PUBLICATIONS. Journal of Geophysical Research: Oceans

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1 PUBLICATIONS Journl of Geophysicl Reserch: Ocens RESEARCH ARTICLE./3JC96 Key Points: Roust reltionship etween: density, locl WSC, nd AMOC Density fluctutions drive the UMO trnsport t sesonl/internnul time scles Correspondence to: A. Duchez, A.Duchez@noc.c.uk Cittion: Duchez, A., E. Frjk-Willims, N. Cstro, J. Hirschi, nd A. Cowrd (), Sesonl to internnul vriility in density round the Cnry Islnds nd their influence on the Atlntic meridionl overturning circultion t 6 N, J. Geophys. Res. Ocens, 9, 3 6, doi:./ 3JC96. Received 9 SEP 3 Accepted FEB Accepted rticle online 5 FEB Pulished online 3 MAR Sesonl to internnul vriility in density round the Cnry Islnds nd their influence on the Atlntic meridionl overturning circultion t 6 N Aurelie Duchez, Elenor Frjk-Willims, Ntli Cstro,Jo el Hirschi, nd Andrew Cowrd Deprtment of Mrine Physics nd Ocen Climte, Ntionl Ocenogrphy Centre, Southmpton, UK Astrct The meridionl interior flow otined from the RAPID rry is determined y horizontl density fluctutions t the estern nd western oundry of 6 N. The physicl cuses of these density vritions re responsile for fluctutions in the Atlntic Meridionl Overturning Circultion (AMOC) nd through it, the meridionl het trnsport of the Atlntic. In this modeling study, high-resolution ocen model is used to investigte the source nd origin of the AMOC vriility ssocited with the density fluctutions t the estern oundry. The AMOC in the model is in good greement with the RAPID oservtions nd ppers to dequtely represent the smller scle fetures of vriility round the Cnry Islnds. In this pper, we identify roust reltionship etween the density structure south of the Cnry Islnds, the locl wind stress curl (WSC) round these islnds nd the AMOC using n empiricl orthogonl functions nlysis, wvelet trnsform, nd wvelet coherence. We find tht the deep density fluctutions t the estern oundry of 6 N rise from the pumping effect of the sptil pttern of WSC south of the islnds. These deep density fluctutions drive the AMOC oth on sesonl nd internnul time scles, through their influence on the sinwide tilt of the thermocline. At sesonl time scles, the density fluctutions south of the islnds re driven y the WSC nd directly influence the AMOC. At internnul time scles, significnt coherence is found etween the density fluctution nd the southwrd Upper Mid-Ocen (UMO) trnsport lthough the origin of these density fluctutions is not explined y the direct pumping cused y the WSC.. Introduction The ocen nd tmosphere redistriute het round the Erth. At 6 N, the Atlntic Meridionl Overturning Circultion (AMOC) ccounts for most of the totl northwrd ocen het trnsport in the Atlntic ( PW) [Johns et l., ], more thn 3% of the totl het trnsport from the tropics to the poles [Gnchud nd Wunsch, ; Hll nd Bryden, 9; Trenerth nd Solomon, 99]. Chnges in the AMOC nd ssocited het trnsport could hve severe consequences for Europe s climte [Velling nd Wood, ]. The AMOC trnsport vries on ll time scles. In the short term, while modeling studies hve shown resonle representtion of the AMOC, the vriility tends to e underestimted either on sesonl or internnul time scles [Mtei et l., ; McCrthy et l., ; Roerts et l., 3]. Long-term climte chnge projections of the AMOC strength show wide rnge of possile ehviors, perhps due to imperfect representtion of AMOC driving mechnisms [Bigg et l., 3; Stouffer et l., 5; Zickefeld et l., 7] nd lck of dt records to quntify long-term vriility [Knzow et l., ]. On millennil time scles, Gnopolski nd Rhmstorf [] suggested tht the vriility could e ssocited with unstle ice sheets in the pst. On multidecdl time scles, the AMOC het trnsport vriility is linked with the North Atlntic Oscilltion (NAO), which is responsile for the tmospheric het flux vriility [Delworth nd Grettch, ; Eden nd Willernd, ]. On internnul nd shorter time scles, the AMOC vritions re cused y oth fluctutions in the density field nd in the wind stress [Hirschi nd Mrotzke, 7; Chidichimo et l., ; Knzow et l., ]. On very short (sudily) time scles, the AMOC my undergo lrge oscilltions due to ner-inertil grvity wves [Blker et l., ]. In order to identify fluctutions on decdl or longer time scles, we must first understnd the short term vriility of the AMOC. The RAPID-WATCH/MOCHA rry (herefter referred to s the RAPID rry) hs een monitoring the AMOC t 6 N since, where the AMOC is computed s the sum of the Florid Strits trnsport (FST), DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 3

