General Circulation of the Mid-Latitude Ocean: Coupled Effects of Variable Wind Forcings and Bottom Topography Roughness on the Mean and Eddy Circulation.

387

GENERAL CIRCULATION OF THE MID-LATITUDE OCEAN: COUPLED EFFECTS OF VARIABLE WIND FORCINGS AND BOTTOM TOPOGRAPHY ROUGHNESS ON THE MEAN AND EDDY CIRCULATION.

B. BARNIER and C. LE PROVOST lnstitut de Mkanique de Grenoble, BP 53,38402 Saint Martin dHhres, France.

ABSTRACT The aim of the present study is to compare a set of numerical experiments which simulate the general circulation of a subtropical gyre within the same overall conditions. The basic simulation is a double-gyre winddriven experiment conducted with a multi-layer EGCM-QG model. The different process studies include a stochastic wind forcing superimposedon theclassicalantisymmetricdouble gyre wind, and a mesoscale random bottom topography. The results identify the effects of the bottom roughness and the role of a variable wind on the mesoscale eddies and the mean circulation. INTRODUCTION The roleof the bottom topography is recognizedto be important in the dynamics of theturbulent flows which govern theoceaniccirculations.Onemajor effect is thedevelopment of steadycurrents lockedtothetopography, as demonstrated by Bretherton and Haidvogel (1976), Herring (1977), and Holloway (1978). Another major effect, demonstrated by Rhines (1977) in free-decaying turbulence experiments, is the tendency of the topography to act against the nonlinear energy cascade toward large barotropic scales, by transferring energy toward smaller horizontal and vertical scales. This process seems important inthedynamicsof the MODEeddies (Owens and Bretherton, 1978). However, a recent study of Treguier and Hua (1988) shows that, in the case of quasi-geostrophic oceanic turbulence forced by large scale surface wind stress fluctuations, the transfer from large to small scales occurs mostly for the barotropic mode, and that little energy exchange can be observed between vertical modes. Therefore, it seems that in the presenceof bottom topography, the nature of the forcing of the oceanic turbulence is an important factor to consider when studying the dynamics of the flow. In theocean, a major sourceof eddy variability is thetransferof energy from the mean flow and stratification fields to mesoscaletransient motions by baroclinic and barotropic instability processes. The role of these eddies in the general circulation of the Ocean has been intensively studied over the past ten years (Hollandet al., 1983, Evansetal., 1987).HoweverlittlehasbeenreportedontheeffectofbottomtopcgraphyinEddy-resolvedGeneral Circulation Model (EGCM) simulations. Schmitz and Holland (1982) pointed out smaller abyssal energies in an idealized double-gyre EGCM experiment with random small scale topography. Verron et al. (1987) in a similar numerical experiment, and Barnier (1984 and 1988) through analytical and numerical approach, have demonstrated the strong constraint of a large scale, ridge-like, bottom topography, which acts like a polarising barrier to the energy radiated from the intense eddying jet regions, or from wind induced Rossby waves. Very recently, Boning(1988)has reexamined the double gyre case with small scale roughness in a primitive equation model study. One of his conclusions is that such a random bottom topography blocks the energy radiating in the lower layers, leading to a much more depth dependent structure of the flows, especially in areas of weak flows. The goal of the present study is twofold. First, it aims to investigate the influence of a mesoscale random bottom topography on the dynamics of the jet stream itself. To that effect, two classical double-gyre,quasi-

388

(a) Bathymetry

(b) spectrum

Fig. 1. The random bottom topography. (a) The model basin and the bathymetry. The mean depth is 5000 m. Dashed (full) lines indicate levels below (equal or above) the mean depth. Contour interval is 100 m. (b) The wavenumber spectrum of the topography. Wavenumbers are dimensionless, such that k=l corresponds to a 4000 km wavelength. At high wavenumbers the slope is k”.’.

