Separation of the hydrological and tidal components in water heights to estimate discharge in the downstream, tidal, Amazon

The Amazon estuary conveys the largest amount of freshwater to the world ocean (20% of the global runoff). Over the past few years, its discharge exhibited record-breaking anomalies, be it flood events (June 2021 and June 2022) or dry spells (drought of November 2023 and October/November 2024). Assessing quantitatively the imprint of these extremes over the estuarine water level is challenging though, due to the ubiquitous and vigorous tidal signal propagating upstream from the Atlantic Ocean which prevents remote discharge estimates in the estuarine part of the river. We used the multi-mission nadir altimetry dataset composed of J3+S6A, S3A, S3B. Altogether, the satellite tracks encompass the whole estuary from its upstream limit 900 km inland down to the mouths of the Amazon terminal delta, making possible to map synoptically the spatio-temporal evolution of the estuarine water level and compute the separation of the tidal and hydrological contributions into the water surface height. The approach relies on an accurate de-aliasing of the tide in the altimetry records, based on a cross-scale hydrodynamic model of the Amazon estuary purposely developed and duly validated. This model uses the SCHISM ocean circulation code, with resolution of the order of 250 m inside the estuary. It allowed inferring a time-varying tidal atlas, which is utilized to remove the tidal signal from the altimetric anomalies. The altimetric residuals depicts the spatio-temporal pattern of water level anomalies in response to discharge variations, both during the flood and drought periods. For instance, the 2021 and 2022 extreme floods induced an anomaly that lasted about 1 month each time, with water level peaks about 50 cm above the seasonal climatology, extending over the upper 500 km of the 900 km-long estuary. Downstream-ward of this, the imprint of the extreme floods decayed sharply, and reached insignificant magnitude throughout the downstream-most 300 km of the estuary (corresponding roughly to the terminal delta). A mirror conclusion can be drawn for the 2023 drought, with 1 m negative anomaly below the seasonal, mostly restricted to the upper 300 km of the estuary at the peak of the event in November 2023, and with a weak signal further downstream. The magnitude of these anomalies largely exceeds the bounds of the accuracy of our altimetric dataset. We present that it is now possible to derive reliable discharge estimates in the estuarine reach of the Amazon river by converting these tidal-free water levels from altimetry measurements through a classical rating curve, including for the extreme events.