Comparisons between a fine resolution model and observations in the Iberian shelf–slope region

Ž .Journal of Marine Systems 26 2000 53–74www.elsevier.nlrlocaterjmarsys

Comparisons between a fine resolution model and observations inthe Iberian shelf–slope region

Ian Stevens a,), Meike Hamann b,1, John A. Johnson a, Armando F.G. Fiuza bá School of Mathematics, UniÕersity of East Anglia, Norwich, NR4 7TJ, UKb Instituto de Oceanografia UniÕersidade de Lisboa, 1700 Lisbon, Portugal

Received 12 January 1999; accepted 4 February 2000

Abstract

Results from a fine resolution model of the Iberian shelf–slope region are compared with data from cruises conducted inthe same area. The agreement between the model and a cruise carried out in May 1993 is qualitatively good. The model isalso able to represent the seasonal cycle in the area as revealed by cruises carried out in November 1993 and JulyrAugust1994. Quantitative agreement between the model and the observations is less good and this is a result of limitations in thedata used to force the model at the surface and the boundaries. Features of the circulation which are modeled well includeseasonal coastal upwelling, the winter Portugal Coastal Countercurrent and the spreading of Mediterranean Water. Coastalupwelling occurs in summer and is enhanced off Cape Finisterre and Cape Roca. The Portugal Coastal Countercurrent existsin the model as a poleward flow over the upper continental slope which is strongest in winter but persists throughout theyear and in summer is seen as a weak sub-surface feature. Mediterranean Water spreads northwards in two cores with adistinct focussing towards the coast to the north of the Tagus Plateau. The particular form of open boundary conditionsemployed perform well with little evidence of false reflection of outgoing waves. q 2000 Elsevier Science B.V. All rightsreserved.

Keywords: numerical model; coastal upwelling; Mediterranean Water; Portugal Coastal Countercurrent

1. Introduction

The objective of the European Union projectŽ‘MORENA’ Multidisciplinary Research in the East-

.ern Boundary of the North Atlantic was to measure,model and understand shelf–ocean exchange in theeastern boundary of the North Atlantic Ocean. The

) Corresponding author. Tel.: q44-1603-592-600; fax: q44-1603-593-868.

Ž .E-mail address: [email protected] I. Stevens .1 Present address: Institut fur Meereskunde an der Universitat¨ ¨

Kiel, Dusternbrooker Weg 20, 24105 Kiel, Germany.¨

modeling component of the project included a primi-tive equation model of the region aimed at describ-ing the general circulation and hydrography of the

Ž .area. A previous paper, Stevens and Johnson 1997described the model in detail and explored the sensi-tivity of the model to surface and open boundaryforcing. The circulation for a nominal winter andsummer situation agreed qualitatively well with thehistorical view of the circulation in those seasonsalthough no detailed comparison with observationswas carried out. The ocean off the Iberian Peninsularis one of high seasonal variability and is a demand-ing area to model realistically. The geometry of the

0924-7963r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0924-7963 00 00038-5

( )I. SteÕens et al.rJournal of Marine Systems 26 2000 53–7454

region is such that for a model of sufficiently fineresolution the model domain must include three openboundaries. This in itself produces a difficult model-ing problem. However, using a particular form of

Ž .open boundary conditions Stevens, 1990 and ap-propriate surface and boundary forcing the modelproduces realistic results. The model is forced at thesurface by monthly mean wind stress, temperatureand salinity data and is therefore able to reproducethe seasonal cycle in the general circulation pattern.However, the climatological nature of the forcingmeans that the model cannot be expected to provideresults to compare well with a particular cruise car-ried out on any given date.

The observational component of the project in-cluded in-situ measurements obtained on four cruisescarried out in May 1993, November 1993 andJulyrAugust 1994 under MORENA and in Septem-ber 1994 under the Portuguese project ‘FLUXPOR’.The timing of these cruises gives a description of thecirculation throughout the year. An upwelling regimeprevails during the summer, with a southward cur-rent along the Portuguese coast in the upper layer,spreading offshore by filaments and meanders. Dur-ing winter and early spring a poleward current carry-ing warm and salty water is observed by hydrogra-

Ž .phy sections Frouin et al., 1990 , and drifting buoysŽ .Haynes and Barton, 1990 . Below the surface layerCentral water is found composed of water of sub-tropical and subpolar origin. Below a salinity mini-mum the Mediterranean Water is present with anupper core characterised by a temperature maximum

Žand a lower core with a salinity maximum Ambar.and Howe, 1979 spreading along the coast north-

Žwards seemingly with topographic steering Daniault.et al., 1994 .

Of the five MORENA cruises the one carried outŽ .in May 1993 MORENA 1 was the most extensive

in terms of geographical coverage and depth of CTDstations and provided a picture of the spring, pre-up-welling situation. The hydrography and geostrophic

Ž .circulation is described by Fiuza et al. 1998 , here-ínafter referred to as F98. The objective of thecruises MORENA 3A and 3B was to study sub-mesoscale and small scale processes during the up-welling season. Only a few CTD sections werecarried out during 3A and none during 3B. Theremaining two cruises, MORENA 2 in November

1993 and FLUXPORr94 in September 1994 weremore limited in horizontal and vertical extent thanMORENA 1 but do provide information after and atthe end of the upwelling season.

