Journal of Oceanography, Vol. 60, pp. 265 to 268, 2004

Introduction to Special Section: Kuroshio Observation, State Estimation and Prediction This special section of the Journal of Oceanography is dedicated to the observation, state estimation and prediction of the Kuroshio, which were the topics of the recent research program entitled “Kuroshio Fluctuation Prediction Experiment”. The Kuroshio is one of the strongest currents in the world oceans, flowing northeastward south of Japan as the western boundary current (WBC) of the North Pacific Subtropical Gyre (NPSG). Its heat transport is crucial to maintaining the global climate. Its location and flow field affect the fisheries, marine transportation, marine sports, etc. and so the Kuroshio has attracted the attention of many people. The Kuroshio is noted especially for a unique phenomenon: the stationary large meander south of Honshu, Japan. The real flow field in and around the Kuroshio, however, is very complicated due to its interactions with mesoscale eddies, warm rings, cold rings and other fluctuations. It has therefore turned out to be very difficult to predict the Kuroshio. The recent progress in satellite remote sensing for earth observation is quite remarkable; the sea-surface topography, sea-surface temperature, sea-surface wind and other properties have been measured globally and repeatedly. Remarkable progress has been also made in the in situ observation of the Kuroshio; thanks to the field program entitled “Affiliated Surveys of the Kuroshio off Cape Ashizuri” (ASUKA), a time series of Kuroshio transport has become available, for the first time, with a fairly fine temporal resolution covering a relatively long period (Imawaki et al., 2001). In ocean modeling, high-resolution ocean general circulation models have been developed and the technique of data assimilation has also become available to provide dynamically consistent data sets, or state estimation. Based on the recent progress mentioned above, an experiment was carried out to challenge the prediction of fluctuations of the location and transport of the Kuroshio south of Japan, which has been a dream for many years. A research team was organized by scientists from Kyushu Univ., Kagoshima Univ., Hiroshima Univ., Kyoto Univ., Univ. Tokyo, Hokkaido Univ. and Meteorological Research Institute, to develop a system for predicting the Kuroshio fluctuation and to assess the possibility of prediction. This experiment was called “Kuroshio Fluctuation Prediction Experiment” and was carried out for five years from 1997 to 2002 as a research program of the Core Research for Evolutional Science and Technology (CREST) of Japan Science and Technology Corporation (JST). The experiment had four aims: (a) collecting ocean variability data on the Kuroshio and western NPSG regions to better understand the variability and its mechanism as well as to provide the data for validation of the state estimation; (b) developing a data assimilation model to provide dynamically consistent data sets from observation data; (c) developing a high-resolution prediction model using those data sets as initial and boundary conditions; and also (d) developing a practical prediction model by incorporating the data assimilation model and prediction model. Two specific predictions were targeted: the short-range (one month) prediction of the location of the Kuroshio path south of Japan, and the medium-range (one to two years) prediction of the Kuroshio transport south of Japan. The experiment was intended to stimulate understanding the mechanism underlying the Kuroshio fluctuations through these predictions. Path fluctuation The path of the Kuroshio south of Japan is usually classified into two stable paths: the large meander path and the non-large meander path (Kawabe, 1995). Small meanders originating south of Kyushu frequently propagate eastward south of Japan. An occasional small meander happens to develop into the stationary large meander. It has been suggested that mesoscale eddies play an important role in this phenomenon. The Kuroshio did not take the stationary large meander path during the present research program. The following progress was made in the study of the Kuroshio path fluctuations, thanks to this program. The surface flow field obtained by combing the satellite altimeter data and surface drifting-buoy data provided a vivid description of the variability of the Kuroshio and Kuroshio Extension (Uchida and Imawaki, 2003; Imawaki et al., 2003). Based on this flow field, the Kuroshio axis was detected by tracking the location of the locally strongest velocity in the Kuroshio region (Ambe et al., 2004). The superposition of Kuroshio axes south of Japan obtained every 10 days for eight years from 1993 to 2000 is shown on the front page of this special section. This is the first data set to describe the variation of the Kuroshio axis with a fairly fine resolution both in time and space. Further, it was shown that fluctuations of the Kuroshio path in the Tokara Strait are induced both by disturbances propagating from