389 geostrophic, EGCM simulations, one with a flat bottom and one with a mesoscale random bottom topography are compared. Second, this study attempts to see if, in the presenceof the topography, the eddy flows generated by the dynamic of the jet are sensitive to transient wind forcings. An experiment similar to the previous one is conducted with a stochastic wind forcing added to the antisymmetric double gyre forcing, and results are compared. 2 THE NUMERICAL EXPERIMENTS 2.1 The experiments

Three experiments are considered. The first is a reference experiment (referredto as LR64). in which the modelisina basicconfigurationwithaflatbottomandasteadyforcing.Thesecondisatopographicalexperiment (referredtoasTRW), inwhichthemodelisinaconfigurationsimilartoLR64, butamesoscalerandomtopography is introduced. In the third experiment (referredto as TWR64) the model is configured like in the topographical case, but a stochastic wind is addedtothe steady forcing. Each experiment requires 10 years of integration (with a 4 hour time step) to reach a state of statistical equilibrium. Then for each case, a further 1500 day run is conductedfromwhichthestatisticsoftheexperimentsarecalculated.Therefore,inthe following, meanquantities

are time-averaged over 1500 days. The results of the experiments are cornpared in section 3: the influence of the bottom topography and of the variable wind on the gyre circulation can be identified since the three experiments have been performed in the same overall conditions. 2.2 Basic configuration of the model

In its basic configuration (experiment LRW), the ocean model is a six-layer, quasi-geostrophic model, with a rigid top and a flat bottom, driven by a steady wind. Details of the model formulation, the geometry, the wind forcing and the nondimensionalparameters that govern the flow can be found in Schmitz and Holland (1986). The basin is rectangular (Lx = 3600 km, Ly = 3200 km), and the forcing is a steady sinusoidal wind stress (0.75 dyn cm-*)which drives a two-gyre ocean. Linear bottom friction and high order biharmonic lateralfrictionprovide thedissipativemechanisms. Thedepthofthesix layers, the jumpindensityatevery interface, thecorresponding internal radii of deformation and all physical and numerical parameters which define the numerical experiments of the present study are shown in Table 1. (The values of the reducedgravity are such that the main thermocline is the third interface, at 1050 m depth). The governing equations are the quasi-geostrophic, nonlinear potential vorticity equations for every layer, coupled by the continuity equation applied at every interface. The model is an eddy-resolvedgeneralcirculation model in the sense that the geometry is basin-size and that the horizontal resolution is fine (20 km). The model isthus ablelodevelopstrong instabilities,and togiveaturbulent pictureoftheoceancirculation.Inconsequence, calculations must be performed over long periods of time, to let the model reach a statistically steady state in which mean and eddy flows are in mutual balance. 2.3 Mesoscale random bottom topography

In the topographical experiment (TRW), a variable bottom topography is used instead of the flat bottom. whereas all the other parameters remain identical to those of the basic experiment (LR64). Statistical characteristics of the sea floor topography have been presented by Bell (1975). The significant features of the wavenumber spectrum are a k-2slope at high wavenumbers (larger than 2d100 km’), and a flattening at lower wavenumbers indicating a lack of 100-km and larger scales.

390

(a) Wavenumber spectrum

€

(b) Frequency spectrum

(c)ECMWF frequency spectrum

Fig. 2. The stochastic wind forcing. (a) The wavenumber spectrum (white, with a cut-off wavenumber of 2dl50 krn.'). As in Fig. 1, wavenurnbers are dimensionless. (b) The frequency spectrum at the center of the basin produced by the Markovianprocess. (c) The frequency spectrum in the middle North Atlantic from the ECMWF data (Mac Veigh et al., 1986).

