Three Types of Synoptic Events and Their Associated Troposphere-Stratosphere Coupling

Events that perturb the extratropical waveguide have the potential to excite Rossby waves. These Rossby waves can disperse and propagate horizontally, impacting the downstream weather, and vertically inducing extreme stratospheric conditions (e.g., sudden stratospheric warmings [SSWs]), potentially impacting surface weather sometime later. Studies have shown that synoptic-scale blocking events are one such type of waveguide perturbation event that can induce these vertically propagating Rossby waves prior to SSWs.

This work examines the role of blocking and two other types of tropospheric synoptic events, bombing extratropical cyclones and the extratropical transition (ET) of tropical cyclones, that can excite tropopause waveguide perturbations and produce vertically propagating Rossby waves. Utilizing NASA’s MERRA-2 dataset the tropopause-level zonal-mean meridional eddy heat flux anomaly was calculated with respect to the climatological mean for all events that occurred in the Northern Hemisphere winter months from 1980-2015. The event lists included 288 blocking events, 876 bombing events, and 128 ET events. The zonal mean meridional eddy heat flux is utilized as it is directly proportional to the vertical component of the Eliassen-Palm (EP) flux vector. The zonal mean meridional eddy heat flux anomaly thus describes when there is anomalous upward wave propagation. The goal of this study is to understand the characteristics of the subset of synoptic events that are associated with the largest (and smallest) upward EP flux anomaly to elucidate the types of synoptic events that have the potential to precede extreme stratospheric conditions.

The results show that each type of synoptic event can be associated with upward EP flux but there is variability in the magnitude of upward EP flux anomaly between and within the event types. The largest 100-hPa zonal-mean meridional eddy heat flux anomaly (46.5 K m s^-1) averaged over the 11 days following event identification was associated with a blocking event but the smallest 100-hPa zonal-mean meridional eddy heat flux anomaly (−30.4 K m s^-1) averaged over the 11 days following event identification was associated with a bombing event. In a composite sense, blocking events that occurred in Europe (West Pacific) were followed by a statistically significant maximum (minimum) in the 100-hPa zonal-mean meridional eddy heat flux anomaly. Bombing events that occurred in the West Pacific (Atlantic) were followed by a statistically significant maximum (minimum) in the 100-hPa zonal-mean meridional eddy heat flux anomaly. The presented results explore the variability in the synoptic-event associated tropopause zonal-mean meridional eddy heat flux anomaly from a climatological and composite perspective.