Variability of Wind Sea and Swell Waves in the North Atlantic Based on ERA-40 Re-analysis

Transcription

1 Variability of Wind Sea and Sell Wave in the North Atlantic Baed on ERA-4 Re-analyi Alvaro Semedo, Kay Sušelj 3 Anna Rutgeron Department of Earth Science Meteorology Uppala Univerity Uppala Seden alvaro.emedo@met.uu.e Riø - DTU National Laboratory for Sutainable Energy Technical Univerity of Denmark Rokilde - Denmark 3 ForWind, Center for Wind Energy Reearch Carl von Oietzky Univerity Oldenburg - Germany Abtract Thi tudy preent a qualitative aement of the ave field in the North Atlantic (NA) bain, baed on the ERA-4 ave re-analyi from the European Centre for Medium-Range Weather Forecat (ECMWF). The ind ea and ell ignificant ave height and mean ave length, eaonal and geographic ditribution, are tudied. The geographic ditribution and dominance of ind ea and ell ave over the bain, in the inter and in the ummer, are alo tudied. It i hon that the ave field in the NA bain i motly ell dominated, epecially in the ummer. The eaonality of the NA ave field i compared ith the North Pacific. The qualitative characteritic of the ave field in both bain i hon to be different. The center of action of the ind ea and ell ignificant ave height are identified uing empirical orthogonal function (EOF) analyi. Keyord: Wave climate, ignificant ave height, ind ea, ell, North Atlantic. Nomenclature H = ignificant ave height Proceeding of the 8th European Wave and Tidal Energy Conference, Uppala, Seden, 9 H = ignificant ave height of ind ea H = ignificant ave height of ell T m = Mean ave period T m = Mean ave period of ind ea T m = Mean ave period of ell L m = Mean ave length L m = Mean ave length of ind ea L m = Mean ave length of ell θ m = mean ave direction θ m = mean ave direction of ind ave θ m = mean ave direction of ell C p = peak phae peed T p f = peak period = frequency θ = ave direction m = zeroth moment ĉ = ave phae peed U = ind peed at meter φ = ind direction u * = friction velocity 9

2 INTRODUCTION Ocean urface ave are one of the mot obviou and fundamental phenomena preent at the air-ea interface. In term of energy the ocean ave pectrum i dominated by urface gravity ave [], accounting for more than half of the energy carried by all ave in the ocean urface, and urpaing the contribution of tide, tunami, coatal urge, etc. []. There are to type of ave in the ocean urface. During the generation and groing procee, they are deignated a ind ea. A ave propagate aay from their generation area, or hen their phae peed i higher than the ind peed, they are called ell. Sell ave are knon to travel long ditance acro the globe, or even acro ocean bain [3], ith very little attenuation [4-6]. Wind ea ave are generated locally, and receive momentum from the ind in their groing proce, therefore they are trongly coupled to the local ind field. On the other hand ell ave are generated remotely and no longer receive momentum from the local ind in order to gro, and are not directly coupled to the local ind field. For thi reaon ave do not necearily reflect the local ind field characteritic. Since ell propagate from one place to the other, the local ave field i affected by both local and remote ind forcing. In the open ocean the ave field at a given time and place i, therefore, the reult of contribution from ave ith different frequencie and direction, reflecting different origin. Higher frequency ave (ind ea) are generated locally, and loer frequency ave (ell) are generated remotely. A a rule of thumb, the longer the frequency, the further ell ere generated []. The motivation and concern behind previou tudie of ind ea and ell ave have been different. The generation and groth of ave ha been the main focu of the ave modeling community, in vie of the development of ave model, mainly for forecating purpoe. Since ell ave carry mot of the ave energy in the ocean urface, o far the main concern ha been it impact on offhore and coatal infra-tructure. Recently there ha been a reneed interet in the tudy of ell, ranging from ell propagation and attenuation [7], to the ell impact