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Abstract: Short-term flow and water temperature fluctuations in Sagami Bay were examined using mooring and hydrographic data observed during the non-large-meander path (NLM) of the Kuroshio. In the surface layer (<150m), the flow fluctuation is predominant and is excited by the sudden strong eastward flow with speeds greater than 20cms−1 in the central part of the bay. This flow is a part of an intrusion of Kuroshio water via the Oshima West Channel into the bay and/or a cyclonic circulation in the northern part of the bay. The cyclonic circulation, which is approximately balanced under the geostrophic flow relationship, is mainly generated or enhanced by the intrusion, which is accompanied by eastward propagation of the small meander of the Kuroshio south of Japan during the transition from the nearshore non-large-meander path (nNLM) to the offshore non-large-meander path (oNLM), regardless of season. The water temperature fluctuation in the subsurface layer (∼150m) is caused by upwelling of cold deeper water and is closely related to the eastward flow fluctuations in the surface layer in the central part of the bay. Our analysis leads us the conclusion that the upwelling occurs as the response of the subsurface density field to the surface flow field under the Earth’s rotation. Additionally, the intrusion of Kuroshio water via the Oshima West Channel tends to cause, not only cyclonic circulation and upwelling in the northern part of the bay, but also the Kyucho, which is the coastal density current under the Earth’s rotation, in the bay’s coastal area during the non-large-meander path (NLM). [Copyright &y& Elsevier]

Abstract: Hydrographic observations based on XBT, XCTD, CTD, and ADCP were performed around the eastern mouth of the Tsugaru Strait in June 2003. This study aims to describe the hydrographic structure of the eastern mouth focusing on the subarctic Oyashio water occupying the northern part, and to discuss the intrusion pathway of this water and its modification after the intrusion. Residual and absolute geostrophic velocities show a nearly barotropic inflow (0–25cm/s) and outflow (0–80cm/s) structures in the northern and southern parts of the strait mouth, respectively. In the north, the Oyashio water, classified as the near-bottom and modified Oyashio waters in this study, inflows into the strait. It is inferred that the near-bottom water is changed to the modified Oyashio water after the intrusion by diapycnal mixing and outflow as the modified Oyashio water from underneath of the Tsugaru Warm Current (TWC) water in the deepest part of the strait mouth, together with the TWC water. Horizontal salinity distribution suggests that the near-bottom water is supplied along the isopycnal surface from the subsurface of Hidaka Bay deeper than the strait. [Copyright &y& Elsevier]

Abstract: Closed loop mesoscale eddies were identified and tracked in the Ulleung Basin of the southwestern Japan/East Sea (JES) using the winding-angle (WA) methodology, for mapping the absolute geostrophic currents into surface streamlines of flow. The geostrophic velocity used here was the sum of the Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO), time variable velocity and the 1992–2007 mean geostrophic velocity. Local sampling bias was removed using the drifter observations. This WA methodology of deriving the Lagrangian path lines that drifters followed over a 7-day period was validated by individual drifter tracks and it demonstrated closed looping eddy motions. The WA method demonstrated that less than 6% of the closed streamlines appeared when drifters did not show a closed loop in their vicinity, compared to 30% of the excess detection rate by the Okubo–Weiss method of locating closed loop structures. Three groups of eddies were identified: (1) Coastal Cold and Warm Eddies, which appeared in the area between the coast of southern Korea and the East Korean Warm Current (EKWC), when a southward coastal current was present, (2) Frontal Cold and Warm Eddies, which were formed in the region of the seaward extension of the meandering EKWC, north of Ulleung Island and (3) Ulleung Warm Eddies (UWE) and Dok Cold Eddies (DCE), which appeared during meanders of the EKWC, in the Ulleung Basin. No seasonal concentration for eddy generation and eddy population was found. The average radius of eddies was about 38–60km. These were born, moved in an erratic pattern and then died in the vicinity where the EKWC separated from the coast and formed a large meander. The time-mean large meander formed meridionally concentrated bands of positive and negative relative vorticity. The cyclonic (cold) eddies tend to reside within the band of positive time-mean relative vorticity, and the anticyclonic (warm) eddies reside within the bands of negative relative vorticity. Six UWE and four warm eddies, in the Yamato Basin (about 10% of warm eddies), were sustained longer than a year. Because the large meander of the EKWC appeared to be controlled by topography, and the JES is a nearly enclosed basin with rapid flow-out to the east through the narrow Tsugaru Strait, there was little eddy energy propagation to the west. The warm eddies in the southwestern part of the JES appeared to be interacting very locally with the mean flow. [Copyright &y& Elsevier]

Abstract: Inter-annual variability in the magnitude and timing of the spring bloom was investigated for the Oyashio region (40 °-48 °N, 143 °E-152 °E) using 10 years (from 1998 to 2007) of satellite ocean-color data. Geostrophic currents were examined using satellite altimeter data. Early spring blooms (>1.5mgm−3) occurred in early April 2001 and late March 2002. The 2001 bloom continued for one month. Late blooms occurred from mid-May 1999, early June 2004 and late April 2006, continuing for about 1 month, 8 days and 16 days, respectively. A strong bloom (4.7mgm−3) also occurred in mid-April 1998; however, it terminated in early May. We classified the Oyashio region based on the pattern of temporal variation of Chl-a concentration from March to June. The spatio-temporal variability in Chl-a concentration during spring was different among years. The area where Chl-a concentration was highest in April was more extensive in 2001, 2002 and 2006 than usual. In 1999, the area where Chl-a concentration was highest in May was the widest among the 10 years. Mesoscale eddies and currents with high velocity were frequently observed in the area of high Chl-a concentration east of Hokkaido, propagating Coastal Oyashio Water of low salinity and low density into the oceanic region. That strengthened stratification in the surface layer. We suggest that this seaward transfer of coastal water could be one of the important factors for phytoplankton distribution in two ways: (1) horizontal advection of water with high Chl-a concentration and (2) enhancement of stratification in the oceanic region. [Copyright &y& Elsevier]

Abstract: Flow fluctuations inside an anticyclonic eddy in summertime Funka Bay were examined using three moorings and hydrographic data. The flow pattern above a sharp pycnocline with a concave isopycnal structure during the mooring period was characterized by high mean kinetic energy and relatively low eddy kinetic energy. The ratios of eddy to mean kinetic energy were equal to or less than one, and the mean flow field and isopycnal structure indicated the existence of a stable anticyclonic eddy above the sharp pycnocline under approximate geostrophic balance. Larger flow fluctuations with periods longer than 7 days were dominant inside the eddy. The low-frequency flow fluctuations are accompanied by north to northeastward movement of the eddy with deepening of the pycnocline and spin-up of the anticyclonic circulation. The wind field over Funka Bay is characterized by bay-scale wind fluctuations. The bay-scale winds are greatly influenced by the land topography around Funka Bay, resulting in non-uniform structure with significant wind stress curl. The bay-scale wind fluctuations are termed “locally modified wind” in the present study. The locally modified wind has a negative (positive) wind stress curl in the central–northeastern (southwestern) region of Funka Bay. The north to northeastward movement of the eddy is caused by horizontal non-uniform supply of vorticity from the locally modified wind forcing by the Ekman pumping process. Through this process, the anticyclonic circulation is enhanced (weakened) in the central–northeastern (southwestern) part of the eddy, resulting in the eddy moving north to northeastward with the pycnocline deepening and spin-up of the anticyclonic circulation. The locally modified wind forcing induces low-frequency flow fluctuations through the movement of the eddy in summertime Funka Bay. [Copyright &y& Elsevier]