2 ./3JC96 Ekmn (EKM) trnsport cross 6 N, nd the mid-ocen trnsport ove the thermocline (clled Upper Mid- Ocen: UMO trnsport), etween the Bhms nd Africn cost [Ryner et l. ; McCrthy et l., nd references therein]. Since the dvent of the RAPID rry, the sunnul vriility of the AMOC hs een topic of interest. Buoyncy forcing is expected to drive density vritions on sesonl time scles [K ohl, 5], while eddies nd Rossy wves my dd stochstic vriility to the AMOC [Getzlff et l., 5; Knzow et l., 7; Hirschi et l., 3]. Wind stress vriility ffects the AMOC through the trnssin Ekmn trnsport s well s upwelling effects t the oundries. K ohl [5] show tht Ekmn upwelling t the costs ws responsile for more thn 7% of the vriility due to winds. Chidichimo et l. [], however, find tht sesonl density fluctutions were coherent down to m t the estern oundry, well elow the typicl Ekmn upwelling depths. Insted, wind stress curl (WSC) t the estern oundry ws implicted in the genertion of roust sesonl cycle of the AMOC t 6 N[Chidichimo et l., ; Knzow et l., ]. In prticulr, Knzow et l. [] showed tht while oth the estern nd western oundry density nomlies contriute to the sesonl cycle of the AMOC, only the estern oundry sesonl cycle ws consistent in mplitude nd phse. The oserved density fluctutions were in qudrture with loclized surfce wind forcing (i.e., the WSC led the density nomlies y roughly 9 or 3 months: Figure 6 in their pper), which is expected for upwelling [K ohl, 5; Knzow et l., ]. More recently, Mielke et l. [3] show tht high-resolution numericl model dequtely cptured the sesonl cycle of the AMOC t 6 N, when compred with RAPID oservtions, ut tht t N, the modeled nd oserved sesonl cycles were out-of-phse. The current study uilds on the work of Chidichimo et l. [] y investigting the reltionship etween the AMOC nd density fluctutions t the estern oundry of 6 N. Here we use n eddy-resolving numericl ocen model simulting the 97 period to study the estern oundry dynmics seen in the RAPID oservtions in roder sptil nd temporl context. The model is / simultion performed with the Nucleus for Europen Modeling of the Ocen (NEMO) model which is descried in section 3. Section descries the physicl processes t the estern oundry of 6 N, section descries the link etween the density structure t the estern oundry, the WSC, nd the AMOC. Finlly, section 5 summrizes the min results of this pper.. Ocen Circultion in the Region of the Cnry Islnds The Cnry Islnds rchipelgo consists of seven islnds, seprted y smll chnnels in the North Atlntic just off the Africn continent. The NW Africn cost is rodly known s n upwelling region with permnent southwrd winds. The prticulrs of upwelling re determined y topogrphy, wind stress vriility, nd strtifiction [Nykjer nd Cmp, 99; Knoll et l., ; Mrcello et l., ]. The Cnry Islnds ct s prtil rrier to the Cnry Current, n estern oundry current flowing southwrd s prt of the sutropicl gyre, nd re mjor source of mesoscle ctivity [Brton et l., 99; Sngretl., 9]. The Cnry Current continues through the Lnzrote pssge, which hs een source of locl trnsport studies for some time now [Sngretl., 9; Frile-Nuez et l., ; Mrcello et l., ]. These islnds generte smll-scle vriility in the winds nd their curl round the islnds [Chvnne et l., ]. This region is lso chrcterized y the presence of tongue of fresh wter south of the Cnry Islnds, clled the Cpe Juy upwelling filment (Brton et l. [99] nd section.). The genertion of the upwelling filment is not well understood, ut oservtions suggest tht it is relted to the winds [Brton et l., 99], depends on the topogrphy [Brton et l., ], nd directly intercts with the islnd-generted mesoscle eddies [Mrcello et l., ]. Previously, costl upwelling nd the circultion round the Cnry Islnds hve een the suject of mny studies [e.g., Mittelstedt et l., 99; Hernndez-Guerr et l., 993; Arıstegui et l., 99; Brton et l., 99; Knoll et l., ; Brton et l., ; Mchın et l., 6; Sngretl., 9; Frile-Nuez et l., or Mrcello et l., ]. Here, we re concerned with the density fluctutions which my impct the trnssin AMOC rther thn with the locl circultion, though the two re not independent. In this pper, our re of interest is represented on the ottom left hnd corner of Figure nd is centered round the Cnry Islnds. Prticulr ttention will e pid to the costl re south est of the islnds where the four min estern oundry RAPID moorings re locted: EBH (7.6 N,. W, m), EBH3 (7. N, 3.7 W, m), nd EBH/5 (7. N, 3.55 W, ). The loction of these moorings is represented y DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved.

3 ./3JC96 Ltitudes N 3 N N 3 N 9 N N 7 N 6 N 5 N ORCA Atlntic Ocen /5 3 9 W 7 W 5 W 3 W Longitudes Cnry Current 3 W W ORCA 3 N 9 N N 7 N 6 N 5 N 9 W 7 W 5 W 3 W c ORCA5 3 N 9 N N 7 N 6 N 5 N 9 W 7 W 5 W 3 W Figure. Bthymetry of ORCA nd ORCA5 in the sutropicl North Atlntic. () The Cnry current is schemticlly represented y two lck rrows. The picture on the left hnd corner of Figure represents zoom on the re of interest centered round the Cnry Islnds. The three red tringles represent the loction of the four estmost moorings of the RAPID rry: EBH, EBH3, nd EBH/5 (the two different loctions of these moorings re not distinguishle t this scle). The loctions nd depth rnge of these moorings re given in section [Ryner et l., ]. Figures nd c compre the thymetry round the Cnry Islnds in () ORCA nd (c) ORCA5. The mooring positions re lso represented on these figures. red tringles on ech pnel of Figure. This re is strongly influenced y the costl upwelling nd the Cpe Juy upwelling filment. 3. Dt nd Methods 3.. Model Output The glol ocen-se ice model simultion used in this study ws performed with the NEMO code (Mdec nd the NEMO Tem ) in the glol ORCA configurtion set up in the DRAKKAR project ( drkkr-ocen.eu) [Brnier et l., 6; DRAKKAR-Group, 7]. The horizontl resolution of the configurtion grid is / ( grid points). At 6 N, the resolution is pproximtely km, ecoming finer t higher ltitudes. At this resolution, the configurtion is eddy resolving t 6 N. The ORCA configurtion used to run the simultion ws developed from the NEMO 3. version nd uses the ORCA tri-polr grid (one pole is locted in Cnd, the other one in Russi, nd the lst one t the South pole, Mdec nd Imrd [996]). This configurtion hs 75 verticl levels with grid spcing incresing from m ner the surfce to m t 55 m. Bottom topogrphy is represented s prtil steps nd derived from ETOPO (Ntionl Geophysicl Dt Center, Ntionl Ocenic nd Atmospheric Administrtion, U.S. Dept. of Commerce, 6, The simultion used in this study ws run t the Ntionl Ocenogrphy Centre in Southmpton (NOCS) with the NEMO-OPA9 code for the ocen dynmics component nd output is stored s 5 dy verges. This simultion is referred to s ORCA3-N in the DRAKKAR dt set ut will simply e referred to s ORCA in this pper. It ws initilized with Levitus [Atls et l., ] in 97. Here we use the output from 9 to (the two first yers eing removed to reduce the effects of the initiliztion stge). The DFS. (97 5) nd DFS5 (6 ) surfce forcing functions (internnul tmospheric forcings) re used nd hve een developed y the DRAKKAR consortium. They hve horizontl resolution of.5.as detiled in Brodeu et l. [], DFS comines elements from two sources: the CORE forcing dt set [Lrge nd Yeger, ], from which precipittion, downwrd shortwve, nd longwve rdition re extrcted nd the ERA renlysis (for the period 95 ) followed y the ECMWF renlysis (from to ) DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 5

