Near-inertial waves advected by the Kuroshio from observation and simulation

초록

Advection of near-inertial waves (NIWs) by background currents has been predicted using numerical simulations, while supporting in-situ observations were rare. A pair of tall current moorings, deployed around 26°N and 125°E in Okinawa Trough from June 2015 to June 2016, provides a clear evidence exhibiting NIWs advected by the Kuroshio. Energetic NIWs are observed after two-typhoon passages in early August and late September 2015. Both typhoon passages occurred across the Kuroshio east of Taiwan upstream of the observation sites about 250 km southwestward with a difference of about 2-degree in latitude. One site (KCM1), relatively close to the center of the Kuroshio axis, exhibits more energetic NIWs than near the edge of the Kuroshio (KCM2), ~40 km away from KCM1. The frequency of NIWs is shifted to the lower band from the local inertial frequency (0.91f), inferring propagation from the south. A data-assimilating three-dimensional numerical model simulates that local NIW generation is weak at observation sites during and after typhoon passages, while its generation in the Kuroshio upstream is energetic. In addition, the model simulates patterns and characteristics of the advecting NIWs consistent with the observations, strongly supporting that the observed features of NIWs are caused by their advection to the northeastward by the Kuroshio.