| Summary: | The impacts of the Arctic stratospheric polar vortex (SPV) changes on wintertime frontogenesis in the northern middle latitudes are analyzed. Both composite analysis and model simulations reveal that the intensity and frequency of frontogenesis over Siberia, the Mediterranean and southern North Atlantic during weak SPV years are significantly larger than those during strong SPV years, while the frontogenesis over the northern parts of the North Atlantic and North Pacific Oceans are weaker and less occur during weak SPV years. These features are more noticeable in middle January and February. The contributions of resultant deformation changes to frontogenesis intensity changes over most regions of the middle latitudes are larger than those of horizontal divergence changes, and the contribution of stretching deformation and shearing deformation is almost the same with each other. The changes in frontogenesis intensity are attributed to changes in tropospheric circulation and temperature gradient associated with SPV changes. Potential vorticity (PV) anomalies in the upper troposphere and lower stratosphere (UTLS) caused by the weakened and shifted SPV towards Siberia lead to tropospheric cyclonic flows, favoring more cold-air mass transport towards mid-latitude Siberia. Meanwhile, more high-PV air towards Siberia results in steeper tropospheric isentropes during weak SPV years. Consequently, both temperature gradient and frontogenesis over Siberia are enhanced. More southward transport of cold-air mass due to the equatorward shift of the polar jet stream induced by weak SPV enhances frontogenesis over the southern North Atlantic.
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