The impact of rain on the global ocean carbon uptake

Precipitation alters sea surface physical and biogeochemical properties locally. However, due to its high temporal and spatial variations, it has largely been overlooked in studies assessing global ocean carbon uptake. Air-sea CO₂ flux is mainly due to the interfacial exchange of CO₂ molecules between the liquid and gaseous phases media. Rain may impact this interfacial air-sea CO₂ flux by (i) enhancing the turbulence at the air-sea interface and (ii) diluting the CO₂ concentration near the ocean surface. At the same time, rain directly injects into the ocean CO₂ absorbed by the raindrops during their fall. This latter component, known as wet deposition, contributes to the CO₂ flux into the ocean. This study provides the first comprehensive global estimate of these effects and their combined influence on the global ocean carbon uptake during the period 2008-2018. We use different representations of the ocean surface response to rain and different rain products with different rain rate distributions (ERA5 and IMERG) to quantify the uncertainty of the global impact of rain on CO₂ sink. We show that rain increases the global ocean carbon sink by +0.14 to +0.19 PgC yr^-1 over 2008-2018, representing an increase of 5 to 7% of the global carbon uptake (2.66 PgC yr^-1). Both interfacial flux and wet deposition have comparable orders of magnitude. Rain mainly increases the CO₂ sink in the tropics, where strong rain rates and weak winds induce noticeable dilution at the ocean surface, in the storm track regions, and in the Southern ocean.