øN is due to the rapid southwaxdturning of the mocline fields continues to increase up to the surface with AC discussed in section 3. Because the recirculation field the region 22øN to 32øN being stronglysurface intensified. calculated from the historical observationsis much broader, Belowthe near- surfacelayer the geostrophicfluxes are quite the maximum in southward temperature flux does not show similar to the primitive equation fluxes. The differencefields up as a narrow peak but is distributed over a larger range are shown in Figure 12c. As discussedearlier, over most of of latitude. Between 26øN and 10øN the model and SI show the region the largest differenceis due to the missingEkman good agreement. The temperature flux continues to drop transport in the geostrophicfields. The upper 35 m showsa off rapidly as the southwardflowing Portugal Current turns positiveflux south of 33øN and a negativeflux north of this to the west and the NEC begins to transport temperature latitude. There are only slight differencesbelow the Ekman to the north. The reversal of the temperature flux occurs layer due to the upper thermocline flow field. In the AC between 18øN and 19øN in the model and at 20øN in SI. At and its recirculation the primitive equation fluxes are a few 10øN the northward flux is a maximum at 0.6 PW in both percent larger than the geostrophicfluxes. the model and SI. It is alsointeresting to look at the differencesbetween the 5.3. Mediterranean Salt Tongue primitive equation and geostrophictemperature flux from the model and compare them to the Ekman transport cal- In this section, the mechanisms of the formation of the culated from the surfacewind stress. The ageostrophiccom- Mediterranean salt tongue will be discussed.Specific ques- ponent of the model flux is due to both the Ekman transport tions to be addressedare: (1) how doesthe salt flux out in the upper ocean and other ageostrophiccontributions be- of the Mediterranean region in the model compare to the low the Ekman layer. The ageostrophicflux in the model observedflux: (2) how doesthis salinityanomaly get into follows very closely the flux due to the Ekman transport, the interior: (3) and why is the modelsalt tonguedifferent curves C and X. North of 32øN the Ekman flux is to the from the observedsalt tongue? south and is much smaller than the subsurface contribution. The Mediterranean basin is a region of net lossof water The similarity of the Ekman flux and the ageostrophiccom- to the atmosphere through evaporation. As a result, there ponent of the model transport indicates that the primary is a larger transport of water into the Mediterranean than ageostrophiccontribution in the model is probably due to returns to the North Atlantic. Because total salt must be the Ekman transport. conserved, the water which flows out of the Mediterranean Estimates of the poleward temperature flux in the North through the Strait of Gibraltar is more saline than the sur- Atlantic have been made by Hall and Bryden [1982]with rounding North Atlantic water, giving rise to the Mediter- the direct method and IGY data at 24ø30'N. Between the ranean salt tongue. Bahamas and Africa they found a geostrophicsouthward There is no exphcit exchangeof water between the North transport of 1.58 PW. At the samelatitude east of 35øW, the Atlantic and the Mediterranean Basin in the model because model produces0.65 PW to the south,or approximately40% the horizontal resolution of the model is not sufficient to of the estimated total southwardtransport. They calculated fully resolve the Strait of Gibraltar. The actual flow into a northward heat flux due to Ekman transport of 0.53 PW the North Atlantic has been estimated to be approximately compared to 0.15 PW in the model east of 35øW. 1 Sv [Brydenand Brady,1988]. This water massinteraction Currents responsiblefor the the integrated temperature hasbeen approximatedthrough the addition of a Newtonian fluxes just discussedwill be further explored by looking at damping term to the tracer equationsin the vicinity of the the distribution in the vertical. The depth dependenceof Gulf of Cadiz, as discussedin section 2. the meridionM temperature flux is shownin Figures 12a, b, The flux of salt relative to 35 ppt has been calculated and c for the primitive equation, geostrophic,and difference through the boundaries of the control box shown in Fig- fields. These figures indicate that the field is meridionally ure 2b. Becauseof the dampingterm, flow leaving the re- inhomogeneousand strongly baroclinic. In Figure 12a there gion is generally of higher salinity than flow entering the are two distinct regimesin the vertical. In the upper 35 m region,giving rise to a net gain in salinity. Over the range there is a layer of very strong temperature flux due to the of Mediterranean water in this region, 500 m to 1500 m, Ekman transport. The PortugalCurrent (at 40øN) trans- the net flux of salinityanomaly is 5.3 x 10•2 kg/year.The ports temperature to the south with local maxima at 75 m anomalousflux of salt through the Strait of Gibraltar has and 400 m. Below 800 m the flux due to the Portugal Cur- beenestimated by Brydenand Brady [1988] to be 50 x 10TM rent is very small. The southward turning of the Azores kg/year. The net salt flux in the modelis only 10% of the es- Current is clearly visible as two maxima in southward flux timated flux out of the Mediterranean. Although the New- at 29øN, one at the surfaceand one at 350 m. Below 350 m, tonian damping term does a good job of reproducingthe the temperature flux due to the AC is strongly baroclinic. observedtemperature and salinity characteristicsin the im- SPALL:•TtON IN• C,•'•Am'BASIN: A ]VJODF/,/rDATA ANALYSIS 9625

