Aircraft observations off Enderby Land, Antarctica, verified that a coastal polynya forms even in severe winters and that new ice production continuously occurs there. In the polynya, water temperature profiles were measured for the first time using the aircraft-launched expendable bathythermograph (AXBT). The AXBT data show that the thickness of the winter mixed layer is 350 m or more and that the layer temperatures are near the freezing point. According to the water mass analysis of the winter mixed layer the active haline convection by the high ice production in the winter polynya contributes to the formation of more homogeneous and saline mixed layer than that under the fast ice cover, where the ice grows slowly and convective mixing is calm. Furthermore, the active convection leads to entrainment of the oxygen-poor deep water under Winter Water. Consequently, in the polynya located over the continental shelf break, the oxygen content of the mixed layer is somewhat lower than that in a coastal polynya that forms on the continental shelf, where the haline convection reaches to the sea bottom. The oxygen undersaturation of the thick mixed layer in the shelf break polynya suggests that the deep-water entrainment rate is more intense than that under the pack ice region in the Weddell Sea with a relatively thin mixed layer. It is, therefore, important to consider such polynya processes which may contribute to water mass modification in the Antarctic coastal region. ¿ 1999 American Geophysical Union