Anticyclonic Eddy Revealing Low Sea Surface Temperature in the Sea South of Japan: Case Study of the Eddy Observed in

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1 Journal of Oceanography, Vol. 6, pp. 663 to 671, 4 Anticyclonic Eddy Revealing Low Sea Surface Temperature in the Sea South of Japan: Case Study of the Eddy Observed in 1999 KOHTARO HOSODA 1 * and KIMIO HANAWA 2 1 Center for Atmospheric and Oceanic Studies, Graduate School Science, Tohoku University, Aoba, Sendai , Japan 2 Department of Geophysics, Graduate School of Science, Tohoku University, Aoba, Sendai , Japan (Received 17 September 2; in revised form September 3; accepted 1 October 3) Various kinds of datasets, such as satellite-derived sea surface temperature (SST), sea surface height, surface velocity produced by combining surface drifter and satellite altimeter data, and hydrographic data, led to the discovery of an anticyclonic eddy with lower SST than those of surrounding waters in the Kuroshio recirculation region south of Shikoku, as if the eddy were cyclonic. This anticyclonic eddy was formed east of Kyushu in late August to early September 1999 from the merger of two anticyclonic eddies which had migrated in the recirculation region to the sea south of Japan from the east. After the merger, the anticyclonic eddy strengthened abruptly and began to exhibit the low SST. In October, this eddy coalesced with the Kuroshio and moved swiftly eastward, accompanied by an amplitude growth of the Kuroshio meander. In mid November, off the Kii Peninsula, the eddy detached from the meandering Kuroshio. It then moved southwestward and again slowly propagated westward along the 3 N line. During this period, at least from late October 1999 to January, SSTs over the anticyclonic eddy were found to be continuously lower than those of surrounding waters. This case tells us that we have to pay careful attention to the interpretation of mesoscale SST distributions. Keywords: Anticyclonic eddy, SST, SSH, the Kuroshio, recirculation region. 1. Introduction There are many mesoscale eddies in the ocean and they play an important role in transporting momentum, heat, salt and other substances. In general, by using horizontal distributions of temperature at a depth of several hundreds meters, we can easily judge which eddies are anticyclonic or cyclonic. However, the vertical temperature cross-section of the anticyclonic eddy sometimes displays a sea surface temperature (SST) signal just above the warm core that is lower than those of the surrounding waters. Figure 1 shows an example of such a case, observed in August This large warm eddy detached from the Kuroshio Extension east of Japan and migrated to the north in the Mixed Water Region, off the east coast of northern Honshu, Japan (Tomosada, 1978). This figure clearly shows that the warm eddy has lower SSTs in * Corresponding author. hosoda@eorc.jaxa.go.jp Present address: Earth Observation Research Center, Japan Aerospace Exploration Agency, Chuo-ku, Tokyo , Japan. Copyright The Oceanographic Society of Japan. its central region than those of surrounding waters, which accompanies the upward doming of the seasonal thermocline just above the warm core. These features of low SST and the upward doming of the seasonal thermocline can also be observed in other cross-sections cutting this eddy, taken by the same cruise (not shown here). Satellite-derived SST maps have regularly been released by operational agencies and institutions, and many users have utilized such maps for various purposes. The merits of satellite-derived SST data, with high spatial and temporal resolution, have facilitated many investigations of the formation, evolution, migration and decay of eddies (e.g., Kawamura et al., 1986; Aoki et al., 1995; Sakaida and Kawamura, 1998). Most of the eddies in these cases are warm (cold) eddies located in the northern (southern) area of the extensions of subtropical western boundary currents such as the Kuroshio, Gulf Stream and East Australian Current. For instance, satellite-derived SST images of such eddies can be found in the textbook by Tomczak and Godfrey (1994). 663