in the marine atmopheric boundary layer [8-]. Having in mind the different nature and impact of ind ea and ell ave, [] and [] complemented exiting ave climatologie (e.g. [3] and [4]), by eparating the global ave climate in the to ave regime. The tudy from [] ued a year long data et, obtained from a combination of remote ening and model hindcat data, and [] ued a 4 year long time erie of ave obervation from voluntary oberving hip (VOS). Thee tudie, although complementary, ued different ind ea-ell claification cheme. The method ued for the retrieval of the mot common ave parameter, the ignificant ave height H ere alo different and baed on empirical aumption. Additionally, one of the mot important quetion concerning the reliability of the VOS ave data i, in fact, the uncertainty of the eparation beteen ind ea and ell, hich i dependent on the human oberver ubjective judgment and experience the ailor eye. In the open ocean H varie gradually over the ea urface and throughout the eaon. The main quetion i ho the variation of H and H are related to the variation of H. The bet ay to decribe the variability of H and H i through ave pectra, hich are only available globally, and ith a long enough time erie, from ave model reult, like the European Centre for Medium-Range Weather Forecat (ECMWF) ERA-4 ave re-analyi [5]. In thi tudy e preent a qualitative tudy of the ave field in the north North Atlantic (NA) bain. The inter and ummer climate of H and H, and θ m and θ m are tudied, baed on ERA-4. Taking advantage of the flexibility of the ERA-4 data et, the contribution of the to ave regime to the eaonal and patial ditribution of H i preented. The patial and eaonal pattern of ind ea and ell predominance are alo tudied. Empirical orthogonal function (EOF) analyi i ued to obtain the main variability mode of the H and H in the NA, in the inter and in the ummer. The remainder of thi paper i organized a follo. Section detail the data and method of analyi. Section 3 preent the ind ea and ell ignificant ave height inter and ummer climate and correpondent variability. The paper i cloed ith ome brief concluding remark in ection 4. DATA AND METHODS OF ANALYSIS. ERA-4 Although rather cloe to reality, the analyi produced in operational forecat center uffer from inhomogeneitie. Weather (and ave) forecat model and the analyi procedure are continually improved. Therefore the variability in the analye archived data i dominated by model and data aimilation change, rather than by natural variability, making them unuitable for climate or variability tudie. The aim of the re-analyi i to overcome the inhomogeneity problem related to model change. The bet available model and data aimilation cheme, that do not change in time, are ued to repeat the analyi procedure for the pat, uing all the available obervation. A reanalyi yield, therefore, a complete global gridded data et, that i a temporally homogeneou a poible. Unfortunately, inhomogeneitie due to uneven data coverage and change in obervation ytem till remain [6]. The ERA-4 data et i a re-analyi of meteorological obervation from September 957 to Augut (45 year), produced by the ECMWF. The data et conit of 6-hourly field (at ynoptic time) ith a.5ºx.5º grid reolution, covering the hole globe. Beide global atmopheric variable, it alo

3 include ave parameter. The re-analyi a produced uing the ECMWF Integrated Forecating Sytem (IFS) a to-ay coupled atmophere-ave model ytem. The ERA-4 i the firt global reanalyi produced uing a ave model coupled to a general circulation model. The ave model ued in the coupled ytem i the tate-of-the-art third generation WAM model, developed by the Wave Modeling group [7]. Detail about the WAM model at ECMWF (the EC-WAM) can be found in [8] and [9]. Additional detail about the ERA-4 ave reanalyi are given by [4].. Wave parameter The WAM model output i the to-dimenional ave energy pectrum F( f,θ ), obtained at each grid point by integrating the o called ave energy balance equation []. In it continuou form, F( f,θ ) decribe ho the mean ea-urface elevation variance, due to the preence of ave, i