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Since the beginning of January, an unusually long period of easterly winds has caused the average water level in the Baltic Sea to fall to a historic low. Measurements at the Swedish Landsort-Norra gauge show values that are the lowest since records began in 1886. Researchers at the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) are currently monitoring this development very closely, as it represents a rare oceanographic situation that could lead to a large inflow of saltwater from the North Sea into the Baltic Sea. An inflow of this kind could significantly affect the physical and chemical conditions in the deep basins of the Central Baltic Sea.
The strong easterly winds that have prevailed over large parts of the Baltic Sea since the beginning of January have caused large quantities of water to be pushed through the Belt Sea straits between Germany, Denmark and Sweden towards the North Sea. As a result, the average the Baltic Sea water level has dropped significantly. On February 5, the Landsort-Norra gauge off the Swedish coast measured more than 67 centimetres below the long-term average. This is the lowest value ever recorded since continuous gauge observations began in 1886.
The latest calculations by IOW researchers show that the Baltic Sea is currently missing around 275 cubic kilometres of water compared to the long-term average. In over 140 years of measuring, only five other years have seen similarly low sea levels with deficits of more than 60 centimetres below normal. The last comparable event occurred more than four decades ago, in March 1980 (see below for further information). Sea level measurements on the island of Landsort are continuously carried out and published by the Swedish Meteorological and Hydrological Institute (SMHI).
Prerequisites for saltwater inflows from the North Sea
Levels 20 cm below mean sea level (MSL) in the Baltic Sea are already considered a good precondition for large saltwater inflows from the North Sea. With the current extreme value of over - 67 cm MSL, the conditions are exceptionally favourable for such a major event to occur. IOW researchers are currently focusing not only on the gauge levels, but also on further meteorological developments in the coming days and weeks. In order for an above-average inflow to occur, the current easterly wind situation needs to end and be replaced by sustained westerly winds, which would then push salt- and oxygen-rich North Sea water into the Baltic Sea. This is of major ecological importance, particularly for the deep central basins of the Baltic Sea, where oxygen deficiency in the deep water layers often prevails for years or decades.
“The chances of a major inflow in the coming weeks are higher than they have been for a long time,” explains Michael Naumann, one of the coordinators of the IOW long-term observation programme. “According to the time series calculated here at the IOW on saltwater inflow events in the Baltic Sea, the probability is currently between 80 and 90 percent,” says Naumann.
Using data from the autonomous Darß Sill measuring station in the western Baltic Sea, among other sources, the IOW is able to record the actual occurrence of a saltwater inflow and its extent in detail. The station is one of three MARNET stations operated by the IOW on behalf of the Federal Maritime and Hydrographic Agency (BSH), which continuously record information on currents, salinity and oxygen levels. Data from other Baltic Sea measuring stations and measurements taken by research vessels during the relevant time period provide additional information on the inflow effects as far as the Central Baltic Sea.
Cold winter inflow with particular significance for deep-water areas
A notable feature of a potential large inflow in late winter is the low temperature of the infcoming North Sea water. “If the inflow occurs now, it would have two effects that are significant for the deep Baltic Sea basins,” emphasises Volker Mohrholz, deputy head of the IOW’s Physical Oceanography Department. “Cold water can absorb significantly more oxygen than warm water and would therefore transport an above-average amount of oxygen into the deep basins in relation to its volume. In addition, an inflow of sufficient intensity could end the persistently elevated deep-water temperatures in the central Baltic Sea basins that have now lasted for around two decades,” says Mohrholz, who specialises in the coupling of marine physical and biogeochemical processes.
Currently, the increased temperatures are leading to heightened microbial activity at these depths. This accelerates the decomposition of organic matter, such as died-off, sunken algal blooms, and is accompanied by increased oxygen consumption. Among other things, a lack of oxygen or its complete depletion makes deep water layers uninhabitable for fish and other higher organisms and can also lead to the release of nutrients from the sediment and thus to internal nutrient loading of the Baltic Sea.
“The combination of long-term observations and current measurement data allows us to accurately assess both, the intensity and the potential effects of an inflow,” says Michael Naumann. “That is why we are preparing for extensive measurements specifically tailored to the current developments as part of our long-term observation programme. The IOW ship Elisabeth Mann Borgese will be spending 49 days in relevant sea areas in the next eight weeks, providing additional good measurement opportunities,” the oceanographer concludes.
Further information
The extent of a saltwater inflow from the North Sea into the Baltic Sea is quantified by the amount of salt in the incoming water with a salinity of over 17 g/kg during the main inflow phase into the western Baltic Sea that passed the Darß Sill and the Drogden Sill. If the salt import exceeds one gigaton (GT), this is referred to as a “Major Baltic Inflow”.
Further background information: https://www.iow.de/major-baltic-inflow-statistics-7274.html
Historical levels of more than 60 cm below MSL in the Baltic Sea (Landsort-Norra gauge, source: SMHI) and intensity of subsequent inflow events (salt amount in GT):
Date: Januar 31, 1937 | Gauge level: - 61,5 cm | Amount of salt: 1,76 GT
Date: February 11, 1947 | Gauge level: - 62,0 cm | Amount of salt: 1,57 GT
Date: February 25, 1954 | Gauge level: - 61,2 cm | Amount of salt: 0,64 GT
Date: March 12, 1972 | Gauge level: - 63,9 cm | Amount of salt: 2,77 GT
Date: March 24, 1980 | Gauge level: - 61,2 cm | Amount of salt: 1,96 GT
Date: February 5, 2026 | Gauge level: - 67,4 cm | Amount of salt: ??
IOW Media contact:
Dr. Kristin Beck | Tel.: +49 (0)381 – 5197 135 | presse@iow.de
The Leibniz Institute for Baltic Sea Research Warnemünde (IOW) investigates natural and anthropogenic changes in coastal and marginal seas using a system-wide and interdisciplinary approach, ranging from fundamental to applied research. The Baltic Sea serves as an ideal case study on the institute’s doorstep. An important mission of the IOW is to engage in knowledge-based dialogue with policy-makers, stakeholders, and society, thereby contributing to the sustainable development of coastal seas. https://www.iow.de
Dr. Volker Mohrholz | Phone: +49 (0)381 5197 - 198 | volker.mohrholz@iow.de
Dr. Michael Naumann | Phone: +49 (0)381 5197 - 267 | michael.naumann@iow.de
Volker Mohrholz is an expert on coupled effects of physical and biogeochemical processes in the sea. ...
Quelle: K. Beck
Copyright: IOW
Michael Naumann is one of the coordinators of the IOW long-term observation programme and has specia ...
Copyright: IOW
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