Comparison of the mass circulation and AO indices as indicators of cold air outbreaks in northern winter

Using the daily ERA-Interim reanalysis data set for the 32 winters in 1979-2011, we find that midlatitude cold air outbreaks (CAOs) tend to preferentially occur within a week after simultaneously stronger mass circulations into the Arctic region in upper levels and out of the Arctic region below. The relationship of CAOs with Arctic Oscillation (AO) is less robust because temporal changes of AO are resulted from a small imbalance between the poleward and equatorward branches of the mass circulation. Results indicate that only when the poleward branch leads the equatorward branch (44% of all cases), CAOs tend to take place within a week after a negative phase of AO, whereas when the equatorward branch leads (19%), CAOs tend to occur after a positive phase of AO. In the remaining cases when the two branches are almost in phase, CAOs can be observed during either negative (24%) or positive (13%) phase of AO.