# GRACE/GFO and Swarm Observation Analysis of the 2023-2024 Extreme Drought in the Amazon River Basin
## 1 Introduction  
Droughs persist as a critical threat to global water security, ecosystems, and human societies, with their impacts escalating under climate change [1]. Amplified by global warming and escalating anthropogenic activities, the natural hydrological equilibrium has been disrupted, rendering climate-hydrological systems more fragile and complex. This transformation has triggered more frequent extreme climatic events, leading to droughs that are now more intense, frequent, and destructive than ever [2, 3]. The Amazon River Basin (ARB) faced an unprecedented extreme drought in 2023. Starting in June, the basin experienced a pronounced precipitation (PRE) deficit, withrainfall levels plummeting to a 40-year low between July and September [4]. Concurrently, a sequence of heat waves from August to November drove sustained temperature surges, peaking at a record 41 \({}^{\circ}\)C during Brazil's winter-spring transition, with maximum temperatures from mid-August to mid-November surpassing historical norms [5]. As the drought unfolded, its spatial extent expanded rapidly: initial impacts were confined to the northern region, but by September, aridity had spread across nearly the entire basin. Tributaries such as the Solimoes, Negro, and Madeira rivers saw water levels plummet to 120-year lows [6], triggering cascading consequences: reservoir power generation halted, water scarcity emerged, hundreds of river-dependent communities were isolated, and massive aquatic organism die-offs occurred [7]. This event exemplified the interplay between natural climate phenomena and human-induced factors: while closely linked to the 2023 El Nino event, anthropogenic activities exacerbated its severity [8]. Given the ARB's role as the world's largest tropical rainforest--often termed the \"lungs of the planet\"--understanding this extreme drought offers critical insights into global climate change mechanisms and the teleconnections linking large-scale climate patterns to regional hydrological extremes. Drought is typically a deficit caused by a reduction in the net input of terrestrial water storage due to climatic anomalies. Therefore, accurately quantifying changes in terrestrial water storage is a key indicator for precise drought assessment. Field observations, hydrological models, and satellite remote sensing are commonly used methods for drought monitoring. While ground-based observations can directly measure meteorological and hydrological variables, they are constrained by their point-based measurement nature and lack large-scale coverage. Additionally, observation stations are scarce in sparsely populated regions, particularly in the ARB [9]. Hydrological models can simulate various hydrometeorological variables, but their accuracy depends on the precision of input physical data and the rationality of mathematical models, resulting in significant uncertainties in simulation results [10]. Satellite remote sensing can provide observations with large coverage and high spatiotemporal resolution; however, it typically measures only water levels and shallow soil moisture (SM) [11; 12]. Therefore, accurately quantifying changes in terrestrial water storage has long been a challenge in drought monitoring. The Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On (GFO) satellites are highly sensitive to changes in the Earth's gravitational field caused by terrestrial water storage changes induced by droughts. They can relatively precisely capture such subtle changes, thereby achieving the goal of accurately quantifying terrestrial water storage changes [13; 14; 15].