ditributed a a function of f and θ. From thee pectra everal derived integrated ave parameter can be obtained. Beide thee parameter U and φ are alo ued in thi tudy. The mean variance of the ea-urface elevation (the zeroth moment) i tatitically related to H, and i defined a H = 4 m, here m f F( f,θ )dfdθ =. The T m ued in the preent tudy i the mean ave period from the firt moment, baed on the firt order momentum of the ea urface elevation ( m = f F( f,θ )dfdθ ) and i defined a Tm m / m =. The mean ave length are computed fromt m, here, from linear theory, Lm =.56Tm. By eighting F( f,θ ), θ m i defined in the WAM model a θm = atan(sf /CF ), here the eight are defined a SF = in(θ )F( f,θ )dfdθ an CF = co(θ )F( f,θ )dfdθ. The ignificant ave height, mean period, and mean ave direction of the ind ea and ell ave are computed by eparating the one-dimenional (D) pectrum into ind ea and ell component. The eparation frequency i defined a the frequency correponding to the ave phae peed ĉ here. 8 ( u * / cˆ ) co( θ φ) =. The ind ea and the ell integrated parameter (in the preent cae m and m ) are computed by integrating over the repective D pectral part. The peak ave phae peed, a derived from linear theory, i cp = gt p / π..3 Method of analyi The gridded ave ( H, H, H θ m, ( U m, T m, T m, T m, θ m, θ, and T p ) and atmopheric parameter, φ ) are proceed to yield mean eaonal value, for inter and ummer. Winter and ummer are conidered here a December, January and February (DJF), and June, July, and Augut (JJA), repectively. The zonal averaging of H, H, and H i done from the equator to 7 N. The Black ea, the Mediterranean Sea, and the Baltic Sea, are not taken into account in thi averaging to avoid poible biae due to their particular ave regime. The North Sea and the gulf of Mexico are neverthele included, ince they have a more open communication ith the open ocean ave field. The ave age criterion C p /U i ued to ae the ind ea (or ell) dominance in the ave field. According to thi criterion the ave field i conidered ind ea dominated if C p /U <. and ell dominated if C p /U >. [] and []. The ave age C p /U =. eparation criterion roughly coincide ith the friction velocity dependent criterion C p /u * 33.6, ued by the WAM model. We choe not to include the eighting /co(θm φ) a in the WAM model, ince in certain area, hen the angle θ m φ i high (cloe to 9 ), thi eighting factor produce unrealitically high ave age value. Since taking the time average of C p /U geographic ditribution ill mooth the peak, and ince the goal here i to ditinguihing hich area of the NA are dominated either by loer or high value of C p /U, a probabilitic (frequency of occurrence) approach i ued. The probability of a ind ea dominated ave field i defined a P( C p / U <. ), and the invere, i.e., the probability of a ell dominated ave field, i defined a P( C p / U <. ).The probabilitie are computed for each grid point and for each 6-hour et. 3 WIND SEA AND SWELL IN THE NORTH ATLANTIC The combination of torm frequency, duration, and intenity, and the geographic characteritic of the ocean bain, determine the propertie of it ave field and eaonal variation. The relatively narro geometry of the Atlantic Ocean retrict the propagation of Southern Ocean ell into the Northern Hemiphere. Therefore ave in the NA are highly eaonal, and peak in the inter. The primary ave generation area in the NA i along the prevailing extratropical torm track [3], from the Eat coat of North America toard the Noregian Sea. Thi can be een in figure, along the highet value of U. In thi figure the inter and ummer yearly averaged U patial ditribution over the NA i hon. The maximum mean inter value i m/, and occur South of Greenland, at ~48 N. The maximum mean ummer value i ubtantially loer (~8.5 m/), and i located South-South-Wet of Iceland at ~55 N. In the hore latitude, north of the 3

