1 Direct observations of brine rejection at the source of North Pacific Intermediate Water in the Okhotsk Sea Andrey Y. Shcherbina*, Lynne D. Talley*, and Daniel L. Rudnick* *Scripps Institution of Oceanography, University of California, San Diego The densest water formed in the North Pacific is its intermediate water (NPIW), which is renewed in its northwestern marginal sea, the Okhotsk 1-3 . Brine rejection accompanying ice formation in northern coastal polynyas over the broad shelves had been hypothesised as the formation mechanism of this watermass 2,4 , analogous to formation of deep and intermediate water masses in the Arctic and Antarctic 5-8 . Although NPIW formation at present is relatively weak, it was much more vigorous during the last glacial 9-12 . During that time the northern Okhotsk Sea, which was only partially covered by fast ice 13 , likely retained its importance as the NPIW formation site 11 . Previous estimates of worldwide production of dense shelf waters were indirect, based on heat budget analyses from remote sensing and atmospheric data or on warm-season water column inventories 5,6,14-16 . Here we present the first measurements of brine rejection and dense water formation in the Okhotsk Sea using moored winter observations. To our knowledge this is the only direct record of brine rejection associated with formation of a major water mass. The global thermohaline circulation is primed by dense water formation in high latitudes, which leads to the gradual renewal of the deep ocean. Densest water formation occurs in the North Atlantic and Southern Ocean. Ventilation of the intermediate layer of the ocean, however, is just as important for the global overturning circulation as the deep water formation in terms of heat and volume transport. The densest water formed in the North Pacific is such an intermediate water, North Pacific Intermediate Water (NPIW), characterized at its top by a vertical salinity minimum throughout the North