ARCTlC SHELF-BASIN INTERACTIONS: EXCHANGES AND THROUGHFLOWS
K. Aagaard
Applied Physics Laboratory, University of Washington
During the past 15 years or so the focus in shelf-basin exchange studies in the Arctic has been on halocline and intermediate-depth ventilation. The source of the ventilation is generally argued to be either shelf waters made dense by cooling and brine rejection, or waters from the Norwegian Atlantic Current that are cooled and somewhat freshened during their transit of the Barents Sea. The modified waters have been thought to enter the interior either as plumes or as trains of eddies. We do not know the extent to which interactions between these waters and the boundary current over the slope are important.
It appears that in the shelf seas in which local processes drive the various exchanges between the shelf and the interior ocean, the exchanges are rather small. For example,estimates of the shelf source strength for waters capable of ventilating the halocline range from about 0.1 Sv in the Laptev to one-third that in the East Siberian. In the case of the Laptev especially, this is a consequence of the very large runoff that dilutes the shelf water salinities, offsetting the winter ice formation, which is very large there. Recently it has also been suggested that in coastal polynyas the salinity enhancement of the shelf waters that is possible during freezing is limited dynamically, essentially by leakage of the dense water through an unstable front. If so, then the very high salinities sometimes observed on the shelf suggest repeated events of salinity enhancement, with the salinity increasing serially with each event. It is in any case clear that the density and volume of the final shelf product depends strongly on the initial salinity, so that interannual variability in the preconditioning of the shelf becomes an important consideration.
The most important ventilation by shelf sources appears to occur when the cross-shelf circulation is part of a larger-scale flow, rather than primarily being forced by the local buoyancy flux to the atmosphere, as occurs in the Laptev, for example. In the former case the shelf serves primarily as a shallow conduit for a regional throughflow overlain by a buoyancy sink of variable strength, and perhaps even variable sign. There are two such shelf sources for the Arctic Ocean: the input of the Atlantic across the Barents shelf and that of the Pacific across the Bering-Chukchi shelf.
While recent work points convincingly to the importance of the Barents inflow in ventilating the upper km or so of the Arctic Ocean,it appears that shelf processes do relatively little to increase the density of this water during its transit of the Barents Sea. This is because the considerable cooling, typically 2-3℃, is nearly offset by the freshening, typically 0.2 or so. Therefore, while the source strength is large in the Barents Sea, probably near 2 Sv, the density interval the water will ventilate within the Arctic Ocean is primarily determined by the properties of the water prior to their entry onto the shelf, rather than by modification of the throughflow over the shelf.
The Bering-Chukchi throughflow appears to be more effectively modified by shelf processes, and particularly in having its density enhanced by salt rejection during freezing. I suspect that the extremely wide and shallow shelf is the key to this. Measurements of the water properties in Bering Strait (made together with Tom Weingartner and Andy Roach ) show an annual cycle of salinity as large as 2 per mille peak-to-peak. An example is shown in Figure 1.Both the annual amplitude and the mean vary from year to year, but in 1990-91 the annual mean salinity was near 33, and the observed winter salinity increase corresponds to an ice formation rate of about 5 cm per day. Current measurements show that during the six winter months an average of about 11,000 km3 of water at the freezing point enter the Chukchi Sea, corresponding to an annualized flow of 0.35 Sv. In 1991 the maximum monthly mean salinity of this inflowing water was near 34, and the salinity may have increased further as the water crossed the Chukchi shelf before entering the Arctic Ocean. These winter waters are clearly capable of ventilating the halocline. The means by which they do so is obviously a pivotal issue. Thus far the observational attention has been focused on Barrow Canyon, but it seems likely that the trajectories following Hope Sea Valley across the central Chukchi and meeting
Knut Aagaard, Applied Physics Laboratory,
University of Washington, 1013 NE 40th Street,
Seattle, WA 98105-6698, U.S.A.
e-mail: aagaard @apl.washington.edu
