# GRACE/GFO and Swarm Observation Analysis of the 2023-2024 Extreme Drought in the Amazon River Basin
## 1 Introduction
They also features all-weather and full-coverage capabilities, overcoming the limitations of traditional monitoring methods, and thus are well-suited for drought monitoring in medium- and large-scale river basins  
The GRACE/GFO satellites detected a significant water storage deficit (WSD) in the central ARB in 2005 that was 10% below normal, and the results were validated by the PRE data based on satellite remote sensing and water level data from local hydrological stations [16]. This demonstrated the unique advantage of satellite gravity technology in monitoring large-scale droughts. [PERSON] et al. [17] proposed a quantitative methodology to assess drought occurrence and severity based on GRACE TWSC data and applied this approach to evaluate drought-induced WSD in the ARB from 2003 to 2014. Their results were corroborated by the Global Meteorological Drought Database. [PERSON] et al. [18] used GRACE data to track the entire 2010 extreme drought event in the ARB, finding a strong correlation between the drought and the El Nino-Southern Oscillation (ENSO). [PERSON] et al. [19] compared three extreme droughts in the ARB (2005, 2010, and 2015), showing that the anomalous PRE deficit in 2015 exceeded previous drought events in both intensity and spatial extent, affecting more than 80% of the ARBPRE, with the eastern region being particularly severely impacted. However, the GRACE/GFO mission ended in June 2017, and the GFO mission did not launch until April 2018, creating an 11-month gap (from July 2017 to May 2018) in the TWSC time series, which posed challenges for hydrological research [20]. The Swarm satellites, which cover this gap, have attracted interest due to their ability to detect Earth's time-variable gravity signals through payloads like accelerometers and global navigation satellite system receivers [21; 22]. Several studies have explored the potential and accuracy of using Swarm satellite data to monitor regional TWSCs. These studies found that Swarm satellites are particularly sensitive to long-wave components of gravity signals, making them capable of monitoring regional TWS changes [23; 24; 25; 26; 27]. [PERSON] et al. [28] applied Swarm data to assess TWS deficits from the 2015-2016 drought in the ARB, validating their findings against hydrological models and local station data. [PERSON] et al. [29] identified 11 drought events in the ARB between 2003 and 2020 by integrating GRACE/GFO and Swarm observations. They found that the most severe droughts occurred in 2005, 2010, and 2015-2016, with low PRE being the direct cause and plains areas being more vulnerable to drought than mountain regions. Although the spatial resolution of Swarm satellites is lower than that of GRACE/GFO data and their background noise is higher than that of GRACE/GFO data, previous studies have shown that the spatial resolution of Swarm satellites does not affect the results of drought monitoring in large-scale river basins such as the Amazon River Basin. The impact of background noise can be greatly reduced by using 1000 km fan filtering [28; 29]. When analyzing the impact of ENSO events on droughts, most previous studies simply calculated the correlation between the ENSO index and GRACE/GFO TWSC or drought indices, without elaborating on the operational mechanism. However, this requires interdisciplinary knowledge backgrounds and data integration [30; 31]. Therefore, this study attempted to analyze the atmospheric vertical movement and water vapor transport in the ARB during the 2023-2024 extreme drought by combining vertical velocity data, wind speed, and relative humidity data and to explore the connection with ENSO events.