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The tidal data of Kushimoto and Uragami on flow path of Kuroshio from 2004 to 2005 are investigated and discussed by time-frequency methods and nonlinear methods in this paper. These analyzing methods based on mathematical science show us new findings about the tidal motion observed on Kuroshio flow path. On the time-frequency analysis, 12 hours component and 24 hours component swing during the period of 350 hours and 320 hours respectively. However, any remarkable differences or changes depending on Kuroshio flow path weren’t seen on the result of time-frequency analysis. On the nonlinear analysis, a periodical structure has seen on the mutual information of tidal difference data, while Kuroshio flow is stable. In addition, the mutual information showed a characteristic of randomness and irregularity, while Kuroshio flow is unstable. The important results brought us a new finding such as classification of tidal motion regardless of the flow path of Kuroshio.

Kuroshio is one of the biggest circulations of the North Pacific Ocean. Kuroshio is a large current started from Eastern Luzon Islands to Bhoso Peninsula or Sanriku coast through southern coast of the Japanese Islands. The width of Kuroshio is about 100 km long, and the drift speed is 2 to 4 knots. Kuroshio effects not only to the ship navigation route but also for climate of southern coast of Japan. In addition, Kuroshio flow path often changes and it is set by monitoring of surface temperature that is based on ocean buoys, satellite remote sensor and sea current data from the ocean research ship [

When Kuroshio greatly meanders in the Japanese southern coast, the route doesn’t recover and this meandering continues about a year. This influences on the amount of fish catches and their kind. If Kuroshio flows near the coast line of Kii Peninsula, the sea level rises. Therefore, it may cause a sea disaster by its high tide. The tide level is observed at more than 150 measurement points of tide gauge stations located in all over Japan and its data are opened to the public online system. Among them, stations located at Kushimoto and Uragami in Kii Peninsula are the most important stations for monitoring Kuroshio because the flowing path of Kuroshio can be roughly estimated by analyzing the tidal data observed at Kushimoto and Uragami.

The tide level difference between Kushimoto and Uragami has been discussing about flowing path of Kuroshio by many researchers [

In this paper, the characteristics of tidal motion are discussed by using the methods of mathematical science such as time-series analysis and nonlinear analysis. These methods without moving average bring us new findings about the tidal motion observed on Kuroshio flow path. The new finding in this paper is that the tidal motion has two different aspects in detail. The mutual information representing the characteristics of tidal motion changes the aspect between stability and irregularities. This transition process shows a cyclic loop in the classification map. The loop looks like an orbital connected at a saddle point between the side of meandering path of Kuroshio and the side of straight path.

As shown in

System which is provided by the Japan Oceano-graphic Data Center (JODC) [

Beginning of meandering | Ending of meandering | Duration time |
---|---|---|

Aug. 1975 | Mar.1980 | 4 years and 8 months |

Nov. 1981 | May. 1984 | 2 years and 7 months |

Dec. 1986 | Jul. 1988 | 1 year and 8 months |

Dec. 1989 | Dec. 1990 | 1 year and 1 month |

Jul. 2004 | Aug. 2005 | 1 year and 2 months |

references [

Six specific months during 2004 to 2005 are selected to discuss the tidal motion in this paper.

The tidal difference has a component of 12 hours as shown in

and has sub peaks between them. This feature looks like one of

Tidal data observed from 2004 to 2005 at Kushimoto and Uragami were analyzed by

time-frequency methods and nonlinear methods. On time-frequency analysis, 12 hours component and 24 hours component swing during the period of 350 hours and 320 hours respectively. However, any remarkable differences or changes between straight path and meandering path of Kuroshio weren’t seen on the result of time-frequency analysis. On the tidal difference data between Kushimoto and Uragami, the power spectrum has 1/f spectrum and a component of 12 hours period. Nonlinear analysis revealed a periodical structure of tidal difference that is seen in a fluctuation of mutual information while Kuroshio flow is stable. The mutual information while Kuroshio flow is unstable shows a character of randomness and irregularity of tidal motion. However, these results show that there is a stable period and unstable period regardless of the path of Kuroshio. In this research, not only bias effect of tidal motion shown by moving average of the tidal difference data but the tidal motion of a regularity and periodicity and also that of an irregularity and randomness were reviled by time-frequency methods and nonlinear methods.

Kirimoto, K. (2016) Time-Frequency and Nonlinear Analysis of Tidal Data Observed on the Kuroshio Path. International Journal of Modern Nonlinear Theory and Application, 5, 147-159. http://dx.doi.org/10.4236/ijmnta.2016.54015