4 Figure : Winter (top) and Summer (bottom) mean U in m and φ (arro) in the NA. trade ind (henceforth deignated jut a trade), U value are loer, particularly in the ummer. In thi eaon the loet value of U are in the hore latitude area and cloe to the equator. Acro the trade the ind peed i conitently in exce of 7 m/ during both eaon. The patial yearly averaged ditribution of H in the inter and in the ummer are hon in figure. The ditribution of the inter and ummer H follo, to a certain degree, the pattern of U hon in figure. Neverthele the H maximum mean inter value (4.5 m) occur Eat on the U maximum, South of Iceland at ~55 N. Thi ituation occur mot likely becaue ell generated in the indiet pot propagated toard Iceland. The um of thi ell and locally generated ind ea ave gave rie to the H inter maximum. The maximum mean ummer value of H (le than m) i ditributed over a large area beteen Ireland, Iceland and Greenland. The difference beteen the mean ummer and inter maxima (more than m) reflect the high eaonality of the ignificant ave height over the NA bain. It i of no urprie that yearly averaged patial ditri- Figure : Winter (top) and ummer (bottom) mean H in m and θ m (arro) in the NA. bution of H and θ m, both in inter and ummer, are highly coincident ith U and φ for the ame period, a can be een in figure 3. The reader i advie that the colorbar cale in figure 3 i different than the one in figure. The maximum mean inter value of H (in exce of.5 m) i ditributed along a large patch, in the centre of the extratropical NA. Thi area i located from ~45 N to South Greenland and Iceland, and to the Wet coat of Ireland and Scotland.Thi patch i coincident ith the mean inter maxima area of U, and ith the predominant extratropical torm track in the NA, here ave are predominantly generated. Cloe to the Wet coat of Ireland and Scotland the value of H are till relatively high, hich i related to the high value of U in that area during the inter. In the South-Wet Europe, off the Iberian Peninula coat, the value of H are loer. A e move South, the value of H gradually decay, folloing the U pattern. The lo value of U acro the hore latitude are related to the alo lo value of H, in both eaon. Coinciding 4

5 ith the trade, in the tropical NA, there i a ell defined patch of.75 m high H, alo in both eaon. In the ummer, hen the extratropical NA i predominantly under the influence of the Azore high, the ind peed drop coniderably, compared ith the inter eaon. The value of H alo decreae in that area. In the inter the area covered by the maximum mean inter value of H i more idely ditributed, in the central NA bain, at ~4 N, and propagating South, parallel to the Iberian Peninula and the North African Wet coat. Alo parallel to the coat of Africa, but more South, a patch of.75 m H can be een. Thee ell ere originated in the extratropical South Atlantic (SA), and propagated North. At thi tage the quetion that arie i ho predominant i one ave regime over the other, i.e., hich ave regime, ind ea or ell, dominate over Figure 3: Winter (top) and ummer (bottom) mean H in m and θ m (arro) in the NA. H compared to. A can be een in figure 4, from 35 N up to Greenland and Iceland, the value of H fall ithin the m range. The Wet coat of Europe, from the Iberian Peninula to Noray, have higher value of H compared to the Eat coat of North America. Thi i related to the predominant propagating direction of ell toard the Eat, aay from the torm track ave generation area. A ith H, in the ummer the patial ditribution of H i alo more uniform than in the inter. The area of maximum mean ummer value of H are till quite praled, ith a branch of.5 m high ell evolving Figure 4: Winter (top) and ummer (bottom) mean H in m and θ m (arro) in the NA. the other and ho thi dominance i ditributed. A can be een in figure 5 and 6 the NA bain i motly ell dominated. The reader i advied that the colorbar cale in different in thee to figure. The degree of thi dominance i, neverthele, eaonal, and ha alo a clearly defined patial variance. In the inter, over the predominant extratropical torm track, the ind ea dominance i higher than in the ummer. In the coatal area of Eat North America, and in the fringe of the ocean ice area off Eat and Wet coat of Greenland, thi dominance i cloe to 5%, reflecting the high ind peed in thoe area. In the Southern part of the bain, even acro the trade ind area, the ind ea dominance i very lo, tending to almot zero toard the equator, and reflecting the light ind regime there. 3 5

