How the intraseasonal oscillations generate the heatwave over the Yangtze River Valley: a numerical study

The Yangtze River Valley (YRV) is the core region for the occurrence of heat wave (HW) in China. HW over the YRV have been reported to be associated with intraseasonal oscillations (ISOs) from tropical/extra-tropical regions. However, the specific roles of these ISOs on the generation of HW cannot be separated using the observational data. This study identified the impact of ISOs from tropic/extra-tropic on the period of HW development through partial lateral forcing (PLF) experiments using the WRF model for a typical ISO-related HW event over the YRV in 2012. The observations show that a remarkable eastward/westward extension of South Asian high (SAH)/western North Pacific subtropical high (WNPSH) accompanied with the HW occurrence. Accordingly this anomalous deep high pressure produced the HW through adiabatic heating and horizontal warm advection, which was related to a lower-level westward-propagating meridional dipole anomaly and an upper-level eastward-migrating wave train from the extra-tropical region. The PLF experiments confirmed that this HW event was caused by the propagation of intraseasonal perturbations from the surrounding regions. The tropical westward-propagating meridional dipole anomaly directly generated a lower-level anomalous anticyclone in the early period of HW development, whereas extra-tropical intraseasonal forcing primarily contributed to the eastward extension of the SAH in the late, which jointly established the deep high pressure and caused the HW. The PLF experiments also indicate plausible tropical-extra-tropical interaction of intraseasonal perturbations over the YRV. Tracing the linkage between ISOs and HW may facilitate a better understanding and improve short- to medium-range forecast of HW.