The bottom water of the northern Baltic Sea usually is well oxygenated. We used a combined approach of numerical model simulations and in situ observations to investigate processes responsible for a regular ventilation of the Bothnian Bay. Surface water masses from the Bothnian Sea and the Bothnian Bay mix at the link between both regions. In winter, when water temperature is low, the resulting density is large enough that the water descends and replaces old bottom water.