# GRACE/GFO and Swarm Observation Analysis of the 2023-2024 Extreme Drought in the Amazon River Basin
## 4 Results
### Associated SST and Atmospheric Anomalies
Figure 14: Temporal evolution of Nino 3.4 index (**a**) and GRACE-DSI (**b**) during January 2003 and February 2024. Based on Figure 14, we can observe that positive anomalies of VV existed in most areas of the ARB from June to July 2023, mainly concentrated in the western and eastern regions. In June, the positive anomalies in the eastern region were relatively strong. Combined with Figure 15a, the upward motion in the eastern region was restricted and suppressed in June, which led to less PRE in the eastern region compared to normal years. The western region itself was under the control of downward motion, and the positive anomalies indicate that the downward motion in this region was enhanced, meaning that the PRE in this region was less and the temperature was higher than in normal years. Therefore, the western region was more prone to drought than the eastern region. The results in Figure 9a-c show that the drought started precisely from the western region. In July, the range of positive anomalies of VV expanded. Analysis combined with Figure 15b reveals that the downward motion in the central and western regions was stronger than in previous years, causing the drought to gradually spread from the western to the central region. Although the range of positive anomalies of VV contracted briefly in August, by September, the positive anomalies almost controlled most areas of the ARB. Figure 10 shows that the DAR reached 60% in September, after which the drought gradually spread to the entire basin. The positive and negative values of moisture flux divergence are closely related to PRE. Negative divergence (water vapor convergence) indicates a net inflow of water vapor in the region. The sufficient accumulation of water vapor provides the material basis for PRE, and when combined with upward motion, it significantly promotes the formation and enhancement of PRE. Positive divergence (water vapor divergence) indicates a net outflow of water vapor from the region. The loss of water vapor leads to insufficient water vapor supply in the region, making it difficult to form effective PRE even if upward motion exists [68; 69]. By comparing Figures 15 and 16, we found that the spatial distribution of divergence and VV is similar: the western part is dominated by positive divergence, corresponding to downward motion, while the eastern part is dominated by negative divergence, corresponding to upward motion. In Figure 16, positive divergence is mainly concentrated in the northwestern and southern parts of the ARB (in June). At this time, the VIWVF shows a west-to-east movement trend, with arrows in the northern part deflecting toward the Caribbean Sea and those in the southern part deflecting toward the Brazilian Plateau (Figure 16a). This causes the water vapor in the ARB to be transported from inland to the ocean. Contrary to the usual water vapor transport from the ocean to inland, this leads  
Figure 15: Spatial and temporal evolution of 500 Pha VV (**a–d**) and its anomalies (**e–h**) in the ARB from June to September 2023. Positive values indicate downward motion, and negative values indicate upward motion. Black dots indicate the presence of significant anomalies. to a reduction in water vapor over the ARB. This is caused by the stronger-than-normal subtropical high in the South Atlantic and the development of anticyclones around the ARB, which result in the weakening of the easterlies. From July to September (Figure 16b-d), the transport direction of the VIWVF generally maintains this trend, while the western and southern parts remain under the control of positive divergence. This makes it difficult for water vapor to condense, resulting in a persistent lack of PRE. This is consistent with the fact that the drought in the ARB shown in Figure 9 is mainly concentrated in the western and southern parts.