Moisture transport to a typical transitional climate zone in North China forced by atmospheric and oceanic internal variability under the background of global warming

摘要

North Central China (NCC) (34 degrees-42 degrees N, 95 degrees-107 degrees E), a typical transitional climate zone between westerlies and monsoon, shows multiple time-scale variations in precipitation under the background of global warming. Thus, NCC moisture transport (NMT) was analysed based on reanalysis data from 1979-2015. By using the spatially unbounded dynamic recycling model, main NMT pathways and moisture sources were identified for the summer rainfall of NCC. The trend pattern of NMT manifests as a seesaw pattern with a weakening northwesterly transport but an enhancing southwesterly transport, suggesting an interaction between the mid-latitude westerlies and the Indian monsoon. The temporal evolution of the NMT trend pattern exhibits interannual and multidecadal variability superimposed on long-term trends, which correspond to the atmospheric and oceanic physical processes, respectively. The partial least squares regression analysis demonstrated that the temporal evolution of NMT trend patterns can be well explained by atmospheric and oceanic internal climate variability (ICV). At interannual time scales, the atmospheric ICV, composed of the circumglobal teleconnection (CGT) and East Atlantic (EA) and East Atlantic/Western Russia (EAWR) teleconnections, forms a Eurasian wave train that weakens westerly transport via the Europe blocking flow and enhances southwesterly transport via local circulation anomalies with a zonal dipole structure. However, oceanic ICV, composed of the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO), exerts influence on a multidecadal time scale to decelerate the mid-latitude westerly jet over the North Atlantic, providing favourable upstream background conditions for the formation of the Europe blocking and further maintaining the atmospheric ICV-induced Eurasian wave train. Thus, the internal oceanic and atmospheric processes at different time scales couple to contribute to long-term changes in precipitation over NCC under the background of global warming.