The plan of the paper is as follows. A shortdescription of the model is given in Section 2. InSection 3 the performance of the model is assessedby comparing the model results for May with theMORENA 1 cruise data. Seasonal variability is dis-cussed in Section 4 by comparing the model resultsfor November and August with data from theMORENA 2 and 3A cruises. Section 5 concludeswith a discussion of the model performance andpossible improvements.

2. Model description

A full description of the model is given in StevensŽ .and Johnson 1997 . A review of the model configu-

ration is given here. The model is a primitive equa-tion level model which uses a finite difference methodto solve the equations of motion on an Arakawa ‘B’

Ž .grid, following Cox 1984 . The domain covered is37.58N to 43.758N and 13.678W to 8.58W. The openboundaries are positioned to the north, south andwest in areas of relatively uncomplicated bottomtopography. The bottom topography is taken from

Žthe DBDB5 dataset US Naval Oceanographic Of-.fice, 1983 and is illustrated in Fig. 1. Prominent

coastal features include Cape Finisterre at 438N andCape Roca at 388N. The coastline to the north ofCape Finisterre is modified to be aligned north–southto ensure that the eastern boundary is composedentirely of land points. Smoothing of the bottomtopography is carried out with one pass of a filterdesigned to eliminate two-grid point variation. Thedashed line in Fig. 1 encloses the more limitedregion for which horizontal sections are presentedlater and the dotted line is at 418N along which zonalsections are illustrated. The horizontal resolution has

Žbeen chosen as 1r128=1r128 approximately 9 km.in the meridional direction and 7 km zonally and is

approximately half the typical Rossby radius of 15–20 km. The number of vertical levels is chosen as 32with thickness ranging from 20.3 m at the surface to233 m at depth. This gives good resolution near the

( )I. SteÕens et al.rJournal of Marine Systems 26 2000 53–74 55

Fig. 1. Model domain and topography. Depths in meters. Thedashed line indicates the boundary of the region illustrated in laterhorizontal sections and the dotted line indicates 418N zonal sec-tion.

surface and a maximum depth of 5400 m that iscomparable with depths over the Iberia Abyssal Plain.

Sub grid scale mixing is parameterised using Lapla-cian friction on co-ordinate surfaces with constantvalues for the eddy diffusion co-efficients. For amodel of limited area the use of constant co-effi-cients is appropriate. More complicated schemes suchas biharmonic or isopycnal mixing and mixed layerdynamics are not included as they are not compatibleat present with the open boundary code. The modeluses eddy viscosity coefficients of 10y4 m2 sy1 forvertical mixing and 80 m2 sy1 for horizontal mixing.The diffusion coefficients for tracers including tem-perature and salinity are 5=10y5 m2 sy1 verticallyand 80 m2 sy1 horizontally. The horizontal mixingco-efficients are chosen as low as possible to allowdevelopment of the eddy field while ensuring themodel does not go unstable.

The surface heat and salt fluxes are incorporatedby relaxing the tracers at level 1 to climatological

Ž .data. These are taken from Levitus et al. 1994 forŽ .salinity and Levitus and Boyer 1994 for tempera-

ture. The relaxation fields are a linear interpolationin time between the monthly mean datasets. Thesurface wind stress is obtained as a linear interpola-tion between the monthly mean datasets from

Ž .ECMWF 1993 .

Ž . Ž . Ž . Ž .Fig. 2. Annual time series of temperature from Levitus and Boyer 1994 dashed line and salinity from Levitus et al. 1994 solid line atŽ . Ž .408N, 128W. Data are plotted for the mid date of the month from January 1 to December 12 .


The open boundary conditions are essentially thoseŽ .proposed by Stevens 1990 . The baroclinic veloci-

ties are calculated using a linear form of their inte-rior predictive equations. This provides a reasonableapproximation if the boundaries are placed where thenon-linear terms are small. The boundary tracers arecalculated using their interior equations but allowingadvection only perpendicular to the boundary andwith the addition of a phase speed calculated at theprevious time step. This allows outgoing waves toleave the model domain without reflection. The orig-

Ž .inal Stevens 1990 scheme included a relaxation toclimatological values for the boundary tracers only atthose points where there was inflow. This was suc-cessful for coarse resolution models but caused insta-

bility at the resolution used here due to large pres-sure gradients developing on the boundary whereinflow and outflow persisted at adjacent points. Herewe use a weak relaxation to climatology at allboundary points. The open boundary conditions re-quire that a streamfunction is specified. This is taken

Ž .from the Community Modeling Effort CME modelŽ .of the North Atlantic Willebrand et al., 1994 . The

four seasonal average datasets of the streamfunctionfrom the CME model were interpolated onto theboundary of the MORENA model, and a linearinterpolation in time between these datasets providesthe required boundary streamfunction. Results fromthis model show no spurious barotropic velocitiesalong the open boundaries which gives confidence

Ž . Ž . Ž . Ž .Fig. 3. a Temperature at level 3 57 m on 18 May, year 3. b Salinity at level 3 57 m on 18 May, year 3.