4 ./3JC96 Trnsport (Sv) Trnsport (Sv) RAPID ORCA ORCA5 ORCA Jn Fe Mr Apr My Jun Jul Aug Sep Oct Nov Dec RAPID ORCA ORCA5 AMOC Climtology, RAPID period Jn Fe Mr Apr My Jun Jul UMO Climtology, RAPID period Aug Sep Oct Nov Figure. () Comprison etween the sesonl cycle of the AMOC in RAPID (in lck), in ORCA (in red), ORCA5 (in green), nd ORCA (in lue). () Comprison etween the sesonl cycle of the UMO trnsport in RAPID (in lck), in ORCA (in red), nd ORCA5 (in green). Monthly men nomlies re represented in these three figures over the period April to Decemer for ORCA nd the RAPID oservtions (April to Decemer 7 for ORCA). which provides m wind, m ir humidity, nd m ir temperture. To compute turulent ir/ se nd ir/se-ice fluxes during model integrtion, the ulk formule proposed y Lrge nd Yeger [] re used. The frequency of DFS is monthly for precipittion, dily for rdition, nd 6 h for turulent vriles. The two lower resolution simultions used in this pper (Figure ) re lso extrcted from the model NEMO, use n internnul tmospheric forcing, nd were run t the NOCS. The / simultion ws run using the ORCA5 configurtion nd is referred to s ORCA5-VN6ERA in the DRAKKAR dt set (it will e referred to s ORCA5 in this pper). This simultion covers the period Jnury 99 to Mrch nd output is stored s 5 dy mens. The surfce forcing for this simultion is supplied y the ERA Interim forcing [Dee et l., ] with horizontl resolution of the tmosphere of.5. The lowest resolution simultion is clled ORCA in this pper nd is referred to s ORCA-N3 is the DRAKKAR dt set. It covers the period Jnury 9 to Decemer 7 nd output is stored s monthly mens. It is forced y the tmospheric forcing CORE [Lrge nd Yeger, ] with the sme horizontl resolution of.5 nd monthly men vlues re ville. To compre the sesonl cycle of the AMOC nd UMO trnsport (Figure ) in these three NEMO simultions, the period April to Decemer is used for ORCA, ORCA5, nd the RAPID oservtions. Model output from April to Decemer 7 is used for ORCA (Figure ). The sesonl cycle ws clculting using monthly mens, with the exception of the sesonl cycle of density nomlies displyed in Figure 3, which is using 5 dy increments. Dec 3.. Model Vlidtion We hypothesize tht to represent the fine-scle circultion round the Cnry Islnds, high-resolution eddy resolving numericl simultion is needed. The islnds of the Cnry rchipelgo s well s the thymetry ner the Africn cost re etter reproduced in the ORCA thymetry thn t corser resolutions. Figure compres the representtion of the thymetry round the Cnry Islnds in ORCA ( nd ) nd ORCA5 (c). The Cnry Islnds re sent from the thymetry of the ORCA configurtion (not shown in this pper). Using three NEMO simultions with different horizontl resolutions: /, /, nd (Figure ), we find tht the sesonl cycle of the AMOC is in good greement etween RAPID nd NEMO. We notice tht the sesonl cycle in the ORCA simultion is in etter greement with the oservtions thn the two lower resolution simultions. In the three NEMO simultions s well s RAPID, the sesonl cycle of the AMOC is chrcterized y minimum round Ferury Mrch nd plteu from July to the end of the yer. During this second prt of the yer, the AMOC is mximum in August in the oservtions nd month efore in the NEMO runs. Although locl mximum is lso found in the simultion in July, we notice tht its sesonl cycle peks in Decemer. Detils on the influence of the model horizontl resolution on the AMOC vriility will e ddressed further in the discussion. The min difference etween the sesonl cycle of DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 6

5 ./3JC96 Depth (m) Depth (m) 6 6 EBH/5 EBH3 Jn Apr Jul Oct EBH/5 EBH3 Dec Jn Fe Mr Apr My Jun Jul Aug Time (months) Sep Oct Nov Dec Figure 3. Sesonl cycle of in situ density nomlies (in kg m 3 ) for () RAPID oservtions nd () the equivlent for ORCA. The period is used for RAPID oservtions while dt from 9 to re used for ORCA. In oth cses, the climtologies re computed using 5 dy mens. For oth pnels, the density profile is mde of densities from the moorings EBH/5 (7. N, 3.55 W) for the top m, deeper densities (down to m) come from the mooring EBH3 (7. N, 3.75 W). the AMOC in RAPID nd NEMO is the mplitude of the minimum in Ferury Mrch, which is underestimted (y out Sv) in the NEMO simultions. Although the AMOC trnsport in the RAPID oservtions is minimum in Mrch, this minimum is reched in Ferury in the three model simultions, which corresponds to sme minimum in the Ekmn trnsport in Ferury in the model output nd Mrch in the RAPID oservtions. This difference in the Ekmn trnsport is prtly due to the use of different wind products in RAPID (ERA interim product: Dee et l. []) nd in the NEMO simultions (DFS-5). Figure compres the sesonl cycle of the UMO trnsport in the RAPID oservtions nd in the / nd / NEMO simultions. The UMO trnsport ws not clculted for the simultion, since Figure did not show ny prticulr improvement etween the sesonl cycle of the AMOC in the / nd simultions. The UMO trnsport is the southwrd meridionl trnsport flowing ove the thermocline est of the Bhms nd west of the Cnries nd is defined s the geostrophic flow in the upper m. It is primrily governed y the density vritions t the western nd estern oundries of the sin. In prticulr, vritions in the density t the estern oundry round the Cnry Islnds re directly ssocited with fluctutions in the UMO trnsport, nd through it, the AMOC [Knzow et l., ]. Using the model simultion, the UMO trnsport is inferred from zonl density grdients nd the therml wind reltion [Hirschi et l., 3]. The full density section, rther thn oundry densities, is used in the clcultion, ut Hirschi nd Mrotzke [7] hve shown tht this mkes no difference to the clculted geostrophic flow. Depthverged velocity is removed to get verticl sher. The result is then integrted zonlly nd verticlly. More DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 7

