The Baltic Sea is a marginal sea in northern Europe. Anthropogenic eutrophication and related deoxygenation are among its pressing environmental problems. In this context, North Sea water plays a crucial role in maintaining the ecological balance as so called Major Baltic Inflows from the North Sea are the only mechanism to ventilate the deep waters of the Baltic with fresh oxygen. Based on machine learning we could identify two types of inflows depending on the driving temporal evolution of atmospheric synoptics – either the inflowing water is relatively salty or fresh which directly affects its capability to oxygenate the deep Baltic Sea..