Ž .Fig. 3 continued .

that the CME model streamfunction is appropriate asa boundary streamfunction for this model. For allruns the model was initialised to a state of rest andwith values for the temperature and salinity from the

Ž .annual mean Levitus 1982 climatology. This data,which is on a 18=18 grid is first extrapolated hori-zontally to cover the model domain including landpoints, then interpolated horizontally and verticallyonto the model grid points. Attempts to use the

Ž . Ž .Levitus et al. 1994 and Levitus and Boyer 1994data to initalise the model were unsuccessful due tothe model quickly going unstable. The extrapolationof the 18=18 Levitus data over the steep topographyof the continental slope resulted in large bottompressure torques, slow convergence of the stream-

function and large barotropic velocities. The LevitusŽ .1982 data is much smoother and initialising themodel with this data caused no stability problems.The tracer relaxation data at the open boundaries arethe initial temperature and salinity at the three openboundaries with a relaxation timescale of 360 days.

It is appropriate here to consider the forcing data.The wind stress is taken from ECMWF analyses

Žaveraged for the years 1986 to 1988 ECMWF,.1993 . This data is on a 1.1258 grid and is interpo-

lated horizontally to all model grid points. In Januaryand February the winds are generally from the west,becoming southeasterlies near Cape Finisterre andnorthwesterlies around Cape Roca. In March andApril the winds over the whole region turn north-


westerly and by May they are generally northerlywith wind stresses near the coast at 418N of 0.03 Pa.This pattern of northerlies remains unchangedthrough June, July and August with maximum windstresses in July of 0.1 Pa. In September, the windsrelax in strength and by October a pattern similar toJanuary appears. November is characterised by beingalmost windless except in the southwest corner ofthe region which has light northerlies. December haslight easterlies to the south of Cape Roca and moder-ate southerlies further north. This seasonal cycle ofwind forcing is an average of only 3 years analyses.However, with the exception of November the pat-tern of strong northerly trade winds in the summerand a weaker westerly flow in winter agrees with thehistorical picture.

The seasonal cycle of surface relaxation fields at408N, 128W is illustrated in Fig. 2 for temperature

Ž .from Levitus and Boyer 1994 and for salinity fromŽ .Levitus et al. 1994 . The temperature follows a

smooth annual cycle with a maximum in July and aminimum in January. The salinity, however, has nosimple cycle with a minimum in September and themaximum in October. It is unclear how representa-tive such a cycle is and it may be that the relativelack of salinity observations contributes to the spikydata. There is nothing special about 408N, 128W;similar patterns emerge over the whole region. Thelimitations of restoring boundary conditions were

Ž .highlighted by Pierce 1996 and a method proposedto reduce the phase and amplitude errors in themodel surface tracer fields. For a tracer, which fol-lows an annual cycle and a restoring timescale of 30days, the phase errors are significantly reduced byrelaxing the model to fields approximately 30 daysin the future. This procedure is followed by relaxingthe level 1 temperature to fields which lead by 1month. The errors in the amplitude of the surfacetracer fields are less significant and no correction ismade to the amplitude of the relaxation fields. Thesalinity relaxation fields do not follow a simpleannual cycle and they are relaxed towards the currentmonth.

The model is initialised to annual mean data andthe open boundary relaxation of temperature andsalinity is to the same data. Therefore, there can beno seasonal cycle in the baroclinic inflow and out-flow at the boundary. The seasonal forcing for the

boundary streamfunction does however provide aseasonal variation in the barotropic flow at theboundary. The model is spun up from rest for 2years. At the end of this period the total kineticenergy has settled down and the model velocity fieldis in dynamic balance with the density field. In someareas, the model sharpens the initially smooth Levi-

Ž .tus 1982 tracer fields although overall, the originalpattern remains. During year 3 the model tracer andvelocity fields are output for comparison with theobservations. Running the model for a further yearresults in a similar seasonal cycle to year 3.

The model described here is essentially the caseŽ .‘D’ of Stevens and Johnson 1997 with the Pierce

Ž .1996 scheme for reducing phase errors in the sur-face tracer fields and lower values for the horizontalmixing co-efficients. The sensitivity study of Stevens

Ž .and Johnson 1997 considered only the qualitativeclimatological picture of the circulation in determin-ing the most appropriate boundary forcing. In thispaper, we compare the detailed data from actualcruises with the model results in a quantitative fash-ion.