391 The bottomtopography used in experiment TR64 aims to represent a random mesoscalebottomroughness Fig. 1(b). It roughlyhasthesame (Fig.l(a)). It hasbeenrandomlygeneratedfromtheisotropicspectrumshownin spectral features as pointedout by Bell (1975). The spectrum is flat for wavenumbers (k) rangingfrom 261000 kml to 2d250 kml, and has a k 1 slope for k rangingfrom 26250 to 2d120 km-l.There is no variability at scales largerthan1000kmbecausewe believethat inthis rangeof wavenumbers, thebottomtopography is best defined by deterministic patterns (like the mid-oceanridge), rather than by random features. The short wavelength cut-

off is 120 km. It is larger than for the real sea floor (Bell 1975), but is imposed by our computational grid size of 20 km. Thus we have six grid points to resolve the smallest topographic scale. The rms topographic height is 120 m, which is less than the observations given by Bell (1975), (around 200 m or more), but is required to be consistent with the quasi-geostrophic approximation. TABLE 1 Model parameters. Parameters are common to all experiments, except for those relative to the bottom topography and the time dependent wind forcing. (Symbols are similar to Schmitz and Holland, 1986). Grid scale: Ax = 20 km Zonal Lx = 3600 km Meridional Ly = 3200 km Basin size: p = 2x10-l1m 1sec-’ Bottom friction: E = set' Coriolis parameter: f, = 9 . 3 ~O1 5 secl Steady sinusoidal wind forcing: T, = 0 . 7 5 ~ 1 0m2 ~ secz Biharmonic Friction: A, = 4x10lo m4secl Stratification (Layer number): Layer depth (m): Reduced gravity ( l o 3m sec2): Radius of deformation (km):

1

300

12 38.8

2 350

8.08 18.7

3 400

4

5.24 12.6

500

5 6 1350 2100 4.99 1.17 10.2 9.2

~

Rms topographic height (TR64 and TWR64): 120 m

Rms wind stress curt (TWR64): Z X ~ OPa - ~m-l

2.4 Stochastic wind forcing In experiments LR64 and TR64, the forcing is the curl of a steady sinusoidal wind stress. Inthe experiment with variable wind forcing (TWRM), we add a time dependent curl to the mean curl. All the other paremeters are thoseofthetopographicalcaseTR64,inorderto investigatethecombined effectsof a random bottomtopography

and an unsteady wind forcing on the gyre circulation. The variable wind stress curl is generated with a Markovian process, from an isotropic white wavenumber spectrum (Treguier and Hua, 1988). The spectral features of the variable wind stress curl are shown in Fig. 2. The cut-off wavelength is 150 km. The integral time scale of the Markovian process is 10 days, which yields a white frequency spectrum at periods longer than 30 days, and a w2decay at shorter periods. The wind stress curl rms, determined after Mac Veigh et al. (1986), is 2x10’ Pa m1and gives a spectral levelat low wavenumbers which is quite comparable to that shown by these authors with the ECMWF data (see Fig. 2(c)).

3 MAIN RESULTS OF THE COMPARATIVE EXPERIMENTS 3.1 Instantaneous flow This section comments the main features of the time dependent circulation in the three experiments from instantaneous maps of the upper and lower layer streamfunctions. -0(

.Theupperlayercirculation(Fig.B(a))showstheclassicaldouble

gyre. The jet stream penetrates more than 2000 km in the interior ocean. It shows meanders whose lengthscale

392

LR64

SIRERHFUFICIIBN LAYER 6

Fig.3. Instantaneousmapsof thesurface(a) and bottom(b) layer streamfunctionsforthe flat bottomexperiment (LR64). Contour interval is 2x10' m*sec' for the surface layer, and 6000 m2sec-'for the bottom layer. Dashed contours indicate negative values.

393

(4 TRM

STRERMFUNCTIBN LRYER 6

Fig. 4. Instantaneous maps of the surface (a) and bottom (b) layer streamfunctions for the topographical experiment (TR64). Contour intervalis 2x104m2sec1forthe surface layer, and 6000 rn2sec"forthe bottomlayer. Dashed contours indicate negative values.