2 Fig. 1. Temperature cross-section of the warm core ring observed in the Mixed Water Region in August (a) SST by bucket method, (b) temperature from the sea surface to depth m, and (c) that of the whole cross-section to depth 1 m. The original data were provided by Dr. Akira Tomosada. Satellite-derived sea surface height (SSH) data are also powerful means for the detection of mesoscale oceanic variabilities. For example, Ichikawa and Imawaki (1994) and Aoki et al. (1995) described the behavior of cold rings (cyclonic eddies) in the Kuroshio Extension region using Geosat altimeter data. Recently, Ebuchi and Hanawa () pointed out the existence of both anticyclonic and cyclonic eddies in the Kuroshio recirculation region south of Japan, using data from satellite altimetry, the Acoustic Doppler Current Profiler (ADCP) installed on a ferry, and expendable bathythermograph (XBT). These eddies are circular in shape with a typical diameter of km and move westward with a propagation speed of about 6.8 cm/s. The typical maximum orbital speed and SSH anomaly associated with eddies are 15 cm/s and 15 cm, respectively. Furthermore, Ebuchi and Hanawa (1, 3) investigated the trajectories of these eddies, and pointed out that the coalescence of eddies with the Kuroshio can sometimes trigger small meanders which propagate eastward along the southern coast of Shikoku and Honshu The present authors have discovered that an anticyclonic eddy, designated by Ebuchi and Hanawa (3) as Eddy No. 14 (see their figure 4) displayed a lower SST than the surrounding waters in late 1999 and early, the same as a warm core ring observed in the Mixed Water Region mentioned above. In the present paper, we give a detailed description of this eddy, together with its formation and movement, and the amplitude growth of the Kuroshio meander due to the interaction with this eddy. The remainder of the paper is organized as follows. Section 2 describes the data used. Using images of SST and SSH together with surface velocity data, in Section 3 we show that the target eddy was accompanied by lower SSTs than those of surrounding waters. Section 4 briefly describes the formation and movement of the anticyclonic eddy and the amplitude growth of the Kuroshio meander. Section 5 summarizes our findings. 2. Data In the present study we use a SST dataset derived from several satellites, a SSH dataset derived from the TOPEX/POSEIDON altimeter, and a surface velocity dataset produced by combining surface drifter and SSH data. The SST dataset used is called New Generation SST version 1. (NGSST: Guan and Kawamura, 4), which is a merged product created by an objective mapping method using the data measured by the TRMM/TMI, NOAA/AVHRR and GMS/S-VISSR. Basically, NGSST is produced mostly based on A-HIGHERS derived from NOAA-AVHRR (see Sakaida et al., ). NGSST has almost the same characteristics in terms of accuracy, temporal and spatial resolution, except for a small cloud cover area, as A-HIGHERS. In the Kuroshio region (1 E 16 E, N 38 N), the spatial resolution of this dataset is.5.5 (lat. long.). We used the daily mean NGSST dataset, which is currently available for the period from October 1999 to September. Hosoda and Kawamura (4) evaluated this dataset, comparing it with in-situ SSTs in the Kuroshio recirculation region, and found that NGSSTs had high accuracy. We also use A-HIGHERS for a description of SST before October We use two SSH datasets. The first is that produced by the Japan Meteorological Agency (JMA) as a part of the Subarctic Gyre Experiment (SAGE) for the period This dataset is composed of SSH anomaly (SSHA) from the mean, and dynamic height at the sea surface (SSDH). The former is obtained by optimum interpolation of the TOPEX/POSEIDON altimeter data in space-time coordinates (Kuragano and Kamachi, ). The latter is a sum of SSHA and the climatological SSDH averaged for the period (Kuragano and Shibata, 1997). In both datasets, values are given every K. Hosoda and K. Hanawa