In Section 4 comparisons are made between thisŽmodel and the DYNAMO level model DYNAMO

.Group, 1997 . Although the DYNAMO project wasa comparison between three primitive equation mod-els with different methods of vertical discretisation,the level model is based on the GFDL MOM codeŽ .Pacanowski, 1995 , which itself is derived from the

Ž .Cox 1984 code used here. It thus provides a usefulcomparison between a relatively coarse resolutionbasin scale model and a fine resolution regionalmodel. The DYNAMO level model covers the wholeNorth Atlantic ocean at a resolution of 1r38, has 30levels in the vertical and includes biharmonic hori-zontal mixing. The MORENA model uses openboundaries to achieve a resolution of 1r128, has 32vertical levels and employs harmonic horizontal mix-ing. Both models use harmonic mixing in the verti-cal. A sensitivity experiment is also carried out withthe DYNAMO model with the inclusion of aKrauss–Turner mixed layer. The initial conditionsand the wind forcing are the same for both models.For the basic run the major differences between theDYNAMO level model and the MORENA modelare the horizontal resolution and the parameterisationof sub grid scale horizontal mixing.


3. Spring data

For comparison with the MORENA 1 cruise,model data is output on 18th May in year 3, themid-date of the cruise. Due to the monthly nature ofthe forcing a snapshot is representative of the circu-lation over a period of a few weeks. The horizontalsections presented cover an area from 398N to 43.58Nand 12.58W to 8.58W. This is a more limited areathan the model domain and focusses on the regioncovered by the cruises. The near surface distributionŽ .57 m of temperature and salinity, shown in Fig. 3aand b indicates the presence of a relatively warm andsaline tongue from around 108W extending north-wards over the slope region. The temperature andsalinity over a more limited area and at the slightlyshallower 50-dbar level from the MORENA 1 cruiseis shown in Fig. 4a and b. The cruise data show asimilar pattern to the model with a warm saline

tongue penetrating northwards above the continentalslope. At 418N, 108W the model is 18C cooler and0.25 fresher than the observations. This is not unrea-sonable as the model is forced at the surface byclimatological data and cannot be expected to repro-duce absolute values found on any particular cruise.In addition, the near surface layer would be expectedto be strongly influenced by the wind and thermalforcing regime in the period leading up to the cruise.The cruise was carried out during a period of strongwest to south-west winds. In contrast, the ECMWFwinds used to force the model are light northerlies inMay.

The velocity field at a depth of 32 m, correspond-ing to model level 2 is shown in Fig. 5. This level isrepresentative of the near surface circulation withoutincluding Ekman effects. On the shelf is a south-wards flow, a result of the northerly winds over theregion during May. This early onset of upwelling

Ž . Ž .Fig. 4. a Temperature at 50 dbar from the MORENA 1 cruise. Bold lines are temperature contours and faint lines bottom topography. bSalinity at 50 dbar from the MORENA 1 cruise. Bold lines are salinity contours and faint lines bottom topography.


Ž .Fig. 5. Velocity field at level 2 32 m on 18 May, year 3.

favourable winds is a feature of the ECMWF dataset.The remains of the poleward Portugal Coastal Coun-tercurrent transporting warm, salty water northwardsis evident offshore of the shelf break although bythis time of year it has become rather broad andweak. Further offshore, beyond 10.58W is the gen-eral equatorward flow of the subtropical gyre. Thegeostrophic flow at 50 dB relative to 350 dB in Fig.6c of F98 also shows a meandering poleward currentbetween 108W and the coast. There is no equator-ward flow over the shelf in this figure, but the windsduring the cruise were generally from the west andsouthwest, in contrast to the ECMWF data.

The vertical structure along a zonal section at418N is shown in Fig. 6. The maximum depth shownin this and subsequent sections is 1287 m rather than

the full depth of the model to highlight features inthe upper ocean to compare with the cruise data. Thevelocity section illustrates the broad, weak near sur-face poleward flow between 10.258W and the shelfbreak at 9.58W with velocities less than 2 cm sy1

and an equatorward flow over the shelf exceeding 5cm sy1. The poleward flowing current carries atransport of 0.12 Sv. At depths greater than 400 mthe flow is also polewards associated with the salin-

Ž .ity maximum of Mediterranean Water MW . Thepoleward transport of MW with salinity greater than35.95 is 1.2 Sv. At depths around 350 m velocitiesare generally small indicating that it is appropriate asa reference depth used in F98 for calculatinggeostrophic currents. Fig. 6 also illustrates well thewater mass structure of the region. The surface layer


Ž .Fig. 6. Zonal sections of northwards V velocity, salinity and temperature at 418N on 18 May, year 3. Northwards velocities are shaded.The dashed lines indicate isopycnal levels.

has a shallow seasonal thermocline of depth 40 m,Ž .also seen in the cruise data F98, Fig. 4a and d .

However, below that there is no remnant of thewinter mixed layer in the model in contrast to the


observations. The question of deep winter mixing isaddressed in Section 4 in relation to the seasonalcycle. The model has a salinity maximum only overthe slope at a depth of 30 m in association with thepoleward current. In contrast, the cruise data has asub-surface salinity maximum throughout the region.It is likely that the limitations of the surface salinityforcing means that the model is unable to reproducethe salinity structure in the surface layer. The isopyc-nal surface s s26.95 was chosen by F98 as theu

lower limit of the surface layer and although in themodel it is not associated with a salinity maximum,it lies at a depth of 100–120 m, comparable to theobservations.

