Progress in Oceanography

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1 Aville online t Progress in Ocenogrphy 75 (2007) Progress in Ocenogrphy Intrinsic nd forced internnul vriility of the Gulf of Alsk mesoscle circultion Vincent Comes *, Emnuele Di Lorenzo School of Erth nd Atmospheric Sciences, Georgi Institute of Technology, Atlnt, GA, United Sttes Aville online 25 August 2007 Astrct The response of the Gulf of Alsk (GOA) circultion to lrge-scle North Pcific climte vriility is explored using three high resolution (15 km) regionl ocen model ensemles over the period On internnul nd decdl timescles the men circultion is strongly modulted y chnges in the lrge scle climte forcing ssocited with PDO nd ENSO. Intensifiction of the model gyre scle circultion occurs fter the climte shift, s well s during nd From the model dynmicl udgets we find tht when the GOA experiences stronger southesterly winds, typicl during the positive phse of the PDO nd ENSO, there is net lrge-scle Ekmn convergence in the centrl nd estern costl oundry. The geostrophic djustment to higher se surfce height (SSH) nd lower isopycnls led to stronger cyclonic gyre scle circultion. The opposite sitution occurs during stronger northwesterly winds (negtive phse of the PDO). Along the estern side of the GOA sin, internnul chnges in the surfce winds lso modulte the sesonl development of high mplitude nticyclonic eddies (e.g. Hïd nd Sitk eddies). Lrge internnul eddy events during winter-spring, re phse-locked with the sesonl cycle. The initil eddy dynmics re consistent with qusi-liner Rossy wve response to positive SSH nomlies forced y stronger downwelling fvorle winds (e.g. southwesterly during El Niño). However, ecuse of the fst growth rte of roclinic instility nd the geogrphicl focusing ssocited with the costl geometry, most of the perturtion energy in the Rossy wve is loclly trpped until converted into lrge scle nonliner coherent eddies. Costlly trpped wves of tropicl origin my lso contriute to positive SSH nomlies tht led to higher mplitude eddies. However, their presence does not pper essentil. The model ensemles, which do not include the effects of equtoril costlly trpped wves, cpture the lrge Hïd nd Sitk eddy events oserved during 1982 nd 1997 nd explin etween 40% nd 70% of the tidl guges vrince long the GOA cost. In the western side of the GOA sin, internnul eddy vriility locted south of the Alskn Strem is not correlted with lrge scle forcing nd ppers to e intrinsic. A comprison of the three model ensemles forced y NCEP winds nd multi-century-long integrtion forced only with the sesonl cycle, shows tht the internl vriility lone explins most of the eddy vrince. The symmetry etween the eddy forced regime in the estern sin, nd the intrinsic regime in the western sin, hs importnt implictions for predicting the GOA response to climte chnge. If future climte chnge results in stronger wintertime winds nd incresed downwelling in the estern sin, then incresed mesoscle ctivity (perhps more or lrger eddies) might occur in this region. Conversely, the chnges in the western sin re not predictle sed on environmentl forcing. Estern eddies trnsport importnt iogeochemicl quntities such s iron, oxygen * Corresponding uthor. Tel.: ; fx: E-mil ddress: Vincent.Comes@es.gtech.edu (V. Comes) /$ - see front mtter Ó 2007 Elsevier Ltd. All rights reserved. doi: /j.pocen

2 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) nd chlorophyll- into the gyre interior, therefore hving potentil upscle effects on the GOA high-nutrient-low-chlorophyll region. Ó 2007 Elsevier Ltd. All rights reserved. 1. Introduction Oservtionl nd modeling studies suggest tht decdl-scle vriility in the Gulf of Alsk (GOA) gyre circultion is strongly controlled y chnges in the tmospheric forcing (Lgerloef, 1995; Cummins nd Lgerloef, 2004). Decdl nd internnul vriility in wind stress over the GOA is correlted with the Pcific Decdl Oscilltion (PDO) nd the Southern Oscilltion. Intensifiction of the Aleutin low during , lso referred to s the climte shift (Miller et l., 1994) modified the surfce ocen circultion of the GOA (Cpotondi et l., 2005; Miller et l., 2005). More specificlly, Miller et l. (2005) suggest tht n intensifiction of the Alskn Strem occurred fter the climte shift, lthough the Alsk Current nd eddies generted on the estern GOA remin mostly unchnged. In ddition to locl chnges in surfce winds, costlly trpped Kelvin wves generted in the tropics during the El Niño Southern Oscilltion (ENSO) ffect the GOA circultion (Meyers nd Bsu, 1999). Despite the numerous oservtionl studies over the northest Pcific Ocen (Meyers nd Bsu, 1999; Crwford, 2002; Stru nd Jmes, 2002; Brickley nd Thoms, 2004; Ldd et l., 2005), importnt spects of the GOA dynmics (e.g., mesoscle ctivity) hve een rrely studied or modeled, nd thus re not well understood. Some modelers hve nevertheless focused their studies on mesoscle circultion in the GOA, such s Hïd or Sitk eddies (Melsom et l., 1999; Di Lorenzo et l., 2005). Okkonen et l. (2001) hve lso suggested tht eddy vriility is dominted y the locl response to wind forcing long the Alsk Current (estern GOA sin) nd is non-deterministic in the Alskn Strem (western GOA sin). Eddies in the GOA hve lredy een studied for severl decdes using ville oservtions (Tt, 1982). In the estern GOA sin, three mjor groups of eddies hve een identified ccording to the loction of their formtion, Ykutt, Sitk nd Hïd (Fig. 1). These three eddy groups shre common fetures, such s n nti-cyclonic rottion (up to 0.4 m se surfce height nomly) with wrmer nd less sline core ove 100 m depth. (Tt, 1982; Crwford, 2002). The genertion nd migrtion of these eddies, which cn survive for more thn three yers, is well documented in stellite oservtions (Thomson nd Gower, 1998; Crwford, 2002). Hïd nd Sitk eddies mostly propgte offshore while the Ykutt eddies propgte longshore long the Alskn Strem. While previous studies ttriute the genertion of these lrge eddies to roclinic instility of the costl currents (Thomson nd Gower, 1998; Murry et l., 2001), more recent numericl investigtion y Di Lorenzo et l. (2005) suggests tht the most southern eddy, the Hïd eddy, cn develop without instilities y simple dvection of uoynt wter msses round Cpe St. Jmes (southern tip of Queen Chrlotte Islnd). These dvected wter msses generte ptches of uoynt flow with nticyclonic circultion which merge to generte lrger eddies when the flow round the cpe is strong. For the more northern Sitk nd Ykutt eddies more detiled modeling study of their formtion hs not een completed. In ddition to the eddies developed in the estern oundry long the Cndin cost, the western sin (here defined s west of 1 W) lso exhiits enhnced eddy ctivity. Along the Alskn Strem, eddies hve een oth oserved (Crwford et l., 2000) nd modeled (Okkonen et l., 2001) nd tend to propgte southwestwrd long the Aleutin peninsul. In the Shelikof se vlley (north GOA sin) Steno nd Hermnn (1996) find good greement etween eddies generted y model nd oservtions, however their study does not dignose the eddy dynmics. Understnding the dynmics of eddies nd other mesoscle physicl processes is criticl for predicting how trcer exchnges etween inshore nd offshore wter msses will respond to chnges in tmospheric nd ocen stte (e.g. ENSO, PDO, glol climte chnge). Eddies hve severl direct implictions on the climte nd iology of the GOA, ecuse of their importnt role in cross-shelf trnsport. A recent study suggest tht out 35 % of northwrd het trnsport during winter is crried offshore y the Hid eddies (Crwford, 2005). Crwford (2005) estimtes tht 15% of the totl river freshwter input etween the Columin River nd 54N is trnsported y these eddies. Nutrient exchnges (Whitney nd Roert, 2002; Ldd et l., 2005) nd iron