394

(a! TWR64

SIRERHFLJNCTIEN LRYER 6

Fig. 5. Instantaneousmaps of the surface (a) and bottom (b) layer streamfunctionsforthe case with the variable wind forcing (MRM). Contour interval is 2x1 O4 m2sec"for the surface layer, and 6000 mzseci for the bottom layer. Dashed contours indicate negative values.

395 is around 500 km and amplitude increases towards the end of the jet. Large eddies are present in the vicinity of the jet. The instantaneous upper layer transport in the jet reaches 54 sv at 500 km off the western boundary. Inthe lower layer (Fig. 3(b)),the jet stream is much weakerand can be identifiedonly over its first 1200 km. Elsewhere, it looks like a turbulent field, with mesoscale eddies and Rossby waves filling the whole basin. However, the eddies seems to be spatially positively correlated with the upper layer eddies, indicating a strong coupling between the eddy flow at all levels. Notice the significant signal in the eastern part of the basin, due to the radiation of barotropic Rossby waves generated by the instabilities of the end of the jet. (ii) p

e lTR6Q . The upper layer circulation (Fig. 4(a)) shows drastic

changes with respect to thr reference experiment. The eastward penetration of the jet is considerably reduced. The jet stream itself is well defined over its first 600 km, then it looks rather chaotic, showing series of mesoscale features, like spliting, eddies and rings, in a much larger number than in LR64. The instantaneous upper layer transport in the jet, near the western boundary, is not significantly changed (50sv). The lower layer circulation (Fig. 4(b)) is also quite different. Significant mesoscale features are now limited to the region of the jet.The correlation with the upper layer eddies is not striking anymore, thus the topography would tend to decouple the upper and lower layers (baroclinisationof the eddy flow). In the eastern basin, the signal is at least three time weakerthan in LR64, which indicates that the random topography is a barrier to the radiation of barotropic waves. It must benotedthatthetopographicscalesseemtobeverymuchpresent inthetimedependentcirculation,

even in the upper layer. (iii)

(TWR64). The upper and lower layer streamfunctions (Fig.

5(a),(b)) are not significantly different lrom the case TR64. The surface jet is still short and chaotic. Concerning the far field (the eastward basinand the regions nearthezonal boundaries), we expect the wind induced currents to modify the local circulation. But this does not appear on the plots presented here, and we need local statistical investigation of the mean and eddy currents to derive quantitative results. 3.2 The mean fields The analysis of the mean fields (streamfunction, mean and eddy energetics, etc...), pointed out several features which illustrate the effectsof the random bottom topography and the stochastic wind forcing on the gyre circulation. In the following we focus on three dynamical features which we believe are most important in the comparison of these three experiments. They are the penetrationof the jet, the baroclinisation or barotropisation of the eddy and mean flows, and the evolution of the far field.

. The upper layer mean streamfunctions for cases LR64 and TR64 are 0) shown in Fig. 6(a) and Fig. 7(a). They illustrate the fact that the bottom topography drastically reduces the eastward penetration of the jet. In the flat bottom case (LR64), the mean jet is straight and reaches the middle

of the basin (Fig. 6(a)), its maximum transport is 52 sv at 500 km off the western boundary, and is still 24 sv 1000 km further. In the topographical case (TR64). the jet is shorter (Fig. 7(a)), and its intensity, as well as the

inertialrecirculation are greatly reduced. The transport at 500 and 1500 km are 24 sv and 10.5 sv, respectively; the maximum transport (33 sv) occurs at 200 km off the western boundary. Notice in Fig. 7(a) the mesoscale meanders inthe mean current, signatureof lower layer mean circulation (Fig. 7(b)).The comparison of the lower layer Streamfunctions for cases LR64 (Fig. 6(b)) and TR64 (Fig. 7(b)) also stresses drastic differences. In the case of a flat bottom, the deep jet is coupled to the surface jet. In the topographic case, the deep circulation is controlled by the mesoscale features of the bottom topography (anticyclonic above hills and cyclonic above

396

Fig. 6. Surface (a) and bottom (b) mean streamfunction forthe flat bottom experiment (LRM). Contour interval is 1O4 m2SeC-'for the surface layer, and 3000 m2sec" for the bottom layer.

