- 著者
- Shion Sekizawa Tsubasa Kohyama
- 出版者
- 公益社団法人 日本気象学会
- 雑誌
- SOLA (ISSN:13496476)
- 巻号頁・発行日
- pp.2022-021, (Released:2022-05-26)
- 被引用文献数
- 13
On January 15, 2022, the volcano Hunga Tonga about 8000-km away from Japan explosively erupted. Following the eruption, tsunami-like sea-level fluctuations were observed in Japan, much earlier than expected based on the oceanic long-wave propagation from Tonga. By contrast, atmospheric pressure disturbance presumably due to the eruption was also observed about 30 minutes before the sea-level change. Therefore, the observed sea-level fluctuations can be considered as meteotsunamis forced by the pressure perturbation rather than tectonically forced by the eruption, but the mechanism is not yet fully understood. This study attempts to understand the nature of this meteotsunami by using a simple one-dimensional shallow-water model. The results show that the time and amplitude of the observed sea-level changes are consistent with the simulated sea-level changes forced by the atmospheric forcing. A set of experiments with different bathymetry profiles also reveals the importance of amplification due to near-Proudman resonance over deep basins and the shoaling effect over the continental slope, while extremely deep and narrow topography such as trenches is of second-order importance.
- 著者
- Shion Sekizawa Tsubasa Kohyama
- 出版者
- 公益社団法人 日本気象学会
- 雑誌
- SOLA (ISSN:13496476)
- 巻号頁・発行日
- vol.18, pp.129-134, 2022 (Released:2022-06-21)
- 参考文献数
- 13
- 被引用文献数
- 13
On 15 January 2022, the volcano Hunga Tonga about 8000-km away from Japan explosively erupted. Following the eruption, tsunami-like sea-level fluctuations were observed in Japan, much earlier than expected based on the oceanic long-wave propagation from Tonga. By contrast, atmospheric pressure disturbance presumably due to the eruption was also observed about 30 minutes before the sea-level change. Therefore, the observed sea-level fluctuations can be considered as meteotsunamis forced by the pressure perturbation rather than tectonically forced by the eruption, but the mechanism is not yet fully understood. This study attempts to understand the nature of this meteotsunami by using a simple one-dimensional shallow-water model. The results show that the time and amplitude of the observed sea-level changes are consistent with the simulated sea-level changes forced by the atmospheric forcing. A set of experiments with different bathymetry profiles also reveals the importance of amplification due to near-Proudman resonance over deep basins and the shoaling effect over the continental slope, while extremely deep and narrow topography such as trenches is of second-order importance.
7 0 0 0 OA Intensive Variability Extraction
- 著者
- Tsubasa Kohyama Hiroaki Miura Shoichiro Kido
- 出版者
- Meteorological Society of Japan
- 雑誌
- SOLA (ISSN:13496476)
- 巻号頁・発行日
- vol.17, pp.246-250, 2021 (Released:2021-12-23)
- 参考文献数
- 14
A modified version of the principal component analysis (PCA) is introduced by reconsidering statistical degrees of freedom in spatial dimensions based on spatial auto-correlations. In the conventional PCA, data points that represent equal areas are assumed to have equal amount of information. In our new method, the intensive variability extraction (IVE), data points correlated with less other data points are weighted more before performing PCA. Hence, variability with independent information is emphasized, even if the variability is confined to small areas.Sea surface temperature (SST) data at each grid in the tropics are shown to have fewer spatial statistical degrees of freedom than that in the extratropics. Tropical SSTs exhibit covariability with large areas, because oceanic equatorial waves and atmospheric gravity waves share temperature information with surrounding areas. As to the extratropics, grids along the western boundaries of oceanic basins are more independent than those in the east, following dynamical requirement of the Earth's rotation.Using IVE, climate modes that involve interscale covariability are extracted. IVE performed for the Pan-Pacific SSTs extracts the Pacific Decadal Oscillation assuming the aforementioned a priori dynamical expectation. Using extratropical SSTs, it is demonstrated that IVE detects synchronicity of small-scale variability between distant narrow regions.
- 著者
- Amane Nakamura Tsubasa Kohyama
- 出版者
- 公益社団法人 日本気象学会
- 雑誌
- SOLA (ISSN:13496476)
- 巻号頁・発行日
- vol.18, pp.236-242, 2022 (Released:2022-10-31)
- 参考文献数
- 18
Analysis of the El Niño-Southern Oscillation (ENSO) phenomena has conventionally been performed using deviations from the mean. However, the question remains as to whether the mean is appropriate as the reference state, since it is sensitive to the existence of a few extreme events. In this study, the validity of the mean and mode as a reference is compared using an idealized simulation model. We show that the mode is not affected by the asymmetry of El Niño and La Niña and is more stable as a reference than the mean. Then, this result is also demonstrated using observed data. Observations also show that the relationship between the ENSO amplitude and the zonal shifts of the ENSO anomalies is more emphasized if the mode is employed. For the variables in the ocean interior, differences between the mode-based and mean-based methods appear throughout all seasons.
- 著者
- Amane Nakamura Tsubasa Kohyama
- 出版者
- 公益社団法人 日本気象学会
- 雑誌
- SOLA (ISSN:13496476)
- 巻号頁・発行日
- pp.2022-038, (Released:2022-10-04)
Analysis of the El Ninño-Southern Oscillation (ENSO) phenomena has conventionally been performed using deviations from the mean. However, the question remains as to whether the mean is appropriate as the reference state, since it is sensitive to the existence of a few extreme events. In this study, the validity of the mean and mode as a reference is compared using an idealized simulation model. We show that the mode is not affected by the asymmetry of El Ninño and La Ninña and is more stable as a reference than the mean. Then, this result is also demonstrated using observed data. Observations also show that the relationship between the ENSO amplitude and the zonal shifts of the ENSO anomalies is more emphasized if the mode is employed. For the variables in the ocean interior, differences between the mode-based and mean-based methods appear throughout all seasons.
3 0 0 0 OA Intensive Variability Extraction
- 著者
- Tsubasa Kohyama Hiroaki Miura Shoichiro Kido
- 出版者
- Meteorological Society of Japan
- 雑誌
- SOLA (ISSN:13496476)
- 巻号頁・発行日
- pp.2021-043, (Released:2021-11-09)
A modified version of the principal component analysis (PCA) is introduced by reconsidering statistical degrees of freedom in spatial dimensions based on spatial auto-correlations. In the conventional PCA, data points that represent equal areas are assumed to have equal amount of information. In our new method, the intensive variability extraction (IVE), data points correlated with less other data points are weighted more before performing PCA. Hence, variability with independent information is emphasized, even if the variability is confined to small areas.Sea surface temperature (SST) data at each grid in the tropics are shown to have fewer spatial statistical degrees of freedom than that in the extratropics. Tropical SSTs exhibit covariability with large areas, because oceanic equatorial waves and atmospheric gravity waves share temperature information with surrounding areas. As to the extratropics, grids along the western boundaries of oceanic basins are more independent than those in the east, following dynamical requirement of the Earth's rotation.Using IVE, climate modes that involve interscale covariability are extracted. IVE performed for the Pan-Pacific SSTs extracts the Pacific Decadal Oscillation assuming the aforementioned a priori dynamical expectation. Using extratropical SSTs, it is demonstrated that IVE detects synchronicity of small-scale variability between distant narrow regions.