Variations of Current Path, Velocity, and Volume Transport of the Kuroshio in Relation with the Large Meander

Abstract

The variations of the Kuroshio Current path and their relations with current velocity, volume transport, and upstream position of the Kuroshio are examined using sea level data. After the long-time, non-large-meander (NLM) path during 1963–75, the Kuroshio takes a large-meander (LM) path during most of 1975–91: 1975–80, 1981–84, 1986–88, and 1989–91. The study of these meanders confirms the description of the path of the Kuroshio based on the three typical paths and the transitions between them, with an exception being the 1981–84 LM path. The formation and decay of the large meander are associated with velocity and position of the Kuroshio in the Tokara Strait south of Kyushu. A small meander, precursor of the large meander, seems to be formed southeast of Kyushu in relation with a temporary increase of Kuroshio velocity and a northward shift of the Kuroshio in the Tokara Strait. An eastward propagation of the small meander, leading to the LM formation, is associated with remaining in the northern position in the Tokara Strait, but not with large velocity except in 1975. Decay of the large meander occurs associated with large velocity and a return to the southern position of the Kuroshio in the Tokara Strait. Unusual separation of a cold eddy from the LM path is also related with a temporary increase of Kuroshio velocity. Volume transport of the Kuroshio shows an interdecadal variation, small before 1975 and large afterward, that is, the transport during the 1975–91 LM-dominant period is larger on average than that during the 1963–75 NLM path. All the LM paths are formed when the transport increases from about 24 Sv (Sv = 106 m3 s−1), while the Kuroshio takes an NLM path whenever the transport is less than 23.5 Sv. Velocity of the Kuroshio main current is mostly correlated with the transport but is different in the decrease after 1981. This shows that the Kuroshio always takes a NLM path for small transport and velocity, but can take either path for their large and medium ranges. A factor for determining the path in these ranges is the position of the Kuroshio in the Tokara Strait; LM paths begin and terminate about four months after the Kuroshio shifts to the north and returns to the south, respectively. The time lags are for the formation and decay processes. In conclusion, LM paths occur when the Kuroshio has large or medium transport and velocity, and is in a northern position in the Tokara Strait. The northern position implies a small curvature and a large vertical inclination of current axis south of Kyushu, which may be related with the dynamics of a large meander.