Simulating Water Residence Time in the Coastal Ocean: A Global Perspective

Exchanges between coastal and oceanic waters shape both coastal ecosystem processes and signatures that they impart on global biogeochemical cycles. The timescales of these exchanges, however, are poorly represented in current‐generation, coarse‐grid climate models. Here we provide a novel global perspective on coastal residence time (CRT) and its spatio‐temporal variability using a new age tracer implemented in global ocean models. Simulated CRTs range widely from several days in narrow boundary currents to multiple years on broader shelves and in semi‐enclosed seas, in agreement with available observations. Overall, CRT is better characterized in high‐resolution models (1/8° and 1/4°) than in the coarser (1° and 1/2°) versions. This is in large part because coastal and open ocean grid cells are more directly connected in coarse models, prone to erroneous coastal flushing and an underestimated CRT. Additionally, we find that geometric enclosure of a coastal system places an important constraint on CRT. Key Points State‐of‐the‐art, high‐resolution global ocean simulations and a new tracer approach are used to estimate coastal residence times globally Coarse models yield a negative residence time bias in 54% of the coastal ocean; this bias is particularly large in narrow boundary currents Geometric enclosure as a function of coastal volume and open ocean boundary area places an important constraint on coastal residence time