River runoff, sea ice meltwater, and Pacific water distribution and mean residence times in the Arctic Ocean

Hydrographic and tracer data collected during ARK IV/3 (FS Polarstern in 19 87), ARCTIC91 (IB Oden), and AOS94 (CCGS Louis S. St-Laurent) expeditions r eveal the evolution of the near-surface waters in the Arctic Ocean during t he late 1980s and early 1990s. Salinity, nutrients, dissolved oxygen, and d elta O-18 data are used to quantify the components of Arctic freshwater: ri ver runoff, sea ice meltwater, and Pacific water. The calculated river runo ff fractions suggest that in 1994 a large portion of water from the Pechora , Oh, Yenisey, Kotuy, and Lena Rivers did not flow off the shelf closest to their river deltas, but remained on the shelf and traveled via cyclonic ci rculation into the Laptev and East Siberian Seas. River runoff flowed off t he shelf at the Lomonosov Ridge and most left the shelf at the Mendeleyev R idge. ARCTIC91 and AOS94 Pacific water fraction estimates of Upper Haloclin e Water, the traditionally defined core of the Pacific water mass, document a decrease in extent compared to historical data. The front between Atlant ic water and Pacific water shifted from the Lomonosov Ridge location in 199 1 to the Mendeleyev Ridge in 1994. The relative age structure of the upper waters is described by using the H-3-He-3 age. The mean H-3-He-3 age measur ed in the halocline within the salinity surface of 33.1 +/- 0.3 is 4.3 +/- 1.7 years and that for the 34.2 +/- 0.2 salinity surface is 9.6 +/- 4.6 yea rs. Lateral variations in the relative age structure within the halocline a nd Atlantic water support the well-known cyclonic boundary current circulat ion.