6 ./3JC96 detils out the clcultion of this trnsport using RAPID oservtions re provided in section 3.3. Since the totl UMO is southwrd, positive UMO nomly corresponds to wekening of this trnsport while negtive nomly corresponds to n incresed UMO trnsport. For oth the oservtions nd the NEMO output, the sesonl cycle of the UMO trnsport hs the wekest southwrd trnsport round Octoer nd the strongest round Mrch or April [Knzow et l., ] lthough the mplitudes of these extrem re underestimted in the model output. As for the AMOC trnsport, we notice tht the mximum in the UMO trnsport is reched in Mrch in the RAPID oservtions ut in April in the model simultions. In order to understnd the verticl structure of the sesonl cycle of deep densities, in situ density nomlies in the wter column re extrcted t single mooring loctions nd the sesonl cycles re presented in Figure 3. In the RAPID clcultion, the density profile in the est is composed of densities from the moorings EBH/5 for the top m; deeper densities (down to m) re from EBH3 (see Figure for mooring positions). As the shllowest instruments of the RAPID moorings re deeper thn m, we compre density nomlies etween m nd m. The longest ville period (9 ) is used for the density profile from ORCA (Figure 3), while the period is used to compute the density profile in the RAPID oservtions (Figure 3). Although the comprison is done t single mooring loction (or single grid point for ORCA), the pttern of density in ORCA (Figure 3) is similr s the RAPID oservtions: stronger density nomlies re found ove 6 m with two mxim, one round April My, the second one round August Septemer, nd minimum round Novemer Decemer. Deeper densities nomlies re lso similr in the two dt sets with positive nomlies round m during the first hlf of the yer followed y negtive nomlies from August to Decemer. Moreover, s found y Chidichimo et l. [] using RAPID oservtions through 7, ORCA shows sesonl cycle in density nomlies with some nomlies coherent down to m t the mooring loctions. The generl greement etween the RAPID oservtions nd the model results provides the sis for further studying the origins of density fluctutions t the estern oundry nd their imprint on the UMO trnsport nd the AMOC Trnsport Clcultion t 6 N In this section, we review the link etween the density round the Cnry Islnds nd the trnssin trnsport clcultions from the RAPID rry. Full detils of the trnsport clcultions cn e found in Ryner et l. []. The component of the AMOC ssocited with the sinwide trnsport (etween the Bhms nd the Cnry Islnds) is clled the UMO trnsport nd is clculted s the sum of three components: ð UMO5 ½T INT ðzþt C ðzþt WBW ðzþšdz; () h MAX where T INT is the interior geostrophic trnsport per unit depth, T C compenstion term which insures zero residul trnsport cross 6 N, nd T WBW (for Western Boundry Wedge), trnsport determined from current meter mesurements ner the Bhms. The limit of integrtion, h MAX, represents the depth of mximum overturning of the AMOC. Here we re concerned with the geostrophic interior trnsport, the depth-integrted T INT, determined s: ð INT5 T INT ðzþdz5 g ð h MAX q f h MAX ð z ½q E ðz Þq W ðz ÞŠdz dz; () h BOT where h BOT represents the depth of the ocen t this estern oundry loction, g is the ccelertion due to grvity, f the Coriolis prmeter, q * reference density, nd q E nd q W, the densities t the estern nd western oundry of the rry, respectively. Eqution () shows tht the trnssin interior trnsport (INT) depends on the densities t the estern oundry etween h MAX (here fixed t m) nd the ottom. The top m trnsport is the integrl of density etween the ottom (h BOT ) nd the region ove it (with the upper limit of the integrl rnging from the surfce to m). In this wy, the densities elow m ffect the sinwide tilt of the isopycnl lyers ove. To explin the link etween the density t the estern oundry, the UMO nd the AMOC, DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved.

7 ./3JC96 we will thus focus on the densities from m to the ottom. Although we re interested in the link etween the density field t the estern oundry (directly linked to the interior trnsport) nd the AMOC t 6 N, the UMO trnsport encompssing the three components descried in eqution () will e further used for comprisons with the density field nd the WSC round the Cnry Islnds.. Results The im of this section is to descrie the hydrogrphic chrcteristics nd temporl vriility of the density field t the estern oundry of 6 N nd relte them to the WSC fluctutions round the Cnry Islnds. The source of these density fluctutions nd their influence on the AMOC vriility on sesonl to internnul time scles will lso e ssessed... Physicl Structure t the Estern Boundry of 6 N in ORCA Surfce wters re generlly fresh nd cold long the Africn cost, indictive of persistent upwelling t these ltitudes. The mximum upwelling occurs in July in this re s the equtorwrd winds pek t this time of the yer [Brton et l., 99]. During this costl upwelling seson, the WSC tends to e cyclonic ner the costl oundry nd nticyclonic in the offshore portions [Bkun nd Nelson, 99]. With lg of 3 months (s shown lter in the result section of this pper, Figure ), this mximum nticyclonic offshore WSC drives minimum in Octoer in the southwrd UMO trnsport (Figure ). Figures nd c show the verged se surfce temperture () nd slinity (c) for the month of Octoer 9. An individul snpshot highlights the higher vriility in the sptil structure of the temperture nd slinity. For this prticulr month (Figures nd c) s well s for the 3 yer verges (Figures nd d), costl upwelling ppers long the cost. This persistent upwelling long the northwest Africn cost modifies the density structure south of the islnds nd contriutes to the formtion of n upwelling filment (Figures nd c). The Cpe Juy upwelling filment ( cold tongue of wter south of the Cnry Islnds) lso ppers in the model round 7.5 N in the long-term verges. This filment intensifies in the utumn, the period of incresed trnsport in the sutropicl gyre nd costl upwelling (not shown here). ORCA represents the fine-scle structures known to e present round the Cnry Islnds (see section ). Both mesoscle eddies s well s the filment re present in the model output, south of the islnds. Mesoscle eddies re visile in the monthly verged SST (Figure ) s well s in the verged velocities in the top m for Octoer 9 (Figure e). The upwelling filment extends westwrd nd is twisted into cyclonic eddy locted from 6.5 Wto.5 W nd 6.5 N to 7.5 N. Wters from the upwelling filment contin mix of wters from the sutropicl gyre (wters crried y the Cnry Current), coming from the north of the islnds, nd wters from the upwelling region in the Northwest Africn cost [Brton et l., ]. Sections through the filment (t.5 W) show its verticl structure nd intensity (Figure 5). The zonl velocities of the filment re strong ( cm s ) westwrd currents etween 7.5 nd N, extending down to pproximtely m (Figure 5). Isopycnls re deflected downwrd just south of the Cnry Islnds, indicting lighter wter in this region thn the re round it. This meridionl density grdient my ffect zonl trnsport through the therml wind reltion (Figure 5). The model shows the presence of permnent cyclonic eddy t 6.5 N (clled C7 in Brton et l. []), with westwrd flow on the north side merging with the westwrd flow of the filment... Sptil Ptterns of Density Fluctutions t 7. N To determine the sptil structure of the dominnt mode of vriility round the Cnries in the 3 yers of model output, we pply empiricl orthogonl function (EOF) nlysis to the density field t 7. N, the ltitude of the RAPID moorings EBH/5 (this section is represented y horizontl lue line in Figure ). Detils of the EOF methodology cn e found in Preisendorfer [9]. Anomlies (with respect to the longterm men) of potentil densities re used to compute these EOFs. A liner trend ws lso removed to reduce signls linked to model drift. Since we re interested in sesonl nd longer time scles, the model output is first smoothed with dy running men efore clculting the EOFs. The first two EOFs explin pproximtely 67% of the vrince in this zonl section. The first mode ccounts for.6% of the vrince (Figure 6). If we focus on the region etween 5.5 W nd the Africn cost, this first mode hs verticl structure chrcterized y three min lyers: the Ekmn lyer (from the surfce to DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 9

