# GRACE/GFO and Swarm Observation Analysis of the 2023-2024 Extreme Drought in the Amazon River Basin
## 4 Results
### Associated SST and Atmospheric Anomalies  
To study the impact of El Nino events on this drought, we compared the time series of the Nino 3.4 index and GRACE-DSI from 2003 to 2024 and marked the El Nino events and droughts within the study period (Figure 14). By comparing Figure 14,b, we found that El Nino events occurred before all drought events within the study period. A total of six droughts happened during the study period. The 2005 drought occurred from June to October 2005, preceded by an El Nino event from August 2004 to January 2005, and similar patterns were observed for the remaining five droughts. Prior to the 2023-2024 extreme drought, an El Nino event began in June 2023 and persisted until February 2024, with its peak reaching 1.95, second only to the 2014-2016 El Nino event. There is a certain correlation between the peak of drought and that of El Nino events: the higher the peak of an El Nino event, the higher the peak of the subsequent drought. For example, the  
Figure 13: The normalized time series of different meteorological and hydrological variable during January 2023 and February 2024. Navy blue line: PRE; red line: ET; light line: surface runoff; orange line: subsurface runoff; green line: SM; purple line: TWSC; the five dotted lines are all 0 scale lines. 2014-2016 El Nino event had a peak value of 2.65 and the subsequent 2015-2016 drought reached a peak of \(-\)1.66, both of which were the highest in the study period. Similarly, the peak of the 2023-2024 drought was \(-\)1.29, the second-highest during the study period, and the preceding El Nino event also had the second-highest peak (1.95). The above phenomena indicate a teleconnection between El Nino events and droughts in the Amazon Basin. The calculation results show that the maximum correlation coefficient between the GRACE-DSI of the ARB and the Nino 3.4 index is 0.48, with a lag of 6 months. A previous study showed that in El Nino years, PRE in the northern region may be reduced by 30% to 50% compared to normal years, and some areas even experience prolonged drought [64]. At the same time, due to the adjustment of atmospheric circulation, the transport of warm and humid air is affected, the distribution of solar radiation energy received by the ARB changes, and the temperature rises. A previous study showed that in years with strong El Nino, the average temperature in the ARB may increase by 1-2 \({}^{\circ}\)C [4]. Atmospheric vertical motion is a key factor influencing PRE. Upward air currents promote the condensation of water vapor, leading to cloud formation and PRE, which are important conditions for the generation of PRE. In contrast, downward air currents cause adiabatic warming of the air, making it difficult for water vapor to condense and suppressing PRE formation. VV anomalies reflect the extent to which this motion deviates from the average state. The regions with significant anomalies indicate that the deviation in vertical motion from the climatic average state reaches a statistically significant level, and their impact on weather and climate is more pronounced. Therefore, we plotted the spatiotemporal evolution of VV and its anomalies in the ARB from June to September 2023. We used the Z-test to conduct a significance test on the anomalies. As shown in Figure 15a-d, most regions of the ARB were under the control of downward motion from June to September 2023, with only a few regions in the east experiencing upward motion. Downward motion causes adiabatic compression and warming of the air, inhibits water vapor condensation, and leads to reduced cloud cover and scarce PRE [65]. For every 100 m the air sinks, the temperature rises by approximately 0.6 \({}^{\circ}\)C, resulting in a significant increase in near-surface air temperature [66]. June to September each year is the winter season in the ARB, during which the region is affected by the edge of the subtropical high-pressure system, with enhanced downward motion, thus forming the dry season [67].