Dynamical Linkage of Cold Air Outbreaks to Intensity Variations of the Meridional Mass Circulation

This study investigates the dynamical linkage between the meridional mass circulation and cold air outbreaks using ERA-Interim data covering the period 1979–2011. It is found that the onset date of continental-scale cold air outbreaks coincides with the peak time of stronger meridional mass circulation events when the net mass transport across 60°N in the warm or cold air branch exceeds ~7.6 trillion tons per day. Associated with weaker meridional mass circulation events, whose intensities are below ~6.2 trillion tons per day, are generally mild conditions over most areas of the mid-latitudes, except the northern part of Western Europe. Composite circulation anomalies during stronger mass circulation events greatly resemble the winter-mean circulation pattern, with the two main routes for cold air outbreaks being along the climatological routes of polar cold air; namely, East Asia and North America. The anomalous circulation pattern is almost reversed when the mass circulation is weaker. The Siberian High shifts westward during stronger mass circulation events, reversing a poleward warm air route in the winter-mean condition to a route of cold air outbreaks through Central Asia. The strengthening of the Icelandic low and Azores high during stronger mass circulation events closes off the climatological mean cold air route passing through Western Europe, responsible for normal temperatures there. The anomalous circulation pattern is almost reversed when the mass circulation is weaker. The weakening of the Icelandic low and Azores high during weaker mass circulation events, or the negative phase of the NAO, leads to the reopening and strengthening of the equatorward cold air route through Western Europe, responsible for cold anomalies there.