Potential Vorticity Circulation Crossing the Boundaries of Northern Troposphere and the East Asian Monsoon in July

The source of potential vorticity (PV) substance (PVS) of the atmosphere is at the earth’s surface, whereas the PVS variation within a limited region depends on the sum of the effective PV flux crossing the boundary surface surrounding the region. Because the PV circulation (PVC) is the time- integral of effective PV flux, the monthly mean gross PVS in the northern troposphere is determined by the sum of PVC penetrating the enveloped three boundaries, e. i., the PVC cross- the equatorial vertical section CEPVC, the PVC cross- the tropopause CUPVC, and the PVC cross- the earth’s surface CBPVC. Because the CBPVC is associated with surface potential temperature and circulation, for an equilibrium gross PVS in the hemisphere, the change in CEPVC or/and CUPVC can induce the climate change near the surface through the adjustment of the atmospheric internal PVC changes.

Based on reanalysis data and by employing statistics and diagnostics, the characteristics of the July mean PVS and PVC in the northern troposphere, and the relationship between the Asian monsoon and the PVC crossing its three boundaries in July are investigated. Results demonstrate that, corresponding to a CBPVC pattern with negative (positive) PVC over the north (south) on the Tibetan Plateau, strong positive PV anomalies and westerly flows develop in the troposphere over the plateau， leading to increased precipitation in the downstream area along the Meiyu front. The combination effects of the CEPVC and CUPVC can trigger the adjustment of the troposphere internal PVC, resulting in PVS and circulation redistribution within NH and forming a meridional seesaw of precipitation over the East Asian summer monsoon area. Attribution diagnoses reveal that the tropical sea surface temperature anomaly contributes significantly to the CEPVC, whereas the meridional gradient of PVS over the northwestern Tibetan Plateau on the tropopause contributes more to the CUPVC.