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

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Abstract
Hydrographic and tracer data collected during ARK IV/3 (FS Polarstern in 1987), ARCTIC91 (IB Oden), and AOS94 (CCGS Louis S. St‐Laurent) expeditions reveal the evolution of the near‐surface waters in the Arctic Ocean during the late 1980s and early 1990s. Salinity, nutrients, dissolved oxygen, and δ18O data are used to quantify the components of Arctic freshwater: river runoff, sea ice meltwater, and Pacific water. The calculated river runoff fractions suggest that in 1994 a large portion of water from the Pechora, Ob, 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 circulation into the Laptev and East Siberian Seas. River runoff flowed off the shelf at the Lomonosov Ridge and most left the shelf at the Mendeleyev Ridge. ARCTIC91 and AOS94 Pacific water fraction estimates of Upper Halocline Water, the traditionally defined core of the Pacific water mass, document a decrease in extent compared to historical data. The front between Atlantic water and Pacific water shifted from the Lomonosov Ridge location in 1991 to the Mendeleyev Ridge in 1994. The relative age structure of the upper waters is described by using the 3H‐3He age. The mean 3H‐3He age measured 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 years. Lateral variations in the relative age structure within the halocline and Atlantic water support the well‐known cyclonic boundary current circulation.

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