Study of the formation of North Pacific Intermediate Water by a general circulation model and the particle‐tracking method: 1. A pitfall of general circulation model studies

Abstract

Formation processes of North Pacific Intermediate Water (NPIW) are examined by a numerical model with fine vertical grids. The model produces a salinity minimum in the North Pacific (calculated NPIW (C‐NPIW)), which is very similar to observed NPIW except for its lighter density. Upon examination with the particle‐tracking method and T‐S diagram, it is determined that C‐NPIW is formed around the Kuroshio Separation, most C‐NPIW is transported by the Kuroshio, and some originates from the subarctic by turbulent mixing. However, C‐NPIW cannot be formed by only a mixture of the Kuroshio and the subarctic waters. Cooling and freshening surface fluxes with a thick mixed layer are generated near the Kuroshio Separation, and less saline C‐NPIW is formed by 4.5 m yr−1 freshening surface flux. While the less saline mixed layer is carried eastward, its upper part becomes saline by the surface flux and a salinity minimum is formed. Thus its density is determined by the surface density at the Kuroshio Separation. These processes of C‐NPIW formation are inconsistent with observations, thus C‐NPIW is not the counterpart of NPIW and should be called “false NPIW.” The false NPIW results from the northward extension of the northwestern North Pacific subtropical gyre (NPSTG) beyond the Subarctic Front. In this situation the surface water near the Kuroshio Separation is the densest in NPSTG, and also, it has lowest salinity in the lower part of the ventilated layer in NPSTG. Hence the less saline water formed at the Kuroshio Separation spreads over NPSTG as false NPIW. The boundary of the subtropical/subarctic gyres expected from the annual mean wind is at the north of the Subarctic Front in the western Pacific, thus the problem will be common to most numerical models. Therefore models must be used that can simulate a realistic Kuroshio Separation in order to reproduce NPIW.