Sensitivity of biogenic carbon export to ocean climate in the Labrador Sea, a deep‐water formation region

Abstract

We used a physical‐biogeochemical model to examine the sensitivity of biogenic carbon export to ocean climate in the Labrador Sea, a subpolar, deep‐water formation region. Documented changes in winter mixed layer depth between the late 1960s and the mid‐1990s were used to construct scenarios of weak, moderate, and strong winter convection that drive the biogeochemical model. The model simulations suggest that the total biogenic carbon export (particle sinking flux + DOC export) is higher under strong winter convection (e.g., during the early 1990s) than under weak winter convection (e.g., during the late 1960s), by ∼70% across the 200‐m isobath and nearly double at 500 m and 1000 m depth. These large variations in total biogenic carbon export are essentially due to the response of DOC export to ocean climate conditions. Sensitivity analyses indicate that the variations in DOC export from the euphotic zone are due to the impact of the convection regime on the development of the microbial food web and on the bacterial consumption of DOC in surface waters. Although DOC downward fluxes within the mesopelagic zone (below ∼500 m) are largely controlled by physical processes, the effect of convection on microbial dynamics can potentially amplify the year‐to‐year variations in the transport of DOC to the deep ocean due to convection.