Characterisation of compound dry and hot events in Inner Mongolia and their relationship with large-scale circulation patterns

In recent decades, frequent compound dry and hot events (CDHEs) have posed a great threat to humans and the ecological environment, especially in Inner Mongolia, which has typical arid and semi-arid characteristics. Therefore, while exploring the characteristics of the spatial and temporal evolution of the CDHEs in Inner Mongolia, and elucidating their teleconnections with large-scale circulation indexes, it can not only provide useful enlightenment for ecological protection, but also provide more detailed information for understanding the corresponding details of CDHEs with large-scale circulation indexes. In this study, monthly temperature and precipitation data from 115 meteorological stations in Inner Mongolia for each summer from 1981 to 2020 were used to establish a standardised dry and hot index (SDHI), characterising the severity of extreme compound events of drought and high temperature in Inner Mongolia. This data was also used to analyse the spatial and temporal characteristics of such compound events over the past 40 years. Thereafter, Pearson's correlation, stepwise multiple regression models, and multivariate wavelet analysis methods were used to assess the relationship between the atmospheric circulation index and SDHI in terms of linear, non-linear, and different combinations of the four large-scale circulation indexes. The results show that SDHI performs well in the assessment of CDHEs in Inner Mongolia and shows great similarities with historical records. The overall trend of CDHEs in Inner Mongolia over the past 40 years has intensified over time. Spatially, abnormal and moderate dry and hot events mainly occurred in Inner Mongolia before the 21st century, and the frequency of occurrence was higher in the eastern region than in the western region. Furthermore, the severity of CDHEs has increased significantly since the 21st century, with severe and extreme dry and hot events occurring more frequently in the western region than in the eastern region. The results of the correlation analysis show that, among the four large-scale circulation indexes, the Atlantic Multidecadal Oscillation (AMO) has the greatest influence on the CDHEs in Inner Mongolia when it is positive, with the highest significance, and when the Pacific Decadal Oscillation (PDO) and North Atlantic Oscillation (NAO) show negative phases, they are more likely to cause dry and hot events in Inner Mongolia. Further, where the number of stations negatively influenced by NAO is higher, the effect of El Nino Southern Oscillation (ENSO) on dry and hot events is usually insignificant. Partial wavelet coherence (PWC) analysis suggests that the independent effects of circulation factors may be weaker but appear to be stronger due to their interdependence with other circulation patterns. Finally, the multiple wavelet coherence (MWC) results show that the changes in SDHI values can be best explained by a combination of two or more factors.