3 268 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) c Fig. 1. () Mp of the domin (Gulf of Alsk: GOA). () Men nd (c) stndrd devition of the se surfce height (SSH). trnsport (Johnson et l., 2005) lso occur verticlly in the core of eddies. The eddy induced trnsport is lso linked to the iologicl productivity in the offshore high nutrient low Chlorophyll- (HNLC) wters of the Alskn gyre (Crwford et l., in press). Links etween eddy trnsport, wter mss properties nd iot hve lso een reported in the GOA western sin (Okkonen et l., 2003) nd estern sin (Mcks nd Glrith, 2002). At the higher trophic levels the link etween mesoscle processes nd species distriutions/undnce hs not een fully estlished. However fish communities hve een found to respond strongly to lrge-scle tmospheric chnges, such s the climte shift (Mntu et l., 1997; Litzow et l., 2006). Some of these chnges my potentilly e relted to chnges in eddy trnsport sttistics s hypothesized y Miller et l. (2005) in study of the Steller se lions decline in the western GOA. Other importnt mechnisms for crossshelf fluxes in the GOA include episodic upwelling, surfce Ekmn trnsport, upwelling sewrd of costl wind jet nd topogrphic steering (Steno et l., 2004). In order to further clrify the men, sesonl nd internnul response of the GOA mesoscle circultion to tmospheric chnges, we conduct numericl investigtions with high-resolution ocen model. Three ensemles, forced y the lst yers of NCEP/NCAR wind stress renlysis (Klny et l., 1996) nd het fluxes corrected using the NOAA extended SST (Smith nd Reynolds, 2004), re used to seprte the intrinsic nd forced vriility in the GOA. The primry gols of this study re to (1) understnd how the surfce circultion nd eddy sttistics re controlled y tmospheric forcing t the sesonl nd internnul timescle, nd (2) seprte the intrinsic nd forced vriility in the GOA.

4 This pper is orgnized s follows. Section 2 descries the model experiments nd the domin used for this study. Section 3 discusses the men circultion in the GOA nd its internnul modultions y chnges in surfce forcing during different phses of the PDO. Section 4 looks t the sesonl nd eddy circultion in the western nd estern sin. A summry is provided in Section Modeling pproch nd domin V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) The Regionl Ocen Modeling System (ROMS, Hidvogel et l., 2000; Shchepetkin nd McWillims, 2005) is used to investigte the forced nd intrinsic vriility of the Gulf of Alsk circultion. ROMS is free-surfce, hydrosttic, eddy-resolving primitive eqution ocen model tht uses stretched, terrin-following coordintes in the verticl nd orthogonl curviliner coordintes in the horizontl. The hydrosttic primitive equtions re solved using split-explicit time-stepping scheme, which llows the seprtion of the rotropic nd roclinic components of the momentum equtions with internl nd externl time steps. Different resolutions nd different domins hve previously een used with ROMS nd provide relile results for the North Pcific Ocen (Mrchesiello et l., 2003; Di Lorenzo et l., 2005; Curchitser et l., 2005; Curchitser et l., 2005). Given the success of these previous ROMS pplictions, this study uses similr model configurtion options. In prticulr we use the KPP scheme for verticl mixing in the surfce oundry (Lrge et l., 1994) nd implicit horizontl mixing ssocited with the third order upstrem is scheme (Shchepetkin nd McWillims, 1998). The grid extends northwrd from 25N to 61N nd westwrd from 111W to 179W. The verge sptil resolution is etween 19 km (South) to 13.4 km (North) with 30 verticl levels (with higher resolution t the upper ocen lyer), tht is to sy totl numer of 1,697,280 points (ltitude longitude depth = ). Although this resolution is of the sme order of the Rossy deformtion Rdius, the model is eddy permitting nd is le to cpture qulittively the lrge-scle Hid nd Sitk eddies (dimeters P 200 km) long the estern side of the GOA nd the eddy centers long the Alskn Strem. The domin of interest for this study includes ll of the Gulf of Alsk from Wshington Stte nd presents open oundries in the south nd west. Along these open oundries we use modified rdition oundry condition (Mrchesiello et l., 2001) together with nudging to oserved monthly men climtologies (Levitus et l., 1994). The nudging is stronger (timescle of 1 dy) if the direction of the flow is inwrd nd weker (timescle of 1 yer) for the outflow. In order to void lrge pressure grdient errors (Mellor et l., 1994), the model thymetry is otined y smooth interpoltion from Dvid T. Sndwell nd Wlter H. F. Smith thymetry (Smith nd Sndwell, 1994; Sndwell nd Smith, 1997). Experiment 1 (Tle 1) consists of three ensemles, ech forced y the lst yers of NCEP/NCAR (Ntionl Center for Environmentl Prediction/Ntionl Center for Atmospheric Reserch) wind stress renlysis (Klny et l., 1996) nd y het fluxes corrected using the NOAA (Ntionl Ocenic & Atmospheric Administrtion) extended SST (Smith nd Reynolds, 2004). The freshwter surfce fluxes re set to corrected monthly climtology derived during n 80 yer long spinup run in which the surfce slinity is nudged to the oserved climtology nd the resulting flux correction is sved. Experiment 2 differs from Experiment 1 in the surfce forcing conditions, which re prescried to the monthly men climtology. The initil nd oundry conditions remin unchnged. We refer to Experiment Tle 1 Tle of experiments Run length Initil conditions Open oundries Surfce forcing (monthly-mens) Experiment 1 Experiment yers ( ) 163 yers ( ) Levitus et l. (1994) Temperture, slinity nd geostrophic velocity derived from Levitus et l. (1994) + nudging yers wind stress of NCEP/NCAR het fluxes corrected with NOAA extended SST fresh wter fluxes from corrected climtology monthly climtologicl forcings derived from Experiment 1 forcing functions

