Cold spells induced by slow and amplified atmospheric waves

Abstract. Cold spells in the Northern Hemisphere mid-latitudes have been linked to planetary waves. Yet the mechanisms by which these waves impact cold-spell formation remain unclear. Here we develop novel metrics to separately measure the amplitude and speed of ridges and troughs, examining the behavior of planetary waves during winter cold spells. Our findings indicate that while the planetary waves across the entire mid-latitudes experience significant changes during cold spells, local wave dynamics play a major role in developing these events. The nearest upstream ridge and downstream trough of the cold-spell region are located in a way that facilitates development of the extreme cold anomaly. This ridge and trough amplify and slow, enhancing and prolonging southward advection of cold air from the Arctic into the cold-spell region. The slow and amplified upstream ridge and downstream trough occur several days before the region’s minimum temperature, suggesting these local wave anomalies induce cold-spell formation.