Evaluation of shelf–basin interaction in the western Arctic by use of short-lived radium isotopes: The importance of mesoscale processes

Shelf–basin exchange in the western Arctic was evaluated by use of water-column analyses of Ra/ Ra ratios and the first measurements of the short-lived Ra (T1/2 1⁄4 3.64 d) in the Arctic. During the 2002 shelf–basin interaction (SBI) program, excess Ra was detected over the shelf but was not found seaward of the shelf-break. Similarly, the Ra/ Ra ratio dropped rapidly from the shelf across the shelf-break. Consequently, the model age gradient (elapsed time since shelf residence) northward across the Chukchi Shelf increased from 1–5 years nearshore to approximately 14 years in surface waters sampled off shelf at the southern margin of the Beaufort Gyre. This steep gradient is consistent with very slow exchange between the Chukchi Shelf and the Beaufort Gyre, whereby Bering Strait inflow is constrained by the Earth’s rotation to follow local isobaths and does not easily move into deeper water. The strong dynamic control inhibiting water that enters the system through Bering Strait from flowing north across isobaths also would lead to a long recirculation time of river water emptied into the Beaufort Gyre. Possible mechanisms that can generate cross-shelf currents that break the topographic constraint to follow isobaths, and thereby transport water (and associated properties) off the shelves include wind-induced upwelling/downwelling, meandering jets, and eddies. Evidence of such a process was found during the ICEX project in the Beaufort Sea in April 2003 when excess Ra was measured over 200 km from any shelf source. This required an NE offshore flow of 40 cm s 1 assuming that the source water derives from the mouth of Barrow Canyon. A weak northeastward flow was measured using an LADCP within the upper 300m of the ocean, but was of lower speed than required by the Raxs at the time of the ICEX occupation. r 2005 Elsevier Ltd. All rights reserved.