Average coastal residence time distribution estimated by a 2-km resolution Japanese coastal model

The average coastal residence time, T_r , which indicates how long water parcels or substances stay in coastal areas, is one of the useful indicators of coastal-offshore water exchange, and has been estimated for some semi-enclosed regions around Japan. We attempted to obtain a spatial distribution of T_r as a first step to comprehend the whole picture of water exchange in the coastal seas of Japan. For this purpose, we devised a new method for both demarcating the coastal retention area and estimating objectively T_r based on a particle-tracking model experiment. Unlike existing methods, this method does not need to assume the retention area in advance, so it can provide the distribution over the entire coastal seas, including semi-enclosed and open regions, rather than targeting a specific region. Appling this method to our coastal ocean model with a horizontal resolution of approximately 2 km, we estimated a spatial distribution of T_r . We were able to differentiate semi-enclosed and open regions around Japan quantitatively. There are 10 semi-enclosed regions, where T_r reached 50–400 days. On the other hand, it was less than 10 days along the coasts exposed to the Kuroshio Current and strong coastal currents. In other coastal regions, it ranged from 10 to several tens of days. In addition, we estimated another timescale of water exchange based on an age-tracer advection–diffusion experiment, and obtained results that are consistent with our particle-tracking estimate. This suggests the validity of the above estimation method for T_r .