Variability of Current Structure Due to Meso-Scale Eddies on the Bottom Slope Southeast of Okinawa Island

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1 Journal of Oceanography, Vol. 61, pp to 1099, 2005 Variability of Current Structure Due to Meso-Scale Eddies on the Bottom Slope Southeast of Okinawa Island MASANORI KONDA 1,2 *, HIROSHI ICHIKAWA 1,3, IN-SEONG HAN 1, XIAO-HUA ZHU 1 and KAORU ICHIKAWA 1,4 1 Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, Natsushima, Yokosuka , Japan 2 Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto , Japan 3 Faculty of Fisheries, Kagoshima University, Kagoshima , Japan 4 Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka , Japan (Received 16 June 2004; in revised form 2 February 2005; accepted 21 April 2005) The relationship between the vertical profile of current on the bottom slope southeast of Okinawa Island and the offshore meso-scale eddy propagated from the east was examined by combined use of the data obtained by a moored upward-looking ADCP (Acoustic Doppler Current Profiler), PIES (Inverted Echo Sounder with Pressure gauge), hydrographic surveys and satellite altimetry during a period from November 2000 to August The variability of current component parallel to the isobath in the layer over 600 m is found to be markedly different from that in the layer below 600 m. The current variability in the upper and the lower layer can be well explained by the first and second modes of the EOF (Empirical Orthogonal Function) decomposition. The PIES and the sea surface height anomaly data suggest that the first mode represents the surface-trapped current associated with the approach of the offshore meso-scale eddy from the east, whereas the second mode has a bottom-intensified structure. The second mode velocity tends to delay to the first mode. The hydrographic data derived from CTD (Conductivity-Temperature-Depth meter) and PIES data along the line across the isobath suggest that the second mode component is generated by the interaction between the meso-scale eddy and the bottom topography. Keywords: Meso-scale eddy, ADCP, Kuroshio, bottom topography, bottom intensification, PIES. 1. Introduction The Kuroshio has been thought to play an important role in North Pacific climate variability due to its large meridional heat transport. This large heat transport should be closely related to the volume transport of the Kuroshio. It is therefore necessary to clarify the mechanism of the variation of the Kuroshio volume transport in order to understand global climate change. The volume transport across the ASUKA line (Affiliated Surveys of the Kuroshio off Cape Ashizuri line) is estimated to be about 42 Sv (1 Sv = m 3 s 1 ), (Imawaki et al., 2001), whereas that in the East China * Corresponding author. konda@kugi.kyoto-u.ac.jp Present address: National Fisheries Research and Development Institute, Busan, Korea. Copyright The Oceanographic Society of Japan. Sea is Sv (Kaneko et al., 1990; Ichikawa and Beardsley, 1993). Lee et al. (2001) estimate that the Kuroshio volume transport in the East China Sea roughly amounts to no more than 60% of the southward interior transport across 24 N in the Pacific. The disagreement of volume transport might be explained by the northeastward current east of the Ryukyu Islands. The northeastward current over the slope southeast of Okinawa Island in the central part of the Ryukyu Islands was first reported by Worthingthon and Kawai (1972). Yuan et al. (1994) estimated the current to the southeast of Okinawa Island by applying their modified inverse method to moored current-meter records and hydrographic data of the onemonth observation in the autumn at In spite of these snapshot observations, the existence of the stable northeastward current in this area has not been confirmed because of the frequent approach of mesoscale eddies from the east, as suggested by Nakano et al. (1998). Akitomo et al. (1996) and Tanaka and Ikeda 1089

