Estimate of the shelf residence time using a 2-km resolution Japanese
coastal model
Abstract
Mean residence time of the seawater in the shelf region, τ, has been
studied in several closed bays and inland seas around Japan. From
previous estimates of 0.69 months in the Ise Bay and 6.4-14.7 months in
the Seto Inland Sea, τ is expected to vary depending on area, but there
is no research that reveals the whole picture of the spatiotemporal
variation of τ around Japan. As the first step, we estimate τ for the
entire coastal regions using the 2-km resolution Japanese coastal model
“MRI.COM-JPN” that we developed for a JMA operational system. The
model reproduces well tides, river inflows and many coastal currents
around Japan, which are necessary to simulate the basic physical
processes of coastal-offshore water exchange. In order to estimate τ, an
experiment was conducted to run an “age tracer” that takes an age
while existing in the shelf region and then resets the age to zero
offshore. The tracer value can be regarded as the mean residence time τ
of the seawater flowing into the shelf region. The model was driven for
9 years, and the results from the last 7 years were used for the
analysis. Results showed that in many coastal areas, τ ranged from 20 to
100 days. In eight closed areas such as the Tokyo Bay and the Seto
Inland Sea, τ reached 100-450 days. In addition, τ was as long as
100-200 days in the downstream of the two coastal currents originating
from continental shelves, the Tsushima Warm Current and the East
Sakhalin Current. On the other hand, τ on the Pacific side, including
the southern coast of Japan where the Kuroshio Current flows offshore,
was as short as several to 20 days. At presentation, we will also
discuss results of a particle tracking experiment.