NPIW

North Pacific Intermediate Water (NPIW) is a water mass in the North Pacific Ocean that occupies intermediate depths, roughly between 700 and 1500 meters below the surface. It is a distinct layer defined by its characteristic temperature, salinity, and chemical properties, setting it apart from surface waters and deeper waters in the region. NPIW plays a key role in the interior circulation of the North Pacific and contributes to the vertical structure and ventilation of the ocean.

Formation of NPIW occurs mainly in the subpolar North Pacific during winter, when surface cooling and sea-ice formation lead to the production of denser water. Brine rejection during sea-ice growth increases salinity and density, allowing the dense fluid to sink to intermediate depths. Principal source regions include areas around the Okhotsk Sea and the western subarctic gyre, with additional contributions from neighboring subpolar regions. The resulting water mass then spreads along midlatitude pathways within the North Pacific.

Properties and circulation: After formation, NPIW flows predominantly eastward and southeastward, residing at depths corresponding to the base of the thermocline. It is characterized by cold temperatures for its depth and a distinct salinity and tracer signature, along with relatively high dissolved oxygen. These features enable it to ventilate intermediate-depth layers and transport properties such as heat, salt, and nutrients between high-latitude formation zones and subtropical regions.

Importance and variability: NPIW is a major component of the North Pacific’s interior circulation, influencing the ocean’s thermohaline structure and nutrient distribution at intermediate depths. Its properties and volume vary with climate patterns, sea-ice extent, and seasonal convection. Observations from hydrographic surveys, tracer studies, and autonomous floats continue to refine understanding of its formation, pathways, and role in the Pacific Ocean system.