ION 20N 3ON 40N LATITUDE LATITUDE

0 L_J_ I I I , ....,k...... i

, 200

400 600

800 - (c}

I000 I ION 20N 3ON 40N

LATITUDE

Fig. 12. Verticalpromes of temperatureaux (contourinterval 5 x 10-3 PW m-•): (a) primitiveequation flux (b) geostrophicflux, and (c) differencebetween primitive equation flux and geostrophicflux.

mediate vicinity of the outflow, it does not result in a suf- the 35.4 ppt contour). Thesefeatures are quite long l]veu, ficient flux of salinity. It is not believed that reducing the at least 2 years. There is a strong correspondencebetween relaxation time would help significantly becausethe salin- these salty filaments and the velocity field, with the high- ity within the damping region is already well represented. salinity water generallyto the right of the maximum velocity Another way to improve the salt flux might be to greatly in- vectors. This circulation may be representative of realistic creasethe size of the damping region. This is not desirable currents associatedwith the formation of the salt tongue or becauseit would reduce the influence of the model dynamics a result of the parameterization of the Mediterranean water and mesoscalevariability, which are the main focus of this outflow. If the climatology of the model oceaninterior is dif- calculation. ferent from the Levitus climatologyto which the model fields There are two different concepts of the mechanism by are being forced in the vicinity of the Mediterranean out- which the large-scale,high-salinity water near the Mediter- flow, a density gradient could result which would generate ranean outflow gets into the interior of the North Atlantic. a geostrophiccurrent between the damped and undamped One theory is that the salinity acts as a passivetracer which regions. Becausethe mean model fields have not reacheda is advected/diffusedinto the interior by the generalcircula- steady state at this depth, it is not possibleto determine if tion field. A second theory is that the Mediterranean water these currents are the result of such an imbalance. is not a passivetracer but has a densityanomaly associated The float data of Price et al. [1986]indicate the existence with it and as a result a geostrophiccurrent which actively of a zonaljet of high-salinitywater in the vicinity of 32'N, advects the Mediterranean water into the interior. 25'W which persistsfor more than I year. Based on these Shownin Figure 13 is the velocity superimposedon the data, they hypothesize that the Mediterranean salt tongue salinityfield at 1125 m in the vicinity of the salt tongue. The does not act as a passivetracer but is dynamically active and field is quite complexwith mesoscalejets and eddiessuper- may have important internal dynamics. From their observa- imposedon the large-scalecirculation. Along 20øW the core tions, it is not clear if the jet originatesfrom the outflow at of the salt tongueprotrudes into the more diluted water to the Strait of Gibraltar or from mesoscaleprocesses within the west in the form of long, mostlyzonal, filaments(i.e., the ocean interior. The model produces a similar structure 9626 S/'An•:•ano/,/l•/a'sm C_•nmc13•Sl•/: A MDDPI./DATAfi•NALYSIS