3 days at.5.5 (lat. long.) grids covering the North Pacific (115 E 95 W, 1 N 6 N). The second one is the along-track SSHA dataset, which is provided from OceanESIP/NASA. This dataset consists of SSHA relative to the average for the period Various kinds of corrections have been applied such as inverse barometric correction, and values are gridded using an exact cubic-spline interpolation to fixed points on the track. Spatial resolution of the grids on the track is approximately 6.2 km. This dataset is available for the global ocean every 1 days, from September 23, 1992 to February 13, 2. In addition to these two types of satellite-derived datasets, we also used the dataset of surface velocity that has recently been produced by Uchida and Imawaki (3). This dataset was derived by combining surface drifter data taken during the World Ocean Circulation Experiment (WOCE) period and satellite (TOPEX/ POSEIDON and ERS-1/2) altimeter data. Spatial and temporal resolutions are quarter degrees in both latitude and longitude and ten days, respectively. The data cover the period during from October 1992 to January 1. See Uchida and Imawaki (3) for a detailed description of this dataset. 3. Anticyclonic Eddy Revealing Lower SST 3.1 Low SST region in the sea south of Japan and surface current field In the period from late 1999 to mid-, in the Kuroshio recirculation region south of Japan, isolated lower SST regions surrounded by higher SSTs had been observed in the NGSST dataset. Figure 2 shows four snapshots of SSTs as an example. The panel of October 28, 1999 shows that the center of the lower SST region is located around 31.5 N and 134 E, and this lower SST region has a horizontal scale of several hundred km. At the center of this region, SST is about 24 C, which is 2 3 C lower than that of the surrounding waters. On the other hand, the panel of May 11, shows that the center of the lower SST region is located around 3.5 N and E and its temperature is less than C. In the panels of December 27, 1999 and February 6,, we also notice the existence of lower SST regions in the recirculation region south of Shikoku. Are these lower SST regions merely a manifestation of a phenomenon that is localized only in the surface layer? Figure 3 shows the surface velocity fields superposed by NGSST on May 13,. The anticyclonic (clockwise) circulation with a horizontal scale of 3 km can be clearly seen around the low SST region centered at 3.5 N, E. It is interesting to note that in the figure, two cyclonic eddies are seen around 28.5 N, E and 31 N, E, and SSTs at these areas are lower than the surrounding waters. This relationship between lower SST and cyclonic eddy is naturally accepted. However, the anticyclonic eddy with low SST is out of our usual expectation and therefore this eddy is the target in the present study. 3.2 Relationship between low SST region and SSH In order to confirm whether it is certain that an anticyclonic eddy accompanies a low SST signal for a certain duration, we examined the relationship between low SST regions and SSH fields. Figure 4 shows four snapshots of SSDH and SST taken in October December In SSDH fields, the existence of an anticyclonic eddy can clearly be seen. On the panel of October 28, 1999, the SSDH maximum greater than 28 cm is situated around 32 N, 136 E and the following three panels show that this anticyclonic eddy revolves clockwise in the Kuroshio recirculation region south of Shikoku. Although the low SST regions shown in Fig. 4 do not exactly correspond to those of SSDH maxima, they are situated close to SSDH maxima. One reason for this difference might be attributed to the differences of temporal and spatial resolutions between the two datasets, viz., the NGSST dataset has high temporal (daily) and spatial (.5 ) resolutions, while those of SSHA are as low as every 5 days in time and.5 in space. In order to further investigate the relationship between the positions of SST minimum and SSH maximum, we use the along-track SSHA dataset, the spatial resolution of which is as high as the NGSST dataset, although the along-track SSHA dataset is available only along the tracks of TOPEX/POSEIDON. Figure 5 shows examples of SST and along-track SSHA distributions along the paths of TOPEX/POSEIDON Lines 11 and 112 taken on December 27, These figures clearly show that higher SSHA regions are located at almost the same positions as lower SST regions. The spatial scale of anticyclonic eddy (peak-to-peak distance in SSTs or trough-to-trough distance in SSHA) can be read as roughly 3 km, and the differences of SSHA and SST between the extremes and those of surrounding waters can be read as about 5 cm and 1 C or greater, respectively. 