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 pr e-WOCE hydrography are used to study the water - mass structure and formati on and transformation of North Pacific Intermediate Water (NPIW). Five neut ral density surfaces are selected and mapped, encompassing NPIW from 400 to 900 m in the subtropical latitudes with a distance of similar to 100 m bet ween a pair of surfaces. NPIW is defined as a subtropical gyre salinity min imum which is well followed by a neutral density surface sigma(N)=26.9. For mation 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 sigma(N)=26.5 neutral surface, in the northwest subpola r gyre and the Okhotsk Sea on sigma(N)=26.9 neutral surface, and mainly in the Okhotsk Sea on the two deep neutral surfaces sigma(N)=27.2 and sigma(N) =27.4. These diffusive regions indicate transformation sources for NPIW. Al ong with additional information of potential vorticity and stream function, it is found that there are two different NPIW formation sources: one in th e Gulf of Alaska characterized by high potential vorticity and the ether in the Okhotsk Sea characterized by low potential vorticity. The former lies shallower at sigma(N)=26.2-26.5, but its effect deepens to NPIW core densit y level at sigma(N)=26.8 on the basis of potential vorticity distribution. The latter includes the influence of the northwest subpolar gyre and extend s much deeper to sigma(N)=27.3. We call them Gulf of Alaska Intermediate Wa ter (GAIW) and Okhotsk Intermediate Water (OIW), respectively. GAIW contrib utes 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 f low stream function mapped on neutral surfaces shows that the contribution of GAIW to NPIW occurs mainly in the wintertime, because in winter a signif icant northward shift of zero wind stress curl makes the Gulf of Alaska an additional source for NPIW.