NUMERICAL STUDY OF WAVE-CURRENT INTERACTION USING HIGH RESOLUTION COUPLED MODEL IN THE KUROSHIO REGION

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1 NUMERICAL STUDY OF WAVE-CURRENT INTERACTION USING HIGH RESOLUTION COUPLED MODEL IN THE KUROSHIO REGION Hitoshi TAMURA Frontier Research Center for Global Change/JAMSTEC 373- Showaachi, Kanazawa-ku, Yokohaa, Kanagawa 3-, JAPAN e-ail: Takuji WASEDA Frontier Research Center for Global Change/JAMSTEC Departent of Environental & Ocean Engineering/The Univ. of Tokyo 7-3- Hongo, Bunkyo-ku, Tokyo 3-, JAPAN e-ail: Yasuasa MIYAZAWA Frontier Research Center for Global Change/JAMSTEC 373- Showaachi, Kanazawa-ku, Yokohaa, Kanagawa 3-, JAPAN e-ail: Kosei KOMATSU National Research Institute of Fisheries Science/Fisheries Research Agency -- Fukuura, Kanazawa-ku, Yokohaa, Kanagawa 3-, JAPAN e-ail: INTRODUCTION Global and regional wave forecasts are routinely conducted nowadays thanks to advances in nuerical weather prediction, satellite scatteroeters and coputers. The deand for further iproveents in such odels coes fro engineering counities, as well as cliate research counities. A nuber of researchers have suggested the possibility of wave concentration caused by the ocean current. But because of the lack of high resolution current forecast in the open ocean, ost wave forecast odels do not include the wave current interaction. There are a few exaples deonstrating the significance of wave-current interactions. Janssen et al. () have investigated the ipact of currents on the significant wave height with a global odel. The onthly ean wave height difference with and without current highlights the ajor current of the ocean. A ore regional study was conducted for the strong western boundary currents. Tolan et al. (99) showed that the wave height was enhanced for wave traveling against the Gulf Strea and currents had a distinct effect on the wave spectru. Siilarly, Wolf (3) has deonstrated the ipact of the eandering Kuroshio with a /3 degree resolution wave odel coupled to the ocean odel OCCAM. WAVE-JCOPE project ais to establish a realistic high-resolution coupled current-wave prediction odel in the Kuroshio region. The current field is prescribed by the Japan Coastal Ocean Predictability Experient (JCOPE) odel developed at Frontier Research Center for Global

2 Change (Guo et al., 3; Miyazawa et al., ). In this study, we focus on the influence of ocean current upon waves around the strong jet of the Kuroshio, by using high resolution wave odel and existing reanalysis products of wind and current.. NUMERICAL MODEL The wave odel eployed in the wave-current coupling is based on the third-generation wave odel WAVEWATCH-III (Tolan, ). The source ters of WW-III use the wind input expressed Snyder et al. (9), the nonlinear wave wave interaction of Hasselann and Hasselann (9), the dissipation by Janssen (99), and the botto friction based on the JONSWAP paraeterization (Hasselann et al., 973). The wave odel is driven by hourly wind stress (/ - / degree resolution) estiated fro Japan Meteorological Agency Meso-scale Spectral Model and the -day ean current field fro the JCOPE reanalysis linearly interpolated at every integration tie steps. JCOPE odel is based on the Princeton Ocean Model (POM); a high-resolution regional odel (7E to E and N to N, at / degree resolution) is ebedded in a low-resolution basin-wide odel (/ degree resolution) and assiilates Sea Surface Height, Argo and T & S fro various sources. The odel has successfully predicted the appearance of a large eander of the Kuroshio in. The two-onth forecasts as well as the reanalysis data are available fro the web page which is updated weekly ( Figure shows the coputational doain of the wave odel( o -9 o E 3 o - o N). The spatial resolution is x = y = / o and the odel has 3 7 grid eleents. The wave spectru is discretized using 3 directions ( θ = o ) and frequencies extending fro. to. Hz with a logarithic increent. o E 3 o E o E 9 o E o N :easureent locations (NOWPHAS) Measureent o N 3 o N 3 o N 3 East China Sea Sea of Japan Muroto Oct. 9 : Wajia Oct. : Typhoon TOKAGE Oct. : Kuroshio ain axis Pacific Ocean Fig.. Study site and the coputational doain. Significant wave height () Model Fig.. Coparison between odel analysis and easureent data. Muroto Significant wave height s ean wave period In this study, we present a case study of the October which was an unusual onth with 3 consecutive typhoons (Typhoon, 3, ) struck Japan. We pay special attention to the Typhoon Mean wave period (s) Model // // // // // // //3 s Wajia Significant wave height ean wave period No. No.3 :eas. No. :eas. :cop. :cop. // // // // // // //3 Fig. 3. Tie variations of significant wave height and ean wave period copared with easureent data at two locations.

3 (a) (b) Fig.. The onthly-average spatial distribution of the significant wave height (a) with and (b) without current 3, nicknae TOKAGE, because one of the buoy located at Muroto (shown in Fig.) has indicated an occurrence of an extree wave of over wave height. 3. MODEL RESULTS For the validation of the odel, NOWPHAS (Nationwide Ocean Wave inforation network for Ports and HArbourS; data were copared, which were obtained at 9 stations (in Fig. ) where water depth was larger than. Figure shows the coparison of significant wave height and ean wave period between odel analysis and easureent data. It is clear that the present odel can reproduce the easured results for relatively low wave height conditions. Figure 3 shows the tie series of easureent data and corresponding coputational results of significant wave height and ean wave period at two locations. These are the three typhoons causing large anoalies in the record.. DISCUSSION To investigate the ipact of wave-current Fig.. The onthly-averaged current field and the difference of the significant wave height interaction, the siulation has been perfored with two nuerical experients with and without current. Figure (a), (b) and show the onthly-average spatial distribution of the significant wave height with and without current and the difference between the, respectively. The difference of the significant wave height over the Kuroshio is quite einent. To clarify the cause of such difference, we investigated the teporal evolution of the wave height difference between the cases with and without current. Figure 7 represents the wave height difference and surface wind vector when Typhoon TOKAGE passed around JAPAN.

