A study was made of the upper ocean region separating the winter water from the warm deep water of the northwestern Weddell Sea, using a series of 25 conductivity-temperature-depth casts. From 90 to 600 m the temperature and salinity increase by about 2¿C and 0.2%, respectively, while fine structure variations on both large and small scales are evident. The large-scale variations appear to be primarily due to isopycnal mixing (horizontal interleaving) below about 450 m and primarily due to internal waves of 8-to 10-hour period above 300 m. There is also evidence of isopycnal mixing in the upper region, especially in the latter casts where inversions appear, and some evidence of internal waves in the lower region. The small-scale structure, which shows steps in temperature, salinity, and density, is shown not be wave induced. The steplike structures are most evident in the region of strongest vertical temperature and salinity gradients, and double-diffusive effects appear to be important. Estimates of the upward heat and salt fluxes, obtained by comparison of the present data with previous laboratory experiments, are 7.65¿10-5 cal cm-2 s-1 and 3.78¿10-9 g cm-2 s-1, respectively. Additional measurements taken in the central Weddell Sea show larger step structures and an upward displacement of isotherms relative to the present data, supporting earlier conclusions of upwelling toward the gyre center.