The s s26.95 isopycnal represents the upperu

boundary of the Eastern North Atlantic Central Wa-

Ž .ter ENACW which occupies the main thermocline.This weakly stratified water mass consists of twobranches, one of subtropical origin, ENACW over-st

lying one of subpolar origin, ENACW . The transi-sp

tion between these two branches is defined by F98 atthe s s27.1 isopycnal level. The base of the EN-u

ACW occurs at the s s27.2 isopycnal. In thesp u

model the s s27.1 isopycnal surface at 418N,u

10.58W is at a depth of 230 m, corresponding wellŽ .with the observations in F98 Fig. 8b where it is at

the same depth. Both model and observations havethis surface deepening towards the continental slopein association with the poleward current. The agree-ment is also good for the s s27.2 isopycnal whichu

lies at depths around 430 m in both the model andobservations. At a depth of 393 m which is within

Ž . Ž . Ž . Ž .Fig. 7. a Salinity at level 13 1087 m on 18 May, year 3. b Velocity field at level 13 1087 m on 18 May, year 3.



the ENACW the model velocity field shows a slowsp

southwards spreading of this water mass with speedsless than 2 cm sy1 in accordance with historical dataŽ .Pollard and Pu, 1985 .

In Fig. 6, at depths around 1000 m the highsalinity is the lower core of Mediterranean WaterMW . The salinity maximum is at a depth of 1087 mL

corresponding to model level 13. This compares withthe salinity maximum found at depths of 1150 m at40.258N and 1000 m at 438N by F98. At thesedepths, the model level thicknesses are around 200 mand therefore with this vertical resolution closeragreement with observation cannot be expected. Thehorizontal distribution of salinity at a depth of 1087

Ž .m model level 13 is presented in Fig. 7a. Thesalinity from the cruise at MW level is shown inL

Fig. 8. Over the region illustrated, the salinity maxi-mum occurs at depths between 950 and 1150 m,comparable to the 1087 m depth of the model level13. The salinity distributions in Figs. 7a and 8 showa similar pattern with the higher salinities confinedclose to the continental slope. A focussing of thehigh salinity protrusion along the coast occurs inboth the model results at 40.758N and the cruise dataat 41.258N. However, in the south of the region theactual values do not compare well with observedsalinities being 0.1 higher than the model values.Along a section at 9.758W the meridional salinitygradient is y0.02r100 km in the model comparedwith y0.06r100 km in the cruise data. The modelsalinities at the open boundaries are determined from

Ž .data taken from the annual mean Levitus 1982


Fig. 8. Salinity at the level of the MW core from the MORENAL

1 cruise. Bold lines are salinity contours and faint lines bottomtopography.

dataset. The boundary relaxation data along 37.58Nhas a salinity at 1087 m of 36.2 whereas the observa-tions have a 36.2 isohaline at 40.58N. With the MWspreading northwards in the model and the salinitysignature becoming weaker as it mixes away themodel cannot reproduce the observed salinity valueswith the given open boundary data. The model veloc-ity field at a depth of 1087 m is shown in Fig. 7b.This clearly shows that between 12.58W and 10.58Wthe Mediterranean Water flows northwards beforeturning eastwards to the north of the Tagus plateaucreating the focussing of the higher salinities seen inFig. 7a. The maximum speed in the core of thiscurrent at 408N is 4 cm sy1 rather less than the 6.5

y1 Ž .cm s found by Daniault et al. 1994 from currentmeter data at the same latitude but closer inshore.

Ž .The upper core of Mediterranean Water MWU

appears in the model as a temperature maximum at a

Ž .depth of 700 m Fig. 6 . This core is concentratedcloser to the slope than the lower core and is also

Ž .seen in observations F98; Zenk and Armi, 1990 . AŽ .horizontal section at level 11 700 m in Fig. 9a

illustrates the northwards spreading. The cruise datain Fig. 10 shows the temperature at the MW levelU

which occurs at depths between 650 and 850 m.Both Figs. 9a and 10 show the poleward decrease intemperature with the focussing occurring at 40.758Nin the model and at 41.58N in the observations. Thisfeature which is also present in the lower core is aresult of the northwards flow crossing the TagusPlateau at 398N and veering eastwards to conservepotential vorticity as the water depth increases. Thedepth of the model MW is slightly shallower thanU

observed but is again determined to a certain extentby the choice of model levels. The temperaturemaximum in the model southern boundary data is11.68C at 37.58N and again, because this temperaturemaximum is being eroded as the MW spreadsU

northwards the model cannot have a temperaturemaximum of 11.98C at 408N as in the observations.The velocity field at a depth of 700 m in Fig. 9bshows a similar pattern to the lower core in Fig. 7b.The velocities in the lower core near 408N have amaximum of 2 cm sy1, again rather smaller than the

y1 Ž .3.5 cm s found by Daniault et al. 1994 . Themodel velocities for both the upper and lower coresare about two thirds of the current meter valuesalthough they follow the same pattern with the lowercore velocity being higher.

