Irina Statnaia, a researcher at the Finnish Meteorological Institute, has published a study, “Factors influencing subseasonal predictability of northern Eurasian cold spells”, in the Quarterly Journal of Royal Meteorological Society.
The report reveals that forecasts of cold weather made three-to-four weeks ahead are more likely to be correct when high-pressure systems are already present over Scandinavia or the Northern Atlantic, and the weather in Northern Eurasia is already cold. On the other hand, these cold spell forecasts were found more likely to be incorrect when strong convective clouds develop over the tropical Indian ocean.
Tropical and stratospheric phenomena affect European winter forecasts
Statnaia and her team looked into the windows of forecast opportunity that occasionally emerge and enable forecast models to predict weather three to four weeks ahead. According to the researchers, these opportunities arise when local weather is influenced by remote regions with more predictable weather conditions.During winter, forecast opportunities are mostly associated with certain tropical and stratospheric phenomena.
The researchers knew that when the stratospheric polar vortex is weakening, there is a higher chance for cold spells to develop. However, when they divided all the cold spell forecasts made during several previous winters into those that were correct and those that failed, they found out that the models predict cold spells after a weak polar vortex too often, resulting in incorrect forecasts.
Madden-Jullian oscillation
The researchers also highlight that the Madden-Jullian Oscillation (MJO) tropical phenomenon can affect weather in Europe and sometimes lead to cold spells. MJO is a characterized by an eastward propagation of convective cloud and rain systems from the equatorial Indian Ocean toward the equatorial Pacific. The researchers found that while the development of convective clouds over the eastern Indian Ocean can often result in failed European cold spell forecasts, the forecasts made when these clouds develop over the western tropical Pacific are more likely to be correct.
The findings are expected to help improve subseasonal forecasts and also be used to improve forecast models. The research is based on forecasts by five different forecast models and reanalysis data for the winter seasons 1993-2020.
In related news, the Finnish Meteorological Institute (FMI) has released research dedicated to developing improvements to monitoring the freeze/thaw (F/T) state with synthetic aperture radar (SAR) satellites. Click here to read the full story.