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the East China Sea (i.e., the upstream region) and by mesoscale eddies propagating from the western part of the NPSG at about 30°N latitude (Ichikawa, 2001). Fluctuations of the Kuroshio path in the East China Sea between the continental slope and the Tokara Strait was also studied in detail (Nakamura et al., 2003). Studies on predicting fluctuations of the Kuroshio path south of Japan were carried out as follows. A reducedgravity model was used to assimilate the satellite altimeter data by an adjoint model, which has an advantage in reproducing non-linear phenomena. Experiments using thus-obtained assimilated data as the initial condition showed that the Kuroshio path can be predicted for about two months (Ishikawa et al., 2004). Further, state estimation was carried out by assimilating the satellite altimeter data into an ocean general circulation model for the whole North Pacific, driven by observed daily wind stress; here a four-dimensional optimal interpolation was used in the assimilation process (Kamachi et al., 2004a). The results obtained compared well with the flow field of the western NPSG observed by a ship-mounted acoustic Doppler current profiler (Kaneko et al., 2001) and moored current meters (Ichikawa, H. et al., 2004). Using this state estimation as the initial condition, a practical prediction model was developed to hindcast locations of the Kuroshio path; a number of experiments were carried out by choosing different starting dates, and the results showed that the prediction was successful for about one month (Kamachi et al., 2004b). The transition mechanism between the large meander path and non-large meander path of the Kuroshio south of Japan was studied. First, the role of the wind stress field of the North Pacific in determining these two stable paths was investigated using an idealized two-layer model (Kurogi and Akitomo, 2003). A sensitivity experiment was then carried out with the same two-layer model, showing that mesoscale eddies located initially south of Kyushu or east of Taiwan induced the transition from non-large meander path to large meander path, but did not induce the opposite transition (Akitomo and Kurogi, 2001). A simulation experiment using a high-resolution ocean general circulation model confirmed that the interaction between the Kuroshio and mesoscale eddies was crucial for the variation of the Kuroshio path south of Japan, including the transition between the two stable paths; a possible mechanism of the generation of small meanders of the Kuroshio south of Kyushu was studied in detail (Masumoto, 2004). Transport fluctuation The transport of the Kuroshio as the WBC of the NPSG is mostly determined by the wind stress over the North Pacific. In the steady state, the transport can be estimated as the transport of WBC compensating the interior Sverdrup transport, which is the wind-induced transport based on the linear theory for an ocean with flat bottom topography; the transport of WBC thus estimated is hereafter called Sverdrup transport for simplicity. The wind stress, however, varies having various time scales, and the corresponding time scales of oceanic response are different. Therefore, the transport of WBC cannot be understood simply as the Sverdrup transport. The bottom topography, such as the IzuOgasawara Ridge, induces interference between the barotropic and baroclinic components of the flow field, and makes the situation more complicated. In practice, excluding the transport associated with mesoscale eddies is also a serious issue. The following progress was made in the study of the Kuroshio transport fluctuations, thanks to the present research program. A very high correlation has been found between the Kuroshio transport (for upper 1,000 m layer) and sea-surface height difference across the Kuroshio (Imawaki et al., 2001), on the basis of intensive ASUKA observations. Using this relationship and TOPEX/POSEIDON satellite altimeter data, the transport of the Kuroshio south of Shikoku, Japan, was estimated for ten years from 1993 to 2002 (Uchida and Imawaki, 2004). This is the first time series of Kuroshio transport obtained for a long period. The mean transport of the eastward-flowing Kuroshio was estimated to be 61 Sv (1 Sv = 10 6 m3/sec), and that of the Kuroshio through-flow was 43 