5 270 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) s the Unforced Run, in tht there is no forcing function with periodicity longer thn the sesonl cycle. In contrst, Experiment 1, which is forced y the time dependent NCEP winds nd NOAA SST, is referred to s the Forced Run. For the initil condition, zonl nd meridionl velocity, se surfce height, temperture, nd slinity re otined from Levitus dt (1994). The initil conditions of the ensemles 2 nd 3 use respectively the finl sttes of ensemle 1 nd ensemle 2. At the open oundries ll model integrtion use the modified rdition condition (Mrchesiello et l., 2001) nd nudging to prescried monthly climtologies for temperture, slinity nd geostrophic velocity derived from Levitus et l. (1994). The geostrophic velocity is computed using the ottom s the reference level of no motion. 3. Men nd internnul circultion The circultion in the GOA is minly descried y the surctic gyre (Fig. 1). On the estern oundry of the GOA sin, the North Pcific Drift diverges into the southwrd Cliforni Current nd the northwrd Alsk Current. The Alsk Current is rich in menders nd eddies, such s Hïd nd Sitk long the estern oundry. As the Alsk Current flows long the Alsk Peninsul it ecomes the Alskn Strem, which hs more developed nd richer eddy field ssocited with the strong instility of the men currents (Melsom et l., 1999). Ner the Kodik Islnd, the Alskn Strem is nrrow (out km) nd strong ( cm s 1 )(Steno et l., 2004). The currents re principlly in geostrophic lnce nd consequently follow lines of constnt dynmic height with the lrgest vlues on the right. The model men circultion, represented in Fig. 1 y the se surfce height (SSH), is mostly consistent with this view descried in the literture. In the estern sin we find rod northwrd current, corresponding to the Alsk Current, nd n intensifiction of surfce currents long the Aleutin Islnds in the loction of the Alskn Strem. A mp of the SSH stndrd devition (Fig. 1c) shows high vrince in regions of intense eddy vriility long the southern edge of the Alskn Strem, long the estern sin, nd in the costl regions where the vriility is directly forced y chnges in the tmospheric winds ( discussion follows in Section 4). On the estern oundry, mxim of stndrd devition re found in the genertion sites of the Hïd nd Sitk eddies. A corresponding region of high vrince is not found t the loction of the Ykutt eddy, which is not well represented in the model. A closer inspection of the model men momentum udget for the surfce lyer confirms (Fig. 2) tht the dominnt lnce is geostrophic, with stronger Alskn strem (0.4 m s 1 ) following the topogrphic slope in the northwestern sin nd wek northwrd flow in the estern sin. The model geostrophic flow long the cost results from the ocen djustment to the men downwelling conditions in the estern sin nd upwelling long the topogrphic slope in the western sin. This is inferred from the model Ekmn currents verged over the surfce lyer (Fig. 2) nd the corresponding verticl fluxes of temperture (Fig. 2c), which show regions of surfce divergence in the west nd convergence in the est. A detiled nlysis of the temperture udget shows tht the dominnt lnce is etween the verticl nd the horizontl flux. The next lrgest term in the udget, lthough still smll compred to the dvection terms, is the surfce het flux (Fig. 2d). On verge the surfce het fluxes re wrming the Gulf of Alsk except in regions of strong downwelling (see lue res in Fig. 2c nd d) where the strong horizontl convergence of surfce wrmer wter tends to het the tmospheric mrine oundry lyer nd cool the ocen surfce. To investigte the internnul modultion of the men circultion we compute the first Empiricl Orthogonl Function (EOF) of the model SSH, where the nomlies re defined y removing the sesonl cycle. The resulting EOF 1 (Fig. 3) explins 30% of the totl vrince nd hs the sme structure s the men circultion. Therefore temporl modultions in the mplitude nd sign of this pttern, which re inferred from the Principl Component 1 (PC 1; Fig. 3e) of the three ensemle memers, reflect internnul nd decdl chnges in the men circultion. The spred of the PCs 1 of the ensemle memers is very smll suggesting tht chnges in the men circultion re minly forced. Indeed strong internnul modultions of the PCs 1 correspond to chnges in the monthly PDO index with significnt correltion of 0.64 (Fig. 3f). Intensifiction of the gyre scle circultion is evident fter the climte shift, s well s during 19 19, nd more grdully during Wekening of the circultions is evident during nd Composite mps of the difference in surfce winds (Fig. 3c nd d) suggest tht the gyre intensifies during nomlous southesterly winds (stronger Aleutin low) nd wekens when the winds re predominntly

6 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) Geostrophic Component ms Ekmn Component ms -1 c Temp Horizontl Flux (positive = divergence) Cdy -1 d Temp Surfce Flux (positive = wrming) Cdy Fig. 2. () Men geostrophic surfce currents, with men SSH contours (white lines). () Men Ekmn trnsport in the model surfce lyer. The red rrows indicte the direction nd intensity of men surfce wind stresses. (c) Temperture horizontl flux, positive regions indicte upwelling. (d) Temperture surfce flux, positive indictes tht the ocen is wrming due to the tmospheric het flux.

