Formation of North Pacific Intermediate Water in Meteorological Research Institute ocean general circulation model: 1. Subgrid‐scale mixing and marginal sea fresh water

Abstract

Formation of North Pacific Intermediate Water (NPIW) is investigated using an ocean general circulation model for the North Pacific with realistic bottom topography. The model has a horizontal resolution of 2° longitude by 1° latitude and 28 vertical levels. Experiments explored the sensitivity of the NPIW property to (1) subgrid‐scale mixing parameterization, (2) surface flux conditions for temperature and salinity, and (3) fresh water sources in marginal seas. A combination of horizontal mixing and sea surface temperature and salinity relaxation toward annual mean climatological values simulates the salinity minimum of the modeled NPIW very poorly. Inclusion of isopycnal subgridscale mixing substantially improves the salinity structure. A low‐salinity tongue associated with the salinity minimum, for example, extends from the subarctic gyre to the subtropical gyre at intermediate depths, while the salinity minimum in the modeled NPIW appears at 26.0–26.2 σθ, compared to 26.8 σθ for the observed NPIW. When surface condition for temperature and salinity restored to perpetual winter is imposed, the isopycnal surface of 26.0–26.2 σθ outcrops in the northwestern subtropical gyre, and salinity increases effectively due to excessive evaporation over precipitation. This allows the NPIW density to increase to 26.4–26.6 σθ. When temperature and salinity are restored to climatological values from the surface to the bottom in the Okhotsk and Bering Seas, less saline water flowing from marginal seas sets the NPIW density to the observed 26.8 σθ. Spatial distribution of modeled NPIW is found sensitive to the isopycnal diffusion. If the isopycnal diffusion is reduced, a low‐salinity tongue of the modeled NPIW disappears in the western North Pacific; relatively large isopycnal diffusion is necessary to simulate a fresh NPIW tongue in this area. Experiments with and without water flux from the northern marginal seas to the North Pacific confirm that the modeled NPIW has two fresh water sources. It is found that the northwestern subarctic sea surface supplies fresh water to the upper NPIW (26.4–26.7 σθ), whereas the Okhotsk Sea supplies fresh water to the lower NPIW (26.8–27.4 σθ).