Underestimation of the anthropogenic carbon inventory of the Western South Atlantic Ocean linked to Antarctic Bottom Water characterization

The western South Atlantic Ocean is an important sink of anthropogenic carbon dioxide (C_ant), whose deepest layers are influenced by Antarctic Bottom Water (AABW) formed at high latitudes, while also receives C_ant transported by the Atlantic Meridional Overturning Circulation (AMOC) towards the Southern Ocean. The mismatch between observation and model-based estimations is a long-standing issue in the current global carbon budget but regional assessments of interior ocean C_ant inventories are still limited. Recent analyses have shown that Global Ocean Biogeochemical Models (GOBMs) predictions underestimate the rates of increase of C_ant in the western South Atlantic, especially in the Brazilian Basin. We contrast full-depth C_ant inventories based on observations (Global Ocean Data Analysis Project, GLODAPv2.2023) against state-of-the-art GOBMs (RECCAP2) and Ocean Circulation Inverse model (OCIM) outputs. Here, we show that most the observation-based approaches, either methods that use transient tracers’ data (TRACE) or marine-carbonate-system data (Φ-method, eMLR(C*)), yield higher C_ant inventories—by up to 50%—than the mean outputs from GOBMs and OCIM. The annual trend in the storage of C_ant per unit area is higher in observation-based approaches (1.08 ± 0.12 mol m^-2 year^-1 for Φ-method, 1.18 ± 0.06 mol m^-2 year^-1 for eMLR(C*) and 0.66 ± 0.03 mol m^-2 year^-1 for TRACE) than the mean outputs from GOBMS and OCIM (0.55 ± 0.01 mol m^-2 year^-1). We identify the inaccurate characterization of AABW C_ant concentration and extent as the main reason of disagreement. Furthermore, a limited advection of C_ant from the North Atlantic to the South Atlantic by the AMOC also affects intermediate layers. This study confirms that GOBMs systematically underestimate C_ant inventory in the western South Atlantic and reaffirm that high-quality deep ocean carbon observations are a requirement to avoid overlook the contribution of AABW to the global budget.