Mesoscale-dependent near-inertial internal waves and microscale turbulence in the Tsushima Warm Current

Abstract

This study examined characteristics of near-inertial internal waves (NIWs) associated with the background mesoscale field near the Tsushima Warm Current. Observational stations off Sado Island were visited recurrently to assess spatiotemporal changes of fine-scale and microscale properties of seawater. Also, NIWs were inspected in terms of relative vorticity and total strain in surface geostrophic motion. During summer expeditions in 2019, current and hydrographic surveys at the rim of an anticyclonic eddy provided clear evidence of downward-travelling NIWs, which were most amplified near the depth of lower pycnocline. The amplification of NIW coincided with elevation in the dissipation rates of turbulent kinetic energy (TKE) and microscale variation of temperature. The fall 2019 expedition found details of wave and turbulence properties associated with the mesoscale structure of paired vortices, where a cyclone and anticyclone were, respectively, adjacent to the east and west. Amplified signals of NIW-related vertical shear and TKE dissipation were found at isopycnals between the dipole cores. From the theoretical perspective of internal wave, the baroclinic term attributable to vertical shear of geostrophic current was interpreted as inducing downward travel of NIW through the lower pycnoclines between the dipole cores. It is also noted that cyclones, passing through the central part of Sea of Japan, can deliver kinetic energy into 10-km scale internal waves as a consequence of interaction between easterly wind and mountainous topography in the Honshu Island.

Amplification of typhoon-generated near-inertial internal waves observed near the Tsushima oceanic front in the Sea of Japan

“…During the shipboard survey near the FATO station in the late October, 2019, we indeed observed apparently weak signals of NIKE within the CE mesoscale structure compared to those in the ambient waters (Fig. 6 of Kawaguchi et al 18 ).…”

“…The FATO mooring station was situated at the geographical point (38.72°N, 137.83°E), roughly 10 km northwest relative to the Sado Island (a mark in Fig. 1 a–c) 15 , 18 . The full water depth at the site is 1770 m.…”

“…That is, the lower propagation limit can be more strictly expressed as follows: where is the gradient Richardson number; is the buoyancy frequency; the gravitational acceleration; is vertical axis pointing upward; and is the density of seawater, which represents the inclination of isopycnal interface 17 . According to their shipboard observations of internal waves and microscale turbulence in the TOF region, Kawaguchi et al (2021) 18 demonstrated that the sloping isopycnals created geostrophic shear and limited the propagation of near-inertial waves toward the deeper layers. From a viewpoint of the wave kinetic energy distribution under the sea, it is worth evaluating the roles and impacts of the baroclinicity of the TOF as a function of the typhoon-induced near-inertial waves.…”

“…In the regions proximate to the Korean Peninsula, the previous studies revealed that the tidal forcing dominantly generated the internal waves and influenced the local turbulent mixing 19 , 20 . In the central part of Sea of Japan, the barotropic tidal currents are negligibly small 6 , 15 , 18 , 21 and are less likely subject to critical corruptions in the current data records by tides. In this sense, the TOF region is an ideal domain for the study of the interaction between oceanic front and wind-induced near-inertial waves.…”

Development of species‐specific multiplex real‐time PCR assays for tracing the small pelagic fishes of North Pacific with environmental DNA

“…Historically, the Atmosphere and Ocean Research Institute (AORI) of the University of Tokyo conducts regular cruises year‐round in the North Pacific. In collaboration with the Japan Agency for Marine‐Earth Science and Technology (JAMSTEC), two vessels, Shinseimaru and Hakuomaru, conduct short‐ and long‐distanced research voyages (Hirai & Tsuda, 2015; Kawaguchi et al, 2021; Nishioka & Obata, 2017; Otosaka, 2017; Shiozaki et al, 2016). Combining the recent advance on eDNA and metabarcoding technologies, the AORI has launched a project called “Ocean DNA” to map the marine lives in the North Pacific.…”