Water volumes equivalent to almost two Lake Vänern have been pushed out of the Baltic Sea this year due to persistent easterly winds and high-pressure blocking. The record-low water levels offer some hope for the oxygen-depleted bottoms in the inland sea. Oceanographer Jörgen Öberg at SMHI warns, however, that improvements require many favorable steps.
A high-pressure blocking has locked in the cold over Sweden in recent weeks, while easterly winds have driven large water masses out of the Baltic Sea via the Öresund and Danish straits. Including water from the Bothnian Bay and Gulf of Finland, the volume amounts to about 300 cubic kilometers, despite storms Johannes and Anna raising levels around New Year. Lake Vänern holds roughly 160 cubic kilometers, making this almost double.
Water levels in the Baltic Sea naturally vary, but such a sharp drop is unusual. Measurement series in the Baltic span over 100 years in many places, and over 200 years in Stockholm, with only a handful of similar instances historically, according to Jörgen Öberg.
In Finland, low-water records have been broken, and Swedish measurement stations are approaching historical lows. The low levels could pave the way for an influx of new water to the oxygen-free bottoms, but Öberg emphasizes the uncertainty: “But it would be like winning five Trisslotter in a row.”
Improvements require westerly winds and low pressure in the form of “autumn weather,” as well as water that is salty, cold, and oxygenated through storms. “It must be water of sufficiently high quality to be useful. It should be salty enough, cold, and preferably have been through a storm so that breaking waves have brought oxygen down into the water,” he says. Several obstacles remain before the water reaches the deepest parts. “One can hope, but there are many steps left. The conditions are better than in a long time, but so far only in the first step of four or five,” Öberg adds.
