09:15 〜 09:30
[ACG44-02] 日本南岸における黒潮大蛇行と沿岸循環の力学的相互作用
キーワード：黒潮大蛇行、下降流、伸張効果、潮岬、JCOPE-T
Introduction
The Kuroshio exhibits three typical path types: the typical large meander (LM) path, the nearshore non-large meander (nNLM) path and the offshore non-large meander (oNLM) path [Kawabe, 1985], exerting important impacts on the coastal marine environment.
Recently, the Kuroshio has been taking the LM path since September of 2017, being the longest-lasting LM in the past few decades. Observations have revealed that during the LM period, a westward countercurrent often occurred along the Enshunada on the shoreside of the meander. The westward countercurrent may interact with the Kuroshio LM near the Cape Shiono-misaki to have some important impacts on the dynamics of the LM, but it has not been investigated yet. Therefore, in this study, we investigate this interaction by using a realistic high-resolution simulation product, aiming at understanding the long-lasting maintenance of the current LM.
Data
A high-resolution data-assimilative product derived from the JCOPE-T (JAMSTEC: Japan Agency for Marine-Earth Science and Technology) [Varlamov et al., 2015] was used in this study. It has a horizontal resolution of 1/36 (~3km) and 46 generalized sigma layers in the vertical direction. This simulation product successfully reproduces the observed Kuroshio variability off the south coast of Japan in the recent years.
Results
It was found that a strong downwelling accompanied with a positive (anti-clockwise) vorticity on its downstream side often occurred near the Cape Shiono-misaki after the formation of the Kuroshio large meander (LM). This downwelling was caused by a strong horizontal convergence between the strong southwestward coastal current and the southeastward Kuroshio main stream near the Cape Shiono-misaki. The strong southwestward coastal current, which was regulated by the Kii Peninsular, was a part of the cyclonic (anti-clockwise) recirculation gyre formed within the Kuroshio LM. The southwestward coastal current impinged almost perpendicularly on the Kuroshio current. This explains why the downwelling near the Cape Shiono-misaki only occurred during the LM period. The strong horizontal convergence was generated in the upper layer, where the downward velocity increased with depth, thereby causing a stretching of the water column to generate the positive vorticity in the upper layer.
After occurring near the Cape Shiono-misaki, the downwelling area and the accompanied positive vorticity area propagated eastward on the Kuroshio axis one after another with a time scale of dozens of days. During this process, the accompanied positive vorticity developed the trough (anti-clockwise circulation) of the Kuroshio meander, and it sometimes induced the offshore detachment of a small eddy from the Kuroshio current. Therefore, it was concluded that the downwelling and the accompanied positive vorticity occurring near the Cape Shiono-misaki contribute to the maintenance and/or the development of the Kuroshio LM.
Reference
Kawabe, M. (1985) : Sea level variations at the Izu Islands and typical stable paths of the Kuroshio. Journal of the Oceanographical Society of Japan, 41 : 307–326.
Varlamov et al. (2015) : M2 baroclinic tide variability modulated by the ocean circulation south of Japan, Journal of Geophysical Research : Oceans, 120 : 3681– 3710.
The Kuroshio exhibits three typical path types: the typical large meander (LM) path, the nearshore non-large meander (nNLM) path and the offshore non-large meander (oNLM) path [Kawabe, 1985], exerting important impacts on the coastal marine environment.
Recently, the Kuroshio has been taking the LM path since September of 2017, being the longest-lasting LM in the past few decades. Observations have revealed that during the LM period, a westward countercurrent often occurred along the Enshunada on the shoreside of the meander. The westward countercurrent may interact with the Kuroshio LM near the Cape Shiono-misaki to have some important impacts on the dynamics of the LM, but it has not been investigated yet. Therefore, in this study, we investigate this interaction by using a realistic high-resolution simulation product, aiming at understanding the long-lasting maintenance of the current LM.
Data
A high-resolution data-assimilative product derived from the JCOPE-T (JAMSTEC: Japan Agency for Marine-Earth Science and Technology) [Varlamov et al., 2015] was used in this study. It has a horizontal resolution of 1/36 (~3km) and 46 generalized sigma layers in the vertical direction. This simulation product successfully reproduces the observed Kuroshio variability off the south coast of Japan in the recent years.
Results
It was found that a strong downwelling accompanied with a positive (anti-clockwise) vorticity on its downstream side often occurred near the Cape Shiono-misaki after the formation of the Kuroshio large meander (LM). This downwelling was caused by a strong horizontal convergence between the strong southwestward coastal current and the southeastward Kuroshio main stream near the Cape Shiono-misaki. The strong southwestward coastal current, which was regulated by the Kii Peninsular, was a part of the cyclonic (anti-clockwise) recirculation gyre formed within the Kuroshio LM. The southwestward coastal current impinged almost perpendicularly on the Kuroshio current. This explains why the downwelling near the Cape Shiono-misaki only occurred during the LM period. The strong horizontal convergence was generated in the upper layer, where the downward velocity increased with depth, thereby causing a stretching of the water column to generate the positive vorticity in the upper layer.
After occurring near the Cape Shiono-misaki, the downwelling area and the accompanied positive vorticity area propagated eastward on the Kuroshio axis one after another with a time scale of dozens of days. During this process, the accompanied positive vorticity developed the trough (anti-clockwise circulation) of the Kuroshio meander, and it sometimes induced the offshore detachment of a small eddy from the Kuroshio current. Therefore, it was concluded that the downwelling and the accompanied positive vorticity occurring near the Cape Shiono-misaki contribute to the maintenance and/or the development of the Kuroshio LM.
Reference
Kawabe, M. (1985) : Sea level variations at the Izu Islands and typical stable paths of the Kuroshio. Journal of the Oceanographical Society of Japan, 41 : 307–326.
Varlamov et al. (2015) : M2 baroclinic tide variability modulated by the ocean circulation south of Japan, Journal of Geophysical Research : Oceans, 120 : 3681– 3710.