No abstract

In this study, we apply causal discovery to analyse causal links among key processes that contribute to Arctic-midlatitude teleconnections. First, we calculate the causal dependencies from observations. We then evaluate climate models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) via a comparison of their causal graphs for the period of 1979-2019 with those derived from observations. Based on observations, we show that the increase (decline) of near-surface Arctic temperature is associated not only with the reduction (increase) of sea ice over the Barents and Kara seas, but also with the strengthening (weakening) of atmospheric blocking over central Asia. We show that the near-surface westerly winds are strongly associated with the phase of the North Atlantic Oscillation (NAO). Observations show that the phase of NAO is connected with the polar vortex (PV), which is affected by the strengthening of the poleward eddy heat flux at 100 hPa. The analysis of CMIP6 historical simulations is in good agreement with the observations but reveals a negative connection between near-surface Arctic temperature and sea ice over Barents and Kara seas, which was not found in observations during December-January-February 1979-2019. Moreover, climate models simulate a more robust link between Arctic temperature and Barents and Kara sea ice towards the end of the century. The analysis of future changes in the Arctic-midlatitude teleconnections during cold seasons 2059-2099 also reveals that the connection between the Aleutian Low and the poleward eddy heat flux is expected to become more robust than in the analysed past.

The ICOsahedral Non-hydrostatic atmospheric model (ICON) has been developed jointly by the German weather service (DWD) and the Max Planck Institute for Meteorology (MPI-M), and is the central piece of the new unified modeling approach in Germany (Bonaventura, 2004; Zängl et al., 2015). ICON currently exists in two main configurations: one for numerical weather predictions by DWD (hereafter ICON-NWP), which has been operational since 2015, and one for climate simulations by MPI-M (ICON-A). Both configurations share the same dynamical core, but differ in their physical packages. The dynamical core has been tested (Zängl et al., 2015), the ICON-NWP forecasts are constantly verified by DWD and ICON-A has been described (Giorgetta et al., 2018) and evaluated (Crueger et al., 2018), but there is no extensive study on the evaluation of Northern Hemisphere (NH) stratospheric winter circulation in ICON.

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.