2 Table 1. Locations and depths of MADCP and PIES sites. Site location Depth (m) MADCP N, E 909 PIES N, E 526 PIES N, E 1523 PIES N, E 2376 PIES N, E 6104 PIES N, E 5029 PIES N, E 734 PIES N, E 1512 PIES N, E 2334 PIES N, E 2220 Fig. 1. Location of observation sites. Open and closed circles indicate the mooring site of upward-looking ADCP and PIESs, respectively. OK line indicates the repeated hydrographic survey line maintained by Nagasaki Marine Observatory, JMA. (2004) showed in their numerical experiments that baroclinic Rossby waves excited by the interannual variation of the wind stress curl in the central Pacific can reach the western boundary and change the volume transport of the Kuroshio south of Japan. Tanaka and Ikeda (2004) indicated that the variation due to eddy accounts for up to 60% of the Sverdrup transport variability across 24 N. Recently, based on the data obtained by a moored ADCP (Acoustic Doppler Current Profiler) and 5 PIES (Inverted Echo Sounder with Pressure gauge) in the period November 2000 to August 2001, Zhu et al. (2003) estimated that the mean volume transport in the top 2000 m southeast of Okinawa Island is 5.4 Sv towards the north. Recently, Zhu et al. (2004) obtained the fluctuation of the northeastward geostrophic volume transport from 10.4 Sv to 30.0 Sv, using satellite altimeter and tide gauge data. From the moored current-meter observations at three or four mooring stations from December 1998 to October 2002, Ichikawa et al. (2004) estimated that the mean northeastward volume transport in the top 1500 m southeast of Amami-Ohshima Island is 16 Sv. These studies indicate the existence of a persistent northward current in the east of the Ryukyu Islands. In addition, some studies have pointed out the baroclinic structure of this current. Yuan et al. (1994) showed the existence of a core current with a maximum velocity of 20 cm s 1 centered around 600 m depth over the area of maximum bottom slope. From the moored current meter observation to the southeast of Amami- Ohshima Island, Ichikawa et al. (2004) indicated the persistent existence of a northeastward undercurrent with a maximum core velocity of 23 cm s 1 at 600 m depth over the shelf slope and volume transport of 4 Sv. Zhu et al. (2005) pointed out that this current core originates from the northeastward current southeast of Okinawa. Kawabe (2001) computed that the transport of the subsurface current is 31.5 Sv to the east of the Ryukyu Islands, being the remainder after the passage of the upper part of the first baroclinic mode over the Ryukyu ridge. These analyses strongly suggest the existence of a northeastward subsurface core current to the east of the Ryukyu Islands. However, it is not evident how the vertical structure is modified by the approach of the mesoscale eddies, which can affect the Kuroshio transport south of the central Japan. LaCasce (1998) showed that the deformation of the initially barotropic vortex on the slope leads to the perturbed vorticity near the bottom in the two layer system. Zavala Sanson and van Heijst (2000), from their laboratory experiments and numerical simulations, suggested that the westward propagating barotropic eddy causes the current variations over the shelf slope. Such phenomena might occur on the shelf slope east of the Ryukyu Islands. However, the change of the current on the continental slope caused by the approach of an eddy has not been captured because of the lack of long term observations of the vertical structure on the slope. We have conducted moored current meter observations with the PIES and moored ADCP and hydrographic observations using CTD (Conductivity-Temperature- Depth meter) on the shelf slope southeast of Okinawa Island. Using the data obtained by these observations, we 1090 M. Konda et al.