7 272 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) c d e f Fig. 3. () First empiricl orthogonl function (EOF 1) of the se surfce height fter removing the sesonl cycle. EOF1 explins 30.0% of the totl vrince. () Men surfce NCEP wind stress. (c) nd (d) show the difference in surfce wind stress for selected periods (lck horizontl lines in (e)). (e) PC 1 for the three forced model ensemles. (f) Comprison etween PDO index nd PC1 ensemle verge. nomlous northwesterly. A detiled nlysis of the model momentum nd trcer udgets revels tht these extrem in the men circultion re indeed forced y chnges in the surfce wind stresses (Fig. 4 illustrtes n exmple for the 1976 shift). In prticulr we find strengthening of the gyre when the Ekmn trnsport tends

8 m V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) Geostrophic Component Ekmn Component ms -1 c d Temp Horizontl Flux (positive = divergence) Temp Surfce Flux (positive = wrming) C dy -1 Cdy -1 Fig. 4. Difference mps for the periods ( ) minus ( ). () Difference in SSH nd Geostrophic surfce currents. () Difference in Ekmn trnsport in the model surfce lyer. (c) Difference in temperture horizontl flux, negtive regions indicte stronger downwelling conditions fter (d) Difference in temperture surfce flux, positive indictes tht the ocen is wrming due to the tmospheric het flux.

9 274 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) to converge mss towrds the costl oundry in the centrl Gulf of Alsk (Fig. 4). This condition is lso ssocited with stronger tht usul downwelling long the estern oundry (Fig. 4c) nd upwelling long the Alskn Strem. The contriution of surfce het fluxes is smll compred to the mechnicl forcing. The djustment process of the gyre scle circultion in response to the internnul nd decdl chnges of the PDO nd ENSO cn e summrized s follows. When the GOA experiences stronger nomlous southesterly winds (positive phse of the PDO s well s ENSO) there is net convergence of surfce mss (downwelling conditions) in the centrl nd estern costl oundry induced y Ekmn trnsport (Fig. 4). This genertes higher costl SSH (Fig. 4) nd lowers the costl isopycnls. The resulting geostrophiclly djusted currents (Fig. 4) intensify the gyre scle circultion. The opposite sitution occurs during stronger nomlous northwesterly winds (negtive phse of the PDO). In the next section we continue to explore the ocen djustment process to chnges in the forcing y investigting how the chnges in the winds re reflected in the eddy scle circultion. 4. The sesonl nd internnul eddy field To gin confidence in the degree of relism of the model simultion, we compre the model sesonl SSH nomlies to the one derived from the AVISO stellite mps (Fig. 5). These mps ( merge dt from T/P or JASON-1+ERS-1/2 nd Envist stellites to produce se level nomlies t weekly resolution from Octoer 1992 to Jnury 2005 on 1/3 1/3 Merctor grid. An nnul oscilltion of the SSH long the cost is evident in oth the model nd stellite dt. Consistent with Stru nd Jmes (2002) oservtionl results, the SSH long the cost is positive during the winter months nd negtive in summer, leding to strong SSH grdient (stronger downwelling current) in winter nd wek SSH grdient (wek current) in summer. From the nlysis of the model udget terms, we find tht this oscilltion is driven from the sesonlity in the wind stress nd consequent Ekmn currents. Following the winter downwelling conditions long the cost in the estern sin, cler westwrd propgtion of the SSH is evident in oth model nd stellite during Mrch through July. As the positive SSH detches from the cost rich eddy field develops off the shelf. Anticyclonic eddies ecome prominent feture, prticulrly in the model where the signture of the sesonl development of the Hïd nd Sitk eddies is stronger when compred to stellite oservtions. In the western sin we lso find stronger mesoscle signture in July nd August. Although oth model nd AVISO sesonl SSH mps use the sme time rnge, from Jnury 1993 to Decemer 2004, the stellite mps revel n overll higher SSH nomlies in the gyre interior etween August nd Octoer nd lower etween Jnury nd My. The sesonl propgtion of estern eddy field is lso reflected in the eddy kinetic energy (EKE) phse mp (Fig. 6). Along the cost the sesonl mxim is reched in Jnury when the stronger current develop on the shelf in response to the stronger downwelling conditions. During lte spring/summer the EKE pek is found further wy from the cost in the estern sin nd reflects the sesonl development of the eddy field. During the fll the sesonl pek moves further offshore in the gyre interior. The mplitude of the sesonl EKE is strongest on the shelf where the currents re directly forced y the winds (Fig. 6) Forced regime in the Alsk current region To etter isolte the development of the lrge nticyclone long the estern oundry we perform n nlysis of the model SSH EOFs 1 nd 2 long the est cost (Fig. 7). Consistent with the sesonl mps, the EOFs shows sptilly coherent excittion of SSH long the cost (EOF 1) followed y offshore propgtion (EOF2). The mjor nticyclonic eddies in EOF 2, Hïd nd Sitk, re cptured y the model nd exhiit sustntil internnul vriility s evident y the corresponding PCs (Fig. 7c). Though the two EOFs re uncorrelted, 1 yer low pss filter of PC1 nd PC2 (Fig. 7c) shows 200 dys lg verge, which confirms tht the development of the two nticyclonic cores in EOF 2 (corresponding to Sitk nd Hïd eddies) follow the SSH of EOF 1. The two EOFs explin more thn 54% of the vrince. The sme EOF nlysis is performed for the specific region round the Queen Chrlotte Islnds, where Hïd eddies re generted (figures not shown here). From this nlysis we infer the speed of eddy propgtion of 1.3 km dy 1, which is consistent with previous estimtes of 1.12 ± 0.35 km dy 1 y Gower nd Tt (1993). The PCs show

