Variability of Kuroshio Velocity Assessed from the Sea-Level Difference between Naze and Nishinoomote

Abstract

Variations of current velocity of the Kuroshio are examined using the 1965-1983 sea-level difference between Naze and Nishinoomote, located on the offshore and onshore sides of the Kuroshio in the Tokara Strait south of Kyûshû.
Interannual variations of Kuroshio velocity are large, especially at periods longer than five years and around 2.1 years. They are almost determined by those of sea level on the offshore side of the Kuroshio.They are highly coherent with the offshore sea level at periods longer than 1.7 years, andincoherent with the onshore sea level at periods longer than 2.8 years.
The mean seasonal variation averaged for 19 years is at its maximum in July and at its minimum in the second half of October, with a sharp decrease in August and September. However, such a variation does not repeat every year. Amplitude, dominant period and phase are greatly differentby year, and they can be roughly divided into four groups: small-amplitude group, semiannual-period group, and two annual-period groups with different phases. The only feature found in almost all years is a weak velocity from September to December.
The amplitude of seasonal variation tends to be large in the formation years of the large meander (LM) of the Kuroshio and small during the LM period. It is also large in the years preceding El Nino, and diminishes remarkably in El Niño years.
Kuroshio velocity in the Tokara Strait is incoherent with position of the Kuroshio axis over the Izu Ridge, but highly coherent with 70-day variations of coastal sea levels which are dominant during the LM period.