Background
The ocean circulation in the Arctic Ocean is to a large degree controlled by a comparatively large freshwater input from river runoff and precipitation. When the salty Atlantic water mixes with the freshwater a buoyant layer of cold, low-saline water is formed in the surface. The process of ice freezing, however, also extracts salt from the fresh layer and separates it into ice and saltier water. Hence the Arctic modifies the Atlantic water into both a lighter, less saline water type but also dense, cold water with a range of different salinities. As a consequence the ocean circulation in the Arctic may be viewed as both estuarine - driven by the positive buoyancy flux supplied by the freshwater - and also that of a reversed estuary, driven by the buoyancy loss from cooling and draining of salt during ice formation.

The estuarine part of the circulation, similar in many ways to that of smaller scale fjords, was first described by Stigebrandt (1981). Given the freshwater flux, the Bering Strait import, and the ice export, the approximate thickness and salinity of the fresh layer can be estimated. In particular it is possible to predict the effects of a change in the freshwater flux on the surface layer. This requires that either the flow of Atlantic water into the Arctic or the outflow of Arctic Surface water is known. Furthermore the entrainment ratio of Atlantic water into the freshwater layer must be parameterized.

The original assumption used in Stigebrandt (1981) was that the outflow of Arctic Surface water is geostrophically balanced and purely baroclinic. The outflow rate then increases with layer thickness and with salinity difference between the layers, i.e. with increasing freshwater flux. More recent research however (Spall, 2004; Walin et al, 2004) shows that a baroclinic boundary current that looses buoyancy does not decrease its total transport, but instead transfers it to a barotropic core on the shelf. These findings indicate that the outflow rate may not be purely baroclinic as previously assumed. Since the estuarine system is highly sensitive to the outflow rate the proposed field work will provide new knowledge that is valuable for understanding the circulation system in the Arctic, and its sensitivity in a changing climate.

Largely because of a pronounced lack of field data, there exists no unified theory for how the Atlantic water gets entrained into the fresh surface layer. Observations show that the freshwater that is released in the Arctic entrains Atlantic water until its volume flux has been increased by a factor of ten. Lab experiments show that the freshwater layer can entrain underlying water up to a factor of 20 (Pierce and Rhines, 1996), or down to a few percent (J. Mullarney, 2006, pers. comm.) in which case the freshwater input appears to affect the circulation only marginally. The proposed fieldwork design monitors the inflow as well as the outflow and will provide valuable new knowledge about the entrainment mechanism.

Proposed activity
Here we will investigate these simple models discussed above using data gathered in WP 1 to study water mass modification, entrainment rates and transfer between the baroclinic and barotropic flow field.