Boreal winter stratospheric variability in EC-EARTH: High-Top versus Low-Top

The European Consortium EC-EARTH climate model version 3.1 is used to assess the role of a well-resolved stratosphere on stratospheric teleconnections in the Northern Hemisphere winter. Two simulations of 100 years with constant radiative forcing, one with top at 0.01 hPa (L91, High-Top) and other with top at 5 hPa (L62, Low-Top) are compared. Results show how High-Top is able to generate a realistic Quasi-Biennial Oscillation in the tropical stratosphere, and to capture its teleconnection to the extratropics. On the other hand, EC-EARTH properly simulates the leading modes of variability (EOFs) of the polar stratosphere in both High-Top and Low-Top configurations, although the El Nino-related wave-like teleconnection dominates the signal of EOF3 (wavenumber 1 at 50 hPa) in High-Top, as in reanalysis, while the tropics-unrelated Pacific/North America pattern does it in Low-Top. At subseasonal time-scale, High-Top shows larger sudden stratospheric warming (SSW) occurrence in mid-winter, which is close to the documented in observational records, while SSW occurrence peaks by late-winter in Low-Top, despite both show a similar SSW decadal frequency (i.e. around 8 events per decade). Examination of the climatological eddy heat flux shows a misrepresentation of the seasonal cycle in Low-Top, with a peak in late-winter that explains the timing of SSW increase. This peak of the eddy heat flux can be traced to the wave injection over Central Siberia in the lower stratosphere, which appears to be dominated by small-scale waves (wavenumbers 3-4). It follows that simulating realistic eddy heat flux seasonality is fundamental to obtain a realistic SSW seasonal cycle.