Shin-Ichiro Umatani

Satellite altimeter monitoring the Kuroshio Transport south of Japan

In 1993–1995, we carried out observations of the Kuroshio south of Japan, including direct current measurements and repeated hydrographic surveys along a satellite track of the TOPEX/POSEIDON altimeter. The velocity field of the Kuroshio is determined by geostrophic calculation using the repeated hydrographic survey data, referenced to velocities observed at mid and abyssal depths. The volume transport of the Kuroshio is estimated from this velocity field. The estimated transports of the Kuroshio have a high correlation with sea-surface height differences across the Kuroshio. Having this relationship and using the altimeter data, we obtained a time series of the Kuroshio transport over seven years at ten-day intervals. The Kuroshio transport, excluding contributions by local recirculations, is estimated to be 42 × 106 m³/sec on average. The correlation between sea-surface height difference and transport provides a practical method of long-term monitoring of the Kuroshio transport using satellite altimetry.

Response of the Eastern Tropical Pacific to Meridional Migration of the ITCZ: The Generation of the Costa Rica Dome

Abstract A regional ocean circulation model with fine horizontal resolution has been developed in order to obtain a coherent seasonal picture of the eastern tropical Pacific off Central America. The Costa Rice Dome with a cyclonic circulation grows rapidly in late spring off the Gulf of Papagayo and matures in summer and early fall in accord with the northward migration of the ITCZ. In winter strong northern converging in the southernmost ITCZ from three passes in Central America excite warm anticyclones nonlinear eddies confined in the upper layer. Those anticyclones are identified as the intermediate geostrophic eddies by Matsuura and Yamagata. The Costa Rica Dome is eroded in winter and early spring by the westward propagating warm anticyclones but, at the same time, a new embryo of the dome begins to evolve from the westward propagating cold cyclonic eddy excited off the Gulf of Papagayo by the northers. The Costa Rica Dome can be maintained by the winds with the cyclonic wind stress curl in summer on...

Geometry-Forced Coherent Structures as a Model of the Kuroshio Large Meander

Abstract The bimodality of the Kuroshio path south of Japan is presented from a new viewpoint of direct interaction of current with local coastal geometry. By solving the barotropic quasi-geostrophic equation in a channel with steplike coastal geometry, we demonstrate that the model Kuroshio can actually show the localized, bimodal behavior for a reasonable range of inlet current speed. The amplitude of the large meander is approximately given by 2Umax/β. In contrast to all “nonlocal” model results, our local coherent structures have nothing to do with the basin-size geometry such as Kyushu and the Izu-Ogasawara Ridge. In general, the present study suggests that even a small feature of coastline geometry may trigger a big change in a nearshore current.

Evolution of an isolated eddy near a coast and its relevance to the “Kyucho”

The evolution of an isolated meso-scale eddy near a coast is studied numerically. In particular, it is found that the translation speed of the adjusted eddy is estimated well by the mutual induction mechanism adapted to a rotating stratified fluid. The nonlinear Kelvin wave excited during the adjustment process is also discussed in connection with the “Kyucho”, the sudden warming of coastal waters associated with swift currents.

Observations of an intrusion of a warmer and less saline water mass into a bay

Abstract Detailed hydrographic observations were made in Wakasa Bay, Japan, in August 1979 as the first of a series on the topic of bay intrusions. An anomalous water tongue, evident at a depth of 50 m, was observed to move eastward into the bay at a speed of about 10 km day −1 . The width of the tongue was about 20 to 30 km in agreement with the Rossby internal radius of deformation. The results of current meter measurements and the observed temporal and spatial evolution of the temperature field near the front of the anomalous water tongue have suggested that ageostrophic cross-frontal motion, in balance with the along-front acceleration, may be important in our understanding of intrusion processes.

Geostrophy in the Intermediate and Deep Layers of the Kuroshio and Its Recirculation Regions South of Japan

Theoretically, the geostrophic approximation holds for the low-frequency flow field, but no detailed examination has been done on how well the estimated geostrophic velocity corresponds with the observed velocity. Intensive surveys were carried out during 1993–1995 in the Kuroshio and its recirculation regions south of Shikoku, Japan, including repeated hydrographic surveys and direct current measurements at nominal depths of 700, 1500 and 3000 m. For these depth intervals, vertical differences of estimated geostrophic velocity are compared with those of observed velocity. For the intermediate layer (between 700 and 1500 m depths), the slope of the regression line is 0.99, correlation coefficient is 0.98, and the root-mean-square of difference from geostrophic balance is 2.8 cm/s which is close to the estimated error of 2.1 cm/s. For the deep layer (between 1500 and 3000 m depths), the corresponding values are 0.82, 0.93, 1.2 cm/s and 2.0 cm/s, respectively. The results indicate that the estimated geostrophic velocity compares well with the observed velocity in these regions.

Further study of synoptic variability in Wakasa Bay, Japan

Abstract Observations of synoptic variability from CTD and current meter measurements in Wakasa Bay, Japan in summer of 1980 and 1981 are compared with the results of 1979 reported by Yamagata, Umatani, Masunaga and Matsuura (1984). It is suggested that the speed and direction of propagation can basically be explained in terms of shelf wave dynamics. In the 1980 event, a dense (colder and more saline) water advanced eastward along the north coast at about 10 km day−1. The lateral scale of the phenomenon was about 30 to 40 km, in agreement with the Rossby internal radius of deformation. The T-S and current data suggest that the 1980 cold event was dominated by phase propagation. In the 1981 event, a light (warmer and less saline) water area advanced eastward at the speed similar to the 1980 cold event, but the T-S and current data suggest that Lagrangian drift of water particles associated with strong eddy motions was not negligible.