8 ./3JC96 Octoer 9 3 yer verged dt N 3 N 9 N 9 N SST Ltitudes N 7 N N 7 N 6 N 6 N 5 N 5 N W 7 W 6 W 5 W W 3 W W 7 W 6 W 5 W W 3 W N 3 N 9 N 9 N SSS Ltitudes N 7 N N 7 N 6 N 6 N 5 N c 5 N d W 7 W 6 W 5 W W 3 W W 7 W 6 W 5 W W 3 W 3.7 cm/s cm/s 3 N 3 N m verged velocities Ltitudes 9 N N 7 N 6 N 9 N N 7 N 6 N 5 N e 5 N f 9.5 W.5 W 7.5 W6.5 W 5.5 W.5 W 3.5 W 9.5 W.5 W 7.5 W 6.5 W 5.5 W.5 W 3.5 W Longitudes Longitudes Figure. ( nd ) SST (in C), (c nd d) SSS, nd (e nd f) velocities (in m s ) verged over the top m in ORCA. Figures, c, nd e show n verge over the month of Octoer 9 while Figures, d, nd f represent 3 yer verged dt (from 9 to ). The grey ckground on the velocity pictures represents the thymetry of ORCA with chnges in the colorscle every m. The colorrs of the SSS figures re the sme for the snpshot nd the 3 yer verged dt. Two different colorrs re used for the SST figures. pproximtely m), nother lyer etween m nd 7 m, nd the lst one from 7 m to the ottom. The first nd third lyers hve the sme sign of vriility. This sptil EOF pttern lso exhiits the costl upwelling round 3.5 W s denser wters from round m upwell long the continentl slope DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 5

9 ./3JC96 Depth (m) Depth (m) N 7 N N 9 N 3 N Meridionl velocity t.5 W, 3 yer verged dt 6 N 7 N N 9 N 3 N Potentil density t.5 W, 3 yer verged dt Figure 5. () Meridionl section (.5 W) of 3 yer verged zonl velocities (in m s ) from the / model output. The loction of this section is represented in Figure y lue verticl line. () Sme meridionl section of 3 yer verged potentil density t.5 W (in m 3 s ). The Cnry Islnds pper on this section s verticl protrusion of lnd round N. A zoom on the first 7 m is shown in Figure 5 to highlight the presence of n eddy in the top first m. up to the surfce minly during the summer. The second mode, ccounting for % of the vrince, shows opposite signs ove nd elow 5 m (Figure 6) in resonle greement with the density structure identified y Chidichimo et l. [] (Figure 3 compred to Figure 5 in their pper). The principl components ssocited with the first two EOFs re shown in Figure 6e. The sesonl cycle domintes the vriility of the first principl component, with wek internnul signl, while the second mode shows more low-frequency vriility with mxim t the eginning nd end, nd minimum round This indictes tht the vriility in the density structure in this re is dominted y sesonl fluctutions lthough some internnul fluctutions re found in the two first EOF. The wvelet nlysis (Figure 9c) confirms tht most of the energy in the first mode of vriility of density is found t sesonl time scles. Some energy is lso found for periods round yers lthough they re not significnt t the 95% level. The percentge of vrince given for ech mode in the legends of Figures 6 nd 6 ccounts for the vriility of the full section. To highlight the center of ction of ech mode, homogeneous correltion mps hve een computed (Figures 6c nd 6d). These mps re otined from the correltion etween the principl component nd the time series of the originl density dt t ech grid cell. Ares covered with lck cross show correltions significnt t the 95% level (Figures 6c nd 6d). The extrem ssocited with mode re locted in the top hundred meters of the Ekmn lyer s well s elow 7 m (with 95% significnt correltions round 5 m) nd show tht the deeper density nomlies hve the strongest signl. Mode is focused on the lyer from the ottom of the Ekmn lyer down to 5 m. As mode does not show ny significnt correltion with the originl density dt in our re of interest nd explins smll frction of the totl vriility, mode will e used to explin the vriility in potentil density..3. Sptil Ptterns of the Wind Stress Curl Around the Cnry Islnds To explore the link etween the density nomlies nd the WSC s suggested y the RAPID dt [Chidichimo et l., ; Knzow et l., ], we compute EOFs of the sptil mps of WSC (Figure 7) using the model output for the sme time period (9 ) nd smoothing the time series y dy filter. The first mode of vriility displys structure with ptches of lternting sign, minly south of the islnds, etween 9 W nd 5 W (Figure 7), the middle ptch including the Cnry Islnds. This pttern of vriility is chrcteristic of estern oundry current regions [Bkun nd Nelson, 99]. The second mode of vriility lso displys similr sptil structure with ptches of lternting sign minly north of the islnds with opposite signs compred with the first EOF. These two first modes ccount for 3.% nd.6% of the vrince, respectively. The homogeneous correltion mps (Figures 7c nd 7d) show tht the ptterns of DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 5