3 (a) (b) (c) Fig. 2. Contour plots of vertical profiles of (a) temperature, (b) SVA and (c) buoyancy frequency derived by the GEM method versus sound travel time. Temperature and the SVA field are computed according to Zhu et al. (2003). Buoyancy frequency is computed from the SVA field. have examined the variability of vertical profile of current related to the meso-scale eddy in detail. Section 2 presents the observation methods and data processing. We demonstrate the results of data analysis in Section 3, with discussions and conclusion in Section Description of Observations The locations of mooring stations of a moored ADCP (75 khz, RDI Workhorse type) and PIESs (University of Rhode Island model 6.1) are shown in Table 1 and Fig. 1. The eddy activity is observed by the satellite-derived SSHA (Sea Surface Height Anomaly) along the Ryukyu Islands in the latitude band of the array (Qiu, 1999; Ebuchi and Hanawa, 2001). The mooring observations conducted for about 9 months from November 2000 to August The moored ADCP was set at 100 m above the bottom on the slope of 909 m depth. It obtained the current velocity from 182 m to 790 m with 16 m vertical intervals and the temperature at the moored depth. The current velocity obtained is decomposed into two components, u and v. Here, u is perpendicular to the isobath and v parallel to it. The value of u and v is positive in the offshore direction (135 from north) and northeastward direction (45 from north), respectively. Variations of u and v shorter than 10 days period are removed by a low-pass filter. The PIES emits a group of twenty-four 10 khz sound pulses per hour at 15 second intervals. It records the round-trip travel time from bottom to surface and the bottom pressure. The spikes, tidal effect and seasonal variation in the round-trip travel time records are removed by the method given in Fields et al. (1991). In addition, short period variations in the round-trip travel time records are removed by a 48-hour low-pass filter. After these procedures, the vertical profiles of temperature and salinity and therefore SVA (specific volume anomaly) are obtained by the GEM (Gravest Empirical Mode) method (Meinen and Watts, 2000). Figure 2 displays the temperature and SVA field versus the travel time in the realistic range. The rightmost panel of Fig. 2 shows the buoyancy frequency obtained by the GEM field in this region as proxy estimates of the density profile. The GEM field used in this study represents the mean and the realistic change of the vertical profiles (Zhu et al., 2003). Dynamic height anomaly at the PIES mooring sites is obtained from the specific volume anomaly calculated by these profiles. The method used to analyze the PIES data is described in detail in Zhu et al. (2003). To identify the temporal and spatial distribution of meso-scale eddies, SSHA data are derived from the TOPEX/Poseidon and ERS-1 and 2 satellite altimetry data. Altimetry data are calculated by optimal interpolation with approximately 150 km and 15 days smoothing scales at grid points with 0.25 degree intervals on every 5 days. The hydrographic observations by CTD were also repeated four times by R/V Chofu-maru on the OK-line Current Structure Southeast of Okinawa 1091

4 Fig. 3. Depth-time plots of u (upper panel) and v (middle panel) in the depth range from 214 m to 790 m, and the variation of temperature at 809 m depth (lower panel) obtained by the moored ADCP. Contour interval of the current velocity is 5 cm s 1. maintained by the Nagasaki Marine Observatory (Fig. 1), and the geostrophic current sections are calculated. In addition to SSHA, SSL (Sea Surface Level) data at the tidal station of Japan Geographical Survey Institute at N, E, are derived after the removal of the atmospheric pressure effect. Variations of the SSL shorter than 50-day period and longer than 300-day period are removed by a band-pass filter. 3. Results Temporal variations of u and v between 214 m and 790 m and temperature at 809 m depth obtained by the moored ADCP are shown in Fig. 3. The variation of v at a period of about 100 days is distinctive. The cross-slope velocity in the surface level shows a frequent reversal with a similar period. The large variation of v is mainly observed in two subsurface layers, i.e., one above 600 m and the other below 600 m. In contrast, the variation of u is weak and is concentrated in the upper layer. The change of variability across 600 m is expected to associate with the density profile in this region. The temperature profile displayed in Fig. 2(a), which has relatively dense contours near m, suggests a large density change. Moreover, the vertical structure of the buoyancy frequency displayed in Fig. 2(c) also changes at the depth Fig. 4. Time-longitude plot of SSHA obtained by satellite altimetry data from 127 E to 137 E along 26 N. Arrows indicate the westward propagation of the individual eddies. Thick broken line roughly indicates the nearest location of Okinawa Island on this latitude M. Konda et al.

