Abstract
Formation and circulation of mode waters in the subtropical North Pacific are investigated by performing a numerical simulation with a high-resolution general circulation model. In the model, important features for forming mode waters, such as separation of western boundary currents, frontal systems, mixed layer processes, and eddy activities, are well reproduced. Annual subduction rates, calculated by following the mixed layer base, have major peaks in three density ranges and are related to the deep winter mixed layer at the south of the fronts in the western North Pacific, that is, the Kuroshio Extension with a winter surface density of 25.5 σθ, the Kuroshio bifurcation front (26.2 σθ), and the subarctic front (26.5 σθ). The lightest peak corresponds to the Subtropical Mode Water (STMW). It is formed at the south of the Kuroshio Extension and enters the main pycnocline as it crosses the mixed layer depth (MLD) front to the south. The remaining two peaks correspond to the Central Mode Water (CMW). The lighter one may be called the lighter variety of CMW (LCMW); it is formed at the south of the Kuroshio bifurcation front and enters the main pycnocline as it crosses the MLD front to the east. The heavier one may be called the denser variety of CMW (DCMW); it is formed at the south of the subarctic front and enters the main pycnocline as it crosses the MLD front to the east. A large heat loss at the sea surface in the western North Pacific is important for STMW formation, while low stability in the subsurface layer, as well as a large heat loss, is important for CMW formation. Among the waters that are subducted in the subtropics, the cores of STMW and LCMW circulate around the wind-driven subtropical gyre, while those at the outer edge of the gyre, which include DCMW, enter the Tropics via the western half of the tropical–subtropical gyre boundary. They amount to about 26 Sv (Sv ≡ 106 m3 s−1), which is determined by the northward surface Ekman transport at the boundary.
Corresponding author address: Dr. Hiroyuki Tsujino, Oceanographic Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan. Email: htsujino@mri-jma.go.jp
