Hindcast and predictability of sporadic Kuroshio‐water intrusion (kyucho in the Bungo Channel) into the shelf and coastal waters

Abstract

[1] Prerequisite(s) for ocean circulation models capable of hindcasting "kyucho" occurrence (a sudden coastal temperature rise induced by Kuroshio frontal waves) in the Bungo Channel, Japan, is investigated using long-term observed temperature and sea level time series, archived hydrographic data, and reanalysis data provided by the Japanese Coastal Ocean Predictability Experiment (JCOPE) group. Anticyclonic mesoscale eddies impinging on the Kuroshio front south of the Bungo Channel enhance the frontal sharpness, frontal wave growth, and activity of kyucho phenomena. A kyucho hindcast is carried out to examine the reliability of a numerical model including realistic anticyclonic eddies propagating south of Japan. The Finite Volume Coastal Ocean Model (FVCOM) for which boundary conditions are given by daily JCOPE2 reanalysis data is adopted in the present study. This numerical model does a reasonable job of hindcasting kyucho occurrence in 2003. It is therefore considered that forecasts of kyucho occurrence up to 2 or 3 months ahead are possible by using the FVCOM in conjunction with JCOPE2 forecast data.Citation: Isobe, A., X. Guo, and H. Takeoka (2010), Hindcast and predictability of sporadic Kuroshio-water intrusion (kyucho in the Bungo Channel) into the shelf and coastal waters,

Development of an operational coastal model of the Seto Inland Sea, Japan

“…Model studies are especially active near the Seto Inland Sea, and models with a horizontal resolution of 1-2 km have been presented as follows (see Fig. 2 for place names): Chang et al (2009) used a POM model with a horizontal resolution of 1 km to show seasonal variations of surface circulations within the Seto Inland Sea; Isobe et al (2010) and Isobe et al (2012) used a FVCOM model with a horizontal resolution of 1 km to forecast the Kuroshio water intrusion in the Bungo Channel; Miyazawa et al (2012) used a POM model with a horizontal resolution of 2 km (1/36 • ) to reproduce the frontal structure in the Kii Channel; and Kuroda et al (2013Kuroda et al ( , 2014) used a ROMS model with a horizontal resolution of 2 km to reveal circulations and formation processes of the winter mixed layer in the Tosa Bay.…”

Ensemble numerical forecasts of the sporadic Kuroshio water intrusion (kyucho) into shelf and coastal waters

“…The present study employs the finite volume coastal ocean model (FVCOM; Chen et al 2003) for resolving complex topography in shallow waters using triangular cell grids. In fact, a hindcast computation using JCOPE2 data as boundary conditions of the FVCOM well reproduces the kyucho events in 2003 (Isobe et al 2010) because mesoscale warm eddies impinging on the Kuroshio south of Japan trigger the kyucho occurrence and these eddies are well reproduced in the JCOPE2 analysis. In addition, the JCOPE2 model successfully forecasts Kuroshio meanders after removing the assimilation schemes incorporated into the model (Miyazawa et al 2005), and FVCOM computations using JCOPE2 forecast data as lateral boundary conditions are therefore expected to forecast kyucho events as they occur in the actual ocean.…”

“…However, it should be noted that kyucho events comparable to those in the first period occur in the midsummer and that the temperature rises by about 3°C twice during the period July through September. Isobe et al (2010) point out that the kyucho events in midsummer 2003 in the Bungo Channel were triggered by mesoscale warm eddies impinging on the Kuroshio south of the channel. Indeed, the SSH maps on days a, b, and c ( Fig.…”

Open and coastal seas interactions south of Japan represented by an ensemble Kalman filter

“…This assumption is acceptable for typical mesoscale variations such as the Kuroshio path variations and mesoscale eddies with temporal and spatial scales of O (10 days) and O (100 km), respectively (Kuragano and Kamachi 2000). However, recent efforts of downscale modeling have allowed us to simulate the open and coastal seas interactions with highly variable temporal and anisotropic spatial scales due to coexistence of multi-scales phenomena (Isobe et al 2010). In particular, the interactions between the currents and coastal topography (Miyama and Miyazawa 2010) enhance the anisotropic features of horizontal scales of the phenomena.…”

“…Recent development of downscaled models allows simulating smaller scales phenomena associated with the Kuroshio and coastal topography interactions (Miyama and Miyazawa 2010). The open and coastal seas interactions involve significant variations of temporal and spatial scales of the relevant phenomena (e.g., Isobe et al 2010), and the associated error covariance is also highly variable. Dynamic representation of error covariance by EnKF is hence required to effectively reproduce the open and coastal seas interactions.…”