10 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) Fig. 5. Model () nd AVISO () SSH sesonl nomlies derived from 1993 to tht some of the lrgest events over the entire record correspond to strong El Nin o yers, nd A tle showing the yer nd month when lrge eddy events re found (Fig. 7d) revels tht lrge Hı d nd Sitk eddies re minly generted during winter months (Decemer, Jnury, Ferury, Mrch) nd re phse locked with the sesonl cycle. Therefore, internnul chnges in the mplitude of the generted eddy re reflected in internnul modultion of the sesonl cycle. To further clrify the reltionship etween the modeled eddies nd El Nin o yers, we perform the sme EOF nlysis for the three model ensemles nd isolte the forcing ptterns corresponding to lrge eddy events (Fig. 8). We find tht PCs 1 of ll ensemles re correlted ove 0.9 (Fig. 8 nd h) corroorting the hypothesis of strongly forced regime. For the Hı d region, composite nlysis of the winter wind ptterns during times when PC1 is greter thn 1 nd less thn 1, shows tht the winds re predominntly southwesterly (Fig. 8c nd d). However if we tke the differences etween the composites nd the men winds we find tht during times of higher mplitude eddies (PC 1 P 1) the winds re strongly downwelling (Fig. 8e) nd therefore cple of exciting stronger SSH. In contrst during times of weker mplitude eddies the winds hve upwelling fvorle nomlies (Fig. 8f). The sme nlysis performed for the Sitk region leds to the sme results (Fig. 8g l), minly tht stronger eddies re excited y stronger winter downwelling winds. An inspection of the composite mp of the winter wind stress nomlies during the 1982 nd 1997 El Nin os (Fig. 9) confirm the existence of stronger downwelling winds, therefore explining the oserved correltion etween El Nin o nd lrge eddy events. In contrst the L-Nin events of 1970 nd 1988 show upwelling fvorle winds nd reduced eddy mplitude. Excittion of positive SSH long the costl oundry cn lso e ttriuted to costlly trpped Kelvin wves of equtoril origin. Indeed Meyers nd Bsu (1999) indicte tht Kelvin wves re source of

11 276 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) EKE Annul Amplitude m 2 S EKE Sesonl Cycle Phse jul my mr jn nov Fig. 6. () Annul mplitude of the eddy kinetic energy (EKE) nd () EKE sesonl cycle phse. internnul vriility in the costl current long western North Americ. However, in this study we used monthly men climtology southern open oundry condition from Levitus et l. (1994), which does not include these tropiclly generted Kelvin-like wves. Nevertheless comprison of the model costl se level with in situ records nd stellite dt in vrious costl loctions (Sitk, Neh By, Sewrd, Ykutt, Kodik nd Snd Point; Fig. 10) indicte tht the model hindcst is le to explin very lrge frction of the in situ se level vrince. For exmple t Sitk the model explins 41% of the in situ oservtions vrince, which is more thn the stellite (21%). At other loctions, except Ykutt, the model generlly compres more fvorly thn the stellite with in situ dt explining etween 40% nd 70% of the tidl guges vrince. The poor performnce of the stellite dt in the nershore region hs een reported efore y Stru nd Jmes (2002) who performed nlysis of costl tide guge nd ltimeter dt to generte merged se level nomly product for the Northest Pcific. At Ykutt the correltion is not very high s the model does not reproduce Ykutt eddies very well. High correltions with in situ dt re lso found t costl loctions long the Alskn Strem. From these results, which re consistent with Stru nd Jmes (2002) oservtionl results, we conclude tht most of the internnul vriility in the estern oundry eddies nd over the shelf, is loclly forced y the winds, lthough costlly trpped wves my still contriute to dditionl vrince. We now discuss dynmicl frmework to understnd the reltionship etween the forcing nd the development of lrge scle eddies in the estern oundry. Previous studies suggest tht the sesonl nd internnul development of the mesoscle eddy field results from instility ssocited with the sesonl men costl currents nd uoyncy grdients (Thomson nd Gower, 1998; Melsom et l., 1999; Okkonen et l., 2001; Murry et l., 2001). However other studies suggest lterntive generting mechnisms tht rely on liner dynmics (Di Lorenzo et l., 2005). For this reson we ttempt to refrme the current nd previous findings in the context of instility of liner Rossy wves. This follows from the oservtion tht EOF 1 nd EOF 2 (Fig. 7) re consistent with liner excittion nd propgtion of Rossy wve. LCsce nd Pedlosky (2004) show using n idelized setup, tht Rossy wves excited t the estern costl oundry re suject to roclinic instility, in prticulr when the growth rte of the instility is fst compred to the time it tkes for the wve to cross the ocen sin. Becuse the phse speed of Rossy wves decreses with

12 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) c d Fig. 7. () EOF 1 nd () EOF 2 of SSH. (c) The red curve (green curve) is the principl component ssocited to EOF1 (EOF2), using 1 yr lowpss filter. (d) Mtrix showing the times when lrge Hid nd Sitk eddies (EOF2) re formed in the model simultion. incresing ltitude, first mode roclinic Rossy wve phse speeds re extremely slow in the Gulf of Alsk (out 1 2 km/dy from Chelton nd Schlx (1996)). Indeed during winter, following the strong downwelling condition, the liner djustment process to the positive se level perturtions (sesonl in Fig. 5, internnul in Fig. 7) leds to rdition of Rossy wves tht would tke out 6 months to propgte wy from the costl oundry where they re excited. This timescle grees with the oserved nd modeled SSH propgtion wy from the est cost during winter nd spring (Fig. 7). However, LCsce nd Pedlosky (2004) theory predicts tht the excited Rossy wves ecome unstle t higher ltitudes nd will not hve time to coherently propgte in the gyre interior. Both model nd oservtions (Chelton nd Schlx, 1996) support this view nd do not show meridionlly coherent westwrd phse propgtion. To verify if the LCsce nd Pedlosky (2004) theory is pplicle for the Gulf of Alsk, we compre the propgtion timescle of the wves with the typicl growth rte of roclinic instility in the Gulf s inferred y the Edy index (Lindzen nd Frrell, 1980) defined: r E ¼ 0:31 f du N dz where f is the Coriolis prmeter (s 1 ), N 2 ¼ g dq represents the sttic stility q o dz (s 2 )ndqthe density of wter (kg m 3 ). In the computtion, we consider u s the current speed. The Edy index, r E, is then integrted etween the surfce nd 100 m meters depth. Although r E descries the roclinic instility of zonl sic-stte flow (Hoskins nd Vldes, 1990) it still provides rough estimte of the typicl growth rte of the instilities nd is similr to the criteri used y LCsce nd Pedlosky (2004) which use the rtio etween verticl sher nd Rossy deformtion rdius. Fig. 11 shows the model monthly men mps of the Edy index, which exhiit cler