6 .5 75 o N o N o N o N o N.5 3 o N o W 6 o W 4 o W o W o o E.5 8 o W 6 o W 4 o W o W o o E o N o N o N o N o N.5 3 o N o W 6 o W 4 o W o W o o E Figure 5: Winter (top) and ummer (bottom) probability of having a ind ea dominated ave field. In the Summer the probability of having a ind ea dominated ave field i loer over the torm track area. No ignificant change occur in the Southern part of the bain. Even in the inter, the probability of ell dominance i almot alay higher than 7% over the all bain, a can be een in figure 6. It i loer along the extratropical torm track, but till higher than the probability of ind ea dominance for the ame period. The probability of ell dominance increae to value higher than 9% South of thi area. In the ummer the ell dominance increae to even higher value, ince U decreae almot over the all bain. Alo in the ummer, off the coat of the Iberian Peninula and the Wet coat of Morocco, the ell dominance i not a high a in other area. Thi i due to the meocale coatal ind feature mentioned before. The tropical torm activity off the Gulf of Mexico alo produce a le dominated ell ave field there. There i a harp tranition, producing almot a ea a feature in the NA, beteen the probability of ell dominance over the extratropical torm track and the Southern part of the bain. The yearly averaged patial ditribution of the inter and ummer L m and L m are hon in figure 7 and 8. The reader i advie that the colorbar cale in figure 3 i different than the one in figure. Thi parameter i of great important, for ave energy purpoe, ince ome ave energy poer device are 8 o W 6 o W 4 o W o W o o E Figure 6: Winter (top) and ummer (bottom) probability of having a ell dominated ave field. tuned to be in reonance ith the ave length. Therefore the climatological knoledge of L m and L m i crucial, ince ell ave carry mot of the energy in the ave pectra, a een from the difference beteen H and.75 H. The longet inter ind ea ave (in exce of 5 m) are located along the ave generation area, coinciding ith the NA extratropical torm track. Thi area extend from the et coat of the North America continent, all the ay to the Noregian ea. Although eaker in the inter, in the trade ind latitude L m ha a regional maxima there (~3 m). In the ummer, the NA ind ea mean ave length exhibit a quai-ymmetric tripole, ith longer ind ea ave in the extratropical and ubtropical area. Folloing the eatard ave propagation track, the longet inter ell cover a ider area than the ind ea. The longer ell are located et of Ireland (~7 m). The patch of longer ell (in exce of 5 m) cover almot the all multitudinal NA, extending from North Africa to the North Sea. In the Summer, the pattern of L m i more complex reflecting the combination of NA and SA ell, generated in the outhern extratropical latitude, a can be een by θ m. Thee long ell ave are located in the Eater part of the NA bain, along the tropical et coat of Africa and the et coat of Europe. 4 6

7 Figure 7: Winter (top) and ummer (bottom) mean L m in m and θ m (arro) in the NA. A mentioned in the earlier in thi ection, and hon in figure -6, H, H and H in the NA have a very high eaonality. Thi eaonality ha alo a trong North-South geographic ignal. In figure 9 a North-South cro ection of H, H and H i hon. Thee meridional profile (henceforth deignated jut a profile) are the reult of a zonal averaging of the H, H and H H field in the inter and ummer. A imilar averaging a done for the North Pacific (NP). The NP profile are hon in the ame figure for comparion purpoe ith the NA profile. A expected, the eaonality of the ave field characteritic in the NA i alo reflected in the ignificant ave height profile. The H i higher in the inter than in the ummer over the all NA. It peak in the inter, along the extratropical latitude (~5 N), reflecting the high ind peed regime acro the torm track, hich occur around that latitude. In the ummer the ituation i rather different. A