10 ./3JC Depth (m) 5 Depth (m) Longitude (W).5 EOF:.6% vrince Longitude (W).5 EOF: % vrince 3.5 c d Depth (m) 5 Depth (m) omogeneous correltion mp st mode 5 3 omogeneous correltion mp nd mode Longitudes (W) Longitudes (W) e.5 EOF:.6% vrince EOF: % vrince Normlized units Yers Figure 6. Empiricl orthogonl functions (EOFs) nlysis for the zonl nd verticl structure in potentil density nomly t 7. N in the / model output. This ltitude corresponds to the ltitude of the RAPID moorings EBH/5 nd is represented y lue horizontl line in Figure. Figures 6 nd 6 represent the sptil pttern of the two first modes of vriility. Figures 6c nd 6d represent homogeneous correltion mps, tht is, the correltion etween the time series of the EOF mode nd the time series of the originl density dt t ech point. Stippled in lck re the res where the correltions re 95% significnt. Figure 6e represents the principl components ssocited with the two first EOFs. vriility south of the islnds in the first mode of WSC re 95% significnt. Figure 7d shows tht pttern of vriility south of Tenerife (etween 6.5 nd N) is lso 95% significnt, which mens tht the second mode of vriility does not only descrie the vriility north of the islnds. In the following, we will however still consider tht the second mode of vriility minly descries the WSC vriility north of the islnds. This different vriility north nd south of the islnds s well s the sptil ptterns chrcterized y three ptches of lternting sign on ech side of the islnds confirm the result shown y Jimenez et l. [] using nnul WSC dt, tht the presence of the islnds influences the wind ptterns over this re nd hence, impct the sptil vriility of the WSC. The principl components of the first two EOFs of the WSC (Figure 7e) re dominted y the sesonl cycle. These two modes lso disply internnul vriility with significnt chnge in mplitude from nd DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 5

11 ./3JC Ltitude (N) 7 Ltitude (N) EOF: 3.% vrince 5 EOF:.6% vrince Longitude (W) Longitude (W) 3 c d Ltitudes (N) Ltitudes (N) Homogeneous correltion mp WSC, st mode Homogeneous correltion mp WSC, nd mode Longitudes (W) Longitudes (W) e.5 EOF WSC: 3.% vrince EOF WSC:.6% vrince Normlized units Time (yers) Figure 7. Empiricl orthogonl functions (EOFs) nlysis for the wind stress curl round the Cnry Islnds in the / model output. Figures 7 nd 7 represent the sptil vriility of the first two modes. Figures 7c nd 7d represent the ssocited homogeneous correltion mps. Stippled in lck re the res where the correltions re 95% significnt. Figure 7e shows the principl components ssocited with the two first EOFs. Hundred dy smoothed dt re represented in Figure 7e. nother one from 6, lthough the nomly in is more pronounced in the second mode. As explined in the section descriing the simultion used in this study, the nomly in is due to chnge in the wind forcing (from ERA to ECMWF). The nomly in 6 is due to chnge in the tmospheric forcing (from DFS. to DFS5). The wvelet nlysis (Figures 9 nd 9) confirms tht most of the energy in the first two modes of vriility in WSC is found t sesonl time scles. While the first mode hs the strongest sesonl cycle nd no significnt energy t internnul time scles, the second mode of vriility shows energy t period of yers nd longer, mostly ssocited with the chnge in the wind forcing previously descried. The EOF nlysis nd wvelets of the principl component time series were lso clculted for the period 9 only. The wvelet results (not shown) were not visily ltered over the 9 period (Figure ). To e le to nlyze the longest time series ville, nd not only ssess the sesonl ut lso internnul vriility in the density nd WSC fields, model output is still nlyzed up to. The wvelet coherence nlysis llows us to clerly identify the impct of the chnge in the tmospheric forcing in while working with dt throughout (Figures 9 nd ). DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 53

12 ./3JC96 Normlized units.5 Sesonl cycle, 9 Density, EOF WSC, EOF noumo 6 r=.96 WSC.5 r=.95 UMO Normlized units.5 Sesonl cycle, 9.5 r=.6 WSC.5 r=.95 UMO.5 Apr Jul Oct Time (months) Density.5 Density.5 Density, EOF WSC, EOF noumo Apr Jul Oct Time (months).5 Density.5 Density Figure. Comprison etween the sesonl cycle of the principl components of potentil density t 7. N (red), time-integrted wind stress curl (lue), nd UMO trnsport t 6 N (lck) for the period 9. Figure compres the first modes of density nd wind stress curl while Figure compres the first mode of density nd second mode of wind stress curl. To the right of the time series, the sctter plot nd liner regression re shown etween density nd wind stress curl (second plot) nd density nd UMO trnsport (third plot). The third figure compring the wind stress curl nd UMO re identicl in oth pnels... Relting the Wind Stress Curl to Density Fluctutions To e le to ssess the link etween the WSC nd the density t 7. N t sesonl nd internnul time scles, their principl components hve een compred. For the sesonl vriility, their principl components re directly compred (Figure ) nd the WSC time series hs een integrted in time to represent the expected effect of the curl on the locl density field (the WSC is only time-integrted in Figure ). At internnul time scles, wvelet coherence is used to investigte the link etween the density nd the WSC not only in the frequency ut lso time domin. Only coherence t 95% level of significnce will e descried in this section.... At Sesonl Time Scles Amplitudes of the first modes for density nd time-integrted WSC re significntly correlted with correltion of.96 from 9 to (Figure ) nd.95 if dt from 9 to re used (not shown here). Hence, the time series of density nd WSC re 9 out of phse with the WSC preceding the density mening tht the WSC nomlies led density nomlies south of the islnds (this is expected s the WSC is time-integrted in Figure ). The wvelet coherence nlysis confirms these results with lrge common power etween the first EOF of WSC nd density (Figure ) t sesonl time scles. The rrows in Figure indicte the lg t which they occur. If n rrow is pointing to the right, the two time series re in phse, if it is pointing to the left, they re considered s nti-phse. The deflection of the rrows indictes the lg (e.g., 9 clockwise deflection for period of yer mens tht the first time series is leding the second one y 3 months). In Figure nd t sesonl time scles (period round yer), the rrows pointing downwrd show the 3 month lg etween these two time series. At sesonl time scles, the second mode of vriility of time-integrted WSC is significntly correlted with the first mode of vriility of density with correltion of.9 only if dt from 9 to re used (not shown here). Indeed, the strong nomly in the WSC in leds to n nticorreltion etween these two time series if dt from 9 to re considered (Figure ). The coherence nlysis shows tht the WSC nd the density hve similr power t sesonl time scles for the period 95 with the density leding the WSC fluctutions y months (Figure ). For the period 7, significnt coherence is lso found etween these dt sets with the WSC leding the density nomlies y 3 months. This chnge in the reltionship etween these two EOFs is not considered to e significnt s it is only due to the chnge in the wind product used to force the simultion. A link etween the WSC, the verticl velocities t m, nd the vortex stretching term dw/dz (where w is the verticl velocity) ws investigted y Sinh et l. [3] down to 65 m. The results here re in good greement with Sinh et l. [3], showing the sme limit round 7 m (Figures 6 nd e). In ddition, we show here tht the WSC influences the density structure down to the ottom t m.... At Internnul Time Scles Given the rnge of time scles present in the principl components, we use wvelet coherence to identify the links etween the WSC nd the density. For the period from 9 to, wvelet coherence nlyses DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 5

