When and How Can the Stratosphere Modify the Midlatitude Cold Air Outbreaks in Northern Winter: An Isentropic Meridional Mass Circulation View

Abstract

Cold air outbreaks (CAOs) in northern winter can be attributed to the variations in the lower‐tropospheric equatorward cold branch (CB) of the isentropic meridional mass circulation (IMMC) across the subpolar latitudes. Understanding when and how the variations of CB are related to the stratosphere is crucial for improving the extended‐range forecast skills of CAOs in winter. Investigation on the vertical coupling of the stratospheric and tropospheric branches of IMMC suggests that the CB (or CAOs) exhibits two major timescales: sub‐monthly (10–30 days) and monthly‐to‐seasonal (>30 days). The CB can be modified by the stratospheric variability that is represented by the stratospheric poleward warm branch (WB‐ST) of the IMMC at monthly‐to‐seasonal timescales via the two‐way stratosphere‐troposphere coupling. At sub‐monthly timescales, however, the upward one‐way coupling from the troposphere to the stratosphere dominates. While changes of the CB always precede those of the WB‐ST at both timescales, indicating the upward tropospheric wave forcing into the stratosphere, the downward impacts of the WB‐ST on the CB (particularly CAOs in North America) are observed only at monthly‐to‐seasonal timescales. Specifically, the accumulated effect of the monthly‐to‐seasonal mass transport by the poleward WB‐ST can be strong enough to dominate the Arctic total column airmass changes, producing not only a barotropic structure of the Northern Annular Mode but also direct changes to the low‐level ageostrophic zonal‐mean meridional flow. The anomalous low‐level ageostrophic flow, in turn, plays a greater role in intensifying (weakening) the equatorward CB following a stronger (weaker) WB‐ST than tropospheric wave activities.