Scale Interactions and Deep Tropical Potential Vorticity Intrusions During Sudden Stratospheric Warmings

Using reanalysis data we examine extratropical-to-tropical potential vorticity (PV) intrusions in the upper troposphere-lower stratosphere (UTLS) in the time period surrounding major sudden stratospheric warmings (SSWs). Our analysis reveals that PV intrusion events that are associated with SSWs are notably different in both geographic location and magnitude when compared to their climatological counterparts. In particular, while PV intrusions during non-SSW time periods are concentrated in the eastern Pacific and Atlantic Ocean basins where the presence of climatological ‘westerly ducts' supports the propagation of wave energy deep into the tropics, PV intrusion events that are associated with SSWs have unique geographic centers of action that are dependent on the type of SSW involved (i.e. split versus displacement). Our results indicate that these geographic patterns are associated with two mechanisms. First, during the highly amplified prewarming period, the stratospheric surf zone extends downward and equatorward into the subtropical and tropical upper troposphere; this represents the dominant mechanism for supplying high PV air to the subtropical UTLS. However, the SSW-related amplified planetary waves and associated material PV deformations also induce zonally asymmetric shifts in jet structure that deform the UTLS synoptic-scale wave guide; this secondary mechanism ducts additional wave energy equatorward. Together these two mechanisms produce PV intrusions that are significantly stronger than their climatological counterparts. In light of these results, we consider the implications for how the PV intrusions and associated wave breaking events may modulate tropical convection and the mixing of trace constituents between the extratropical and tropical UTLS.