4 Typhoon TOKAGE K K K K3 Fig.. The teporal evolution of the wave height difference between the cases with and without current. Associated with the changes of both the surface wind and the current, the wave-current interactions see rather coplicated. During the early stage, Typhoon TOKAGE passed south part of Kuroshio ain axis. In contrast, during the late stage, it passed north part. Corresponding to these relative positions between Typhoon path and Kuroshio ain axis, the convergence of wave propagation in the countercurrent region and wave divergence in the following current region are recognized. Moreover, these differences are extending for quite a distance away fro the Kuroshio downwind. This indicates that the effect of the wave-current interaction is not liited locally but extends further away fro where the interactions actually occur. As deonstrated here, the effect of wave-current interaction is highly sensitive to the sall current structure, and therefore the realistic representation of the current field is very iportant for high-resolution wave forecast. Figure 7 represents the tie series of the wave height difference at locations indicated in Fig. The difference of the significant wave height increases along the Kuroshio ain axis and reaches. (This value is about 3% of the significant wave height) at K. Figure (a) and (b) indicate the directional wave spectra calculated with and without wave current interactions at K (Oct. :). One obvious effect is that wave energy to south-southwestward direction is added to the south-westward spectru for Fig. (a) and it creates a second peak in the spectru, increasing the directional spreading. This coponent ay be considered as the trapped wave by Kuroshio. Another effect is the veering of the wave by the current. The wave corresponding to the ain peak of the spectral is directed clockwise by the Kuroshio. The difference of the significant wave height and the

5 .. K K // z // z K K3 -. / /7 / /9 / / / /3 / x Fig. 7. Tie series of the wave height difference at locations (a) (b) Fig.. Directional wave spectra calculated (a) with and (b) without wave current interactions at K (a) T=h U =/s E(f) Sin Snl Sds E(f) Sin Snl Sds profile of the wave spectral over the Kuroshio are quite einent and are in accord with the earlier work of Holthuijsen & Tolan (99).. CONCLUSION AND DISCUSSION Hs:.3 In this study, we have presented the hindcast results as the preliinary results of WAVE-JCOPE project. Coparing the onthly-averages with and without current, the difference of the spatial distribution of the significant wave height, over the Kuroshio is quite einent. Moreover, instantaneous differences extend for quite a distance away fro the Kuroshio downwind suggesting that the effect of the wave-current interaction is not liited locally to where the interactions actually occur. The shapes of the directional wave spectra calculated with and without wave current interactions are also quite different x Hs: Fig. 9. One-diensional frequency spectru and the energy source ters calculated by (a) DIA and (b) SRIAM as the non-linear interactions. (b) T=h U =/s In the presentation, we will discuss the balance of the energy source ters with and without current, and apply different nuerical schees such as RIAM (Koatsu and Masuda, 99) for calculating the accurate nonlinear energy transfer, to iprove the estiation of the wave-current interaction. The following ethods for coputing the nonlinear source ters were copared; DIA, WRT, RIAM and S-RIAM. While DIA and WRT are standard odules of WWIII, both RIAM and S-RIAM were ipleented in this study as new odules for the WWIII. We conclude the discussion by showing one result fro the case of fetch-liited wave. Figure 9 indicates the one-diensional frequency spectru and the energy source ters calculated by (a) DIA and (b) SRIAM as the non-linear interactions. The CPU tie of SRIAM is about ties longer than DIA. It is ephasized that energy spectral level for peak frequencies and frequency

6 bandwidths are copletely different for two cases. In addition to the effect of wave-current interactions for spectral shape, these differences results in the odification of the average steepness and the frequency bandwidth that are the two iportant paraeters considered to alter the wave statistics fro the Rayleigh distribution. The paraeter often used these days is called the Benjain-Feir Index and this is in the interest of identifying seas with high chances of extree event. REFERENCES Guo, X., H. Hukuda, Y. Miyazawa and T. Yaagata(3): A triply nested ocean odels -Roles of horizontal resolution on JEBAR-, J. Phys. Oceanogr. 33, -9. Holthuijsen L. H. & Tolan H. L. (99): Effects of the Gulf Strea on Ocean Waves, JGR, 9,,7-,77 Janssen P., Jean-Rayond Bidlot, Saleh Abdalla and Hans Hersbach (): Progress in ocean wave forecasting at ECMWF, ECMWF Technical eorandu, 7 Koatsu K. and Masuda A.(99): A new schee of nonlinear energy transfer aong wind waves, RIAM ethod. -Algorith and perforance-, J. Oceanography,, 9-37 Miyazawa, Y., X. Guo and T. Yaagata (): Roles of eso-scale eddies in the Kuroshio paths, J. Phys. Oceanogr. 3, 3-. Tolan H. L. (99): Wave-current interactions, Application to the open ocean, Dynaics and Modelling of Ocean Waves (G. J. Koen), Cabridge University Press. Tolan H. L. (): User anual and syste docuentation of WAVEWATCH-III version.. NOAA / NWS / NCEP / MMAB Technical Note, 33 pp. Wolf J. (3): Wave-Current Modelling : Phase (Kuroshio) (WP 3), Technical Note, Along Track Interferoetry for Ocean Currents and Swell, No.