Estonia's winters increasingly mild in the wake of westerly winds
Nothing good ever comes from the east, at least not in the winter months. A recent study now shows that Estonia is seeing fewer and fewer crisp, freezing winters brought by easterly winds. Instead, mild weather carried in by westerlies is becoming more common, bringing slush rather than biting frost.
Piia Post, a researcher at the University of Tartu's Institute of Physics, and Andreas Lehmann, a researcher at the Helmholtz Center for Ocean Research Kiel, published a detailed analysis on the subject in the Journal of the European Meteorological Society. Their study confirms that weather patterns over the Atlantic Ocean have changed significantly over the past 72 years. Compared with earlier decades, these patterns now favor westerly winds, which bring milder and more humid air to the Baltic Sea region.
Post and Lehmann used data collected between 1950 and 2022. With the help of machine learning, they analyzed more than 70 years of daily weather maps to determine which large-scale weather systems have the greatest impact on winters in Estonia.
The results show that the frequency of weather conditions linked to westerly winds has increased by nearly 14 percent during that period, while the frequency of easterly winds associated with cold air has dropped by more than 11 percent. In other words, the number of mild winter days driven by westerlies has grown, while the number of days shaped by icy easterlies has declined.
The shift is directly reflected in the Baltic Sea. Stronger westerlies are linked to greater inflows of water into the sea, raising its level and influencing water exchange with the North Sea. The paths of the Baltic's major currents are also changing.
The cause lies in the large-scale weather systems spanning the Atlantic Ocean. Estonia's winters are shaped mainly by the interplay between two pressure systems: the Icelandic low and the Azores high. At times, the pressure difference between them is large — lower than usual over Iceland and higher than usual over the Azores. When that happens, it creates a kind of express route for warm, humid air from the Atlantic, ensuring milder winters here.
The study shows that this westerly-favoring setup has become increasingly dominant in recent decades. At the same time, the systems that produce cold easterlies have weakened.
That does not mean the Icelandic system has disappeared altogether. Rather, the balance has tilted to one side. Post and Lehmann note that during the positive phase of the North Atlantic Oscillation, the number of mild winter days has increased noticeably in recent decades, especially since the 1980s.
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Editor: Airika Harrik, Jaan-Juhan Oidermaa, Marcus Turovski