5 (correlation coefficient) : 127~ 128 : 128~ 129 : 129~ 130 : 130~ 131 : 131~ 132 : 132~ 133 : 133~ (time-lag;days) Fig. 5. Cross correlation function between the SSL at Okinawa Island and the longitudinal mean SSHA from 127 E to 134 E with one-degree spatial interval. 99% confidence level is given by the thin gray line. of m, as suggested from the velocity structure. The velocity maximum of v below 600 m tends to appear after the surface velocity reaches a maximum. This lagged correlation suggests some association between the surface and the bottom velocity. In order to examine the variation of current structure on the bottom slope southeast of Okinawa Island, the approach of the offshore meso-scale eddy to the ADCP observation station is drawn (Fig. 4). It is obvious in this figure that the strong anticyclonic eddy in the offshore region, where SSHA is larger than 30 cm, reaches to the southeast of Okinawa Island in July Eddies gradually diminish as they are transferred westward. Nevertheless, we should be cautious about the values near the coast or the islands, as the SSHA data used here undergo spatial smoothing due to the optimal interpolation. Okinawa Island and the continental shelf ridge lie near 128 E on this latitude, as shown by the thick broken line in Fig. 4. The island and the shallow ocean over the continental shelf might affect the SSHA, causing it to decrease there. Arrows in this figure illustrate the westward propagation of cyclonic and anticyclonic eddies in the offshore region of this area during the observational period. We estimate that the westward phase speed of these eddies is about 7 8 cm s 1 from the time lag of the maximum correlation coefficients between the band-passed SSL at Okinawa Island and the longitudinal mean SSHA (Fig. 5). This value of phase speed is consistent with the westward phase speed of about 6 7 cm s 1 estimated by Ebuchi and Hanawa (2001) westward of the Kuroshio upstream region. Previous studies reported current variability with a similar period in the upstream region. Zhang et al. (2001) and Lee et al. (2001) found a 100-day peak in the current variability observed by moored current meters to the east of Taiwan. They reported that the 100-day variability is associated with the approach of westward eddies. Although this study cannot evaluate the effect of the upstream variability, the change of a velocity synchronized with the approach of the westward eddies strongly suggests the similar effect of eddies on the current variability. If one compares Fig. 3 with Fig. 4, it is evident that the temporal variation of v is closely related to the approach of meso-scale eddies. The strong northeastward current is found simultaneously with the approach of anticyclonic eddies in January, March and July. The approach of the cyclonic eddy in April corresponds to the southwestward current that appears in the upper layer (Fig. 3). It also suggests that the temperature change is caused by the approach of the anticyclonic eddy. The temperature increase is accompanied by a notable increase of the northeastward current. An EOF (Empirical Orthogonal Function) analysis is performed in order to examine the predominant vertical structure and their temporal variations in v. The EOF modes are calculated by a cross-covariance matrix with v at 39 observed levels between 182 m and 790 m. The first mode and the second mode, respectively, explain 82.4% and 14.2% of the variance, and 96.6% in total. As shown in Fig. 6(a), the first mode represents the profile with the velocity trapped in the surface layer. The second mode shows the bottom-intensified structure, the sign of which changes at about 500 m depth. Time coefficients of the first and the second modes are also shown in Fig. 6(b). Temporal variation of the first mode shows semi-periodic cycles at about 100 days. The first mode is Current Structure Southeast of Okinawa 1093

6 Fig. 6. (a) Vertical distributions of the first (blue line) and the second mode (red line) of the EOF decomposition of v and (b) temporal variations of time coefficients with the first mode (blue line) and the second mode (red line) from v obtained by the moored ADCP are displayed. The southeastward component of SSHA gradient near the mooring site (between grid of 26.0 N, E and grid of N, E) (thin black line) is superimposed (thin black line) on panel (b). dominant during the whole observational period rather than the second one. The variation of the second mode tends to be amplified after the increase of the first mode, while this is not the case for the southward acceleration of the second mode at the end of June. This relationship between the first and the second mode is also noticeable in Fig. 3. The velocity maximum below 