Linking warm Arctic winters, Rossby waves, and cold spells: an idealized numerical study

Abstract : The changes of midlatitude Rossby waves and cold extreme temperature events (cold spells) during warm Arctic winters are analyzed using a dry three-level quasigeostrophic model on the sphere. Two long-term simulations are compared: the first run has the observed wintertime climatology, while the second run includes the composite of the global anomalies associated with the six hottest Arctic winters. A spectral analysis shows a large increase in wave amplitude for near-zero and westward phase speeds and a more moderate decrease for high eastward phase speeds. The increase in low-frequency variability (periods greater than a week) associated with the power shift to slower waves is largely responsible for an increase in midlatitude long-lasting cold spells. In midlatitude regions, in the presence of a mean warming, that increase in low-frequency variance compensates the increase of the mean temperature, resulting at places in a frequency of cold spells that remains by and large unaltered. In presence of mean cooling, both the increase in variance and the decrease in the mean temperature participate in an increased frequency of cold spells. Sensitivity experiments show that the power shift to slower waves is mainly due to the tropical anomalies that developed during those particular winters and less importantly to changes in the background flow at higher latitudes associated with the Arctic amplification pattern.