3-DIMENSIONAL STRUCTURE OF THE WIND-DRIVEN CIRCULATION IN THE SUBTROPICAL NORTH PACIFIC

The subduction rate is calculated for the North Pacific based on Levit us climatology data and Hellerman and Rosenstein wind stress data. Bec ause the period of effective subduction is rather short, subduction ra tes calculated in Eulerian and Lagrangian coordinates are very close. The subduction rate defined in the Lagrangian sense consists of two pa rts. The first part is due to the vertical pumping along the one-year trajectory, and the second part is due to the difference in the winter mixed layer depth over the one-year trajectory. Since the mixed layer is relatively shallow in the North Pacific, the vertical pumping term is very close to the Ekman pumping, while the sloping mixed layer bas e enhances subduction, especially near the Kuroshio Extension. For mos t of the subtropical North Pacific, the subduction rate is no more tha n 75 m yr-1, slightly larger than the Ekman pumping. The water mass vo lume and total amount of ventilation integrated for each interval of 0 .2sigma unit is computed. The corresponding renewal time for each wate r mass is obtained. The inferred renewal time is 5-6 years for the sha llow water masses (sigma = 23.0-25.0), and about 10 years for the subt ropical mode water (sigma = 25.2-25.4). Within the subtropical gyre th e total amount of Ekman pumping is 28.8 Sv (Sv = 10(6) m3 s-1) and the total subduction rate is 33.1 Sv, which is slightly larger than the E kman pumping rate. To this 33.1 Sv, the vertical pumping contributes 2 4.1 Sv and the lateral induction 9 Sv. The maximum barotropic mass flu x of the subtropical gyre is about 46 Sv (east of 135-degrees-E). This mass flux is partitioned as follows. The total horizontal mass flux i n the ventilated thermocline, the seasonal thermocline, and the Ekman layer is about 30 Sv, and the remaining 16 Sv is in the unventilated t hermocline. Thus, about one-third of the mass flux in the wind-driven gyre is sheltered from direct air-sea interaction.