397

Fig. 7. Surface (a) and bottom (b) mean streamfunction for the topographical experiment (TR64).Contour interval is 10' m*secl lor the surface layer, and 3000 m2sec1lor the bottom layer.

398

TWR64

lUR64

HERN STRERMFUNCTIBN

MEAN SIRERMFUNCTIBN

LAYER NLMBER I

LAYER NUMBER 6

Fig.8. Surface(a)andbottom(b) meanstreamfunctionforthecasewithvariablewindforcing(TWR64). Contour interval is 10' rn2Sec1lor the surface layer, and 3000 m2sec-'for the bottom layer.

399 hollows), and feeds back the meanders of the surface jet. The analysis of global and local energetics is necessary to understand the new equilibrium reached by the jet in the presence of bottom topography. Global (spatially averaged over the basin) energy budgets are presented in Fig. 9. Comparedto the flat bottom case, the topographic case presents the following discrepancies which may explain the shortening of the jet. For the surface layers (1 to 3), the energy input by the wind in the mean circulation, the kinetic and potential energy of the mean flow (MKE and MPE) are weaker, but the mean kinetic energy of the eddy flow (EKE) is aboutthe same. (In fact, EKE is larger in the jet incaseTR64, but because the jet is shorter, the area of large values of EKE is smaller, and yields an integralvalue of the same order as for case LR64). Furthermore,the energy transfer rate related to the barotropic instability (EKE t MKE) is larger in all three surface layers. Therefore, the jet is problably reduced because it transfers more of its mean kinetic energy to the eddies, via horizontal shear (barotropic) instability. To complete this picture, we need to look at the energetics of the lower layers for case TR64. As shown in the plot of the mean streamfunction of layer 6 (Fig. 7(b)), the bottom flow is controlled by the topography. The eddies generated at the surface by barotropic instability propagate energy downward. They interact with the topography, generating rectified mean mesoscale currents (in Fig. 9 rectification is the transfer MKE, t EKE,). Thus the mean energy is (in case TR64) scattered at smaller scales than in the flat bottom case, and is more efficiently removed by lateral friction (note in Fig. 9that the dissipation of the mean energy in the bottom layers is always largerinthetopographiccase).Theupwardtransferofmeankineticenergyfromthelowerlayerstoward the surface (MKE, t MKE, t MKE, in Fig. 9). is peculiar to the topographical experiment and indicates that the rectified deep currents have a strong barotropic component which constrains the surface flow (the meanders of Fig. 4(a)). We believe that it is the interaction of the bottom flow (controlled by the mesoscale topography), with the surfaceflow (drivenby a basin scale wind forcing), which favours the barotropic instability of the jet stream. The main consequence is a weakening of the jet and of its recirculation, but the level of eddy energy remains high. The case with stochastic wind forcing (TWR64) presents no drastic discrepancy compared to the case with the topography alone. Upper and lower layer mean streamfunctions (Fig. 8(a),(b))are quite similar, differences appearingonly locally. Thevariable wind problably amplifies, near majortopographic features, the eddy currents, and thus enhances the rectification of the deep currents, and therefore modifies the upper mean flow. But those changes are small (the maximum transport in the jet is 34 sv in case TWR64, an increase of 1 sv compared to case TR64). (ii) v