3.3 Horizontal and vertical temperature distributions of the target eddy In order to reveal the vertical temperature section of anticyclonic eddy described in the present paper, we searched the hydrographic data archived by the Japan Oceanographic Data Center (JODC). Fortunately, we found that the XBT/CTD data taken in November 8 9, 1999 are available to show the horizontal and vertical temperature distributions to some degree. As deduced from the panels dated October 28 and November 17, 1999 of Low SST Anticyclonic Eddy 665

4 Fig. 2. Four examples of NGSST images in the sea south of Japan. Contour interval is.5 C. 1m/s May 13, Fig. 3. Surface velocity fields (arrows) on May 13, superposed by NGSST (shading). The arrow length of 1 m/s is shown in the upper right corner. Note that the areas where arrows are not attached show regions where surface velocity data are not updated. Fig. 4, at this time the anticyclonic eddy was situated south of Kii Peninsula. Figure 6 shows the horizontal temperature distribution at the sea surface and the depth of 3 m, and two vertical sections (Lines A and B). Unfortunately, although the data are not distributed just around the center of the target eddy, we can see in the vertical sections that the anticyclonic eddy is situated close to the Kuroshio, which is clearly shown by the depth of the C isotherm. We can also see the upward doming of the sea666 K. Hosoda and K. Hanawa Fig. 4. Four examples of SSDH distributions (left panels) and NGSST images (right) in the sea south of Japan. Contour intervals are 5 cm in SSDH and.5 C in SST. sonal thermocline over the eddy and this doming structure is very similar to that in Fig. 1. If one compares the temperature maps of sea surface and 3 m, it seems that the low SST center at the sea surface and the center of the warm core of the eddy is somewhat different in position, with a westward, upward tilt. This westward tilting of the eddy is a common feature of anticyclonic eddies in the open ocean (Roemmich and Gilson, 1). Due to the inadequacies of the data distribution, however, we can not make any concrete statement on this matter. Regardless of the inadequacy of the data, we could confirm by independent hydrographic data that the target eddy is anticyclonic and has a lower SST field over the core.

5 14 SSHA (cm) SSHA (cm) Day: Dec. 27, Latitude ( N) 35 3 Line: Latitude ( N) Line: Fig. 5. Profiles of SST (solid line) and SSHA (broken) along the tracks of TOPEX/POSEIDON Lines 11 (top panel) and 112 (middle), taken on December 27, Bottom panel shows the positions of Lines 11 and 112, on which NGSST is superposed. 4. Formation and Trajectory of the Low SST Eddy, and the Kuroshio Path Variation In this section we investigate when the anticyclonic eddy with low SST signal was formed, how this eddy migrated to the area south of Shikoku, and what kind of effect the eddy had on the Kuroshio path variation. Information on these items would be very useful in considering the formation mechanism of low SST accompanying an anticyclonic eddy. 4.1 Trajectory analysis of the low SST eddy In order to answer the above questions, we first analyzed the trajectory of the anticyclonic eddy using both SSHA and NGSST datasets. In both datasets, we detected positions of SSHA maximum and SST minimum in the 1 1 (lat. long.) box. If multiple points have the same maximum or minimum value, then the center (average position) of these points is regarded as the position of the eddy. In addition, we selected the extremes of SST or SST ( C) SST ( C) 34 N 32 N Line A Line B 3 N m 134 E 136 E 138 E km 16 Line A L 135 E 136 E 137 E SSHA, which can be traced from the target eddy situated east of Kyushu in late October 1999 (see the panel of October 28, 1999 of Figs. 2 and 4). Here, to reject the pseudo-positions obtained mechanically by the above method, we applied the condition that the speed of the eddy s motion does not exceed cm/s, since