A mean vertical profile of temperature and salinity over the Arctic Ocean reveals several features. There is a low salinity surface layer, with temperatures near the salinity-adjusted freezing point. Below this surface layer, extending to about 200-300 m depth, is a rapid increase in salinity. This is attended by an increase in temperature to maximum (and above-freezing) values at around 300-500m depth. Although temperature falls off at greater water depths, from about 400 m downward salinity stays nearly uniform at 34.5-35.0psu (practical salinity units). The layer of rapid salinity increase is termed a halocline. The layer of rapid temperature change is termed a thermocline. Over most of the global ocean, a stable upper-ocean stratification (less dense water at the top) is maintained by higher temperatures closer to the surface. The situation in the Arctic is quite different. At the low water temperatures of the Arctic, the density is determined by salinity. Consequently, the halocline is associated with a strong increase in density with depth (a pycnocline). This means that the upper Arctic Ocean is very stably stratified. The warmer water at depth, if brought to the surface, would quickly melt the sea-ice cover. Suppression of vertical mixing by the 'cold Arctic halocline' is one of the key features of the Arctic (along with low winter air temperatures) that allows sea ice to form and persist.
The fresh surface layer is maintained primarily by river runoff, the influx of relatively low salinity waters from the Pacific into the Arctic Ocean through the Bering Strait and net precipitation over the Arctic Ocean. It is also influenced by the growth and melt of the sea-ice cover. As sea ice forms, brine is rejected. The density of the surface layer increases as does the depth of vertical mixing. As ice melts in summer, the surface layer becomes fresher and vertical mixing is inhibited. The temperature maximum layer at about 300-500m depth manifests the inflow of Atlantic-derived waters. This is provided by two branches, one west of Spitzbergen (the West Spitzbergen Current) and one through the Barents Sea (the Barents Sea Branch). It appears that this Atlantic inflow has changed.
As summarized by Dickson et al. (2000), results from a number of oceanographic cruises indicate that, in comparison with earlier climatologies, the Arctic Ocean in the 1990s was characterized by a more intense and widespread influence of Atlantic-derived waters. The Atlantic-derived sublayer warmed 1-2°C compared with Russian climatologies of the 1940s to 1970s and the subsurface temperature maximum shoaled (to about 200 m in from some observations). The boundary between waters of Atlantic and Pacific origin spread west, resulting in an extension of the
Atlantic water range by nearly 20%. Evidence also arose of a weakening of the cold halocline in the Eurasian Basin (Steele & Boyd, 1998).
The change in the cold halocline is attributed to eastward diversion of Russian river inflow in response to changes in the atmospheric circulation. The Atlantic layer changes are attributed primarily to changes in the Atlantic inflow in the early 1990s, and some warming of this inflow. These changes have been modelled successfully, and are viewed as a response to changes in surface winds associated with increasing dominance of the positive phase of the NAO/AO. As argued by Maslowski et al. (2000), this enhanced inflow of warm, Atlantic-derived waters promotes a stronger upward oceanic heat flux, which may be contributing to the observed sea-ice decline. Subsequent work (Maslowski et al., 2001) suggests an additional role of a warmer inflow of Pacific waters through Bering Strait. Whether changes in the cold halo-cline may also be involved is not clear.
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