How do different pathways connect the stratospheric polar vortex to its tropospheric precursors?

Abstract. Processes involving troposphere-stratosphere coupling have been identified as important contributors to an improved subseasonal to seasonal prediction in mid-latitudes. However, there is only a very vague understanding of the localised coupling mechanisms and involved timescales, in particular when it comes to connecting tropospheric precursor patterns to the strength of the stratospheric polar vortex. Based on a novel approach in this study, we use ERA5 reanalysis data and ensemble simulations with the ICOsahedral Non-hydrostatic atmospheric model (ICON) to investigate tropospheric precursor patterns, localised troposphere-stratosphere coupling mechanisms and the involved timescales of these processes in Northern Hemisphere extended winter. We identify two precursor regions: Mean sea level pressure in the Ural region is negatively correlated to the strength of the stratospheric polar vortex for the following 5–55 days with a maximum at 25–45 days, and the pressure in the extended Aleutian region is positively correlated to the strength of the stratospheric polar vortex the following 10–50 days with a maximum at 20–30 days. A simple precursor index based on the mean pressure difference of these two regions is very strongly linked to the strength of the stratospheric polar vortex in the following month. The pathways connecting these two regions to the strength of the stratospheric polar vortex, however, differ from one another. Whereas a vortex weakening can be connected to prior increased vertical planetary wave forcing due to high-pressure anomalies in the Ural region, this is not the case for the extended Aleutian region. A low-pressure anomaly in this region can trigger a Pacific/North American (PNA) related pattern leading to geopotential anomalies of the opposite sign in the mid-troposphere over central North America. This positive geopotential anomaly travels upward and westward in time directly penetrating into the stratosphere and thereby strengthening the stratospheric Aleutian High, a pattern linked to the displacement towards Eurasia and subsequent weakening of the stratospheric polar vortex. Overall, this study emphasises the importance of the non-zonally-averaged picture for an in-depth understanding of troposphere-stratosphere coupling mechanisms. Additionally, this study demonstrates that these coupling mechanisms are realistically reproduced by the global atmosphere model ICON.