Barents-Kara Sea Ice Variability Drives Stronger Tropospheric Anomalies over East Asia after 2000 due to Weakened Stratospheric Polar Vortex

This study investigates the midlatitude tropospheric teleconnection associated with February Barents-Kara Sea (BKS) sea ice during 1979–2019 and finds evidence for its significant shift around the year 2000. Before 2000, BKS sea ice reduction is associated with an enhanced stratospheric wavenumber 1, but the relationship between BKS sea ice reduction and downstream tropospheric circulation is insignificant. After 2000, BKS sea ice reduction shows a weaker relationship with stratospheric wavenumber 1 but is accompanied by a significant strengthening of the East Asian trough in the troposphere. Both observational and modeling evidences suggest that this shift is partly attributable to the weaker background stratospheric polar vortex after 2000 compared to earlier decades. Before 2000 (under a stronger vortex state), the late-winter lower stratosphere provides favorable environment for planetary wave propagation, promoting the vertical wave propagation triggered by BKS sea ice reduction, which enhances stratospheric wavenumber 1 but weakens the horizontal wave propagation in the troposphere, causing the East Asian trough to be relatively insensitive to sea ice changes. After 2000 (under a weaker vortex state), the lower stratosphere becomes unfavorable for upward wave propagation, and waves triggered by BKS sea ice reduction are largely confined to horizontal propagation in the troposphere. As a result, the East Asian trough becomes more sensitive to sea ice changes, while the stratospheric wavenumber 1 response is insignificant. We propose that the background state of the stratospheric polar vortex plays a crucial role in modulating the causal relationship between BKS sea ice variability and East Asian tropospheric circulation.