. . n of the &. The diagrams of Fig. 10 and Fig. 11 show the variations

with depth of the kinetic energy of the mean flow (MKE) and of the eddy flow (EKE). Fig. lO(a) sketches local values of MKE at one point located in the most active part of the jet, for each of the six layers. In the flat bottom case, the jet is strongly baroclinic, most of the mean energy being above the thermocline (MKE, is 1200 cm2/sec2 and MKE, is 200 cm2/sec2).In the topographic case, we observe a barotropisation of the jet, in the sense that the mean energy is significantly reduced at the surface and is increased at the bottom (MKE, and MKE, have comparable values, around 500 cm2/sec2).When the variable wind is added, the mean energy increases only in the surface layers. Thus the variable wind seems to reinforce the baroclinic character of the jet stream. Fig.lO(b) is similar to Fig. 1O(a). but for EKE. No drastic modification in the vertical distribution of EKE in the jet is noted between the three cases, except for an increase in the upper layers in the cases with topography. The physical processes responsible for the features of Fig. 10 have been commented in the latest subsection.

400 WINO

2.57

2-05

EKE, 5.04 4.87

0.20

zo 0.50

106.14

se.12

Fig.9. Theglobalenergy diagram for the flat bottomexperiment (italic) andthe topographicalexperiment (bold). Units are in rn(nPsec-*) for the energies (boxes) and lo8 rn(m2sw2)hecfor the energy transfer rates (arrows).Transfersbelow lod are noted 0.00. Negative numbers indicate transfers opposite to the direction of thearrows. Inthat diagram, all quantities are basin-averaged,mean (averagedover 1500days) quantities.MKE and MPE stand for the kinetic and potential energy of the mean flow, and EKE and EPE for the kinetic and potential energy of the eddy flow.

401

Thelargervaluesof MKE inthebottom layers incasesTR64andTWR64areduetotherectifcationofthebottom flow by the topography. The reduction of MKE and the increase of

EKE in the surface layers in cases with

topography are explained by the same phenomenomas for the shorter penetration of the jet. The mean current is reduced becauseit transfers more of its energy to the eddies, via horizontal shear instability, resulting in larger values of EKE and smaller values of MKE. The diagrams of Fig . 11 illustrate the variation with depth of the basin-averaged values of MKE and EKE (denotedJMKEand IEKE in Fig. 11). They must be interpreted with caution, since they average quantities which are not homogeneously distributed in space. These plots confirm that, compared to the flat bottom case, the topographical cases present MKE values which are larger in the bottom layers and lower in the surface layers (Fig. 11(a)). An important feature of Fig. 11(b) is the reduction of the eddy kinetic energy in the bottom layers in the topographical cases. Since this feature is not present in Fig. 10(b),we deduce it is characteristic of the flow outside the region of the jet. Therefore we can say that outside the jet itself, the topography produces a baroclinisation of the eddy flow, in the sense that the eddy activity is reduced under the thermocline. (iii) Evolution of the

.

. The far field is the part of the basin which is far away from the jet and its

recirculation. Basically it covers the eastern basin and the regions adjacent to the southern and northern boundaries. In the flat bottom case (LR64), the variability of the eastern basin is too weak to be realistic; This is a well known failure of the type of model used in this study. As noted in section 3.1, the main effect of the topography on the far field is to block the eastward energy radiation due to the barotropic Rossby waves generated at the extremity of the jet (see Fig. 3 and Fig. 4). Therefore, the level of EKE of the eastern basin is lower in case TR64 than in case LR64, which shows that the random topographic roughness alone cannot compensate for the lack of variance in that region. When the variable wind is added to the random topography (case TWR64). the eddy kinetic energy of the surface layers reaches a level higher than in case LR64, but still too low comparedto observations of the North-Eastem Atlantic. However, complete local energy diagnostics of

caseTWR64are presently undertakenin ordertoquantifythe importanceof thevariable wind and the mesoscale topography in generating mesoscale variance in the eastern basin. Fig. Sshowedthat in the topographical case (TR64).theglobal rateof the baroclinic instability is considerably

reduced (transfer EPE t MPE). Infact, whereasincaseLR64thetransferbetweenthepdentialenergiesis from mean to eddy (indicating dynamics dominated by the baroclinic instability), it is from eddy to mean at every interface, except for the first one in TR64 (indicatingeddy driven dynamics). Howeverthese results are averaged over the basin, and significant discrepanciescan be observed dependingon the area we consider. In experiment