4. Seasonal variability

In this section we compare model output withobservational data to investigate how well the modelcan reproduce seasonal variability in the hydrogra-phy and circulation.

By year 3 the near surface response of the modelto the seasonally varying surface heat flux has settledinto a repeating pattern. At a position away from thecoast at 408N, 128W a winter mixed layer of depth63 m exists from the beginning of the year until 21st

Ž .March day 80 when levels 1, 2 and 3 becomestably stratified. Convective mixing occurs between


Ž .levels 1 and 2 on November 23rd day 327 resultingin a mixed layer depth of 41 m, increasing to 63 m

Ž .on 27th December day 361 when level 3 mixes.The MORENA 1 cruise in May 1993 found rem-nants of the winter mixed layer down to 150 m andthe MORENA 2 cruise in November 1993 already

Žfound a mixed layer of depth 60–80 m Hamann et.al., 1996 . The shallowness of the model winter

mixed layer may be partially due to a poor represen-tation of the surface forcing, in particular salinity. Inaddition, the DYNAMO model results from a posi-tion in the North–East Atlantic at 408N, 158W has awinter mixed layer depth of 93 m with the constantK vertical mixing scheme but when a Krauss–Turnermixed layer is implemented the mixed layer depthbecomes 180 m.

4.1. Summer situation

Turning now to the upwelling season, the circula-Ž .tion at level 2 32 m on 2 August, the mid date of

the MORENA 3A cruise is shown in Fig. 11. TheECMWF winds are at their strongest from the northduring the months of June, July and August withwind stresses of 0.1 Pa over the region. The modelresponse to the winds includes a generally equator-ward flow between the open boundary at 13.678Wand 10.58W, the Portugal Current which forms partof the subtropical gyre and also a much strongerequatorward flow over the shelf with velocities of 15cm sy1. Separating these two regions is an area ofweak poleward flow. A zonal section at 418N in Fig.12 illustrates this feature. The poleward flow isconcentrated at a depth of 180 m and centred on9.758W, offshore of the shelf break. The polewardflow over the slope in Fig. 12 is continuous from thelevel of the MW to the surface. This contrasts withthe spring situation in Fig. 6 where the polewardflow of MW is separated from the near surfacepoleward countercurrent by an area of equatorwardflow between 400 and 550 m. Examination of thevelocity field at a depth of 393 m throughout theyear reveals that offshore of 10.58W there is alwaysa slow southwards flow of ENACW but betweensp

10.58W and the coast the flow is generally polewardbetween late summer and early spring and equator-ward at other times. The temperature and salinity

sections show upwelling of the s s26.95 isopycnalu

particularly over the shelf, but also the downwelleds s27.1 isopycnal between 9.758W and the conti-u

nental slope associated with the poleward current.This poleward flow and the downwelled isopycnalare not seen in the cruise data, which has upwellingof all the isopycnals between the surface and 500 mand equatorward flow everywhere near the surface.This may be a result of the climatological nature ofthe surface forcing in the model. The original views

Ž .of the poleward current Frouin et al., 1990 was thatit existed only in winter. The MORENA cruisesrevealed that it existed as late as May and becameestablished as early as November. Rather than beinga winter feature it was a non-summer feature. In themodel, the current is most apparent at the surface inwinter but it exists all year round as a weak subsur-face feature.

An average of the model level 2 velocity field forthe months of July, August and September is qualita-tively similar to Fig. 11 but with a maximum equa-torward flow over the shelf of 8 cm sy1. This patternis also similar to the summer climatological surface

Žcurrent in the DYNAMO level model DYNAMO.Group, 1997, Fig. 9.2a but which has an equator-

ward flow over the shelf of only 3 cm sy1. Bothmodels use the same wind forcing and the morerealistic velocities in this model may be due to thebetter horizontal resolution.

Ž .The temperature at a depth of 10 m level 1 on 2August is shown in Fig. 13. This is close to the peakof the upwelling season in the model as measured bythe time of the coldest surface water at the coast. Thecool water is evident all along the coast with theisotherms aligned north–south between 408N and428N where the bottom topography is regular. Offboth Cape Finisterre at 438N and particularly CapeRoca at 398N the upwelling is enhanced. Both areas

Žare positions where cool filaments form Sousa and.Fiuza, 1994 and although the model resolution is´

not sufficient to generate upwelling filaments theeffect of the capes is to enhance the upwelling. At418N the temperatures close to the coast are 38Ccooler than those at 108W in the deep ocean. Thiscompares well with a difference of 3.0–3.58C at the

Žtime of the FLUXPORr94 cruise Hamann et al.,.1996 . Taking an average of the surface temperature

field over the summer months this model gives a


difference of 2.758C between the coast and 108W.The coarser resolution DYNAMO level model gives

Ža poorer result with a difference of 1.58C DY-.NAMO Group, 1997, Fig. 9.3a . The results are not

directly comparable although the DYNAMO modeluses biharmonic horizontal mixing which should pre-serve frontal features.

