Abstract
This study investigates the out-of-phase change in the Siberian High (SH) between December and January (stronger than normal in December and weaker than normal in January, and vice versa). The results show that the monthly reversal frequency of the SH between December and January increases significantly after 2000 from 30% (1981–2000) to 63% (2001-2019). Correspondingly, the influence of November snow cover over Siberia on the phase reversal of the SH has intensified after 2000. The reasons may be as follows. Higher snow depth over Siberia (SSD) in November corresponds to stronger diabatic cooling and increased snow accumulation over Siberia in November and December, which may strengthen the SH in December via the positive feedback of snow albedo. The dynamic mechanisms between the higher SSD in November and weaker SH in January are further investigated from the perspective of troposphere–stratosphere interaction. Such anomalously higher SSD with strong upward heat flux induces the upward-propagating wave activity flux in November and December over the Urals and Siberia, leading to a weaker and warmer stratospheric polar vortex in January. Subsequently, the anomalies of the stratospheric polar vortex signal propagate downwards, giving rise to a negative Arctic Oscillation–like structure in the troposphere and a weakening of the SH in January. This mechanism can be partly reproduced in CMIP6. Additionally, the variability of the September–October Arctic sea ice mainly leads to coherent variations of the SH in December and January via the eddy–mean flow interaction before 2000. Furthermore, the preceding November snow cover over Siberia enhances the intraseasonal prediction skill for the winter SH after 2000. Meanwhile, considering the previous November SSD, the prediction accuracy for the out-of-phase change in the SH between December and January increases from 16% (outputs of the NCEP’s Climate Forecast System, version 2) to 75%.
Highlights
The Siberian high (SH), a center of seasonal activity, is formed in the lower troposphere over Eurasia during the wintertime as a result of surface radiative cooling and large-scale descending motion (Ding and Krishnamurti, 1987; Sahsamanoglou et al, 1991; Xie et al, 1992)
While the precipitation over the Urals increases in December, it correspondingly decreases over the Urals in January, and vice versa (Figures 1F,G)
With the higher Siberian SD (SSD), the snow albedo increases over Siberia, which tends to results in strong radiation cooling and a lowering of the surface air temperature (SAT) over Siberia in the preceding November
Summary
The Siberian high (SH), a center of seasonal activity, is formed in the lower troposphere over Eurasia during the wintertime as a result of surface radiative cooling and large-scale descending motion (Ding and Krishnamurti, 1987; Sahsamanoglou et al, 1991; Xie et al, 1992). Cohen et al (2001) investigated the connection between the Eurasian snow-cover extent (SCE) and variabilities of the SH. Li and Lan (2017) pointed out that a warmer sea surface temperature (SST) in the North Atlantic noticeably intensifies the Ural blocking ridge, resulting in a stronger winter SH. Based on these significant factors and relatively low prediction skill of the winter SH, a highly efficient prediction model was constructed (Yang and Lu, 2014; Yang and Fan, 2021). The wintertime average SH is very important, it is easy for it to mask the characteristics of intraseasonal variation, meaning intraseasonal predictions of the SH are more challenging
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