the mean speed of mesoscale eddies in the Kuroshio recirculation region is about 6.8 cm/s (Ebuchi and Hanawa, ). Henceforth, we call the low SST eddy described above Eddy A, for convenience. Figure 7 shows the eddy trajectories detected from SSHA (from January to December 1999: in the panel the trajectory is shown until August, except for October ) and NGSST (from late October 1999 to January ). We could trace the target eddy almost continuously in the above period, except for the period from late August to late October. In this period, since the eddy has an elongated shape, as shown later, the exact center of the eddy could not be fixed. It was also found that it was relatively difficult to trace the eddy from NGSST after January, since strong cooling due to the outbreak of the winter monsoon sometimes lowered the SST fields homogeneously in the study area. After determining the trajectories, we calculated zonal and meridional speeds of motion from these trajectories (not shown here), and read peak values of eddy SSHA during propagation, as shown in Fig. 8. Interestingly, from this trajectory analysis we found that Eddy A was formed by the merger of two eddies, which occurred in late August to early September In January 1999, one eddy (Eddy B) was situated to the 16 H 134 E 136 E 138 E m 1km 8 Line B 32 N 33 N 34 N Fig. 6. Horizontal temperature distributions at sea surface and depth 3 m (upper panels) and vertical temperature sections from sea surface to depth 45m along Lines A and B, which are shown in the left upper panel (lower panels). XBT/ CTD data taken in November 8 9, 1999 are used, provided by the Japan Oceanographic Data Center Low SST Anticyclonic Eddy 667

6 - Oct. -3 Aug Aug Eddy C May 28 Mar. 24 Jan. 1 Mar Eddy B Jan Aug. 1 Aug. Sep. 1 Sep. 3 3 Oct. 24 Nov. 1 Jan. 31 Eddy A Nov. 24 Fig. 7. Trajectories of anticyclonic eddies, traced from the low SST eddy found in the east of Kyushu in late October (Top panel) Trajectories obtained from SSHA from January to October Here, since it is difficult to specify the center of the merged eddy from September to late October, only the positions on August 31 and October were plotted and outlined by dots. (Bottom) Positions (dots) of minima of low SST region. Solid line represents low-pass filtered trajectory using a Gaussian filter with an e-folding scale of 2.3 days. Approximate dates are shown in the trajectory. SSHA(cm) Eddy B Eddy C l l l l l l l l l l l l J F M A M J J A S O N D J 1999 Fig. 8. Time series of peak SSHA of the centers of the two anticyclonic eddies (until late August) and merged eddy detected from SSHA (since mid September). Lines have the same meaning as Fig. 7. east of Izu Ridge, and another eddy (Eddy C) was situated to the west of the Ridge. Eddy B then propagated westward along the 29 N line at a speed of about 7 8 cm/s, and moved northward around the Izu Ridge in March April 1999, propagating westward again from May along the 31 N line to the south of Shikoku. On the other hand, Eddy C was trapped at the west side of the Izu Ridge until March 1999, and then propagated roughly along the Fig. 9. Temporal changes of SSHA fields every days in August September SST diff ( C) Eddy B Eddy C Eddy A l l l l l l l l l l l l J F M A M J J A S O N D J 1999 Fig. 1. Timeseries of SST differences between mean SSTs calculated in the region of.5.5 (lat. long.) centered at the SSHA maxima and those calculated in the region of 2 2 centered at the SSHA maxima. SST differences of Eddy A are calculated in the same way as Eddies B and C for the minimum of low SST region using the NGSST dataset. Two thin lines show the root-mean-squared difference of SSTs calculated in the area of 3 5 (lat. long.) centered at SSHA maxima. 28 N line at a speed of 8 9 cm/s to the south of Shikoku. Figure 9 shows temporal changes of SSHA every days in August and September It was found that the coalescence (merger) of Eddies B and C took place in late August to early September 1999 around the position 31 N, 134 E. Although, before the merger, both eddies were fairly circular in shape (see the panel of August 1 in Fig. 9), the merged eddy (Eddy A) has an elongated shape along the Kuroshio (see the panels of September 1 and 3). At the same time, SSHA at the center of Eddy A abruptly reached 35 cm in height from cm prior to the merger (Fig. 8). After the merger, Eddy A stagnated in that region, and from late October it moved northeastward and then southeastward along the Kuroshio to the Kii Peninsula at a speed of 1 cm/s. 668 K. Hosoda and K. Hanawa

