3:30 PM - 3:45 PM
[AOS14-06] Overview and preliminary results of Four-dimensional Variational Ocean Reanalysis for the seas around Japan over 60 years (FORA-JPN60)
Keywords:ocean reanalysis, seas around Japan, coastal model, ocean data assimilation system
Realistic reproduction of long-term ocean variations of the major currents such as Kuroshio, Oyashio, and Tsushima Warm Current which flows around Japan is important for a wide range of scientific fields, including oceanography and climate science as well as air-sea interaction in the mid-latitude and fishery science. In particular, long-term ocean datasets with high-resolution grids provide an opportunity to investigate the abrupt coastal strong currents and the mesoscale eddy activities including the long-term variability in the Kuroshio Extension regions.
The MEXT-Program for the advanced studies of climate change projection aims to develop the prediction system (SENTAN) and analyze the mechanism for climate change around Japan. It is critical to validate the representation performance of the prediction system for the current climate and requires a reference dataset. In this study, we develop the high-resolution long-term ocean reanalysis for the seas around Japan over 60 years with the resolution of 2 km (FORA-JPN60, hereafter JPN60) as the reference dataset and show an overview and preliminary results.
FORA-JPN60 is the successor to the ocean reanalysis FORA-WNP30 (Usui et al., 2017, hereafter WNP30) and is produced by using an operational coastal forecasting system in JMA (Hirose et al., 2019). The ocean model is based MRI.COM v5.0 and consists of the Japanese coastal model (2 km resolution), a North Pacific model (10 km resolution), and a global model (100 km resolution). The three models are coupled by using a two-way on-line double nesting method (Sakamoto et al., 2019). Surface boundary conditions are forced by JRA-3Q (Kosaka et al., 2024). To improve coastal processes, tidal forcing and barometric effect are incorporated and river runoff data based on the JMA runoff index (Urakawa et al., 2016) is applied.
The data assimilation scheme applied to the North Pacific and global models is based on MOVE-4DVAR (Usui et al. 2015) and MOVE-3DVAR, respectively. By using analyzed temperature and salinity fields, the three models are corrected by the incremental analysis updates scheme (Bloom et al. 1996). Sea surface temperature used is MGDSST (Kurihara et al., 2006). Along-track sea level anomaly correcting barystatic sea level is assimilated. In-situ temperature and salinity data are combined with other datasets from around the world into EN4 dataset (Good et al., 2013). Increased observation datasets include FRESCO, KODC, in-situ observation in the Sea of Okhotsk in February, water quality data in the Seto-Inland Sea. Sea ice concentration is assimilated by a nudging method and observations for assimilation are microwave radiometer (SSMI, SSMIS) and manually derived sea ice analysis data produced operationally by the JMA. The period of the reanalysis experiment is from 1960 to 2020 (now completed until 2007). We summarize the major improvements of JPN60 against WNP30; finer horizontal resolution from 10 km to 2 km, longer data period from 30 years to 60 years, implementation of physical scheme (tide, sea level pressure, river runoff), improvement of assimilation scheme (assimilation of sea level anomaly removing barystatic sea level), and increasing number of observations.
Preliminary results of JPN60 are as follows. The Kuroshio south of Japan takes three types of path pattern; straight path close to coasts, meander path flowing southward off the Kii Peninsula, and the middle of the straight and meander paths. The mender paths occur in early 1960s, late 1970s, and after 2017 recently. The current axis of the Kuroshio in JPN60 is consistent with the previous studies. Surface circulation in the Sea of Japan is driven by the volume transport through the straits connected with neighbor seas. The volume transport through the straits is proportional to the sea level difference in the strait. Estimates of volume transport based on the sea level difference in the Tsushima Strait (Shin et al., 2022) and the Soya Strait (Ohshima & Kuga, 2023) are used for validation. A bias of the volume transport through the Tsushima Strait (Soya Strait) in JPN60 from 1982 to 2007 reduced to -0.11 (-0.17) Sv from -0.21 (-0.24) Sv in WNP30. The coastal sea level in JPN60 is improved compared with WNP30 due to increasing of correlation coefficient and decreasing of RMSE. Increasing of the horizontal resolution in JPN60 contributed to the result due to the finer bottom topography in the strait (Han et al., 2018). The presentation will show a validation of the dataset for the entire period from 1960 to 2020.