600 m in January, March, June and July should correspond to the peaks of the second mode shown in Fig. 6(b). Considering the correspondences found in this 9-month observation, the decomposition of the velocity into the two types of the vertical structure seems to represent the variability of the v component velocity in the upper and lower layers. On the other hand, the southward current at the beginning of July near the bottom seems to conflict with the regular relationship. As this velocity appears between the peaks of the 100-day variation, it might be generated by a different mechanism. However, a detailed investigation of this is beyond the objective of this study. To examine the relation between the approach of the meso-scale eddy and the change of the velocity structure, we compared the temporal variation of the southeastward component of SSHA gradient near the moored ADCP with time coefficients of the first and the second mode of the v component. The SSHA gradient near the mooring site is superimposed on Fig. 6(b). The figure shows that the time evolution of the surface inclination is similar to the first mode of the velocity. Moreover, cross correlation functions between the SSHA gradient and respective time coefficients (Fig. 7) shows that the SSHA gradient is closely correlated with the time coefficient of the first mode. The approach of eddies would induce the change of v near the surface without a time lag. In contrast, the correlation between the SSHA gradient and the time coefficient of the second mode is not so evident either in Fig. 6(b) or Fig. 7. We conducted moored PIES observation on the slope southeast of Okinawa Island for more detailed analysis of the relation between the current section and the approaching eddies. Temporal variation of dynamic height anomalies is obtained by five PIESs (P1 P5) referred to 2000 dbar, along the OK line. The values divided by the acceleration due to gravity are shown in Fig. 8 for easier comparison with the SSHA. The ADCP site is located almost in the middle of the OK line and the line that consists of three PIES (P6 P8). In order to concentrate the analysis on the variation associated with the approach of eddies, we do not here discuss the difference between the two adjacent lines. The dynamic height anomaly variability computed by the PIES observation clearly shows the westward propagation of offshore eddies as well as the SSHA from the satellite altimetry. Figure 8 indicates the propagation speed changes near the bottom slope. We roughly estimate the speed of the eddy on the slope to be down to 3 cm s 1 from the peak-to-peak time difference. This is not 1094 M. Konda et al.

7 (correlation coefficient) (time lag; days) Fig. 7. Temporal variation of the cross correlation functions between SSHA gradient and time coefficients of the first (black line) and the second (gray line) mode from v obtained by the moored ADCP. 99% confidence level is given by the thin black line Stn.1 Stn.2 Stn.3 Stn.4 Stn.5 (cm) DEC JAN FEB MAR APR MAY JUN JUL AUG Fig. 8. Temporal variations of dynamic height anomaly obtained by PIESs. Values in the figure are divided by the acceleration of gravity. very different from the phase speed of the first baroclinic Rossby wave (1.8 cm s 1 ) computed from the GEM field representative of the average condition of this region, assuming that the wavelength is 200 km. Figure 8 also shows that the surface elevation decreases on the bottom slope. Close inspection of Fig. 8 in association with the bottom topography in Fig. 1, allows one to conclude that the dynamic height anomalies measured by PIES arrays suggest the surface elevation associated with the approach of the offshore meso-scale eddy diminishes on the bottom slope. The SSHA measured by the altimeter demonstrated in Fig. 4 also attains a maximum value near 129 E. The smoothed SSHA may be qualitatively consistent with the surface elevation change estimated by the PIES measurement. The southeastward dynamic height anomaly gradients derived by the PIES are compared with time coefficients of the first and the second modes velocity in Fig. 9. The first mode is almost synchronized with the change of the dynamic height anomaly, whereas the second mode is less correlated with it. The variation of the first mode with the surface trapped velocity structure probably responds to the meso-scale eddy (Fig. 9(a)). In contrast, the second mode, which has a bottom-intensified structure, does not show any evident lead-lag correlation with the sequential change of the surface elevation due to the Current Structure Southeast of Okinawa 1095