13 278 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) c d e f g h i j k l Fig. 8. () EOF 1 nd () PC 1 of SSH in the region of formtion of Hid eddies. (c) Wind stress composite mps for PC 1 > 1, (d) for PC 1 < 1. (e) Wind stress nomly composite mps for PC 1 > 1, (f) for PC 1 < 1. (g-l) sme s (-f) ut for Sitk region.

14 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) c d Fig. 9. NCEP surfce wind stress nomly (with respect to the period) for the El Niño winters () , (c) nd () L Niñ winters , (d) sesonl cycle with higher roclinicity during winter. Between Octoer nd April long the cost where the SSH is pertured y the sesonl pek of downwelling winds, the Edy index hs vlues etween 4 nd s 1. The corresponding timescle for the growth rte of the instility is 2 6 dys, which is much too fst for the costl disturnces to rdite wy s Rossy wves. The reoccurrence of lrge nticyclones t the sme regionl loctions (e.g. Hïd, Sitk nd Ykutt) is not explicitly ccounted for y the LCsce nd Pedlosky (2004) theory. Nevertheless it is likely tht during the liner phse of the ocen djustment process to the wind induced isopycnl perturtions (EOF 1 nd 2 Fig. 7), the positive SSH pttern is strongly influenced y the costl nd topogrphic geometry. This geogrphicl focusing comined with the LCsce nd Pedlosky (2004) theory could explin why the mplitude of the estern sin eddy field is proportionl to the perturtion energy of the downwelling winds. Specificlly during downwelling conditions in winter, the erly response of the costl ocen is liner with the onset of stronger polewrd currents ssocited with the high costl SSH, nd with the initil rdition of Rossy nd topogrphiclly trpped wves. However ecuse of the fst growth rte of the instility nd the geogrphicl focusing, most of the perturtion energy is loclly trpped until converted into the eddy field. Although more detiled studies re needed, this energy trpping mechnism would explin the strong reltionship etween the perturtion mplitude of the winds nd eddies Intrinsic regime in the Alskn strem region While the development of mesoscle eddies on the estern GOA sin is modulted y chnges in the wind stress, eddies long the Alskn Strem (western sin) do not correlte with tmospheric forcing nd their vriility my e intrinsic (Okkonen et l., 2001). To seprte the intrinsic nd forced vriility in the GOA we conduct n dditionl long-term (300 yers) model run forced y climtologicl monthly men surfce fluxes (momentum, het nd freshwter; Experiment 2; Tle 1; Unforced run ). We then seprte the SSH vrince of the Forced Run nd Unforced Run into sesonl (defined s ll frequencies P sesonl) nd internnul (ll frequencies lower thn sesonl) (Fig. 12,, d nd e). First inspection of Fig. 12 c shows tht oth runs exhiit similr sesonl vrinces with higher vlues long the cost where the sesonl winds drive the costl current. A mp of the difference in sesonl vrinces (Fig. 12c) shows tht the

15 280 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) c d e f g Fig. 10. Comprison of the costl desesonlized SSH for Snd point, Kodik, Sewrd, Ykutt, Sitk, nd Neh By etween the model output (green line), AVISO/TOPEX (lck line) nd GLOSS fst delivery dt (red). Unforced Run hs higher vrince t the genertion sites of Hïd nd Sitk eddies. This is expected ecuse the lrge internnul eddy events in these regions re phse locked with the sesonl cycle (see previous discussion of Fig. 7), therefore some of the sesonl vrince is shifted to the lower frequencies in the Forced Run. If we now compre the mps for the internnul vrince we find tht the Unforced Run (Fig. 12e) hs strong vrince in regions of high eddy ctivity in oth the western nd estern sin. Since this model run hs no forcing on periodicities higher thn sesonl, regions of high vrince must e ssocited with internl ocen (intrinsic) vriility. To ssess the degree of intrinsic vrince we tke the difference in vrince etween the Forced nd Unforced (Fig. 12f). Three importnt fetures cn e isolted. First we oserve tht in the western sin (Fig. 12f) there is no significnt difference in internnul vrince etween the Forced nd Unforced run. This confirms tht most of the vrince in the region of high eddy ctivity long the Alskn Strem is intrinsic in nture. A second importnt feture in the difference mp is in the shelf region long the western sin costline. Here the Unforced Run hs much less vrince thn the Forced Run. This confirms tht internnul chnges in shelf circultion re mostly forced y the winds nd it explins why the model Forced Run ensemles re so fvorly correlted with in situ tide guges (s shown efore in the discussion of Fig. 10). The third feture is in the estern sin, where the Forced Run hs higher vrince long the estern oundry. This result grees with the findings of the previous section showing tht internnul chnges in the winds force lrge nticyclones eddy such s Hïd nd Sitk. To further isolte the intrinsic nd forced vriility we compre timeseries from ll Forced model ensemles in four selected regions of eddy ctivity, two in the western sin nd two in the estern sin (Fig. 13, c,

