Determination of Positions

Kuroshio Extension and Northern Branch

The main path of the Kuroshio Extension is identified by the 9°C isotherm at 450 m depth and the main path of its Northern Branch by the 9°C isotherm at 250 m depth.

The 9°C isotherm at 500 and 450 m is in the area of maximum isothermal slope associated with the Kuroshio Extension at 144°E and 165°E in the vertical sections of temperature. Moreover, the 9°C isotherm at 250 m depth is still in the area of the maximum temperature gradient corresponding to the Kuroshio Extension Front (KF) at 144°E. However, it is associated with a northern front, the Kuroshio Bifurcation Front (KBF), at 165°E.

Vertical sections of temperature at 144°E and 165°E from CTD data obtained by the R.V. Kofu Maru on July, 1995 and the R.V. Ryofu Maru on June, 1998, respectively. The red line indicates the 9°C isotherm. The temperature interval is 2°C.

Bifurcation Point

We define the Bifurcation Point as the location where the meridional distance between 9°C at 450 and 250 m has a minimum value followed by a sudden and persistent increase of this distance to the east.

Assuming that upstream of the Bifurcation Point, the location of the 9°C at 450 and 250 m are inside the Kuroshio Extension front and that downstream of the bifurcation 9°C at 450 is still inside the Kuroshio Extension front but 9°C at 250 is inside the Kuroshio Bifurcation Front (Figure 3), this distance, derived from upper layer thickness maps, can provide the location of the Bifurcation Point.