7 Figure 1 shows time series of differences between mean SSTs calculated in the region of.5.5 (lat. long.) centered on the SSHA maxima corresponding to Eddies B and C, and those in the region of 2 2 centered on the SSHA maxima. It can be seen that SSTs of both Eddies B and C fluctuate around the zero line and do not show any significant deviation until the merger. We also observed all A-HIGHERS images by eye and confirmed that Eddies B and C had accompanied no marked low SSTs in their vicinities. It can also be seen that, even after the merger, when SSTs are calculated around the region of SSHA maximum, the deviations are not significant. On the other hand, when mean SSTs are calculated in the region of SST minima, the deviations amount to about 2 C. It is considered that, since Eddy A is closely situated around the Kuroshio axis, the eddy center detected by SSHA does not necessarily coincide with the actual center of the eddy. Although NGSST is not available prior to October 1999, it would seem acceptable that low SST accompanying an anticyclonic eddy appears after the merger of Eddies B and C. 4.2 Clockwise revolution of the eddy Figure 11 shows snapshots of SST every two days taken in November These sequential images show that Eddy A detached from the Kuroshio around mid- November and then propagated southwestward and then westward along 3 N. In other words, Eddy A revolved clockwise in the Kuroshio recirculation region south of Shikoku, which can also be clearly seen in the trajectory obtained using NGSST, as shown in Fig. 7. Although the trajectory of Eddy A could not be traced sequentially after January from the NGSST dataset, for the reason mentioned earlier, it was found that such clockwise revolution was repeated at least three times during the period from October 1999 to May (see Fig. 2), using the SSHA dataset. That is, the period of revolution can be roughly estimated as about 2 3 months. This clockwise revolution coincides with the finding of Ebuchi and Hanawa (3), although the revolution period of 2 3 months found here is rather short compared with the 5 6 months they found. This might be due to the small size of the Kuroshio recirculation gyre off Shikoku (that is, the Large Warm Eddy off Shikoku) compared with that during the period of the straight path of the Kuroshio. It can thus be concluded that the movement of Eddy A described here is a common feature of anticyclonic eddies found in the Kuroshio recirculation region south of Shikoku. 4.3 The eddy and the Kuroshio path variation As we have already described, from late October, Eddy A swiftly moved with the Kuroshio and detached from the Kuroshio in mid-november. Here, we focus our Fig. 11. Snapshots of SSTs in the sea south of Honshu every two days in November The contour interval is.5 C. The range of color bar, [Tmax, Tmin], is 4 C, but changing in time so as Tmax = 27..1d and Tmin = 23..1d, where d is the day counted from November 1, attention on the relationship between the eddy motion and the Kuroshio path variation. Figure 12 shows temporal changes of the Kuroshio path every ten days in September-November 1999, cited from the Monthly Marine Report issued by JMA (). This figure shows that the Kuroshio began to take a meander path with larger amplitude around the third ten-day period of October to the first ten-day period of November. As mentioned earlier, since Eddy A was originally situated in the sea east of Kyushu and stagnated close to Low SST Anticyclonic Eddy 669

