Intermittent Displacement of the Kuroshio Extension Front from the Current Axis Revealed from Simultaneous Observations By Three Research Vessels and an Eddy-Resolving Ocean General Circulation Model

Abstract:

Intensive observations of the ocean and atmosphere were conducted in the Kuroshio Extension (KE) region during 2-6 July in 2012, as parts of the research project "Hot Spot in Climate System" in Japan. Three research vessels were placed along 143E across the KE and carried out simultaneous observations. For the ocean, they conducted XCTD observations across the KE at about 9 locations every 0.25 degee in latitude once a day for 6 days in a row. The present study analyzes the XCTD observations and also the North Pacific hindcast simulation by OFES (OGCM for the Earth Simulator), to explore how the KE and its accompanying front vary on day-to-day timescales and what relationships there are between the changes in the KE and front.

The observations show that the mean axis of the density front deviates to the north from the mean current axis above the main pycnocline by about 20-30 km, due to the KE-carrying warm water, indicating that the surface KE current is basically anchored by the fronts below the main pycnocline. The changes in the current and front axes are characterized by a vertically uniform displacement; they both moved northward from 2nd to 5th by about 35km, and then slightly turned back southward afterwards. The front considerably deviated to the north of the current axis in the upper 200m depth from 5th to 6th when the KE started turning to the south, with warm Kuroshio water spread to the north across the current axis. The OFES shows the similar northward displacements of the front, which occur at the time when the KE veers away to the south. The meridional displacement of the KE is associated with the passing of mesoscale frontal waves propagated from the upstream and has a periodicity of about 7-15 days. The OFES suggests that the KE-carrying warm water overshoots to the north and departs from the current axis at the crest of frontal waves, causing the front to move northward above the main pycnocline.