In late 2023, the Amazon River Basin experienced its most extreme drought, putting its population and ecosystem at major risks. Gauges that were still functioning measured the lowest river water levels (RWL) on record. Here, satellite observations, including Surface Water Ocean Topography (SWOT), reveal the spread and timing of extremely low RWL across the entire river system. Nadir altimeter observations show that the 2023 minimum RWL in Central Amazon were more than 3 m below their annual average, representing two to three times its mean variability. Additionally, SWOT enables the first basin-scale characterization of the reduction in RWL with a spatial resolution of 200 m and how it propagates with time. Large-scale evaluation with gauges suggest that SWOT outperforms classical altimetry in estimating RWL, even if observed biases need further investigations. SWOT offers a new opportunity to understand hydroclimatic extremes and their broad impacts on the environment of the Amazon.

Arctic sea ice thickness (SIT) have been mostly retrieved from microwaved and visible altimeters since the 2000s. However, the repeatability of altimeters and their spatial coverage limit SIT estimates spatially and temporally. On the other hand, the passive microwave (PMW) radiometer have daily basin-scale coverage of the Arctic. In this study, we proposed a SIT retrieval from PMW observations, based on a statistical inversion technique. It is based on the evidence of hig correlations between PMW observations and existing altimetric satellite-derived SIT, especially at 36 GHz. Lidar ICESat-2 SIT products were used to train a neural network with multiple combinations of brightness temperatures between 1.4 and 36 GHz as inputs over the 2018-2019 time period. The PMW retrieved SIT can mimic the lidar SIT product over the full winter over the Arctic, with a correlation of 0.85, and a RMSE of 0.54 cm. Results were also compared with the altimeter CS2SMOS and the Nucleus for European Modelling of the Ocean (NEMO) SIT products and with the Operation IceBridge QuickLook SIT measurements. The Neural Network (NN) SIT retrieval with all frequencies from 1.4 to 36 GHz has good performance, a correlation of 0.72 and a RMSE of 57 cm when compared to OIB-QL measurements, for large sea ice thickness (mostly above 3 m), under multi-year ice environments. The NN SIT retrieval using only 18 and 36 GHz has also shown satisfactory performances, paving the way for the creation of long time series, these two microwave channels being available since the 1980s.