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

Abstract:

This study investigates the dynamical linkage between the meridional mass circulation and cold air outbreaks using the ERA-Interim data covering the period 1979–2011. It is found that the onset date of continental-scale cold air outbreaks coincides well 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. During weaker mass circulation events when the net mass transport across 60°N is below ~6.2 trillion tons per day, most areas of the mid-latitudes are generally in mild condition except the northern part of Western Europe. Composite pattern of circulation anomalies during stronger mass circulation events greatly resemble that of the winter-mean, with the two main routes of anomalous cold air outbreaks being along the climatological routes of polar cold air; namely, via East Asia and North America. The Siberian High shifts westward during stronger mass circulation events, opening up a third route of cold air outbreaks through Eastern Europe, where lies the poleward warm air route in winter-mean condition. The strengthening of the Icelandic low and Azores high during stronger mass circulation events acts to close off the climatological mean cold air route via Western Europe, responsible for the comparatively normal temperature there. The composite pattern for weaker mass circulation events is generally reversed, where the weakening of the Icelandic low and Azores high corresponding to negative phase of the NAO, leads to the reopening and strengthening of the equatorward cold air route through Western Europe, responsible for the cold anomalies there.