13 ./3JC96 Period (yers) EOF WSC Time (yers) / / / /6 /3 / Period (yers) EOF WSC 995 Time (yers) / / / /6 /3 c Period (yers) EOF density Time (yers) / / / /6 /3 /6 d Period (yers) UMO Time (yers) / / / /6 /3 /6 Figure 9. Wvelet nlysis for the () first nd () second EOF of wind stress curl, nd (c) first EOF of density. Figure 9d shows the wvelet nlysis for the UMO trnsport. Model output from 9 to is used to perform this nlysis. Blck contours represent 95% significnce. do not revel continuous significnt coherence etween the WSC nd the density fluctutions t fixed internnul time scle (Figure ). For shorter periods of time however, s during , significnt coherence is found with period of 3 yers etween the first modes of WSC nd density, with the density leding the WSC fluctutions y 6 months (upwrd rrows on Figure ). Likewise, the second mode of vriility of WSC (Figure ) does not led to density fluctutions t internnul time scles consistently for the whole period 9. For shorter periods of time, however, significnt coherence is found etween the WSC north of the islnds (EOF ) nd the first mode of vriility of density, with the WSC leding the density fluctutions (for exmple etween nd 99 5, downwrd rrows on Figure ). At internnul time scles, lthough no consistent significnt coherence is found etween the WSC nd the density section t 7. N, intermittent significnt reltionships were found etween the ptterns of vriility of WSC south nd north of the islnds nd the first mode of vriility of density. During these periods of time, WSC fluctutions do not necessrily led density fluctutions, which mens tht the density fluctutions south of the islnds re not primrily driven y the direct pumping from the WSC. Insted, other processes, not yet determined, re responsile for internnul vriility in density..5. Link With the Atlntic Meridionl Overturning Circultion One of the primry motivtions for this work is to shed light on the processes responsile for the reltionship etween the winds nd the AMOC. In previous oservtionlly sed studies of the sesonl cycle of the AMOC t 6 N, single point loction ws used for WSC vriility, to relte to the single (different) loction of the moorings t m [Chidichimo et l., ; Knzow et l., ]. From our EOF nlysis of WSC vriility, we find tht if different point loction hd een used for the WSC time series, the reltionship etween WSC nd density t the mooring loction would hve chnged. In section., we relte the sptil ptterns of WSC to density nomlies, nd we will now mke the connection etween the density nd the AMOC. From eqution (), the lrgest southwrd trnsport of UMO is expected when the west to est thermocline tilt deep to shllow is strongest. The roust sesonl cycle in the UMO trnsport cn e ttriuted to the sesonl cycle of WSC, where enhnced cyclonic (nticyclonic) curl in the winter (summer) uplifts DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 55

14 ./3JC96 Period (yers) WTC: EOF WSC EOF Density Time (yers) WTC: EOF WSC EOF Density.35 Period (yers) Time (yers) c.35 WTC: EOF WSC UMO d.35 WTC: EOF WSC UMO Period (yers) Period (yers) Time (yers) Time (yers).. e.35 WTC: EOF Density UMO Period (yers) Time (yers) 5. Figure. Wvelet trnsform coherence (WTC) nlysis for the period 9, etween () the first EOF of wind stress curl (WSC) nd the first EOF of density, () the second EOF of WSC nd the first EOF of density, (c) the first EOF of WSC nd the UMO trnsport, (d) the second EOF of WSC nd the UMO trnsport, nd (e) the UMO trnsport nd the first EOF of density using the pckge of Grinsted et l. []. The shding shows the highest correltions etween the time series (lck contours: 95% confidence level). Arrows indicte the lg t which they occur. If n rrow is pointing to the right, the two time series re in phse, if it is pointing to the left, they re considered s in nti-phse. The deflection of the rrows indictes the lg. (depresses) isopycnl surfces, resulting in pek displcement roughly 3 months lter in spring (fll). When isopycnls re t their shllowest t the estern oundry (i.e., wter t m is denser thn usul), the UMO trnsport reches its mximum southwrd vlues. The sptil pttern ssocited with the first EOF of density (Figure 6) shows negtive density nomlies etween 7 m nd m. The sesonl cycle of the first EOF of density (Figure ) shows positive pek round Octoer nd negtive pek round Mrch April. These two plots show tht in Octoer, the wter from 7 m to m is lighter thn usul, while the wter etween nd 7 m is denser thn usul. In contrst, in Mrch April, the mplitude of the sesonl cycle is negtive, so the wter etween 7 m nd m is denser thn usul. Figure shows tht the UMO trnsport peks t the sme time, illustrting the link etween the density t the estern oundry nd the UMO trnsport. DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 56

