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

The subduction rate is calculated for the North Pacific based on Levitus climatology data and Hellerman and Rosenstein wind stress data. Because the period of effective subduction is rather short, subduction rates calculated in Eulerian and Lagrangian coordinates are very close. The subduction rate defined in the Lagrangian sense consists of two parts. 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 base enhances subduction, especially near the Kuroshio Extension. For most of the subtropical North Pacific, the subduction rate is no more than 75 m yr-1, slightly larger than the Ekman pumping. The water mass volume and total amount of ventilation integrated for each interval of 0.2sigma unit is computed. The corresponding renewal time for each water mass is obtained. The inferred renewal time is 5-6 years for the shallow water masses (sigma = 23.0-25.0), and about 10 years for the subtropical mode water (sigma = 25.2-25.4). Within the subtropical gyre the 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 Ekman pumping rate. To this 33.1 Sv, the vertical pumping contributes 24.1 Sv and the lateral induction 9 Sv. The maximum barotropic mass flux 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 in the ventilated thermocline, the seasonal thermocline, and the Ekman layer is about 30 Sv, and the remaining 16 Sv is in the unventilated thermocline. Thus, about one-third of the mass flux in the wind-driven gyre is sheltered from direct air-sea interaction.