FORMATION AND EVOLUTION OF THE SURFACE MIXED-LAYER AND HALOCLINE OF THE ARCTIC-OCEAN

Fresh water from summer ice melt and the total freshwater content of t he 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 abov e 1 m and is moderately uniform over the observation area. Regions of lower melting are seen over the Morris Jesup Plateau. The freshwater c ontent is calculated relative to the salinity just above the thermocli ne, 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 fr eshwater content are separated by fronts over the Nansen-Gakkel Ridge, over the Lomonosov Ridge, and in the western Eurasian Basin between w aters derived from the Eurasian and Canadian Basins. Denser water, hom ogenized north of the Barents Sea, is recognized by a temperature mini mum layer. The absence of the temperature minimum near the Nansen-Gakk el Ridge indicates that heat is transferred from the Atlantic Layer ov er a longer time than the shortest route would allow. This observation can be explained if the layer circulates together with the Atlantic L ayer, i.e., toward the east and returns above the Nansen-Gakkel Ridge and along the Amundsen Basin. North of the Laptev Sea, this water form ed north of the Barents Sea becomes covered by low-salinity shelf wate r. The increased freshwater content limits the winter convection, so i t no longer reaches the thermocline and an intermediate halocline is f ormed. The halocline in the Eurasian Basin consists of water originati ng from winter convection in the Arctic Ocean north of the Barents Sea , which then circulates around the basin. Such a formation mechanism a lso explains the observed distribution of low NO water. The strong den sity increase limits vertical exchange, and the vertical diffusion coe fficient in the halocline is small (similar to 1 x 10(-6) m(2) s(-1)). The increased temperature of the halocline shows that the heat lost u pward by the Atlantic Layer, mainly by double-diffusive convection, is trapped below the mixed layer.