Observed heatwave changes in arid northwest China: Physical mechanism and long-term trend

Arid regions suffer seriously from frequent and severe heatwaves. As compared to other regions, however, the synoptic-scale characteristics and mechanism of heatwave events in arid region are less understood and warrant in-depth investigations. Using observational and reanalysis datasets, here we investigate the heatwave behaviors in arid northwest China (ANC) during 1961-2014. Results reveal that heatwaves in ANC are accompanied by a hot and dry air column. In particular, the main synoptic systems are characterized by a warm-core low pressure in the lower troposphere and a deep warm high pressure in the middle and upper troposphere over the ANC and nearby regions, resulting in the heatwaves there. In the lower troposphere, the prevailing southerly flow induced by the warm-core low pressure increases the near-surface air temperature via warm advection. In the middle and upper troposphere, the deep warm high pressure is accompanied by sinking motion over the ANC surrounding region, leading to clear-sky conditions and less precipitation. These pressure configurations facilitate temperature increases by radiative heating and subsidence warming. Notably, the deep warm high pressure is closely associated with the evolution and eastward movement of a wave-like pattern over the northern midlatitudes. Over the long-term period, the heatwave in ANC exhibits significant intensifying trends in terms of increasing frequency (0.40 events decade(-1)), prolonging duration (1.67 days decade(-1)), and strengthening amplitude (0.32 degrees C decade(-1)). Also, we find that heatwaves in ANC tend to commence earlier and end later during recent decades. These findings provide scientific evidence from the synoptic and climatic perspectives for understanding the formation mechanism and forecasting of the heatwaves in arid regions.