8 Fig. 9. Cross correlation functions between southeastward dynamic height anomaly gradients from P1 to P5 obtained by PIESs and time coefficient of (a) first mode and (b) second mode. 99% confidence level is given by the gray line. (a) (b) (b) A B 10 Fig. 10. (a) Vertical section of geostrophic current referred to the maximum common depth of two adjacent CTD observations on the OK-line from April 30 to May 1, Contour interval is 10 cm s 1. Thick contour indicates 0 Sv. Positive and negative values of geostrophic velocity indicate northeastward and southwestward, respectively. (b) Spatial distribution of the SSHA obtained by satellite altimeter data on 29 April Contour interval of the SSHA is 5 cm. Thick line in panel (b) indicates the OK line. approach of the meso-scale eddy (Fig. 9(b)). Considering the close correlation between the first and the second mode velocities, the arrival of offshore eddies should have some impact on the variability of these eddies. If these eddies have vertically deep coherence in the offshore region, they might be affected by the inclination of the slope southeast of Okinawa Islands through bottom intensification (Rhines, 1970). Although the bottom intensification does not generate the typical baroclinic structure like the second mode velocity here, the reversal of the surface velocity of the second mode extracted by the simple EOF can be influenced by the decrease of the surface velocity synchronized with the increase of the bottomintensified velocity. In order to see the vertical velocity structure of the offshore eddy, we computed the geostrophic current along the OK line using the hydrographic data obtained in early May (Fig. 10(a)). We assume the reference level to be the maximum common depth between two adjacent CTD observations. The spatial distribution of the SSHA obtained by the altimeter data is also shown in Fig. 10(b). The northern peak of the cyclonic eddy marked A in 1096 M. Konda et al.

9 the center of Fig. 10(b) corresponds to that seen in April in Fig. 4. There must be an anticyclonic eddy to the east of this cyclonic eddy, although it is out of range of the figure. In Fig. 10, the northeastward and the southwestward currents over 20 cm s 1 are notable above 600 m, corresponding to the anticyclonic and cyclonic eddies, respectively. We can conjecture that the velocity structure in Fig. 10(a) propagates westward with these marked eddies. We can see that the geostrophic current normal to the OK line is completely consistent with the SSHA. A vertically coherent velocity exists down to 800 m. A strong surface current exists across the line. In contrast, there is no significant velocity maximum below 600 m in the offshore region. In the onshore region, the strong surface velocity should correspond to the first mode decomposed by the moored ADCP observation in Fig. 6. There is a noticeable velocity shear near the bottom slope, which may correspond to the second mode. It is well known that the geostrophic current structure depends strongly on the definition of reference level, particularly on the bottom slope, so some uncertainty might attach to the amplitude of the near-bottom velocity. For example, an inconsistency is found between the near-bottom velocity in Fig. 10(a) and the ADCP velocity demonstrated in Fig. 3, which indicates a slightly southward current over the slope at this time. However, the time sequence of the first and the second mode displayed in Fig. 6(b) shows that the northward velocity of the second mode is decreased after a little delay to the southward acceleration of the first mode. This suggests that the velocity component near the bottom is accelerated in the same direction as the first mode due to the topographic beta effect. Considering the possibility that the mean current velocity is included in the velocity measured by the ADCP, the geostrophic velocity structure over the bottom slope should correspond to the composition of the first and the second modes. A vertical shear is considered to exist near the bottom slope between the two adjacent CTD observations. The advection of the effect of the adjacent eddy, like B in Fig. 9, should not play a direct role here, judging from the fact that the structure of the second mode velocity is concentrated in the lower layer. 