LR64,the regionsof the far field near the zonal boundariesare regionswhere the baroclinic instability is the main source of eddyvariability. The introductionof the randomtopography reducesthe rate of instability. It seems that the topography scatters at mesoscale the available MPE, making the development of the instabilities more difficult, since a growing perturbation might randomly meet favorable or unfavorable conditions of growth over a characteristic topographic length scale (a few 100 km). However this process remains the dominant source of eddy variability in that region. Again, detailed energetics of that region are presently undertaken in order to precisely describe the effect of the mesoscaletopography and transient wind forcings on thedevelopment of the baroclinic instability.

402

Fig. 10. Comparison (in the three experiments)of the vertical distribution of, (a)the mean kinetic energy (MKE), and (b)the mean eddy kinetic energy (EKE), at a point located in the axis of the jet at a distance X of the western boundary. To make the comparison meaningful X has been chosen in each experiment, such that the kinetic energy is maximum in the upper layer. Dashed horizontal lines indicate the interfaces between the layers of the model.

403

0

30

60

JMKE (cm/sec)2 90 120

150

180

150

180

1000-

D 2000E P T 3000H

e

0 TR64

(m)4000- D)

+ TWR64

500G

(4

J EKE 30

0

60

(cm/sec) 2 90 120

1000

D 2000 E P

*

Q

t

0

T 3000H (m)4000.

0 TR64

+ TWR64

5000

Fig. 11. Comparison (in the threeexperiments)oftheverticaldistributionofthe basin-averaged, (a) mean kinetic energy (IMKE), and (b) mean eddy kinetic energy (IEKE). Dashed horizontal lines indicate the interfaces between the layers of the model.

404

4 CONCLUSION

Theresultsofthe present study identify several aspectsofthedrasticinfluenceofa mesoscaletopography roughness on the dynamics of a subtropical ocean gyre model driven by a steady wind. It appears that, underthe thermocline, the flow is controlled by the interaction between the eddies and the

mesoscalefeaturesof the bottomtopography (generatingsignificant mean rectified currents), whereas theupper flow is driven by the large scale wind forcing and the eddies generated by internal instabilities. We believe that the interaction betweenthe surface and the deep flows favours the barotropic (horizontal shear) instability of the upper layer jet stream. In consequence, the eastward penetration of the jet is considerably reduced,the intensity of the jet and its recirculation are weakened, but the level of eddy energy above the thermocline remains high. Therefore, we notice a barotropisation of the jet itself, in the sense that the mean flow is reduced (by shear instability) at the surface and is increased (by rectification) in the bottom layers. Outside the region of the jet itself, an effect of the mesoscale bottom roughness is to reduce the eddy energy inthedeep layers, resulting in a baroclinisationof theeddy flow. This is another indication (afterwunsch, 1981 ; Schmitz and Holland, 1982; and Boning, 1988) of the possible contribution of the bottom topography to

