# Desertification Mitigation in Northern China Was Promoted by Climate Drivers after 2000
## 4 Discussion
### Atmospheric Circulation and Internal Variability  
Precipitation was demonstrated to be a dominant factor for desertification by the above analysis, as its variations were tightly linked to changes in atmospheric circulation. To discuss the mechanism behind precipitation variations, we focus on the rainy season from April to September, which constitutes 87% of the total annual precipitation (Figure S9). The sources and transportation of water vapor were examined at 500 hPa and 850 hPa (Figure 9). Precipitation in the Juggar region is influenced by northwestward airflow during the rainy season (Figure 9A,B). Generally, little water vapor reaches the inland arid region through advection, so Juggar primarily relies on the obstruction posed by the tall mountain range for precipitation, which primarily occurs on the northern slopes of the Tianshan Mountain [98]. The Tarim region, encircled by mountain ranges to the west, south, and north, experiences restricted access to water vapor from the westerlies, which consequently leads to limited precipitation. It mainly relies on water vapor transported by low-level (850 hPa) easterly flows from the east side of the Tarim region (Figure 9A). As it is situated in the northeastern part of the Tibetan Plateau, Qinghai primarily receives water vapor for precipitation from the Eurasian landmass via the westerlies and from the northern Indian subcontinent via the Indian summer monsoons [99; 100]. The precipitation in Ala Shan, Hexi Corridor, and Ordos is influenced by the westerly trough, which is characterized by a prevailing northwest airflow and low-level (850 hPa) water vapor from the southwest monsoon winds. The Hulunbuir, Oitindag, and Horqin regions are located be-hind an aloft westerly trough, which brings water vapor. Regressions of water vapor flux divergence and horizontal wind at 500 hPa and 850 hPa (Figure 9C,D) show that the anomalous east winds over the region from Northeast China to Mongolia support water vapor convergence over Ala Shan, Hexi Corridor, and Ordos but reduce water vapor transport from the upstream regions to Hulunbuir, Oitindag, and Horqin. Accompanied by the anomalous northeast winds over the north side of Tianshan Mountain, water vapor convergence increases over Juggar. Meanwhile, the anomalous west winds over eastern Tarim restrain water vapor transport from the east side. Thus, water vapor convergence there decreases. These anomalies of water vapor divergence related to atmospheric circulation anomalies are generally consistent with precipitation trends (Figure 8G), which partly illustrate the causes of precipitation changes. Decadal-scale internal climate variability, such as the phases of the PDO and AMO, affects atmospheric circulation patterns. After the 1990s, the AMO shifted from a cold phase to a warm phase, while the PDO transitioned from a warm phase to a cold phase (Figure 10). These changes in oceanic temperature anomaly patterns triggered adjustments in atmospheric circulation and thereby influenced precipitation in NC [101; 102]. The  
Figure 9: Spatial distributions of specific humidity (**A,B**), water vapor flux divergence (shading) and horizontal wind (vectors) (**C,D**), and their regressions against the precipitation series at 850 hPa (left line) and 500 hPa (right line). The black slashes and blue vectors represent grid values that are significant at the 90% confidence level. The dashed boxes are the different areas in the DPR. AMO transitioned from a negative phase to a positive phase, which triggered a remotely correlated wave train over Eurasia, an atmospheric fluctuation that can produce opposing climate effects in different regions [103; 104]. Over the Mongolian Plateau, this wave train manifests as an anti-cyclonic anomaly [105], which enhances the water vapor transported to the arid region of Northwest China, resulting in increased precipitation [106]. In addition, there is a strong correlation between the annual precipitation in Northwest China and the AMO [107].