Seasonality In The Response Of East Asian Westerly Jet To The Mid-Holocene Forcing

The East Asian westerly jet (EAWJ) features large seasonal variations in its intensity and position and exerts profound influences on local precipitation. In this study, we found the amplified seasonality of the EAWJ in response to the mid-Holocene forcing. Compared with the preindustrial, the summer EAWJ significantly weakens by 20 +/- 4% and shifts northward by 4 +/- 2 degrees during the mid-Holocene, while the winter EAWJ slightly strengthens by less than 5%. Two different mechanisms are proposed to explain the significant summer weakening. From the perspective of stationary waves, the deficient precipitation over the tropical northwest Pacific produces the anomalous cyclonic circulations in the upper troposphere. The easterly winds north of the cyclone act to decelerate the EAWJ. In terms of transient processes, the mid-Holocene reduced midlatitude meridional temperature gradient suppresses the synoptic-scale disturbances through changes in the atmospheric baroclinicity. The subsequent decrease in kinetic energy transformation from transient eddies to the mean flow contributes to the weakening of the EAWJ. The reduction of the meridional temperature gradient is related to the subtropical cooling, which is possibly caused by the excessive monsoon rainfall and enhanced convection over East and South Asia. Both mechanisms involve the influence of low-latitude precipitation on the midlatitude jet stream behavior, which deserves more attention in future studies of the jet stream dynamics.