4.2. Autumn situation

The near surface circulation at a depth of 32 mŽ .level 2 on 24 November, the mid date of theMORENA 2 cruise is shown in Fig. 14. The PortugalCoastal Countercurrent is already well established.The largest velocities which reach 6 cm sy1 are over

the shelf break and extend the whole length of themodel domain. A zonal section along 418N in Fig.15 shows well the countercurrent which extendsfrom the coast to 9.758W and from the surface to 350m. The isopycnal surface s s27.1 shows a pro-u

nounced deepening between 9.758W and the conti-nental slope in association with this poleward cur-rent. This poleward current was first observed by

Ž .Frouin et al. 1990 using data from the MEDPORr2cruise and subsequently by Haynes and BartonŽ . Ž .1990 . According to Frouin et al. 1990 only aboutone fifth of the observed transport of this current canbe accounted for by onshore Ekman convergenceinduced by southerly winds which predominate dur-ing the winter. The remainder of the transport is

Ž . Ž . Ž . Ž .Fig. 9. a Temperature at level 11 700 m on 18 May, year 3. b Velocity field at level 11 700 m on 18 May, year 3.



postulated to be due to a mechanism which is similarŽ .to that proposed by McCreary et al. 1986 for the

Leeuwin Current off western Australia. The pole-ward decrease in sea surface temperature causes ameridional density gradient and consequently a de-crease in surface dynamic height towards the pole.This in turn drives an eastwards flow which inducesdownwelling at the coast and an associated polewardcurrent. The Portugal Coastal Countercurrent is ob-served only in winter as the equatorward winds insummer are strong enough to drive an equatorwardcurrent which overwhelms the poleward current. Thismechanism has been reproduced in the model of

Ž .Stevens and Johnson 1997 , essentially the samemodel as used here. If the model is forced at thesurface with annual mean temperature and salinity

the near surface circulation is generally equatorwardforming part of the subtropical gyre. The result isthat water having the characteristics of the relaxationdata at the northern boundary is advected south-wards. This relatively cool fresh water eventuallyfills the upper ;300 m of the model domain andhas little meridional gradient in temperature or salin-ity. No Portugal Coastal Countercurrent forms in thiscase after the first year. If the surface relaxation is tomonthly mean data the effect of the seasonal cycle isto produce and maintain a meridional pressure gradi-ent in the upper part of the water column. In thiscase, a poleward current is formed over the slope andis maintained in a run of 5 years.

From data obtained on the MEDPORr2 cruise inthe winter of 1983r84 geostrophic transports for the


Fig. 10. Temperature at the level of the MW core from theU

MORENA 1 cruise. Bold lines are temperature contours and faintlines bottom topography.

Ž .current are calculated by Frouin et al. 1990 atselected sections between 38.58N and 41.758N. Thetransports are calculated by integrating the velocitycomponent normal to the section from the surface to300 m and laterally between selected stations. Thehorizontal extent is chosen to capture the core of the

Ž .current and varied from two stations 11 km atŽ .38.58N to five stations 45 km at 41.358N. The

transports showed a trend of generally increasingpolewards from ;0.3 Sv in the south to ;0.6 Sv atthe northernmost sections. The exception to thistrend is a spuriously low value of 0.1 Sv at 408N.Maximum geostrophic velocities were in the range20–30 cm sy1. During the MEDPORr2 cruise thewinds were generally southerly and assuming thatwind stress contributes 20% of the total alongshoretransport these estimates should be reduced by thatamount to compare with the model results here

which do not include any wind stress generatedcomponent due to the almost windless conditions inthe ECMWF dataset for November.

Transports in the model are calculated by integrat-ing the northwards component of velocity over asection spanning 18 in longitude and from the surfaceto 333 m. At 398N the transport is 0.14 Sv, increas-ing to 0.28 Sv at 418N and 0.37 Sv at 438N. Thesevalues are approximately two thirds the geostrophic

Ž .transports calculated by Frouin et al. 1990 . They dohowever follow the same pattern of increasing north-wards. Part of the discrepancy between the modeland the observations can be accounted for in the lackof a wind stress generated component in the modelresults. After allowing for this, the transports in themodel are all approximately 0.15 Sv low comparedto the observations. This may be due to a weakerbarotropic component in the model because of theimposed boundary streamfunction. Also, due to themodel horizontal resolution, the current is not wellresolved with velocities a factor of five smaller thanobserved although this is compensated for to someextent by the current being rather broader in themodel. In addition, the poor surface salinity forcingin the model may lead to a reduced meridionalpressure gradient and hence a weaker poleward cur-rent. However, these results, in addition to the mod-

Ž .elling of Stevens and Johnson 1997 confirm thatthe generation mechanism proposed by Frouin et al.Ž .1990 is correct.

The DYNAMO level model winter climatologicalmean velocity field also shows a poleward current ofwidth ;90 km and maximum speed 3 cm sy1

Žextending the length of the Iberian Peninsula, DY-.NAMO Group, 1997, Fig. 9.4a . This figure only

shows speeds greater than 2 cm sy1 and using thesame criterion, the MORENA model winter meanvelocity field gives a current width of ;60 km andmaximum speed 4 cm sy1. This again reinforces theview that resolution is important in modelling coastalfeatures.

