Baltic Sea: Easterly winds cause historically low water levels - and an opportunity

The Baltic Sea is an inland sea with very little water exchange. As too many nutrients enter the Baltic Sea via the rivers, these accumulate and ensure that the oxygen is completely depleted in some deep areas - life is then no longer possible here, which scientists refer to as dead zones. This makes it all the more important that fresh and oxygen-rich North Sea water can regularly flow in via the straits between Germany, Sweden and Denmark, but this is only the case in special and very rare weather conditions - and this could be the case right now.

Easterly wind pushes water out of the Baltic Sea

The reason for this is the easterly winds that have been blowing since the beginning of January, forcing large quantities of Baltic Sea water westwards and out over the Belts. How the Leibnitz Institute for Baltic Sea Research in Warnemünde (IOW) reported, measurements at the Landsort-Norra gauge in Sweden confirm water levels that are lower than ever before since records began in 1886. According to the IOW, a daily average of more than 67 centimetres below the long-term average water level was measured on 5 February. This was the lowest value recorded since continuous water level observations began in 1886. According to current calculations, the Baltic Sea is currently lacking around 275 cubic kilometres of water compared to the long-term average. In the 140-year series of measurements, similarly low water levels with deviations of more than 60 centimetres below sea level have only been measured in five other years. The last comparable event was more than four decades ago and dates back to March 1980.

As the IOW states in a press release, "with the current extreme value of over -67 cm MSL, the starting conditions for a major inflow event are exceptionally good. However, the IOW researchers are currently focusing not only on the current water levels, but also on further meteorological developments in the coming days and weeks. For an above-average inflow of North Sea water into the Baltic Sea to occur, the current easterly wind situation must end and be replaced by persistent westerly winds, which will then push salt- and oxygen-rich North Sea water into the Baltic Sea. This is of great ecological importance, especially for the deep central basins of the Baltic Sea, as there is often a lack of oxygen in the deep water layers for years or decades, according to the IOW.

"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 of saltwater inflow events into the Baltic Sea calculated here at the IOW, the probability is currently 80 to 90 per cent."

The long-lasting easterly wind could therefore lead to a noticeable improvement in ecological conditions in the Baltic Sea.