Presentation Information
[AOS25-P05]Comparison of mean dynamic topography derived from tide station observations and the ocean reanalysis dataset
*Masahiro Nambu1, Hiromichi Rinno1, Noritsune Seo1, Chikara Tsuchiya1, Yoshiya Irie1 (1.Hydrographic and Oceanographic Department, Japan Coast Guard)
Keywords:
dynamic sea surface topography,tide stations,mean sea level,a geoid model,a hydrographic chart datum
To establish a hydrographic chart datum, it is necessary to understand the spatial distribution of mean sea surface referenced to the geoid (Mean dynamic topography, MDT) in coastal areas. Along the Japanese coast, the MDT differs by up to approximately 1 m depending on the region, producing gradients that are non-negligible for hydrographic surveying.
Observations of the MDT are conducted by sea levels measurement at tide stations and conversion of the datum using GNSS or direct leveling surveys. However, because tide stations are distributed unequally and absent offshore, it is difficult to interpolate the gradient of MDT using tide gauge observations alone.
In recent years, the Japanese coastal ocean reanalysis dataset “MOVE/MRI.COM-JPN Dataset” (hereafter referred to as the MOVE reanalysis) has been developed by the Meteorological Research Institute of the Japan Meteorological Agency. This high-resolution model, with a horizontal resolution of approximately 2 km and assimilating satellite altimetry data, accurately reproduces coastal processes such as the path of geostrophic currents. The MOVE reanalysis is expected to realistically represent the gradient of mean sea level in coastal regions and thus has potential for estimating the spatial distribution of the MDT.
In this study, to evaluate the applicability of the MOVE reanalysis for estimating the distribution of MDT, we compared observed MDT at tide stations and MDT derived from the MOVE reanalysis. Based on this comparison, correction values were estimated to combine the MOVE reanalysis with tide station observations. The averaging period for MDT was set to five years.
At each tide station, observed MDT was obtained as follows. At tide stations where GNSS observations were conducted, mean sea levels referenced to the WGS84 ellipsoid were converted to those referenced to the geoid to using the geoid model “JPGEO2024”. At tide stations where direct leveling surveys were conducted between tide stations and benchmarks, MDT were calculated using the height differences and the elevations of the benchmark. For tide stations where both measurements were available, the maximum likelihood estimate of MDT was calculated by taking into account the variance associated with leveling surveys and GNSS observations. All tide station sea level records were corrected for vertical land movements using the F5 solution of the nearest GNSS continuous observational reference station.
Observations of the MDT are conducted by sea levels measurement at tide stations and conversion of the datum using GNSS or direct leveling surveys. However, because tide stations are distributed unequally and absent offshore, it is difficult to interpolate the gradient of MDT using tide gauge observations alone.
In recent years, the Japanese coastal ocean reanalysis dataset “MOVE/MRI.COM-JPN Dataset” (hereafter referred to as the MOVE reanalysis) has been developed by the Meteorological Research Institute of the Japan Meteorological Agency. This high-resolution model, with a horizontal resolution of approximately 2 km and assimilating satellite altimetry data, accurately reproduces coastal processes such as the path of geostrophic currents. The MOVE reanalysis is expected to realistically represent the gradient of mean sea level in coastal regions and thus has potential for estimating the spatial distribution of the MDT.
In this study, to evaluate the applicability of the MOVE reanalysis for estimating the distribution of MDT, we compared observed MDT at tide stations and MDT derived from the MOVE reanalysis. Based on this comparison, correction values were estimated to combine the MOVE reanalysis with tide station observations. The averaging period for MDT was set to five years.
At each tide station, observed MDT was obtained as follows. At tide stations where GNSS observations were conducted, mean sea levels referenced to the WGS84 ellipsoid were converted to those referenced to the geoid to using the geoid model “JPGEO2024”. At tide stations where direct leveling surveys were conducted between tide stations and benchmarks, MDT were calculated using the height differences and the elevations of the benchmark. For tide stations where both measurements were available, the maximum likelihood estimate of MDT was calculated by taking into account the variance associated with leveling surveys and GNSS observations. All tide station sea level records were corrected for vertical land movements using the F5 solution of the nearest GNSS continuous observational reference station.