40N estimates based on observations. The model dynamics have introduced fine horizontal and vertical scalesto the initially coarseclimatological representation of the temperatureand L• salinity fields. Some aspects of the density fields were im- • 35N proved while others were degraded. The two major defi- ciencies of the model fields are the representation of the Mediterranean salt tongue and eddy kinetic energy values • SON which are 2 to 6 times too low. The Canary Basin has been shown to be important to the general circulation through the meridional flux of tem- perature, formation of the Mediterranean salt tongue, and 25N ventilation of the thermocline. The temperature flux was 30W 20W lOW found to have a meridional structure similar to that of LONGITUDE the geostrophicadvection of the climatologicaltemperature field, although the model generallyhad larger equatorward Fig. 13. Velodty and salinity at 1125 m; salinity contour interval transports. The dominant ageostrophiccontribution was 0.05 ppt. due to the Ekman transport in the upper 35 m. This indi- cates that estimates of subsurfacetemperature fluxes based on a geostrophicbalance is probably sufficient. The Newto- without the explicit outflow from the Mediterranean. The nian damping parameterizationof the Mediterraneanout- source of the water for the model jet is the Azores Cur- flow was found to reproduce the water properties near the rent, seenat this depth enteringfrom the westat 30øN.The outflow quite well but producedonly 10% of the estimated Azores Current does not have an anomalous salinity, but flux of salinity anomalyout of the region. It is believedthat part of this flow interacts with the saltier water to the east this low salt flux is at least partially responsiblefor the dif- before forming the westward jet. Although this similarity ferenceseen between the model salt tongueand climatology. does not eliminate the possibility that the observedjet is Dynamically active zonal filaments of high-salinity water are due to some other process,it does indicate that such a fea- proposedas a mechanismto transport salt from the coastal ture can be formed from the mesoscaleprocesses contained region into the interior. Ventilation of the thermoclinewas in this model. studied using a passiveage tracer in the model. In the Beta Several processesrelated to the formation and mainte- Triangle area, the model age and ventilation rates compare nance of the Mediterranean salt tongue are not accurately quite well with estimates based on tritium observations.At representedin the numerical model. As was discussedearlier 370 m there is a mix of ventilation due to the Ekman pump- in this section,there is no explicit flow through the Strait of ing of the basic ventilated thermocline theory and shoaling Gibraltar and the net flux of salt out of the region is much of distant mixed layers due to mesoscalejets and eddies. less than estimated from measurements. In addition, the The Azores and Portugal currents were both found to be horizontal resolution of the model grid is not sufficient to effective means of ventilation in the model. The implication resolvethe lensesof Mediterraneanwater (Meddies)which is that ventilation is a complicatedand inhomogeneouspro- havebeen observed in the area [Armiet al., 1989;Richard- cessinvolving direct and distant wind forcing and large and sonet al. 1989].The Meddiesare on the orderof 100km in mesoscale circulations. diameter and 1000 m thick. Estimates of Richardson et al. Based on this analysis, several recommendationscan be [1989]place the transportof salinityanomaly due to Med- made to the modeling community. A major deficiencyof dies at approximately 20% of the total flux into the North the model fields is the representation of the Mediterranean Atlantic through the Strait of Gibraltar. It is likely that salt tongue. Alternatives to the parameterization of the these processesare important to the dynamical balance in Mediterranean outflow must be studied, including explicit the Mediterranean salt tongue, and as such, the analysisof water exchange,sensitivity to resolution, and form of the the dynamical balance in the model salt tongue has been open boundary conditions. The second major discrepancy limited to the discussionof the zonal filaments as a possible between the model and observationsis the low value of eddy mechanismfor offshoretransport of Mediterranean water. kinetic energy. The horizontal and vertical viscosity,hori- zontal resolution,and surfaceboundary conditionsare sug- gested as topics for future study. A third shortcomingof 6. CONCLUSIONS the model calculation is the location and transport of the Azores Current. The dominant forcing mechanismsof the The Canary Basinregion of an eddy-resolving,general cir- Azores Current are not known, so this result cannot yet be culation model of the North Atlantic Basin has been studied attributed to any aspect of the numerical model. The com- in terms of its mean velocity and density structure, eddy putational expenseof the model precludesextensive physi- variability, and its role in three fundamental oceanic pro- cal/computationalparameter studieson this scale. To fur- cesses.The model calculation was carried out by Bryan and ther investigate the relation of the model numerics to the Holland [1989]as the first CommunityModeling Effort in physical processesjust discussed,simpler models, both nu- the World Ocean Circulation Experiment. The three ma- merical and analytical, must be used to isolate the dominant jor currents flowing into the Canary Basin are the Azores forcing mechanismsand function of the numerical parame- Current, the Portugal Current, and the North Equatorial terizations. These aspectsof the calculation should be bet- Current. The transports and locations of each of these cur- ter understood before the next step in the evolution of this rents in the mean model fields compare reasonablywell with class model. SPA/•:•anot,/n,/a'sm CaNam'Bash,/: A MOt)m/DATAAN•YSIS 9627