8 Fig. 12. Temporal changes of the Kuroshio path from the first ten days of September to the last ten days of November Cited from the Monthly Ocean Report, No. 85 (January issue) by the Japan Meteorological Agency. the Kuroshio from September, as shown in Fig. 7, it might have some effect at the beginning of the formation of the larger amplitude meander. For example, the Kuroshio might entrain some portion of the waters of Eddy A and its transport might be strengthened as a result. However we cannot at present make any conclusive statement on the direct role of Eddy A on the meander formation mechanism. Nevertheless, we can point out that Eddy A directly enhanced the meander s amplitude. In the panel of November 1, shown in Fig. 11, it can be seen that the tip of the low SST region extending from the coast is situated around 32.5 N, 138 E. As Eddy A approaches the meander part of the Kuroshio, the Kuroshio meander grows in amplitude, as seen in the panels of November 9 and 11. Eddy A then detached around November 17 and 19, and at this time the tip of low SST region extending from the coast was situated at 3.5 N, E. That is, the meander amplitude becomes twice as large as before Eddy A approaches. This suggests that an interaction between Eddy A and the Kuroshio might contribute to the amplitude growth of the meander. Such amplitude growth of the Kuroshio meander due to the eddy was also found in March and May (not shown here). 5. Summary We have described an example of an anticyclonic eddy, which displayed lower SSTs than those of surrounding waters in the recirculation area south of Japan. Using datasets of SST, SSH and surface velocity, it was found that this anticyclonic eddy was formed to the east of Kyushu in late August to early September 1999, by the merger of two anticyclonic eddies which had migrated to the sea south of Japan from the east within the Kuroshio recirculation region. After the merger, the anticyclonic eddy was strengthened and began to exhibit lower SSTs than its surroundings. In October, this eddy coalesced with the Kuroshio and swiftly moved eastward accompanied with a growth in amplitude of the Kuroshio meander. In mid-november, off the Kii Peninsula, the eddy detached from the meandering Kuroshio and then moved southwestward, again slowly propagating westward along the 3 N line. That is, this anticyclonic eddy revolved clockwise in the sea south of Shikoku, as already described by Ebuchi and Hanawa (3). During this period, from late October 1999, (at least) to January, SSTs over the anticyclonic eddy were lower than those of surrounding waters. Since SST signals of this anticyclonic eddy were similar to that of a cold eddy, this discovery tells us that we have to pay careful attention to the interpretation of SST maps. Statistics on the percentage of anticyclonic eddies which display low SST, and mechanisms responsible for the appearance of low SST over the anticyclonic eddy are a very interesting subject. The authors will try to investigate this in future. Acknowledgements The authors wish to express their sincere thanks to members of the Physical Oceanography Group at Tohoku University for useful discussions. Dr. Akira Tomosada kindly provided the original data of Fig. 1 and Miss Kanako Sato prepared the figure. The surface velocity dataset used in Fig. 3 was kindly provided by Dr. Hiroshi Uchida. Two anonymous reviewers gave useful comments on the original manuscript. This study was made as part of NEAR-GOOS project (Chairpersons: Professors K. Taira and M. Kawabe, Ocean Research Institute, University of Tokyo), which was financially supported by the Ministry of Education, Culture, Sports, Science and Technology. References Aoki, S., S. Imawaki and K. Ichikawa (1995): Baroclinic disturbances propagating westward in the Kuroshio Extension region as seen by a satellite altimeter and radiometers. J. Geophys. Res., 1, Ebuchi, N. and K. Hanawa (): Mesoscale eddies observed by TOLEX-ADCP and TOPEX/POSEIDON altimeter in the Kuroshio recirculation region south of Japan. J. Oceanogr., 56, Ebuchi, N. and K. Hanawa (1): Trajectory of mesoscale eddies in the Kuroshio recirculation region. J. Oceanogr., 57, Ebuchi, N. and K. Hanawa (3): Influence of mesoscale eddies on variations of the Kuroshio path south of Japan. J. Oceanogr., 59, 36. Guan, L. and H. Kawamura (4): Merging satellite infrared and microwave SSTs Methodology and evaluation of the new SST. J. Oceanogr. (in press). 67 K. Hosoda and K. Hanawa

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