How are heat waves over Yangtze River valley associated with atmospheric quasi-biweekly oscillation?

Abstract

Over Yangtze River valley (YRV) where heat wave (HW) events most frequently occur in China during 1979–2014, 30 out of 57 HW events (nearly 55%) in July and August is found to be related with the dry phases of atmospheric quasi-biweekly oscillation (QBWO). When a significant low-level anticyclonic anomaly (LAA) associated with QBWO appears over YRV, temperature rises sharply according to the adiabatic heating caused by subsidence and the enhanced downward solar radiation due to decreased clouds. The LAA with subsidence over YRV is primarily generated by quasi-biweekly atmospheric waves, which are classified to three types through case-by-case categorization, named as “mid-latitude wavetrain”, “WNP (western North Pacific) wavetrain” and “double wavetrains”, respectively. The mid-latitude wavetrain QBWO causes the LAA through subsidence induced by upper-level cyclonic vorticity which is associated with an eastward/southeastward migrating wave train from Eastern Europe to WNP in the upper troposphere. The WNP wavetrain QBWO forms LAA through a northwestward migrating lower-tropospheric wave train emanating from tropical WNP to southeastern China. The double wavetrains QBWO triggers LAA through both the low-level shear anticyclonic vorticity provided by a low-level northwestward/westward propagating wave train from tropical WNP to South China Sea and the upper-level positive vorticity associated with an eastward/southeastward migrating wave train from Eastern Europe to southeastern China in the upper troposphere. In all cases, South Asian High extends eastward and WNP subtropical high extends westward during HW events. Tracing these distinct precursory circulation anomalies may facilitate better understanding and short-medium range forecast of HW.