Impact of Westward-Propagating ISO over Mid-High-Latitude Eurasia on SSW during Boreal Winter

Abstract

In this study, the impact of intraseasonal oscillation (ISO) over mid-high-latitude Eurasia on stratosphere sudden warming (SSW) events during boreal winter is examined based on the NCEP–NCAR reanalysis data. It is found that the dominant ISO periodicity is 10–30 days. By extended empirical orthogonal function, two leading ISO modes with opposite directions of westward and eastward propagation are extracted. Since the eastward-propagating mode was involved by previous studies, this study is focused on the westward-propagating one. As revealed, the westward-propagating ISO propagates from the Canadian Arctic Archipelago to western Europe, which is dominated by waves 1–2. The analysis of lead–lag correlation and phase division indicates that SSW events prefer to occur at the time of lagging the westward-propagating ISO phase 2 by 7–9 days. The diagnosis of the dynamic process of ISO impacting SSW suggests that during the westward journey of the ISO, the corresponding pattern leads to sea ice loss over the Barents–Kara Sea, which emanates upward-propagating planetary waves. At the same time, the ISO circulation expands the easterly anomalies, which weakens westerly flow that favors the planetary waves of wave 1 and wave 2 propagating upward into the stratosphere to affect the onset of SSW. By applying the subseasonal-to-seasonal (S2S) reforecast data from the ECMWF model, the probability density functions of zonal wind anomaly verify the impact of westward-propagating ISO on SSW. The intraseasonal oscillation (ISO) is a vital prediction source for the forecast of extreme weather and climate in both the troposphere and the stratosphere, but the relationship between the mid-high-latitude one and stratospheric extreme weather events lacks research. The purpose of this study is to understand how the ISO over mid-high-latitude Eurasia affects the stratosphere sudden warming (SSW). Our results find that the westward-propagating ISO can impact the onset of SSW events by leading to sea ice loss over the Barents–Kara Sea and the expansion of easterly anomalies. This provides a new clue for the extended-range forecast of SSW.