Monitoring polar vortex changes can improve subseasonal winter weather forecasts

Researcher Irene Erner from the Finnish Meteorological Institute examined in her study how changes in the stratosphere, particularly the stratospheric polar vortex, can improve subseasonal winter forecasts in the Northern Hemisphere.

First, the study investigates the case of the Sudden Stratospheric Warming (SSW) event of 2018, which caused a severe cold spell. It was found that accurately predicting this event required correct capturing of a specific sequence of atmospheric events: first, the breaking of an atmospheric wave, followed by persistent high-pressure system over the Urals which led to a record-high transfer of energy to the stratosphere. Getting both the timing and intensity of these events right was crucial for providing a reliable SSW forecast.

Furthermore, the findings show that, on average, a weakened stratospheric polar vortex can add three to five days of extra predictability to cold spell forecasts. However, there's a challenge: sometimes the stratospheric signal can be misleading, causing false alarms for cold spells. This happens when the stratosphere becomes dynamically decoupled from the troposphere, allowing local weather processes closer to the surface to take over.

Overall, the work advances weather prediction methods by highlighting how remote drivers in the atmosphere can influence our weather weeks in advance.

Weakened polar vortex increases probability of cold spells in Northern Eurasia

The polar vortex is a strong band of winds high in the atmosphere. In winter, its strength can fluctuate, and extreme weakening can lead to sudden stratospheric warming events. When the polar vortex weakens, the jet stream near the surface often shows similar changes: it becomes weaker and more distorted, which in turn increases the likelihood of cold spells in Northern Eurasia.

Predicting the weather for the next few weeks is challenging but crucial for sectors like agriculture, energy, and transport. Medium-range forecasts rely on accurate current weather data, while long-term seasonal forecasts use broader climate patterns. This study focused on so-called S2S forecasting (subseasonal-to-seasonal), which refers to predicting the weather from a few weeks to a couple of months ahead. S2S forecasting sits between medium-range weather forecasts and long-term seasonal outlooks and is particularly difficult because it is influenced by both short- and long-term atmospheric processes. However, under certain conditions, remote atmospheric phenomena can provide valuable clues, as this research demonstrates.

Further information:

Researcher Irene Erner, Finnish Meteorological Institute, irene.erner@fmi.fi

The dissertation can be found at Helda digital repository.

Irene Erner defended her PhD thesis titled “Subseasonal Weather Predictions in Northern Hemisphere Winter” on Friday, 21st of February 2025, at the University of Helsinki. The opponent was Professor Gabriele Messori (Uppsala University, Sweden), and the custos was Professor Heikki Järvinen.