a conequence of a eaker extratropical torm activity in the NA H i almot even from North to South, ith mall peak in the extratropical and trade latitude. When looking at the H and H profile, their eaonal and North- South variability ha particular characteritic that Figure 8: Winter (top) and ummer (bottom) mean L m in m and θ m (arro) in the NA. allo a more qualitative vie of the ave field over the NA. In the inter the value of H and H have peak around the ame latitude for the ame reaon a H /H /H [m] H /H /H [m] Latitude [ o ] Latitude [ o ] Figure 9: H (black), H /H /H [m] H /H /H [m] Latitude [ o ] Latitude [ o ] H (blue) and H (red) meridional variation. North Atlantic (right plot, top inter, and bottom ummer) and North Pacific (left plot, top inter, and bottom ummer). H. Around ~3 N (the hore latitude, roughly north of the trade) here the U i eak almot all year 5 7

8 round, H ha rather lo value. Therefore H i the main contributor for the H field there. A e move South toard the equator, the H profile ho a light increae, coinciding ith the trade ind area. Thi increae i not preent in the inter H profile, hich decreae almot conitently from the extratropical latitude to the equator. South of the trade, H decreae, ith a minimum value at the equator, here the relative eight of H in the overall H i highet. In the ummer, in pite of the relative meridional contancy of H, the behavior of the H and H ho intereting feature. The ind ea and ell alo peak in the extratropical latitude, folloing the (mall) H increae there. At the trade latitude H increae, and then decreae a e move South. The minimum value of H can be een around 5 N, and South of thi latitude H increae lightly. Thi minimum roughly coincide ith the JJA intertropical convergence zone (ITCZ). In the ummer the H profile alo ho a decreae from the extratropical latitude to the hore latitude, but no thi decreae i le pronounced than in the inter. A in the inter, in the hore latitude the H i alo dominant. A e move South, the H value increae, herea in the inter the profile hoed an oppoite behavior. Thi increae in H i due to the previouly mentioned intake of ell that a generated in the Southern extratropical latitude, and propagated North. A imilar analyi of the NP ignificant ave height profile (alo in figure 7) ho the difference beteen the ave field in that bain, compared to the NA. In the inter the H peak value i relatively imilar in both bain. The H profile ha a higher peak in the NA extratropical latitude, reflecting the higher ind there, compared to the NP. But the H profile in the NA ha a loer peak value than in the NP. Thi light difference, hich allo for H to be almot equal in both bain, i the reult of the Northbound Southern ocean ell. In DJA (ummer in the Southern hemiphere), ind are till rather trong in the Southern extratropical latitude, and capable of generating ave that propagate a ell toard the the Northern Hemiphere. Thee ell do not ignificantly penetrate in the NA bain, but do o in the NP bain, traveling all the ay to the NA extratropical latitude and contributing to the ave climate there. In the ummer, a in the inter, in the NP bain i le indy than the NA [4]. Therefore the NP H profile ho that thi ave parameter i alay loer than in the NA, ith loer peak in both the extratropical and trade ind latitude. But, due to it geography, the NP i more ell dominated all year round, and therefore the JJA (inter) Southern ocean ell that traveled north allo for higher value in H at the NP extratropical latitude and South from there. Thi fact contribute for a lightly higher peak in the H profile in the NP, compared to the NA bain, at the extratropical latitude, but alo in the more South latitude. Aiming to explain the different patial pattern of the ind ea and ell ignificant ave height, an EOF analyi i ued (ee e.g. [3] for detail). The principal mode of the 6 o N 3 o N 6 o N 3 o N H and H monthly mean, in the 75 o W 5 o W 5 o W o 75 o W 5 o W 5 o W o Figure : Firt EOF of inter H and H. Spatial ditribution and PC time erie

