Advective and Mixing Time Scales for Transport of Denmark Strait Overflow Water from the Labrador Sea to the Western Subtropical Atlantic Ocean Determined from 129I, CFC and Hydrographic Time Series Observations

¹²⁹I released to the North Sea from two nuclear fuel reprocessing plants is transported through the Nordic Seas and the Arctic Ocean and is entering the deep North Atlantic, predominantly in dense Denmark Strait Overflow Water (DSOW). CFCs enter the surface ocean and also become incorporated in DSOW. Measurements of temperature, salinity, CFCs and ¹²⁹I have been made at least annually along WOCE/CLIVAR line AR7W in the Labrador Sea from the mid 1990s to present, along Line W extending from the continental slope southeast of Cape Cod toward Bermuda from 2003 to 2014, and along a single occupation of a line extending from Bermuda southeast across the Bermuda Rise in 2010. The measurements in the Labrador Sea were used as input to DSOW flowing from there to the subtropical western Atlantic Ocean. We compared the temporal changes along Line W to the temporal changes along the AR7W line and applied the boundary current model of Waugh and Hall (J. Phys. Oceanogr. 35,1538-1552, 2005) to the Line W and Bermuda Rise line observations to determine the transit time of DSOW transported to Line W in the Deep Western Boundary Current (DWBC) and transported to the southeastern flank of Bermuda via interior flow paths. The lateral mixing time scale along these two flow paths was also estimated with this model. CFC-11 and ¹²⁹I increase monotonically in the DSOW in the Labrador Sea and salinity oscillates on a 5-year cycle. The boundary current model reproduces all of these trends. The transit time and lateral mixing time constant for DSOW transported to Line W are 7 years (mean flow velocity of 2.1 cm/sec) and 3-6 years, respectively, and for DSOW transported to the southeast flank of Bermuda are 6-10 years and 2-5 years.