4. Discussion and Conclusions The variation of current structure associated with the approach of the meso-scale eddy has been examined from the data obtained by 9 months of observation with moored ADCP, PIES and hydrographic survey with satellite altimetry data. Both PIES observation on the OK line and the SSHA obtained by the altimeter show the change in the SSL according to the approach of the meso-scale eddies. During the mooring period from November 2000 to August 2001, both anticyclonic and cyclonic eddies approach the shelf slope southeast of Okinawa Island at a phase speed of about 7 8 cm s 1. The EOF decomposition of the velocity component along the isobath observed by the ADCP on the slope shows that the time evolution of the first and the second mode matches well with the velocity variations in the layers above and below 600 m, respectively. Temporal variation of the first mode is predominant with a period of about 100 days. Three of four cases of the enhancement of the second mode in the same direction appear after those of the first mode during the mooring period. In this way, the variation of the current structure on the bottom slope can be divided into two major modes. The shape of the eigenfunction indicates that the first mode has a surface-trapped structure, which is directly affected by the approach of the meso-scale eddy, and the second mode shows the bottom-intensified structure. If one compares the first mode velocity component with the SSHA gradient near the mooring site, one observes that the SSHA gradient, which should be changed by the behavior of meso-scale eddy, is closely correlated with time coefficient of the first mode without time lag. The first mode velocity well reflects the current structure in the upper level accompanied by the anticyclonic and cyclonic eddies. In contrast, the second mode current structure does not seem to have any direct correlation with the change of the surface elevation caused by the mesoscale eddy. The deformation of meso-scale eddies is detected by the three PIESs on the bottom slope. It may appear qualitatively in the spatially smoothed SSHA derived by the satellite altimeter. The geostrophic current obtained by the hydrographic observation along the OK line in April 2000 shows that meso-scale eddies in the offshore region where the depth is over 1000 m have a vertically coherent structure, whereas they do not have any velocity maximum at depths below 600 m. Taking into account the vertical structure of these offshore eddies, the velocity maximum below 600 m found on the slope southeast of Okinawa Island is probably generated by the separation from the surface eddy by the bottom intensification on the slope. Zavala Sanson and van Heijst (2000) showed that the approach of the cyclonic eddy would initially produce the southward current on the slope in their numerical and laboratory experiment done under barotropic conditions. As soon as the cyclonic eddy comes onto the slope, the vortex-stretching effect occurs, generating the northeastward jet on the western flank of the eddy. The jet produced by the barotropic vortex-bottom interaction is not evident in our observation. One of the most probable reasons for this is that their experiment was done under barotropic conditions. Our results clearly shows that offshore eddies are separated into surface-trapped and Current Structure Southeast of Okinawa 1097

10 bottom-intensified structures. Although their experiment is consistent with our observation in the point that the current is affected by the arrival of eddies at the bottom slope, there are several differences in terms of the behavior of eddies on the slope. In particular, the contrast between the cyclonic and the anticyclonic eddy is important. They demonstrate that the center of the barotropic anticyclonic eddy cannot enter the strong slope, whereas the cyclonic eddy undergoes the deformation on the slope. The behavior of the cyclonic eddy may be different from that of the anticyclonic eddy in the condition we observed on the slope southeast of Okinawa. Unfortunately, as our current meter observation catches few cyclonic and anticyclonic eddies at one point on the slope, it is difficult to ascribe the individual modifications of the current structure to the difference between the cyclonic and the anticyclonic eddy. On the other hand, LaCasce (1998) characterized the deformation of the eddy on the slope in terms of the strength of the slope, defined as 2 βl ε =, () 1 U where L and U are the length and the velocity scales of the eddy, and the topographic β is based on the expression β = fγ/h with f the Coriolis parameter, γ the inclination of the slope and H the depth of the ocean. The experiment was done for a two layer system without background current. By assuming U = 30 cm s 1, L = 52.9 km, and the topographic β = m 1 s 1, the slope near the MADCP site in our observation has the parameter value ε = 26, which corresponds to the strong slope case according to LaCasce (1998). In this case, the initially barotropic eddy approaching the slope would produce bottom intensification because of the topographic beta effect (Rhines, 1970). A rough estimation of the deformation radius from the GEM field in Fig. 2 indicates that the typical value for the first baroclinic mode is about 80 km with an