Major Baltic Inflows come in different flavours

Abstract The Baltic Sea is a marginal sea in northern Europe. Anthropogenic eutrophication and related deoxygenation are among its pressing environmental problems. To this end intermittent major salt water inflows from the North Sea are of major importance because they replace deep low-oxygenated waters with oxygenated surface water that sinks to depth after passing the shallow Danish Straits. Here we show, guided by machine-learning and backed by geophysical fluid dynamic modelling, that major inflows come in two “flavours” as a result of distinctly-different successions of the driving synoptic atmospheric patterns: (1) Relatively salty inflows, carrying mainly waters of pristine Atlantic origin, preconditioned by moderate high-pressure systems over the Baltic that are ultimately driven by gale force westerly winds. (2) Relatively sweet (fresh) inflows, preconditioned by pronounced high-pressure systems that build up oceanic barotropic pressure gradients across the Danish Straits. The latter, in conjunction with moderate to strong westerly winds, ultimately drive inflows that carry a mix of previously-exported brackish Baltic Sea water and salty Kattegat water - rather than more pristine Atlantic flavours. Our conceptual framework provides, for the first time, an explanation for the question why inflow strength is not directly linked to re-oxygenation of the deep Baltic.