Sv, excluding the local recirculation. The mean seasonal signal of Kuroshio transport estimated from this ten-year long time series was fairly weak compared with that of the Sverdrup transport estimated from the observed wind stress over the North Pacific (Uchida and Imawaki, 2004). To understand the difference, the ocean response to the seasonally varying wind stress was studied using an idealized two-layer ocean model. The results showed that the bottom relief corresponding to the IzuOgasawara Ridge prevents most of the barotropic signal, originated in the interior region, going beyond the relief, and therefore the seasonal signal of Kuroshio transport is much reduced (Isobe and Imawaki, 2002). The seasonal signal in the flow field west of the Ridge was further studied in detail (Isobe et al., 2004). In regard to the interannual fluctuation of Kuroshio transport, the exchange among vertical modes of the flow field due to the Izu-Ogasawara Ridge was found to be important (Tanaka and Ikeda, 2004). Data sets of the wind stress over the North Pacific, used to drive ocean general circulation models, were examined in detail (Yoshinari et al., 2004). An investigation was made on where in the NPSG the major origin of the Kuroshio transport fluctuations having time scales of 2–3 years was located (Wakata et al., 2004). Further, a study on the predictability of the interannual fluctuation of Kuroshio transport was carried out, in the fashion of a hindcast, as follows: An ocean general circula-

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tion model for the North Pacific was driven by an observed time series of wind stress having interannual fluctuations. The wind stress was then fixed at a given time and the model continued to be driven by that fixed wind stress. The Kuroshio transport thus calculated was compared with that in the model driven continuously by the original time series. The study showed that most of the interannual fluctuation of Kuroshio transport can be predicted for one to two years, using only the past record of time series of the wind stress over the North Pacific (Tanaka et al., 2004). A discrepancy had been recognized in the Kuroshio transport, which is about 40 Sv (Imawaki et al., 2001) south of Shikoku, Japan, and about 20 Sv in the East China Sea (Ichikawa and Chaen, 2000). This can be probably accounted for by the northeastward flow over the continental slope east of the Ryukyu Islands (i.e., the upstream region of the Kuroshio south of Japan), which was detected by the moored current meter observations during 1998–2002 and TOPEX/POSEIDON satellite altimeter data (Ichikawa, H. et al., 2004). The flow was found to be confined over the slope and consist of a surface mode and a subsurface-core mode centered at about 600 m depth. The four-year mean transport was estimated to be 16–18 Sv. These results suggest that a new boundary current, which might be called the “Ryukyu Current System”, exists east of the Ryukyu Islands and thus of schematic flow field of the Kuroshio system proposed by Nitani (1972) deserves revision in this light. The flow field of the western NPSG was further measured by a ship-mounted acoustic Doppler current profiler (Kaneko et al., 2001). The measured instantaneous surface velocities were combined with the satellite altimeter data to provide the mean surface velocities along the ship tracks (Ichikawa, K. et al., 2004). The results can be used to monitor the surface flow field continuously using satellite altimeter data. As described above, considerable progress has been made in understanding the fluctuations in the location and transport of the Kuroshio south of Japan, thanks to the present research program. The latest ocean general circulation model incorporated with the data assimilation model was shown to be able to predict those fluctuations to some extent. Further progress is required, however, for those prediction systems to gain acceptance in our daily life. This special section is the collection of the papers resulting from the research program of CREST. It is a pleasure to thank all those who have contributed to this special section and to the research program in general. We thank the authors for writing up their contributions, and the reviewers for their critical reading and reports. We also thank the JST and its staff for their support and encouragement, in particular Prof. Tomio Asai, the supervisor of the program. Guest Editor-in-Chief Shiro Imawaki Kyushu University Guest Editors Hiroshi Ichikawa, Kagoshima University Motoyoshi Ikeda, Hokkaido University Atsuhiko Isobe, Kyushu University Masafumi Kamachi, Meteorological Research Institute

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