[A-CG40] Dynamics of Oceanic and Atmospheric Waves, Vortices, and Circulations

[ACG40-P04] Scale dependence of horizontal diffusion coefficient in the Kuroshio region

*Kosei Komatsu¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:horizontal diffusion, GPS drifter, FSLE, Kuroshio

Particle transport experiments using numerical models are now a fundamental analytical method in fields such as biogeochemistry and fisheries oceanography. In transport experiments, it is common to parameterize the diffusion process, but the parameterization should be applied after careful consideration of its validity according to the spatial resolution of the model. Assuming surface quasi-geostrophic currents, the kinetic energy spectrum of the surface current is known to be Kolmogorov-type in the inertial subrange, proportional to the -5/3 power of the wave number, where the horizontal diffusion coefficient is proportional to the 4/3 power of the horizontal scale L (Richardson's 4/3 power law). On the other hand, it is also known from numerical experiments in the California Current region, where fronts and filamentary structures are prominent, that the kinetic energy spectrum of the surface current is proportional to the -2 power and the horizontal diffusion coefficient is proportional to the 3/2 power of L. Observational studies have accumulated GPS drifting buoy data and presented global maps of horizontal diffusion coefficients, but their horizontal scale dependence for each region is not well understood. Therefore, this study aimed to elucidate the actual scale dependence of horizontal diffusion coefficients at scales ranging from 100 m to 100 km in the Kuroshio region, which is a major transport site of various substances and organisms in the seas around Japan. The analysis used position data from three GPS drifting buoys (drogue depth: 15 m; Zeni Lite Buoy Co., Ltd.) deployed almost simultaneously (average distance between the buoys calculated from their first position transmitted by each buoy: 14 m) at 4:42 (UTC) on November 19, 2015 in the Kuroshio jet (surface current speed: 1.5 m/s) off Cape Ashizuri during the R/V Hakuho-maru KH-15-4 cruise. The positioning interval was 10 minutes for the first three days and 30 minutes thereafter, and data was used for approximately 20 days until 23:00 on December 8, when one of the three buoys lost communication. The buoys all drifted in the downstream direction of the Kuroshio Current at around 1.5 m/s and reached 143E off Joban after 9 days. Horizontal diffusion coefficients were calculated using the traditional method derived from the rate of change over time of the relative variance between buoys and using the finite scale Lyapunov exponent (FSLE). Horizontal diffusion coefficients estimated by both methods were almost identical and about one order of magnitude larger than those of previous studies at each scale, indicating strong dispersion in the Kuroshio region. The regression of the mean diffusion coefficients at each L estimated by FSLE using the least-squares method showed that the horizontal diffusion coefficients were almost proportional to the 4/3 power of L for 100 m < L < 3 km (-5/3 power type spectrum for kinetic energy) and almost proportional to the 3/2 power of L for 3 km < L (-2 power type spectrum).