16 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) Jn Fe Mr Apr My Jun Jul Aug Sep Oct Nov Dec Fig. 11. Sesonl vriility of the Edy index, units in 10 6 s 1. e nd g). These timeseries show tht in the estern sin (Fig. 13 nd c) the spred of the ensemle memers is very low with shred vrince ove 42%. This confirms gin the existence of forced regime. In contrst in the western sin the ensemles memers shre little vrince (Fig. 13e nd g) nd re mostly uncorrelted with ech other. Some low frequency vriility fter 1990 is shred y ll the ensemle memers in the western sin, with n indiction of negtive trend, however the mount of explined vrince y this component is smll. Interestingly comprison with stellite dt (not shown) revels similr negtive trend over the lst 10 yers, however we did not pursue this spect further s the temporl dt coverge is short. Although eddy vriility in the western sin is intrinsic, one my expect tht chnges in vrince from yer to yer cn e relted to chnges in the men ocen conditions or forcing function. To explore this concept we compre the 4-yer running stndrd devition for ll ensemles (Fig. 13, right column). On the estern oundry inspection of the timeseries show strong degree of correltion mong ll ensemles implying tht internnul chnges in eddy vrince re lso forced. In contrst in the western sin the spred mong the ensemle memers is high, lthough there re periods in the record when the ensemles re closer to ech other. This reinforces the ide tht chnges in the forcing hve wek impct in modulting the eddy vrince, long the Alskn Strem, on internnul timescles. We lso investigte if ENSO teleconnected tmospheric forcing my ply ny role in chnging the conditions for roclinic instilities. We quntify the ENSO effect y correlting the instility growth rte (the

17 282 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) d e c f Fig. 12. Model SSH vrinces (m 2 ) for the run forced y monthly NCEP wind stress (first row) nd for the run forced y climtology monthly wind stress (second row). The difference etween the two runs is shown in the 3rd row. The first (second) column shows the sesonl (internnul) vrince. Edy Index) with the NINO34 index (Fig. 14). The sptil pttern of the correltion shows tht long the estern oundry nd northern Gulf of Alsk, n increse in the instility growth rte is positively correlted with positive ENSO phses. In contrst long the Alskn strem in the region of high SSH eddy vrince, nd in the gyre interior, we find negtive correltion indicting tht during El Niño the growth rte is reduced. However the mount of negtive correltion is insignificnt, which grees with the hypothesis of the intrinsic nture of eddy vriility in the western sin. 5. Summry nd conclusions The djustments of the Gulf of Alsk circultion to chnges in lrge scle climte forcing hs een investigted using three high resolution ensemles of regionl ocen model forced with the NCEP/ NCAR winds nd NOAA SST corrected het fluxes. Consistent with previous descriptions of the circultion we find tht the men currents re minly geostrophic (Fig. 2) nd mintined y Ekmn downwelling

18 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) c d e f g h Fig. 13. For the three ensemles (1, 2, 3), (), (c), (e) nd (g) show the SSH respectively in the loction 1, 1 0, 2 nd 2 0 represented on Fig. 1c. (), (d), (f) nd (h) show the 4 yer running stndrd devition ssocited to (), (c), (e) nd (g). (corr = correltion coefficient). conditions in the estern sin nd upwelling long the Alskn Strem (Fig. 2 nd c). On internnul nd decdl timescle the men circultion is strongly modulted y chnges in the lrge scle climte forcing ssocited with PDO nd ENSO vriility (Fig. 3). The leding mode of sptil nd temporl vriility of the men currents in the three model ensemles is significntly correlted with the PDO (0.64) nd shows strong peks during El Niño (e.g ). Intensifiction of the gyre scle circultion is found fter the climte shift, s well s during 19 19, nd more grdully during A

19 284 V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) Fig. 14. Mp of the correltion etween Edy index nd ENSO index detiled nlysis of the model momentum nd trcer udget revels tht when the GOA experiences strong nomlous southesterly winds (positive phse of the PDO fter the 1976 shift nd the 1997 ENSO) there is net Ekmn convergence of surfce mss (downwelling conditions) in the centrl nd estern costl oundry (Fig. 4). This genertes higher SSH (Fig. 4) nd lower isopycnls long the cost. The resulting geostrophiclly djusted currents (Fig. 4) intensify the gyre scle circultion. The opposite sitution occurs during strong nomlous northwesterly winds (negtive phse of the PDO). Internnul chnges in the surfce winds re lso reflected in modultions of the sesonl development of high mplitude nticyclonic eddies long the estern sin, in prticulr t the loction of the Hïd nd Sitk eddies. The genertion of the lrge internnul eddy events during winter-spring, is phse-locked with the sesonl cycle nd is consistent with qusi-liner Rossy wve response to stronger winter southwesterly winds typicl of El Niño (Figs. 7 nd 8). The high correltion etween the mplitude of the estern sin eddy nd the perturtion energy of the downwelling winds is explined y the comined effect of geogrphicl focusing nd instility of Rossy wves t higher ltitudes (LCsce nd Pedlosky, 2004). Specificlly during downwelling nomly conditions, the erly response of the costl ocen is liner with the onset of stronger polewrd currents ssocited with the high costl SSH, nd with the initil rdition of Rossy nd topogrphiclly trpped wves (Fig. 7). However ecuse of the fst growth rte of the instility (Edy index mps Fig. 11) nd the geogrphicl focusing ssocited with the costl geometry, most of the perturtion energy in the Rossy wve response is loclly trpped until converted into the eddy field. In the western sin, the high core of internnul eddy vriility south of the Alskn Strem (Fig. 12d) is not correlted with lrge scle forcing nd ppers to e intrinsic. Indeed comprison of the forced nd internl internnul vriility from the three forced ensemles nd multi-century-long integrtion forced only with the sesonl cycle of winds, het nd freshwter, shows tht the intrinsic vrince lone explins most of the eddy vriility (Fig. 12f). The symmetry etween the eddy forced regime in the estern sin nd the intrinsic regime in the western sin is lso confirmed y timeseries of the ensemle memers SSH. These show tht long the estern oundry the spred is very low with shred vrince ove 42% (forced regime). In contrst in the west, the ensemles memers shre little vrince (Fig. 13e nd g) nd re mostly uncorrelted (intrinsic regime), lthough some low frequency co-vriility is evident fter We lso explored the possiility tht ENSO tmospheric teleconnections my impct the degree of roclinicity in the western sin. However sptil correltion nlysis etween the Edy index, which is mesure of the instility growth rte, nd NINO34 index shows high correltions only long the estern oundry nd northern Gulf of Alsk ut not in the centers of high eddy ctivity in the western sin. Although the eddy vriility in the western sin is intrinsic, internnul chnges in the shelf circultion re forced y the winds (Fig. 12f). A comprison of SSH etween tidl guges nd the three model ensemles forced with the NCEP winds, revels tht the model is le to explin etween 40% nd 70% of tidl guges vrince. This finding suggests tht most of the shelf vriility on internnul scle does not strongly depend on equtorilly trpped wves, ut rther on downwelling (upwelling) conditions ssocited with the lrge scle wind stress ptterns. This study provides dynmicl frmework to interpret nd predict how the GOA circultion hs nd will respond to lrge scle climte forcing. The model nlyses suggest tht the djustment to lrge scle climte

