Northward shift in circulation system over the Asian mid-latitudes linked to an increasing heating anomaly over the northern Tibetan Plateau during the past two decades

摘要

The northward shift in circulation exhibited by a geopotential height of 5700-gpm at 500 hPa and the westerly jet at 200 hPa covers more than 40 degrees of longitude (80 degrees -120 degrees E) in Asia. The northward shift in circulation is also exhibited by a northward shift in eddy, showing the ascending (descending) motion anomalies on the north of the previous convergence (divergence) regions. The latitude position of circulation correlates well with the northern ridge and the central latitude of the South Asian High (SAH), which shows weak negative correlation with the eastern Pacific sea surface temperatures (SSTs), the potential connection with the La Nina SST pattern and the negative Pacific Decadal Oscillation pattern, and positively correlated with the land surface temperatures over North America, Eastern Europe and the region between the Tibetan Plateau (TP) and the Lake Baikal in Asia. The local thermal forcing of the TP (north of 33 degrees N) leads to increasing convergence in the lower troposphere, contributing to a positive latent heat of condensation, higher temperatures in the middle-upper troposphere, a stronger SAH, as well as northward secondary circulation in the middle troposphere. The northward extension of the SAH and a south wind anomaly encourage warming convection towards the north in the middle-upper troposphere, which leads to northward shift in atmospheric baroclinicity, and further be helpful to northward shift in eddy. These coupled thermodynamic and dynamic processes contribute to a northward shift in circulation in the mid-latitudes.