Assessment of the hydrodynamical signature of the record-breaking 2021 flood along the Amazon estuary

Abstract

Since the late 1990s, the hydrological cycle linking the Andes, the Amazon and the Atlantic Ocean has been intensifying. The Amazon estuary is the eastern terminal connection of this hydrosystem, linking the Amazon watershed to the open ocean. At Óbidos, the upstream limit of the estuary, the 2021 May-June flood was recorded as the highest flood in 12 years, with a peak discharge of 260,000 m³.s^-1. The impact of this record flood has not been yet quantified along the estuary. This study aims to quantify its signature on estuarine hydrodynamics, using a cross-scale hydrodynamic model of the Amazonian estuarine continuum based on SCHISM. It turns out that the 2021 discharge anomaly (10 % above the seasonal climatology) has a prominent influence on the Amazon River from 800 km to 380 km inland, inducing water level maxima typically 0.3 m higher than during a normal flood year, and about 1 m higher than during a weak flood year. Analysis of the various hydrodynamic factors conducive to the water level maxima along the estuary (i.e. discharge, oceanic tide and atmospheric forcing) shows that this is largely due to the discharge contributing twice as much in 2021 as in a normal year. In contrast, from 380 km inland to the oceanic mouth, both in the north-western arm (the North Channel) and in the south-eastern arm (the Pará) of the estuary, the 2021 flood has no significant impact on the water level maxima dynamics, as the variability is dominated by the oceanic tide.