The ambient field characteristics for quasi-straight long path and multi-turning path of eastward moving Tibetan Plateau vortex

Composite methods are applied to analyze atmospheric observations and the NCEP/NCAR Final Operational Global Analysis data as well as the Tibetan Plateau vortex (TPV) and shear line yearbooks from 1998 to 2018 to reveal the ambient field characteristics for the groups of the Tibetan Plateau vortices that move eastward following quasi-straight long-path (QSLTPVs) and multi-turning path (MTTPVs), respectively. The leading factors that lead to the TPVs's turning are also discussed. The results show that the ambient field characteristics common for long-lasting QSLTPVs and MTTPVs activities are that there are obvious weather systems affecting TPVs. And the subtropical high is located to the southeast of the TPVs, while the east and west segments of jet stream exist in the upper levels to the north of the TPV. These systems promote positive vorticity advection into the TPVs and there is positive divergence region above the TPVs. Potential vorticity in the upper levels are transported downward to TPVs. The difference in ambient field conditions between the QSLTPVs and the MTTPVs is obvious, too. The MTTPVs are accompanied by strong tropical low-pressure activities. They are blocked and forced to turn under the influences of the sub-tropical high, the westerly wind belt, the topical low-pressure systems and their interaction. The QSLTPVs move eastward in the ambient background field dominated by westerly synoptic systems. And the QSLTPVs are more affected by cold air, southwesterly flow and upper-level front belt than the MTTPVs. These systems lead to stronger positive vorticity advection, larger potential vorticity, stronger baroclinicity and positive divergence in upper levels. The main factors causing the turning of MTTPVs are that the TPVs are weakened and blocked by the ambient field conditions, and the downward transport of high-level potential vorticity to the TPV results in strong positive vorticity in the west of the TPV. Thereby, the TPV moves to the area where it becomes stronger, which explains why the TPV makes turn.