Mechanisms and predictability of Sudden Stratospheric Warming in winter 2018

In this study, we investigate the Sudden Stratospheric Warming that took place on 12 February 2018 (SSW2018), its predictability and teleconnection with the Madden-Julian Oscillation (MJO) by analysing ECMWF ensemble forecast initialised on 1 February 2018. Several days prior to that date MJO was in Phase 6 and had a strong amplitude potentially contributing to triggering the SSW. Two wave trains can be identified in the upper troposphere over the northern Atlantic and Pacific regions. Starting from the 3 February, the amplitude of planetary wave with wavenumber 2 (PW2) started to increase and reached record high values, while the PW1 amplitude decreased.

In order to better understand the sources of uncertainties, we divided the forecast ensemble members into two groups. The first group predicted the SSW onset in time while the second group of ensemble members did not capture the wind reversal at 60°N 10 hPa. The results obtained with the ensemble forecast data were compared with the ECMWF’s reanalysis ERA-Interim (ERA-I). The analysis of the two groups of ensemble forecasts shows that in the first group of forecasts PW2 prevailed with ridges over the Ural and Alaska and troughs over the west Siberia and Canada, as observed. Instead, PW1 is seen in the second group of ensemble members with a broad ridge over Eurasia. Calculations of wave activity fluxes show that there is less zonal wave energy propagation in the second group compared to the first group and ERA-I over Eurasia, which can be associated with the errors in the forecasted location of the Ural high. There is also wave energy propagation towards an area of high pressure over Alaska, as seen in ERA-I. Here, wave energy propagation is similarly underestimated by both groups. Overall, the structure of the geopotential anomalies averaged for 5-7 February for the first group and ERA-I is more consistent with the climatological response from MJO phase 6 taken with lag 5-9 days than that in the second group.