20 forcing of the costl nd gyre-scle men circultions is predictle. On decdl nd internnul timescle the most importnt spect of the forcing is the overll strength of downwelling conditions long the cost, lthough the role of freshwter forcing hs not een fully investigted. River runoff long the GOA is known to contriute to the cross-shore uoyncy grdients, which in turn strengthen the shelf circultion. Therefore decdl vrition in river runoff my lso led to dditionl vriility in the currents nd their stility properties. This study lso supports the ide of n est-west symmetry in the response of the mesoscle eddy circultions. In prticulr eddy ctivity in the estern sin is strongly correlted with chnges in downwelling fvorle winds, while in the western sin the vriility is minly intrinsic. The link etween stronger eddy in the estern sin nd downwelling winds hs importnt climte implictions. It llows predicting chnges in the cross-shore trnsport of importnt iogeochemicl quntities such s iron, oxygen nd chlorophyll-, which in glol climte chnge scenrio my contriute to feedcks in the HNLC region of the GOA. Within the dynmicl nd modeling frmework presented in this study, further nlyses with higher resolution nested models will etter resolve the mixing sttistics ssocited with the eddy field in estern nd western sin. Model representtions of the GOA circultion re in fct very sensitive to the costl nd topogrphic geometry. As such the model results presented so fr should e interpreted more qulittively rther thn quntittively. Acknowledgements The reserch is prt of the Ph.D. disserttion of Vincent Comes nd ws supported y the Ntionl Science Foundtion through the grnts OCE-NSF02654, GLOBEC-NSF06575 nd OCE-NSF This pper ws presented t the PICES/DFO Symposium on Time Series of the N.E. Pcific: A Symposium to mrk the th nniversry of Line P in Victori, BC, Cnd, July References V. Comes, E. Di Lorenzo / Progress in Ocenogrphy 75 (2007) Brickley, P.J., Thoms, A.C., Stellite-mesured sesonl nd inter-nnul chlorophyll vriility in the Northest Pcific nd Costl Gulf of Alsk. Deep-Se Reserch Prt II Topicl Studies in Ocenogrphy 51, Cpotondi, A., Alexnder, M.A., Deser, C., Miller, A.J., Low-frequency pycnocline vriility in the northest Pcific. Journl of Physicl Ocenogrphy 35, Chelton, D.B., Schlx, M.G., Glol oservtions of ocenic Rossy wves. Science 272, Crwford, W.R., Physicl chrcteristics of Hid Eddies. Journl of Ocenogrphy 58, Crwford, W.R., Het nd fresh wter trnsport y eddies into the Gulf of Alsk. Deep-Se Reserch Prt II Topicl Studies in Ocenogrphy 52, Crwford, W.R., Cherniwsky, J.Y., Foremn, M.G.G., Multi-yer menders nd eddies in the Alskn Strem s oserved y TOPEX/Poseidon ltimeter. Geophysicl Reserch Letters 27, Crwford, W.R., Brickley, P.J., Thoms, A.C., Mesoscle eddies dominte surfce phytoplnkton in northern Gulf of Alsk. Progress in Ocenogrphy 75, Cummins, P.F., Lgerloef, G.S.E., Wind-driven internnul vriility over the northest Pcific Ocen. Deep-Se Reserch Prt I Ocenogrphic Reserch Ppers 51, Curchitser, E.N., Hidvogel, D.B., Hermnn, A.J., Doins, E.L., Powell, T.M., Kpln, A., Multi-scle modeling of the North Pcific Ocen: ssessment nd nlysis of simulted sin-scle vriility ( ). Journl of Geophysicl Reserch- Ocens, 110. Di Lorenzo, E., Foremn, M.G.G., Crwford, W.R., Modelling the genertion of Hid Eddies. Deep-Se Reserch Prt II Topicl Studies in Ocenogrphy 52, Di Lorenzo, E., Miller, A.J., Schneider, N., McWillims, J.C., The wrming of the Cliforni current system: dynmics nd ecosystem implictions. Journl of Physicl Ocenogrphy 35, Gower, J.F.R., Tt, S., Mesurement of eddy motion in the north est Pcific using the Geost ltimeter. In: Jones, I.S.F., Sugimori, Y., Stewrt, R.W. (Eds.), Stellite Remote Sensing of the Ocenic Environment. Seiutsu Kenkyush, Tokyo, pp Hidvogel, D.B., Arngo, H.G., Hedstrom, K., Beckmnn, A., Mlnotte-Rizzoli, P., Shchepetkin, A.F., Model evlution experiments in the North Atlntic Bsin: simultions in nonliner terrin-following coordintes. Dynmics of Atmospheres nd Ocens 32, Hoskins, B.J., Vldes, P.J., On the existence of storm-trcks. Journl of the Atmospheric Sciences 47, Johnson, W.K., Miller, L.A., Sutherlnd, N.E., Wong, C.S., Iron trnsport y mesoscle Hid eddies in the Gulf of Alsk. Deep- Se Reserch Prt II Topicl Studies In Ocenogrphy 52, Klny, E., Knmitsu, M., Kistler, R., Collins, W., Deven, D., Gndin, L., Iredell, M., Sh, S., White, G., Woollen, J., Zhu, Y., Chellih, M., Eisuzki, W., Higgins, W., Jnowik, J., Mo, K.C., Ropelewski, C., Wng, J., Leetm, A., Reynolds, R., Jenne, R., Joseph, D., The NCEP/NCAR 40-yer renlysis project. Bulletin of the Americn Meteorologicl Society 77,