the surface-intensified and weak abyssal eddy energies observed in the interior of the subtropical Ocean gyres. A component of the baroclinisation of the flow far away from the region of the jet (the far field), is the blocking, which occurs in the lower layers, of the energy radiating away from the jet (eddies and Rossby waves). This is another indication that the low-frequencyvariabiliies in the western and the eastern basins of the North-Atlantic Ocean do not have the same origin (Wunsch, 1981). The additiin of a transient stochastic wind forcing to the double-gyrewind pattern reveals several features on the combined effects of a mesoscale bottom roughness and unsteady winds on the gyre circulation. In the present study, only preliminary results have been presented, and a more thorough investigation of the experiments remains to be done. The variable wind does not drastically modify the circulation. It amplifies, near major topographic features, the barotropic rectified mesoscale currents. It also slightly increases the kinetic energy of the upper layer mean flow, acting against the topography (which tends to render the flow more barotropic), in reinforcing the baroclinic character of the jet. In the far field, the variable wind increases the eddy energy of the upper layers, but this is not enough to compare favorably with observations of the eastern NorthAtlantic. However, this is an indication of the possible role of the combined effects of the variable wind forcing and the bottom topography for the surface-intensified nature of the eddy field in the eastern North-Atlantic. ACKNOWLEDGMENTS Support for computations was provided by the Conseil Scientifique du Centre de Calcul Vectoriel pour la Recherche in Palaiseau (France). Fundings came from the CNRS and IFREMER support to the lnstitut de Mkanique de Grenoble through the Programme dEtude de la Dynamiquedu Climat. REFERENCES Barnier. B., 1984. Energy transmission by barotropic Rossby waves across large-scale topography. J. Phys. Oceanogr., 14,438-447. Barnier. B.,1988. A numericalstudy on the influence of the Mid-Atlantic ridge on non linear first mode baroclinic Rossby waves generated by seasonal winds. J. Phys. Oceanogr.. 18,417-433. Bell, T. H.. 1975. Statistical features of sea-floor topography. Deep-sea Res., 22, 883492.

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Boning, C., 1988. Influence of a rough bottom topography on flow kinematics in an eddy-resolving circulation model. J. Phys. Oceanogr., submitted. Bretherton, F. P.. and D. B. Haidvogel, 1976. Two dimensional turbulence above topography. J. Fluid Mech., 78,129-154. Evans, J. C., D. B. Haidvogel, and W. R. Holland,1987. A review of numerical ocean modeling (1983-1986): Midlatitude mesoscale and gyre scale. Rev. of Geophysics, 25,235-244. Herring, J. R., 1977. Onthestatisticaltheoryoftwodimensionaltopographicturbulence.J.Atmos. Sci., 34,17311750. Holland, W. R., D. E. Harrison and A. J. Semtner, 1983. Eddy-resolving numerical models of large scale circulation. In Eddies in Marine Science (A. R. Robinson, Ed.). Springer Verlag. Holloway, G. 1978. A spectral theory of non linear barotropic motion above irregular topography. J. Phys. Oceanogr.. 8,414-427. MacVeigh, J. P., 8. Barnier, and C. Le Provost, 1986. Spectral and EOFanalysisof fouryearsof ECMWF wind stress curl over the North Atlantic ocean. J. Geophys. Res., 92,13141-13152. Owens, W. R., and F. P. Bretherton,l978. A numerical study of midlatitude mesoscale eddies. Deep-sea Res., 225, 1-14. Rhines, P. B., 1977. The dynamics of unsteady currents. The Sea, Vol. 6, Marine Modelling, E. D. Goldberg, 1. N. Mc Cane, J. J. O’Brien, and J. H. Steele Editors - Wiley, 189-318. Schmitz, W. J., and W. R. Holland, 1982. Apreliminarycomparison of selected numericaleddy resolvinggeneral circulation experiments with observations. J. Mar. Res.. 40, 75-117. Schmitz, W. J., and W. R. Holland, 1986. Observed and modeled mesoscale variability near the Gulf Stream and Kuroshio extension. J. Geophys. Res.. 91,9624-9638. Treguier, A. M., and B. L. Hua, 1988. Influenceof bottomtopography on stratified quasigeostrophic turbulence in the wean. J. Phys. Oceanogr., submitted. Verron, J., C. Le Provost, and W. R. Holland, 1987. On the effects of a mid-ocean ridge on the general circulation: Numerical simulation with an eddy resolving ocean model. J. Phys. Oceanogr., 17, 301-312. Wunsch, C., 1981. Low frequency variability of the sea. Evolution of Physical Oceanography, MIT Press, 342-374.