The coastal ocean contributes to regulating atmospheric greenhouse gas concentrations by taking up carbon dioxide (CO2) and releasing nitrous oxide (N2O) and methane (CH4). Major advances have improved our understanding of the coastal air-sea exchanges of these three gasses since the first phase of the Regional Carbon Cycle Assessment and Processes (RECCAP in 2013), but a comprehensive view that integrates the three gasses at the global scale is still lacking. In this second phase (RECCAP2), we quantify global coastal ocean fluxes of CO2, N2O and CH4 using an ensemble of global gap-filled observation-based products and ocean biogeochemical models. The global coastal ocean is a net sink of CO2 in both observational products and models, but the magnitude of the median net global coastal uptake is ~60% larger in models (-0.72 vs. -0.44 PgC/yr, 1998-2018, coastal ocean area of 77 million km2). We attribute most of this model-product difference to the seasonality in sea surface CO2 partial pressure at mid- and high-latitudes, where models simulate stronger winter CO2 uptake. The global coastal ocean is a major source of N2O (+0.70 PgCO2-e /yr in observational product and +0.54 PgCO2-e /yr in model median) and of CH4 (+0.21 PgCO2-e /yr in observational product), which offsets a substantial proportion of the net radiative effect of coastal \co uptake (35-58% in CO2-equivalents). Data products and models need improvement to better resolve the spatio-temporal variability and long term trends in CO2, N2O and CH4 in the global coastal ocean.

The observed sea level (SL) around the Japanese coast shows a peculiar multidecadal variation with the peak in the 1950s followed by the gradual fall until the 1970s and the rebound continuing to the present, making the recent SL rise less remarkable in the historical record. An ensemble mean of the historical simulations conducted for the Coupled Model Intercomparison Project phase 6 (CMIP6) using the Meteorological Research Institute Earth System Model version 2.0 (MRI-ESM2.0) reproduces this variability well, implying that this was a forced one. The MRI-ESM2.0 simulations for the Detection and Attribution Model Intercomparison Project suggest that the increase in anthropogenic aerosols caused the SL fall from the 1950s to the 1970s and the increase in greenhouse gases caused the SL rise after that. Additional sensitivity runs indicate that the surface heat loss in the North Pacific due to anthropogenic aerosols plays a dominant role in the SL fall.

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.