春日 悟 (カスガ サトル)
| 地球環境科学研究院 統合環境科学部門 環境地理学分野 | 博士研究員 |
研究者基本情報
■ URLresearchmap URLホームページURL■ ID 各種研究者番号ORCID IDJ-Global ID■ 研究キーワード・分野研究キーワード研究分野研究活動情報
■ 論文MJO phases and cold surges impact on the development of the Borneo vortex: application of the cutoff low detection scheme to the Borneo vortex in the tropicsMiki Hattori; Satoru Kasuga; Jun Matsumoto
Journal of the Meteorological Society of Japan, 2026年07月07日
英語, 研究論文（学術雑誌）,
Abstract The Borneo vortex (BV) is a cyclonic circulation that strongly influences convection and rainfall over the South China Sea and Maritime Continent. This study investigated the frequency, intensity and dynamical characteristics of BVs using 40-year boreal winter (1980–2020) reanalysis data to clarify the respective and combined influences of MJO phases and cold surges (CS) on BV occurrence. BVs were objectively identified using a tropical vortex detection scheme based on a modified mid-latitude cutoff-low detection method. BV frequency was highest in phase 5, with nearly half accompanied by CS. BV intensity was strongest in phases 4, 5, and 8 for the CS cases, and in phases 5 and 6, with phase 8 also relatively strong, for the no-CS cases, indicating strong MJO influence during phases 4–6, while relatively strong BVs still occurred in phase 8 despite suppressed convection. In phases 5 and 8, strong BV-related vorticity was associated with CS northerlies (or easterlies in the no-CS cases) together with MJO-related westerlies in phase 5 and southerly flow from the Southern Hemisphere in phase 8. BV locations closely followed the latitude of the westerly wind band in phase 5. Vorticity budget analysis showed that strong BVs were associated with positive planetary vorticity advection and enhanced vertical stretching. Positive planetary vorticity advection extended farther south in southern-type BVs, whereas relative vorticity stretching dominated, particularly in phase 8 under CS conditions. These results highlight the important roles of phase-dependent MJO circulation, cold surges, and associated low-level flow structures in controlling BV occurrence. Spatial structure of rain pool observed on the Kuroshio and Baiu fronts in the East China SeaTetsutaro Takikawa; Yoshihiro Tachibana; Hirohiko Nakamura; Ayako Nishina; Atsuyoshi Manda; Hatsumi Nishikawa; Satoru Kasuga; Yuta Ando; Teruyuki Kato
Journal of Oceanography, 2025年10月
英語, 研究論文（学術雑誌）,
Abstract Spatial three-dimensional structure of low-salinity water on the Baiu and Kuroshio fronts is shown using intensive observation results by three ships in the East China Sea. The surface low-salinity pool along the Kuroshio front was formed by rainfall of the Baiu front during the observations. The horizontal distribution of the low-salinity pool with about 130 km long, 40 km wide, and 9–12 m thick along the Kuroshio front is consistent with the precipitation map of the weather radar. The relationship between the surface low-salinity water and precipitation was also quantitatively explained by the conservation of volume. Barrier layer below the surface rain pool had a thickness from several meters to a maximum of 46 m. Subsurface (~ 20-m depth) low-salinity patches were observed below the barrier layer. Based on the order of vertical flow, it is considered that the low-salinity patches moved from the sea surface to the subsurface layer in about 1 day, and their origin is presumed to be rainfall several days earlier. Below the low-salinity patches, we detected temperature inversion layer with warm and saline water, which is thought to have an effect similar to the barrier layer. Influence of the Kuroshio System and Its Large Meander on the Incidence and Interannual Variation in Lightning StrokesMisa Kato; Yoshihiro Tachibana; Satoru Kasuga; Hiroko Sato; Erina Shigeta; Michihiro Matsui
Journal of Climate, 2025年07月15日
研究論文（学術雑誌）,
AbstractWarm western boundary currents are associated with increased convective precipitation events, which are, in turn, conducive to the occurrence of lightning. The East Asian monsoon regions, which experience high rainfall, also experience high levels of lightning. Japan is unique in that both of these factors affect the frequency of lightning. However, despite the high population density and potential for natural disasters, the spatiotemporal variations in lightning occurrence around the country have not been clarified to date. Here, we show that the incidence of lightning is influenced by the Kuroshio path system, which conveys tropical heat to the midlatitude regions of the western Pacific and ranks among the world’s strongest warm ocean currents. Specifically, we show that the Kuroshio large meander affects both the mean annual frequency and the interannual variation in lightning occurrence. Lightning is more likely to occur in areas with high sea surface temperatures (SSTs) than in areas with relatively low SSTs. Warm oceans, which supply large amounts of water vapor and heat, favor the development of lightning. The abundance of winter lightning, another peculiarity of Asian winter monsoon regions, is also linked to the Tsushima Warm Current, which flows through the Sea of Japan and has its source in the Kuroshio. The temperature difference between the sea surface and the midtroposphere, an indicator of potential instability associated with vertical deep mixing, is also a good predictor for lightning occurrence and the interannual variations therein. Similar relationships may exist in other oceans, particularly in other western boundary currents.
Significance Statement
The Kuroshio, which conveys tropical heat to the midlatitude western Pacific, ranks among the world’s strongest warm ocean currents. The Kuroshio has a significant influence on the weather and climate of not only East Asia but also on regions as distant as North America. Since 2017, the Kuroshio has exhibited a large meander, which has led to an unprecedented anomalous northward shift since 2023, potentially altering climate and lightning patterns. Despite lightning having a high potential for causing disasters, the meander’s impact has been largely ignored. We show that warm oceanic areas associated with the meander have increased lightning activity, suggesting that ongoing World Ocean warming could trigger an increase in lightning-related disasters.
Seamless Detection of Cutoff Lows and Preexisting TroughsSatoru Kasuga; Meiji Honda; Jinro Ukita; Shozo Yamane; Hiroaki Kawase; Akira Yamazaki
Monthly Weather Review, 2021年06月22日
研究論文（学術雑誌）,
AbstractWe propose a new scheme based on geopotential height fields to detect cutoff lows starting in the preexisting trough stage. The intensity and scale derived from the proposed scheme will allow for a better understanding of the cutoff low life cycle. These cutoff lows often accompany mesoscale disturbances, causing adverse weather-related events, such as intense torrential rainfall and/or tornadoes. The proposed scheme quantifies the geometric features of a depression from its horizontal height profile. The height slope of a line intersecting the depression bottom and the nearest tangential point (optimal slope) locally indicates the intensity and scale of an isolated depression.The strength of the proposed scheme is that, by removing a local background height slope from a geopotential height field, the cutoff low and its preexisting trough are seamlessly detected as an identical depression. The distribution maps for the detected cutoff lows and preexisting troughs are illustrated along with their intensities, sizes, and local background flows estimated from snapshot height fields. We conducted climatological comparisons of cutoff lows to determine the utility of the proposed scheme.
