Origin of waters observed along 137°E

Using the World Ocean Atlas data set, we examine the origins and flow paths of subducted waters observed along the 137°E section in the western North Pacific. The waters studied consist mainly of the water mass known as North Pacific Tropical Water, but includes much of the water midlatitude thermocline. A method is developed to trace these waters from 137°E back through the subtropical gyre to their outcrops. Subducted waters are aged using this technique and found to be between 0.5 and 35 years old by the time they reach 137°E. For this subducted regime, waters on a given isopycnal observed along 137°E increase in age with decreasing latitude, with waters at the southern end of the section being 2-3 times older than waters at the northern end. This estimate of age is consistent with previous estimates calculated from chlorofluorocarbon measurements. It is found that subducted water masses are strongly homogenized by the time they reach 137°E. That is, the originally subducted waters have a wide variation in χ-S characteristics, but by the time they reach 137°E, they form a coherent water mass with a tight θ-S relation. It is shown that isopycnal mixing is not a plausible mechanism for this homogenization but that diapycnal mixing is a more likely process.