9 inter and in the ummer, are invetigated. Figure and ho the pattern of the firt EOF (EOF) of H and H. To facilitate the inter-eaon comparion, the EOF of H and H are organized by eaon in figure 8 (inter) and figure 9 (ummer). The reader i advied that the colorbar cale in different in thee to figure. The EOF of the inter H and H (figure 8), explain.7% and 6.% of the patially averaged variance, repectively. The EOF of H ha a very ell defined reemblance to the North Atlantic Ocillation (NAO) tructure acro the central midlatitudinal NA [4], and a tatitically ignificant poitive trend in it coefficient time erie (PC). Thi poitive trend can be explained by the peritent poitive NAO index trend from the 96 to the early 99, indicating an increae in the Weterlie trength, ith a direct connection to the ind ea component of the ave field. The maximum of the explained poitive variability of H i located along the predominant extratropical torm track, explaining the predominant ave generation area in the NA. A patch of negative variability i located more South, beteen 3-45 N. A light poitive variability i located along the trade, outh of the hore latitude. The EOF of H and H in the inter exhibit a noticeable difference. The maxima of the explained variance of ell ignificant ave height i centered more South, extending all the ay to the Iberian Peninula. Thi pattern clearly repreent ell propagating from the extratropical torm track toard South, and South-South- Eat. The H PC time erie ha a poitive trend alo. Thi trend i related ith the NAO index behavior beteen 96 and 99. The EOF of the ummer H and H (figure 9), explain.8% and 6.5% of the patially averaged variance, repectively. The maximum of the explained poitive variability of the ummer H i located beteen 3-4 N, in the center of the bain. A patch of negative variability i located more South, in an area coinciding ith the hore latitude. Thi negative variability a originated in the et coat of North African and extend Wet into the center of the NA bain. The highet poitive variability i located along the trade, outh of the hore latitude, and North of the equator, here the variability i alo cloe to zero. The ummer H PC time erie ha a light poitive trend until 976, and a negative trend from then after. The maximum poitive variability pattern of H, covering an area from 3 N, extending to Greenland and Iceland. It eam like ell ave generated in the center of the bain, along the H centre of action, have propagated motly North. The variability South of 3 N i till poitive but loer. The ummer H PC 6 o N 3 o N 6 o N 3 o N 75 o W 5 o W 5 o W o 75 o W 5 o W 5 o W o Figure : Firt EOF of ummer H and H. Spatial ditribution and PC time erie. time erie ha a harp peak in 99 and 993. Thi peak i related to an identified error in the ave height data aimilation in the re-analyi proce [4]. The effect of thi error on the variability of H i not invetigated in the preent tudy. 4 CONCLUDING REMARKS A detailed qualitative tudy of the ave regime in the NA bain, baed on the ERA-4 ave re-analyi, a preented. Although everal prior tudie have characterized the H climate in the NA, a more qualitative tudy of the ave field in that bain, mainly of the ind ea and ell regime, a miing. The tudy of the climate and eaonal variability of H and 7 9

10 H, and correponding θ m and θ m, filled thi gap. It ha been hon ho ell dominate the NA bain, even in the inter, hen U peak and ind ave are more preent. The meridional variability of H, H and H a alo tudied. The profile of the ignificant ave height hoed the high eaonality of the characteritic of the ave regime in the NA. A rough comparion ith the NP H, H and H climate hoed the difference beteen the ave regime in both bain, regarding the ind ea and ell regime. While the H climate can be claimed to be relatively imilar in the inter and ummer, in both bain, their qualitative characteritic are in fact different, due to the difference in the ind peed regime, and to the different geographie of thee to bain. The NP i le indy, but it ha a coniderably tronger Southern Ocean ell intake, compared to the NA. Thi concluion i in line ith the finding from [3]. The EOF