These results have described several mesoscale processes In the present mode], the equation of state is representedby not previouslyfound in the observations.If actually present a polynomialfit to the Knudsenformula [Bryan and Cox, in the ocean, many of these mesoscalefeatures may play an 1972]. important role in the fundamentalprocesses studied in this The subgrid-scaleprocesses are parameterized with a paper. As such, and in order to test and improve future second-orderoperator in the vertical and a biharmonic op- model developments,observational programs should con- erator in the horizontal. sider these processesin the design and analysisof future experiments. Does a low-frequency,zonal wave pattern ex- ist superimposedon the Portugal Current? Are dynamically F u = AhX7a(/a)+ A,,t•,, (15) active filaments of salt presentin the salt tongue and, if so, Ah is thehorizontal diffusion coefficient (2.5 x 10•9cm 4 s-•) are they an important mechanismof transport into the in- andAo is the verticaldiffusion coefficient (30cm: s-•). terior? Does a deep, anticyclonic circulation exist over the Canary Abyssal Plain? Is the Central Water Boundary a re- gion of baroclinicinstability? To what extent is the Canary Acknowledgments. The author would like to thamk F. O. Basin ventilated by mesoscalefeatures such as the Azores Bryan and W. R. Holland for helpful discussionsand assistance with the results from the Community Modeling Effort numerical Current? calculation. W. J. Gould is also tha•_ked for providing prelim- inary float data. NCAR is sponsoredby the National Science Foundation APPENDIX A

