Surface-Layer Circulations in Suruga Bay Induced by Intrusions of Kuroshio Branch Water

Takahiro Toyoda,Goro Yamanaka,Norihisa Usui,Yuma Kawakami,Hideyuki Nakano,Takaaki Katsumata,Kensuke K Komatsu,Kiyoshi Tanaka,Daisuke Takahashi,Masato Niki,Toru Miyama,Kunio Kutsuwada,Kei Sakamoto,Nariaki Hirose,L Shogo Urakawa

doi:10.3389/fmars.2021.721500

Abstract

The water mass structure in Suruga Bay is strongly influenced by open-ocean water. In particular, it is suggested that intermittent intrusions of the Kuroshio water generate characteristic circulations in the surface layer of the bay. In this study, we investigated the processes of the intrusions of open-ocean water into the bay and related generation of bay-scale cyclonic and anti-cyclonic circulation patterns. In doing so, we used an ocean simulation product with observational data constraint on meso and larger scales and with a resolution fine enough to resolve the smaller-scale intrusion structure. Cyclonic and anti-cyclonic circulation patterns as suggested by previous observational studies were detected as positive and negative first leading empirical orthogonal function (EOF) modes of the velocity field in Suruga Bay. The time scale of occurrences of these patterns was estimated as about 1 month, which was consistent with short-term Kuroshio fluctuations as reported in previous studies. Conditions favorable for generating these patterns were analyzed for three typical Kuroshio path periods individually. As suggested by previous studies, relatively strong northward flow to the west of Zeni-su generally promoted the open-ocean water intrusions into the eastern bay mouth, leading the cyclonic circulation in Suruga Bay. Our results showed that the correlation of this relation was significant for each Kuroshio path period. The open-ocean water intrusion increased the surface-layer temperature in Suruga Bay by about 0.7°C on average. On the other hand, the anti-cyclonic circulation pattern in Suruga Bay tended to be generated with relatively weak northward flow to the west of Zeni-su during the large meander Kuroshio path period, whereas this relation was rather weak during other periods. These results were mostly supported by available observations and would be useful for integrating our understanding of the influences of the western boundary current fluctuations on the circulation and temperature variations in proximal bays.

Highlights

Suruga Bay, located on the south side of the main island of Japan, is characterized by a widely open bay mouth and a deep bottom canal, the Suruga Trough, leading from the Nankai Trough (Figure 1)
The surface-layer flow patterns and related environmental fields as obtained in this study provided useful information for understanding the circulation variability and the variabilities in temperature and other properties in Suruga Bay
Reproduced SST and coastal sea level distributions during the events were consistent with available satellite and tide-gauge observation data

Summary

Introduction

Suruga Bay, located on the south side of the main island of Japan, is characterized by a widely open bay mouth (about 56 km width) and a deep bottom canal, the Suruga Trough (reaching >1,000 m depth near the bay head), leading from the Nankai Trough (Figure 1). Freshwater discharge mainly from the Fuji, Kano, Abe, and Oi Rivers influences the surface layer in particular near the western coast, where salinity is about 33.0–34.0 (e.g., Nakamura, 1972). The spreading of this low-salinity water varies with the discharge amount (seasonal maximum in summer) and surface wind stress pattern (e.g., Tanaka et al, 2009, 2010; Niki et al, 2011). Beneath the low-salinity water near the surface, warm and salty water originating in the Kuroshio occupies the main thermocline layer (e.g., Iwata et al, 2005). Under the Kuroshio water, there is a salinity minimum layer (

Methods

Results

Conclusion