analyi revealed the non urpriing concluion that the ind peed i the dominating ource of variability of the ind ea ignificant ave height. The center of action of H and H are not coincident, hoing the main traveling ell path. A more detailed tudy of the variability of the H and H field, uing principal component analyi, in needed, o that the geographic origin of the ind ea and ell acro the NA bain can be tracked. Acknoledgement Alvaro Semedo and Kay Sušelj ere funded by the European Commiion through a Marie Curie reearch fellohip under the ModOb project, contract MRTN- CT Special thank are due to Andrea Sterl and Sofia Caire, for valuable uggetion, and to Jean Bidlot, for helping ith the acce to the ERA-4 data. Reference [] Munk, W.H., 95: Origin and generation of ave. Proc. Firt Coatal Engineering Conference, Long Beach, Calif., 4. [] Kinman, B. 965: Wind ave. Prentice-Hall, Engleood Cliff, NJ, p [3] Alve, J.H.G.M, 6: Numerical Modeling of Ocean Sell Contribution to the Global Wind- Wave Climate. Ocean Modelling. Vol. (-), 98-. [4] Munk, W.H., 947: Tracking torm by forerunner of ell. J. Meterol. 4(), pp [5] Munk, W.H., G.R. Miller, F.E. Snodgra and N.F. Barber, 963: Directional recording of ell from ditant torm. Philo. Tran. R. Soc. London, A55, pp [6] Snodgra, F.E.,G.W. Grove, K.F. Haelmann, G.R. Miller, W.H. Munk and W.M. Poer, 966: Propagation of ell acro the Pacific. Philo. Tran. R. Soc. London A59, pp [7] Ardhuin, F. and A. D. Jenkin, 6: On the interaction of urface ave and upper ocean turbulence. J. Phy. Oceanogr., 36, [8] Sullivan, P. P., J. B. Edon, T. Hritov, and J. C. McWilliam, 8: Large-eddy Simulation and Obervation of Atmopheric Marine Boundary Layer above Nonequilibrium Surface Wave. J. Atmo. Sci., 65, [9] Smedman, A.-S.,, A., U. Högtröm, E. Sahleé, W. M. Drennan, K. K. Kahma, H. Petteron, and F. Zhang, 9: Obervational tudy of marine atmopheric boundary layer characteritic during ell. Accepted for publ. in J. Atmo. Sci. [] Semedo, A., Sætra, Ø., Rutgeron, A., Kahma, K., Petteron, H.: Wave induced ind in the marine boundary layer. Accepted for publ. in J. Atmo. Sci. [] Chen, G., Chapron, B., Ezraty, R., and Vandemark, D., : A global vie of ell and ind ea climate in the ocean by atellite altimeter and catterometer. J. Atmo. Oceanic Technol., 9, [] Gulev, S. K., S. K., V. Grigorieva, A. Sterl, and D. Woolf. 3: Aement of the reliability of ave obervation from voluntary oberving hip: Inight from the validation of a global ind ave climatology baed on voluntary oberving hip data. J. Geophy. Re., 8(C7), 336, doi:.9/jc437. [3] Young I. R., 999: Seaonal variability of the global ocean ind and ave climate. Int. J. Climatol., 9, [4] Sterl, A. and S. Caire, 5: Climatology, Variability and Extrema of Ocean Wave - The Web-baed KNMI/ERA-4 Wave Atla. Int. J. Climatology, 5(7), , doi:.9/joc.75. [5] Uppala et al., 5: The ERA-4 Re-analyi, Q.J. Roy. Meteorol. Soc., 3, [6] Sterl A., 4: On the (in-)homogeneity of reanalyi product. Journal of Climate, 7: [7] WAMDI Group, 988: The WAM Model - A Third Generation Ocean Wave Prediction Model. J. Phy. Oceanogr., 8, [8] Janen, P. A. E. M., J. D. Doyle, J. Bidlot, B. Hanen, L. Iaken, and P. Viterbo, : Impact and feedback of ocean ave on the atmophere. Advance in Fluid Mechanic, W. A. Perrie, Ed., Vol., Kluer,

11 [9] Janen, P., 4: The Interaction of Ocean Wave and Wind. Cambridge Univerity Pre. [] Komen, G. J., L. Cavaleri, M. Doneland, K. Haelmann, S. Haelmann, and P. A. E. M. Janen, (Ed.), 994: Dynamic and Modelling of Ocean Wave. Cambridge Univerity Pre. [] Pieron, W. J., 964: The interpretation of ave pectrum in term of the ind profile intead of the ind meaured at a contant height. J. Geophy. Re. 69, [] Pieron, W. J. and Mokoitz, L. 964: A propoed pectral form for fully developed ea baed on the imilarity theory of S. A. Kitaigorodkii. J. Geophy. Re. 69, [3] von Storch, H. and F. W. Zier, 999: Statitical analyi in climatology. Cambridge, Cambridge Univ. Pre. [4] Roger, J.C., 984: The Aociation beteen the North Atlantic Ocillation and the Southern Ocillation in the Northern Hemiphere. Mon. Wea. Rev.,,