Roles of the Okhotsk Sea and Gulf of Alaska in forming the North Pacific Intermediate Water

Recently obtained World Ocean Circulation Experiment (WOCE) sections and pre-WOCE hydrography are used to study the water-mass structure and formation and transformation of North Pacific Intermediate Water (NPIW). Five neutral density surfaces are selected and mapped, encompassing NPIW from 400 to 900 m in the subtropical latitudes with a distance of ∼100 m between a pair of surfaces. NPIW is defined as a subtropical gyre salinity minimum which is well followed by a neutral density surface σ_N = 26.9. Formation and transformation of NPIW is examined by the mapped Turner angle on neutral density surfaces. Apparent diffusive double diffusion is found in the Alaskan gyre on σ_N = 26.5 neutral surface, in the northwest subpolar gyre and the Okhotsk Sea on σ_N = 26.9 neutral surface, and mainly in the Okhotsk Sea on the two deep neutral surfaces σ_N =27.2 and σ_N = 27.4. These diffusive regions indicate transformation sources for NPIW. Along with additional information of potential vorticity and stream function, it is found that there are two different NPIW formation sources: one in the Gulf of Alaska characterized by high potential vorticity and the other in the Okhotsk Sea characterized by low potential vorticity. The former lies shallower at σ_N =26.2-26.5, but its effect deepens to NPIW core density level at σ_N = 26.8 on the basis of potential vorticity distribution. The latter includes the influence of the northwest subpolar gyre and extends much deeper to σ_N = 27.4. We call them Gulf of Alaska Intermediate Water (GAIW) and Okhotsk Intermediate Water (OIW), respectively. GAIW contributes to NPIW in the eastern part of the subtropical gyre east of date line, whilst OIW dominates in the west and entire lower part of NPIW. Seasonal flow stream function mapped on neutral surfaces shows that the contribution of GAIW to NPIW occurs mainly in the wintertime, because in winter a significant northward shift of zero wind stress curl makes the Gulf of Alaska an additional source for NPIW.