The equations of motion solvedin the numerical integra- REFERENCES tion of Bryan and Holland[1989] are presentedin this sec- tion. Only a brief summary will be given here; details of the Armi, L. and D. Haidvogel, Effects of variable and anisotropicdif- fusivities in a steady state diffusion model, J. PhiIS. Oceanogr., calculation can be found in the reference. 1•, 785-794, 1982. It is first usefulto definethe advectiveoperator F(/a) as Armi, L., and H. Stommel, Four views of a portion of the North Atlantic Subtropical Gyre, J. PhiIS. Oceanogr., 13, 828-857, 1982. r(,) = + + Armi, L., D. Herbert, N. Oakey, J. F. Price, P. L. Richardson, H. T. Rossbyand B. Ruddick, Two years in the life of a Mediter- Where u = aAt/rn, v = aSt, rn = sec(t•),t• is the latitude, ranean salt lense, J. Phys. Oceanogr., 19, 354-370, 1989. is the longitude,and a is the radius of the Earth. Bryan, F. O. and W. R. Holland, A high resolution simulation of The A, 6/, and z momentum equations are written as the wind- and thermohaline-driven circulation of the North At- lantic Ocean, Paper presented at Hawaiian Winter Workshop: Parameterizations of small scale processes, Univ. of Hawaii, Jan 17-20, 1989. Sponsored by U.S. Office of Naval Research, ., + r(.)- = + r (7) Hawaiian Inst. of Geophysics,and Dept. of Oceanogr. Univ. of Hawaii. Bryan, K. and M.D. Cox, A numerical investigation of the + r(o) + = + F oceanic general circulation, Tellus, 19, 54-80, 1967. Bryan, K. and M.D. Cox, An appropriate equation of state for numeric• models of ocean circulation, J. PhiIS. Oceanoyr., •, Pz - pg = 0 (9) 510-514, 1972. Bryden, H. Poleward heat flux and conversion of available po- wheref = 2f•sin(8), p0 is the meandensity (taken to be tential energy in the Drake Passage, J. Mar. Res., 37, 1-22, unity), and P is the pressure.F u and F ø are parameteriza- 1979. tions of subgrid-scaleprocesses. Bryden, H. L. and E. C. Brady, R. D. Pillsbury, Flow through the The conservationof massfor an incompressiblefluid gives Strait of Gibraltar, Paper presented at Gibraltar. Seminario Sobre Oceanoyrafia Fisica Del Estrecho De Gibraltar, Madtic, the continuity equation Octubre, •4-•8, 1988, SECEG, MaAric, 166-194. Camp, N. T. and R. L. Elsberry, Oceanic thermal response to strong atmospheric forcing, II, The role of one dimensional process, J. PhiIs. Oceanoyr., 8, 215-224, 1978. Equations are written for the active tracers temperature Dickson, R. R. and W. J. Gould and T. J. Muller and C. Maillard, Estimatesof the mean circulationin the deep(> 2000m)layer (T) and salinity($) and the passiveage tracer (A) as of the eastern North Atlantic, Prog. Oceanogr., 14,, 1985, Dietrich, G. and K. Kalle and W. Krauss and G. Siedler, General OceanoyraphiI, John Wiley, New York, 626 pp, 1980. T, + r(T)= r Gould, W. J., Physical oceanography of the Azores front, Prog. Oceanogr., 14,167-190, 1985. Hall, M. M. and H. L. Bryden, Direct estimates and mech•sms of ocean heat transport, Deep Sea Res. •9, 339-359, 1982. s, + r(s) = r (12) Hah, Y. J., A numerical world ocean general circulation model. Part II, A baroclinic experiment, DiIn. Atmos. Oceans. 8, 141- 172, 1984. At + r(A)= r + Hellerman, S. and M. Rosenstein, Normal monthly wind stress over the world ocean with error estimates, J. PhiIs. Oceanogr., whereF T is the parameterizationof subgrid-scale processes 13 1093-1104, 1983. for each of the tracers. An equation of state is used to close Hellerman, S., Charts of the Variability of the wind stre•s over the Atlantic, Deep Sea Res., •6, 63-75, 1980. the system. Jenkins, W. J., H and He in the beta triangle: Observations of gyre ventilation and oxygen utilization rates, J. PhiIs. p=p(T,S,P) (14) Oceanoyr., 17, 763-783, 1987, 9628 SVAU.,:Ov, cx•'nON IN • CaNAm'BasiN: A ]•ODIV/,/DATAANALYSIS

