5:15 PM - 6:45 PM
[SCG48-P03] Time change of sound speed structure in GNSS-A observation
Keywords:GNSS-A, internal tide, meso-scale eddy
For centimeter-level GNSS-A observations, it is necessary to model the disturbances received along the acoustic signal path between the sea surface station and the seafloor station. Various modeling methods have been attempted in the past. GARPOS (Watanabe et al., 2020, FES) models it as a function of the position of the station on the sea surface and ocean floor. It has been shown that the actual ocean field can be interpreted as a characteristic gradient field from the obtained function and can be evaluated quantitatively (Yokota et al., under review).
SGO-A, operated by the Japan Coast Guard, has accumulated observation data for about 20 years since the start of observation, and from this data it is possible to see various oceanographic locations in the past. In this study, we extract the gradient field parameters of all data in SGO-A and consider their temporal change characteristics.
The quantitatively extracted average sound speed and gradient field changes temporally, and a semidiurnal component as similar with a component represented by the M2 tide clearly occurred. There were also components with smaller amplitudes, and minute phenomena localized in different periodic bands and times occur depending on the observation site. This feature is different from the time-varying feature pointed out by Matsui et al. (2019, MGR), which examined data mainly from fixed point observations, and can be said to be a feature of SGO-A's wide survey line observation. In other words, the semidiurnal component is influenced by a relatively shallow range.
By the average depth of this time-varying gradient fields of past SGO-A data, there were two types depeding on sea areas. One was a shallow type at 100-200m on the Japan Trench side, and another was an about 400m depth type on the Nankai Trough side. This depth is generally shallower than the average gradient field depth (unchanged gradient field depth) after removing temporal changes, which is considered to be a global structure such as the Kuroshio Current. The changing gradient field depth within each sea area (Japan Trench region and Nankai Trough region) is approximately constant regardless of the observation site. This feature suggests that the factors that generate gradient fields in a certain sea area are the same. According to features about depths and strengths, the temporal changes on the Nankai Trough side are thought to be due to the tide effect and internal tidal waves (e.g., Niwa and Hibiya, 2001, JGR), and the shallow change layer on the north side of the Japan Trench is thought to be mainly due to the influence of mesoscale eddies. These phenomena can be quantitatively evaluated using the parameters of GNSS-A.
Achnowledgement: This study was supported by ERI JURP 2022-Y-KOBO25 in Earthquake Research Institute, the University of Tokyo, by SECOM science and technology foundation, and by JSPS KAKENHI Grant Number JP21H05200 in Grant-in-Aid for Transformative Research Areas (A) “Science of Slow-to-Fast Earthquakes.”
SGO-A, operated by the Japan Coast Guard, has accumulated observation data for about 20 years since the start of observation, and from this data it is possible to see various oceanographic locations in the past. In this study, we extract the gradient field parameters of all data in SGO-A and consider their temporal change characteristics.
The quantitatively extracted average sound speed and gradient field changes temporally, and a semidiurnal component as similar with a component represented by the M2 tide clearly occurred. There were also components with smaller amplitudes, and minute phenomena localized in different periodic bands and times occur depending on the observation site. This feature is different from the time-varying feature pointed out by Matsui et al. (2019, MGR), which examined data mainly from fixed point observations, and can be said to be a feature of SGO-A's wide survey line observation. In other words, the semidiurnal component is influenced by a relatively shallow range.
By the average depth of this time-varying gradient fields of past SGO-A data, there were two types depeding on sea areas. One was a shallow type at 100-200m on the Japan Trench side, and another was an about 400m depth type on the Nankai Trough side. This depth is generally shallower than the average gradient field depth (unchanged gradient field depth) after removing temporal changes, which is considered to be a global structure such as the Kuroshio Current. The changing gradient field depth within each sea area (Japan Trench region and Nankai Trough region) is approximately constant regardless of the observation site. This feature suggests that the factors that generate gradient fields in a certain sea area are the same. According to features about depths and strengths, the temporal changes on the Nankai Trough side are thought to be due to the tide effect and internal tidal waves (e.g., Niwa and Hibiya, 2001, JGR), and the shallow change layer on the north side of the Japan Trench is thought to be mainly due to the influence of mesoscale eddies. These phenomena can be quantitatively evaluated using the parameters of GNSS-A.
Achnowledgement: This study was supported by ERI JURP 2022-Y-KOBO25 in Earthquake Research Institute, the University of Tokyo, by SECOM science and technology foundation, and by JSPS KAKENHI Grant Number JP21H05200 in Grant-in-Aid for Transformative Research Areas (A) “Science of Slow-to-Fast Earthquakes.”