The eastward subtropical countercurrent on isopycnal surface in the western North Pacific

The classic Sverdrup theory suggests that the water movement in the central subtropical gyre of North Pa-cific be slowly westward or southwestward.In the late sixties of the20th century,the existence of a peculiar eastward narrow flow between20°N and25°N in spring was theoretically predicted.It was named the Subtropical Countercurrent(STCC),although direct observational evidences were not yet sufficient to con-firm whether or not such eastward flow between20°N and25°N was a persistent…

The eastward subtropical countercurrent on isopycnal surface in the western North Pacific

The monthly circulations on isopycnal surface in the western North Pacific are calculated based on the Navy's Generalized Digital Environmental Model climatological temperature and salinity data on a 1/2°× 1/2° grid using a P-vector method. The eastward Subtropical Countercurrent (STCC) in the central subtropical gyre has been studied with emphasis on its spatial distribution, vertical structure, volume transport and its nearby eddy phenomenon. The results reproduce the observed feature of STCC, and reveal some new phenomena on isopycnal surface, such as: (1) Atσθ= 24.6, there is strong seasonal variability in the distribution, origin and flow status of STCC. There is no STCC in January, February and March. (2) From May to October, a branch of the Kuroshio Countercurrent to the south of Japan merges into STCC, which increases the velocity of STCC and widens its width. (3) The velocity vector field shows that STCC and nearby eddies coexist. (4) The eastward volume transport of STCC across 137.5°E is the strongest in summer with the maximum of 6.9 Sv ( 1 Sv ≡ 106 m3 s-1) in August, and weaker in spring and autumn with 1.7 Sv in April and 1.6 Sv in November. Most of the transports of STCC occur between isopycnal surfaces 24.0σθand 25.0σθ. (5) The distributions of the zonal flow along 137.5°E show that the flow core of STCC is between 24.3σθand 24.7σθ.