This work was conducted under MEXT–program for the advanced studies of climate change projection (SENTAN; JPMXD0722680734).
The MEXT-Program for the advanced studies of climate change projection aims to develop the prediction system (SENTAN) and analyze the mechanism for climate change around Japan. It is critical to validate the representation performance of the prediction system for the current climate and requires a reference dataset. In this study, we develop the high-resolution long-term ocean reanalysis for the seas around Japan over 60 years with the resolution of 2 km (FORA-JPN60, hereafter JPN60) as the reference dataset and show an overview and preliminary results.
FORA-JPN60 is the successor to the ocean reanalysis FORA-WNP30 (Usui et al., 2017, hereafter WNP30) and is produced by using an operational coastal forecasting system in JMA (Hirose et al., 2019). The ocean model is based MRI.COM v5.0 and consists of the Japanese coastal model (2 km resolution), a North Pacific model (10 km resolution), and a global model (100 km resolution). The three models are coupled by using a two-way on-line double nesting method (Sakamoto et al., 2019). Surface boundary conditions are forced by JRA-3Q (Kosaka et al., 2024). To improve coastal processes, tidal forcing and barometric effect are incorporated and river runoff data based on the JMA runoff index (Urakawa et al., 2016) is applied.
The data assimilation scheme applied to the North Pacific and global models is based on MOVE-4DVAR (Usui et al. 2015) and MOVE-3DVAR, respectively. By using analyzed temperature and salinity fields, the three models are corrected by the incremental analysis updates scheme (Bloom et al. 1996). Sea surface temperature used is MGDSST (Kurihara et al., 2006). Along-track sea level anomaly correcting barystatic sea level is assimilated. In-situ temperature and salinity data are combined with other datasets from around the world into EN4 dataset (Good et al., 2013). Increased observation datasets include FRESCO, KODC, in-situ observation in the Sea of Okhotsk in February, water quality data in the Seto-Inland Sea. Sea ice concentration is assimilated by a nudging method and observations for assimilation are microwave radiometer (SSMI, SSMIS) and manually derived sea ice analysis data produced operationally by the JMA. The period of the reanalysis experiment is from 1960 to 2020 (now completed until 2007). We summarize the major improvements of JPN60 against WNP30; finer horizontal resolution from 10 km to 2 km, longer data period from 30 years to 60 years, implementation of physical scheme (tide, sea level pressure, river runoff), improvement of assimilation scheme (assimilation of sea level anomaly removing barystatic sea level), and increasing number of observations.
Preliminary results of JPN60 are as follows. The Kuroshio south of Japan takes three types of path pattern; straight path close to coasts, meander path flowing southward off the Kii Peninsula, and the middle of the straight and meander paths. The mender paths occur in early 1960s, late 1970s, and after 2017 recently. The current axis of the Kuroshio in JPN60 is consistent with the previous studies. Surface circulation in the Sea of Japan is driven by the volume transport through the straits connected with neighbor seas. The volume transport through the straits is proportional to the sea level difference in the strait. Estimates of volume transport based on the sea level difference in the Tsushima Strait (Shin et al., 2022) and the Soya Strait (Ohshima & Kuga, 2023) are used for validation. A bias of the volume transport through the Tsushima Strait (Soya Strait) in JPN60 from 1982 to 2007 reduced to -0.11 (-0.17) Sv from -0.21 (-0.24) Sv in WNP30. The coastal sea level in JPN60 is improved compared with WNP30 due to increasing of correlation coefficient and decreasing of RMSE. Increasing of the horizontal resolution in JPN60 contributed to the result due to the finer bottom topography in the strait (Han et al., 2018). The presentation will show a validation of the dataset for the entire period from 1960 to 2020.
This work was conducted under MEXT–program for the advanced studies of climate change projection (SENTAN; JPMXD0722680734).