Quantitative assessment of the intra-regional precipitation variability in Northwestern China and its driving forces, AD 580-1979

Abstract

Northwestern China (NW China) is characterized by complex regional precipitation pattern, but there are few discussions about the intra-regional precipitation variability (IRPV) in NW China. The purposes of the present study are to quantify the long-term IRPV and to investigate the driving mechanism of IRPV in NW China for the period AD 580-1979. This study focuses on Gansu, Ningxia, Sha’anxi and the eastern part of Qinghai. To improve the climatic significance of the result, the study area is further divided into two sub-regions, namely the arid region (Region A) and the semi-arid region (Region B). Since the transition zone of the Asian Summer Monsoon (ASM) and the Westerlies locates in the study region, the regional precipitation is highly sensitive to the fluctuations of various climatic driving forces and hence, characterized by strong regional precipitation disparity.

In the present study, historical documentary records of drought/ flood events are employed to reconstruct the decadal precipitation index (PI) for Region A (PI_A) and Region B (PI_B). The IRPV index is derived from the differences of precipitation index between the two sub-regions. The verification result shows that the IRPV index is able to reflect the long-term spatial hydro-climatic disparity between regions in NW China for the last millennium. Besides, wavelet analysis is employed to reveal the cyclic components of PIA, PIB and IRPV in NW China. The results show that a distinctive ~120-200 year periodicity band exists during the Little Ice Age (LIA, c. AD 1400-1900), which coincides with the period when the IRPV extremes are frequently observed.

Various climatic factors are examined for the potential driving relationships with IRPV, including ASM, Arctic Oscillation (AO), Atlantic Multi-decadal Oscillation (AMO), El Niño Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), Pacific Decadal Oscillation (PDO) and China temperature. Results of wavelet coherence analysis show that the influences of AO, AMO, NAO and PDO on IRPV vary in Medieval Warm Period (MWP, c. AD 1000-1300) and the LIA. ENSO and China temperature are found to be important long-term driving factors in modulating the IRPV in NW China at the multi-decadal to multi-centennial scales, possibly through the geographic shifting of between ASM and the Westerlies.

In terms of academic significance, the present study provides quantitative assessment that reveals the long-term variations of IRPV in NW China and statistical evidence of the driving mechanisms modulating the IRPV in the last millennium. The present study applies a new method to study the long-term dynamics of IRPV in NW China (i.e. calculating the relative differences in precipitation index), which facilitates the investigation of precipitation pattern and the associated spatial disparity at the regional scale. In terms of practical values, the present study enriches our knowledge about the hydro-climatic changes and the associated driving mechanisms of NW China from pre-industrialization era to modern times, which helps identify the anthropogenic impact on climate change and therefore improve the ability to monitor and forecast the change of regional precipitation regime in the coming decades.