K'•se, R. H. and J.F. Price and P.L. Richardsonand W. Zenk, A Schmitz, W. J. and J. F. Price, P. L. Richardson,Recent moored quasi-synopticsurvey of the thermoclinecirculation and water current meter and SOFAR float observations in the eastern mass distribution within the canary basin, J. Geoph•s. Res., Atlantic near 32øN, J. Mar. Res., 46, 301-319, 1988. 91, 9739-9748, 1986. Siedler, G. and A. Kuhl, W. Zenk, The Madeira Mode water, J. K'•se, R. H. and A. Beckmannand H. Hinrichsen,Observational Ph•ls. Ocean, 17, 1561-1570, 1987. evidence of salt lens formation in the Iberian Basin, journal, Stramma, L. and H. Isemer, Seasonalvariability of meridional J. Geoph•s. Res., 94, 4905-4912, 1989. temperature fluxes in the easternNorth Atlantic, J. Mar. Res., K'•se, R.H. and W. Zenk and T.B. Sanford and W. Hiller, Cur- 4 6, 281-299, 1988, rents, Fronts, and eddy fluxes in the canary basin, journal, Stramma, L. and G. Siedler, Seasonalchanges in the North At- Prog. Oceanogr., 14, 231-257, 1985. lantic subtropical gyre, J. Geoph•ts.Res., 93, 8111-8118, 1988. Kielmann, J. and R.H. K•se, Numerical modelingof meanderand Stramma, L. and H. Isemer, Meridional temperature fluxes in the eddy formation in the Azores current frontal zone, J. Ph•s. subtropical eastern North Atlantic, Deep Sea Res., 33, 209- Oceanogr. 17,, 529-541, 1987. 223, 1986. Levitus, S., Climatologicalatlas of the world ocean,NOAA Prof. Stramma, L., Oeostrophic transport in the warm water sphere Pap., 13, 173pp., 1982. of the eastern subtropical North Atlantic, J. Mar. Res. Luyten, J. R., J. Pedloskyand H. Stommel,The ventilated ther- 537-558, 1984. mocline, J. Ph•ts. Oceanogr., 13, 292-309, 1983, Sverdrup, H. U. and U. W. Johnson, R. H. Fiemining, The Maillard, C. and R. K'•ise,The near sudaceflow in the subtropical Oceans,Prentice-Hall, Englewood Cliffs, N.J., 1087pp.,1942. pyre south of the Azores, J. Geoph•s. Res., 94, 16133-16140, Tomczak, M., An analysis of mixing in the frontal zone of South 1989. and North Atlantic central water off Northwest Africa, Proi. Pedlosky,J., W. Smith and J. R. Luyten, On the dynamicsof the Oceanolaf., 10, 173-192, 1981. coupledmixed layer-thermoclinesystem and the determination Willebrand, J. and J. Meincke, Statistical analysis of fiuxuations of the oceanic surface density, J. Ph•s. Oceanogr., 14, 1159- in the Iceland-Scotland frontal zone, Deep Sea Res. •7,, 1047- 1171, 1984, 1066, 1980. Price, J. F., T. K. McKee, J. R. Valdes, P. L. Richardson, L. Zemanovic, M. E. and P. L. Richardson, J. R. Valdes, J. F. Price, Armi, SOFAR float Mediterranean outflow experiment, 1984- L. Armi, SOFAR float Mediterranean outflow experiment data 1985, Tech. Rep., WHOI-86-31,199pp., Woods Hole Oceanogr. from the second year, 1985-1986", Tech. Rep., WHOI-88-43, Inst., Woods Hole, MA., 1986. 230pp., Woods Hole OceanographicInst., Woods Hole, MA, Richardson, P. L. and K. Mooney, The Mediterranean outflow 1988. -a simple advection-diffusion model, J. Ph•s. Oceanogr., 5, Zenk, W. and B. Klein, M. Schroder, Cape Verde frontal zone, 476-482, 1975. Deep Sea Res., in press, 1989. Richardson, P. L., D. Walsh, L. Armi, M. Schroder, J. F. Zenk, W and T. J. Muller, Sevenyear current meter record in the Price, Tracking three Meddies with SOFAR floats, J. Ph•s. eastern North Atlantic, Deep Sea Res., 35, 1259-1268, 1988. Oceanopt., 19, 371-383, 1989. M.A. Spall, Woods Hole Oceanographic Institution, Sarmiento, J. L., A Tritium box model of the North Atlantic Woods Hole, MA 02543. thermocline, J. Ph•ts. Ocean., 13, 1269-1274, 1983. Saunders, P.M., Circulation in the eastern North Atlantic, J. (ReceivedSeptember 16,1989; Mar. Res., 40, 641-657, 1982. acceptedOctober 14, 1989.)