15 ./3JC96 Although the UMO trnsport is more energetic t periods etween 5 nd yers thn t sesonl time scles (Figure 9d), the first EOF of density is significntly correlted with the sesonl cycle of the UMO trnsport with correltion etween these time series of.95 (for the period 9, Figure ) nd.9 (for the period 9, not shown) t zero lg. The homogeneous correltion mp ssocited with the first EOF of density (Figure 6c) shows tht the most significnt re of this section is etween m nd the ottom. This suggests tht these deep density nomlies re responsile for UMO trnsport nomlies t sesonl time scles. Furthermore, the UMO trnsport nd first mode of vriility of density re lso coherent nd in phse t internnul time scles during t period of 3 yers (Figure e). A significnt coherence is lso found etween the UMO trnsport nd first mode of vriility of density t longer time scles of 5 yers. At these time scles, the UMO trnsport fluctutions precede the density nomlies y out.5 yers. A possile hypothesis tht explins this lg etween the UMO trnsport nd the density fluctutions will e further discussed in the conclusion/discussion prt of this pper. The strongest significnt coherence etween the WSC south of the islnds (EOF ) nd the UMO trnsport is found t sesonl time scles with the WSC leding the UMO y 3 months (Figure c). Figure d suggests tht no continuous link exists etween the UMO trnsport nd second mode of WSC t either sesonl or internnul time scles. This suggests tht t internnul time scles ( 3 yers nd longer thn 5 yers), density fluctutions (first mode) still drive the vriility of the UMO trnsport, ut re not driven y the WSC ptterns in EOF or. 5. Conclusions nd Discussion Using four yers of RAPID oservtions, Chidichimo et l. [] found sesonl cycle in in situ density throughout the upper ocen nd s deep s m. Knzow et l. [] show tht oth the estern nd western oundries contriute to the AMOC vriility, ut tht the estern oundry drives the roust sesonl vriility in the AMOC. However, with only yers of dt nd reltionship etween WSC t single point loction with the densities t individul mooring loctions rises questions of how representtive the results re. Here we use high-resolution numericl simultions to investigte the reltionship etween the density structure south of the Cnry Islnds, the wind stress curl (WSC) round these islnds, nd the AMOC. We find tht the density fluctutions t the estern oundry of 6 N not only drive the UMO trnsport t sesonl ut lso internnul time scles. The / resolution simultion of the model NEMO (ORCA) is in good greement with the RAPID oservtions nd ppers to dequtely represent the smller scle fetures of vriility round the Cnry Islnds including n upwelling filment nd eddies (Figure ). In ORCA s in the RAPID oservtions, the climtologicl verticl structure in in situ density t the estern oundry mooring loctions (moorings clled EBH3 nd EBH/5) is coherent down to roughly m (Figure 3). We use empiricl orthogonl function (EOF) nlysis to understnd the sptil ptterns of vriility in density t 7. N south of the Cnry Islnds (ltitude of the RAPID moorings EBH/5) nd in WSC round the islnds. The first mode of vriility of density explins most of the vriility in our re of interest nd exhiits three centers of ction: the first one from the surfce to the ottom of the Ekmn lyer, the second one from m to 7 m, nd the lst one from 7 m to the ottom. As the first mode is the only significnt mode in our re of interest, it is the only one further nlyzed in this pper. While the first mode of vriility of the WSC explins the vriility south of the Cnry Islnds, the second mode shows the vriility minly t the north of the islnds. These two modes re significnt nd show pttern punctuted y the Cnry Islnds, indicting the importnce of the islnds on the WSC pttern. Although the sesonl cycle domintes the vriility of the first modes of density nd WSC, the second modes show more internnul vriility. At sesonl time scles, the WSC fluctutions south of the islnds (first mode) re significntly correlted nd precede the first mode of density fluctutions y 3 months. The corresponding principl component is lso correlted with the UMO trnsport with the sme lg, nd deep density fluctutions (from 7 m to m) re thus directly linked with the AMOC fluctutions t sesonl time scles. At internnul time scles, during nd for periods of 3 yers, the first mode of vriility of density is significntly coherent nd in phse with the UMO trnsport. For periods etween 5 nd yers, the first mode of vriility of density is lso significntly coherent with the UMO trnsport with DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 57

16 ./3JC96 lg round.5 yers etween these two time series, the UMO trnsport preceding the density nomlies. This lg could e explined y the fct tht the UMO trnsport is driven y density nomlies t the mrgins [Hirschi nd Mrotzke, 7]; the RAPID moorings re lso plced to ensure good coverge of the mrgin densities. However, the EOF of density re clculted using lrger region thn the mrgin itself nd the center of ction of the first mode of density is locted offshore round 5 W (Figure 6). Consequently, if the density signl first ppers long the mrgin efore ecoming visile further wy from the cost, it could explin lg etween the first EOF of density nd the UMO trnsport t internnul time scles (with the UMO trnsport preceding n nomly in the density field). The second mode of vriility of WSC is not continuously significntly coherent with the density field nor the UMO trnsport thorough the period 9. This is possily due to chnge in the wind product used to force the simultion in. A cler reltionship etween the WSC, the density, nd the UMO trnsport hs thus een estlished t sesonl time scles. No continuous link in time (over the 3 yer time period) ws found etween these three fields t internnul time scles, which my e due to n inconsistent wind product used to force the ORCA simultion or could result from different processes driving the circultion over different periods. In the clcultion of the AMOC using RAPID oservtions, the density profile is tken from mooring instruments t single loction. From our results, we cn speculte whether the oserved sesonl cycle of the AMOC depends on the prticulr position of the moorings. To investigte this question, Figure shows the () mplitude, () phse, nd (c) frction of vriility explined y the sesonl cycle, for density t m in the domin chosen. These re determined with hrmonic fit to the time series of density nomly t ech pixel. The estern extent of the mp shows the limit of the m isoth. The mplitude indictes tht the sesonl cycle is most intense t the continentl slope. Furthermore, ll long the m isoth, the phse of the sesonl cycle is constnt. This suggests tht while the sesonl cycle is driven y the sesonl cycle in WSC, which hs vrile sptil structure, tht t m throughout the domin considered, the sesonlity of the density nomly is coherent. Thus, the sesonlity in density t m is not strongly dependent on the position of the moorings, s long s the instrument t m depth is t the m isoth. In other words, if tll, deep mooring were further offshore, the dt t m would not hve the sme sesonl cycle s if it were t the m isoth. The previous nlyses y Chidichimo et l. [] nd Knzow et l. [] used individul loctions for WSC to relte to the density nomlies. Here, however, we hve shown tht the pttern of WSC is sptilly vrile (Figures 7 nd 7), nd tht comprisons sed on individul loctions for the WSC time series my not e roust. Moreover, the sptil pttern of WSC ppers highly dependent on the presence of the Amplitude Phse c Frction of vriility Ltitudes (N) / / / Longitudes (W) 5 6 Longitudes (W) 5 6 Longitudes (W) Figure. () Amplitude, () phse, nd (c) frction of vriility of the sesonl cycle of density t m long the estern oundry, determined with hrmonic fit nd using the 3 yers of model output. The colorscle of Figure is expressed in months nd shows when the sesonl cycle peks. The loctions of the RAPID moorings EBH, EBH3, nd EBH/5 re represented with lue tringles on ech pnel of this figure. DUCHEZ ET AL. VC. Americn Geophysicl Union. All Rights Reserved. 5