equivalent depth of 2.6 m. Eddies with wavelength longer than the deformation radius are converted into topographic waves and the surface-trapped vortex, according to the LaCasce s experiment. The weak correlation of the second mode velocity at the ADCP and the surface elevations should reflect the separation from the surface mode through the bottom intensification. The surface vortex is expected to be weakened by the radiation of the topographic waves. Figure 8 indicates the decrease of the amplitude of the dynamic height anomaly of individual eddies on the slope, as previously mentioned. The amplitude is reduced by about 30 40% near the slope (Station 3). However, quantitative discussion is quite difficult for two main reasons: i.e., LaCasce s experiment was done without any background current; and the GEM field used in this study does not correctly reproduce the second mode velocity (Zhu et al., 2003). On the other hand, Fig. 5(b) indicates that the second mode velocity is accelerated after the first mode velocity in the same direction, except at the beginning of July, as previously mentioned. The case of LaCasce (1998) seems to be consistent with this observational result. The situation may be quantitatively different given the existence of the northward current in the background. Many studies (e.g. Orlanski, 1969; Luther and Bane, 1985; Xue and Mellor, 1993; Sutyrin et al., 2001) investigated the effect of the bottom slope on the evolution of a meander in the jet. They showed that the slope stabilized the along-slope jet in the lower layer because of the small potential vorticity gradient without disturbing the potential vorticity in the upper layer. The low variability of the u component in the lower layer demonstrated in Fig. 3 might reflect this. Taking into account these theoretical studies, the behavior of the current structure demonstrated in this study suggests that the meso-scale eddy approaching the shelf slope southeast of Okinawa Islands undergoes a change of the vertical mode due to the bottom topography. Consequently, the velocity in the upper layer directly reflects the current structure of the eddy, whereas that in the lower layer might be converted into topographic waves to be radiated or advected by the current. This analysis cannot capture such transformation of the vertical structure of the eddy. As a GEM field is the projection of the temperature and density data onto a single vertical mode (Meinen and Watts, 2000), we cannot retrieve the internal mode from it. However, it is possible that the bimodal GEM field is parameterized by an additional physical parameter such as the SSL or the internal pressure, which represents different physical properties from the travel time (Watts et al., 2001; Pérez- Brunius et al., 2004). The pressure gauge data, which are not analyzed here, should give profitable information. More PIES observations, collocated with hydrography measured by CTD and satellite altimetry, may create a GEM field that is expected to reproduce both the first and the second mode. PIES observations for a longer period will be needed to catch the process of transformation, if the GEM in this region can retrieve the individual vertical modes. In the present circumstance, where we have neither the bimodal GEM field nor another current meter either upstream or downstream, we do not evaluate the effect of the background current on the deformation of the eddy on the slope. In addition, the effect of the variability in the upstream region through the advection is not evaluated here. These problems will need further analysis of the temperature profiles in this region and will be addressed in future work M. Konda et al.

11 Acknowledgements We wish to thank Captain, Prof. Y. Akishige and his officers and crewmembers of the training vessel, Kakuyomaru of the Faculty of Fisheries, Nagasaki University for their professional support in the recovery of instruments. Mr. G. Chaplin kindly visited us and helped to maintain the PIESs. Drs. A. Ostrovskii, A. Kaneko, N. Gohda, S. Umatani, J.-H. Park devoted much of their valuable time to the deployment operations. Nagasaki Marine Observatory provided the hydrographic data on the OK line. Finally, special thanks are addressed to Drs. K. Takeuchi, T. Miura and T. Takigawa for their helpful comments on this study. References Akitomo, K., M. Ooi, T. Awaji and K. Kutsuwada (1996): Interannual variability of the Kuroshio transport in response to the wind stress field over the North Pacific: Its relation to the path variation south of Japan. J. Geophys. Res., 101, Ebuchi, N. and K. Hanawa (2001): Trajectory of mesoscale eddies in the Kuroshio recirculation region. J. 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