5. Discussion

A fine resolution model has been constructed ofthe Iberian shelf–slope region and the model output


Ž .Fig. 11. Velocity field at level 2 32m on 2 August, year 3.

compared with three cruises carried out in differentseasons. General patterns of the circulation are wellreproduced by the model. However, actual values oftemperature and salinity do not compare well andthis is due to the climatological nature of the modelforcing. The open boundary conditions perform well.They work in an active mode and allow water massessuch as Mediterranean Water to enter the modeldomain as well as allowing waves to leave withoutreflection. Particular features of the circulation in-cluding the influence of topography on the spreadingof Mediterranean Water and the Portugal CoastalCountercurrent are well modelled. Both cores ofMediterranean Water are present in the model resultsand as they spread northwards become constrained

more closely to the continental slope. The existencein this model of the Portugal Coastal Countercurrentsuggests that the generation mechanism involvingthe oceanic meridional pressure gradient proposed by

Ž .Frouin et al. 1990 is correct. The open boundarydata for temperature and salinity in the model areannual mean and therefore cannot reproduce sea-sonal changes. However, the boundary streamfunc-tion data is seasonal and this is reflected in theseasonal signal in the barotropic flow at the bound-ary. An improvement would be to use monthly meantemperature and salinity data at the boundaries. Such

Ž .data is available as the Levitus et al. 1994 andŽ .Levitus and Boyer 1994 datasets. However, the

observations which these datasets are compiled from


Ž .Fig. 12. Zonal sections of northwards V velocity, salinity and temperature at 418N on 2 August, year 3. Northwards velocities are shaded.The dashed lines indicate isopycnal levels.

are relatively sparse and the degree of smoothingimposed along with the coarse resolution means that

there is no improvement over using annual meanboundary data.


Ž .Fig. 13. Temperature at level 1 10 m on 2 August, year 3.

Another deficiency is in the surface forcing. Thewind data are a climatological monthly mean and atthe date of some of the cruises the wind forcing ofthe model is very different from the winds experi-enced during the cruise. The other surface forcing ofconcern is for tracers. The surface relaxation field fortemperature is the most satisfactory though these arealso monthly mean fields which lose some of theextrema of the real forcing. The salinity relaxationfields cause the greatest problems. Due to the sparcityof salinity observations, little confidence can beplaced in the derived monthly means and the nearsurface response of the model suffers. Of the three,the wind data should be the easiest to improve byusing 6 hourly or 12 hourly winds available from theECMWF. However, initial tests ran into stability

problems at the open boundaries when highly vari-able wind forcing was used.

Improvements to the model code would be toinclude an embedded Krauss–Turner mixed layer atthe surface and biharmonic rather than harmonicmixing for scale selective damping. It is not knownhow either of these options would perform in con-junction with the open boundary code. A furtherimprovement would be to include river runoff. Thisis difficult with an open boundary rigid lid modeland has not been pursued as the effect would belimited to the winter season close to the coast.

The upwelling season off Iberia is characterisedby cool filaments extending far offshore from the

Žcoast as revealed by satellite observations Sousa and.Fiuza, 1994 . The upwelling in the model is en-´


Ž .Fig. 14. Velocity field at level 2 32 m on 24 November, year 3.

hanced off Cape Finisterre and Cape Roca but thereis no evidence of filament like structures. The natureof the baroclinic instability process which generatesthese filaments suggests that even a resolution of1r128 is not sufficient to model these features. Testswith a channel model have generated realistic fila-

Žments if the horizontal resolution is 1r488 ap-proximately 2 km or about 1r10 of the Rossby

.radius in the region .One advantage of models is their cost effective-

ness. For example, the surface temperature signal ofcoastal upwelling can be observed using remotesensing on a daily basis but the vertical structure

which is provided by in-situ observations are expen-sive to obtain and therefore a less comprehensivetemporal coverage results. This paper demonstratesthat models are becoming more realistic and forexample, a day to day simulation of the upwellingprocess is easily obtained. Along with the usualfields diagnostic results such as the vertical velocitycan be generated which may be of interest to thebiological community.

In spite of the limitations the model is verysuccessful at reproducing the major features of thecirculation and including seasonal variability. It islikely that more realistic results would be obtained if


Ž .Fig. 15. Zonal sections of of northwards V velocity, salinity and temperature at 418N on 24 November, year 3. Northwards velocities areshaded. The dashed lines indicate isopycnal levels.

the model had a better vertical resolution and im-provements to the code such as a Krauss–Turner

mixed layer. Along with model developments, amore important and possibly more challenging re-


quirement is for better forcing data. This modelshould therefore be regarded as a step towards morerealistic modelling of regional seas.

Acknowledgements

The MORENA project is financed by the Euro-pean Union MAST II programme under contractnumber MAS2-CT93-0065.

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Documents

Comparisons between a fine resolution model and observations in the Iberian shelf–slope region