Formation and evolution of the surface mixed layer and halocline of the Arctic Ocean

Abstract

Fresh water from summer ice melt and the total freshwater content of the Arctic Ocean water column above the thermocline are estimated from vertical profiles of temperature and salinity observed on the I/B Oden 1991 cruise. The seasonal ice melt ranges from 0.5 m to slightly above 1 m and is moderately uniform over the observation area. Regions of lower melting are seen over the Morris Jesup Plateau. The freshwater content is calculated relative to the salinity just above the thermocline north of the Barents Sea. The freshwater content increases toward the interior of the Arctic Ocean, showing that fresh water is advected from other regions into the observation area. Regions of different freshwater content are separated by fronts over the Nansen‐Gakkel Ridge, over the Lomonosov Ridge, and in the western Eurasian Basin between waters derived from the Eurasian and Canadian Basins. Denser water, homogenized north of the Barents Sea, is recognized by a temperature minimum layer. The absence of the temperature minimum near the Nansen‐Gakkel Ridge indicates that heat is transferred from the Atlantic Layer over a longer time than the shortest route would allow. This observation can be explained if the layer circulates together with the Atlantic Layer, i.e., toward the east, and returns above the Nansen‐Gakkel Ridge and along the Amundsen Basin. North of the Laptev Sea, this water formed north of the Barents Sea becomes covered by low‐salinity shelf water. The increased freshwater content limits the winter convection, so it no longer reaches the thermocline and an intermediate halocline is formed. The halocline in the Eurasian Basin consists of water originating from winter convection in the Arctic Ocean north of the Barents Sea, which then circulates around the basin. Such a formation mechanism also explains the observed distribution of low NO water. The strong density increase limits vertical exchange, and the vertical diffusion coefficient in the halocline is small (∼1 × 10−6 m2 s−1). The increased temperature of the halocline shows that the heat lost upward by the Atlantic Layer, mainly by double‐diffusive convection, is trapped below the mixed layer.

Carbon utilization in the Eurasian sector of the Arctic Ocean

“…General circulation-The ACU distribution along the Barents and Laptev slopes is consistent with the general circulation scheme of intermediate waters, i.e., with a boundary current flowing eastward along the continental slopes (e.g., Rudels et al 1994). In addition to the branch of Atlantic water entering the Arctic Ocean north of Svalbard, Atlantic water that has been modified while traversing the Barents Sea enters at the St. Annas Trough and displaces the Fram Strait branch toward the interior (Schauer et al 1997).…”

“…In addition to the branch of Atlantic water entering the Arctic Ocean north of Svalbard, Atlantic water that has been modified while traversing the Barents Sea enters at the St. Annas Trough and displaces the Fram Strait branch toward the interior (Schauer et al 1997). Part of this subsurface current will deflect and follow the ridge topography poleward north of the Laptev Sea (Rudels et al 1994), while the remaining current continues east into the Canadian Basin. At the Laptev Sea shelf break, low salinity water of high runoff content enters the surface water over the deep interior and follows the Siberian branch of the Transpolar Drift toward the Fram Strait (Anderson et al 1994b).…”

“…The seasonal transformation of surface water along the investigated area (Rudels et al 1996) makes it appropriate to combine the surface mixed-layer and lower halocline waters into the top layer. The lower boundary of layer 2 originates from the depth in the deep basin where rACU approximately equals zero.…”

“…This transport time will be applied for the Fram Strait branch in the two uppermost layers, assuming these layers to flow with the same speed. Rudels (1987) calculated the flow of Atlantic water through Fram Strait from two CTD sections across the strait, assuming geostrophic balance. He concluded that a flux of 1.9 Sv of Atlantic water passed the section of which half recirculated into the northern part of the strait, while the remaining half continued into the Arctic Ocean.…”

Anthropogenic radioactivity in the Arctic Ocean — review of the results from the joint German project

“…Water masses that enter with the WSC turn east and continue below Ž . polar surface waters Rudels et al, 1996 , forming the core of an Atlantic layer which lies at approximately 200᎐600 m depth. This water re-circulates in several cyclonic loops along the major ridges and slopes in the Eurasian and Canadian Basins to exit finally also through Fram Strait below the Ž .…”