A Generation Mechanism for Mesoscale Eddies in the Kuril Basin of the Okhotsk Sea: Baroclinic Instability Caused by Enhanced Tidal Mixing

Mesoscale eddies, particularly anticyclonic ones, are dominant features in the Kuril Basin of the Okhotsk Sea. In 1999, both surface drifter and hydrographic observations caught the same anticyclonic eddy northwest of Bussol’ Strait, which has a diameter of ∼100 km, typical surface velocity of 0.2–0.3 m s⁻¹, and less dense core extending to a depth of ∼1200 m. Based on an idea that the generation of mesoscale eddies is caused by strong tidal mixing in and around Kuril Straits, we have conducted a series of three-dimensional numerical model experiments, in which strong tidal mixing is simply parameterized by increasing coefficients of vertical eddy viscosity and diffusivity along the eastern boundary. Initially, a regular series of disturbances with a wavelength of ∼70 km starts to develop. The disturbances can be clearly explained by a linear instability theory and regarded as the baroclinic instability associated with the near-surface front formed in the region between the enhanced mixing and offshore regions. In the mature phase, the disturbances grow large enough that some eddies pinch off and advect offshore (westward), with the scale of disturbances increasing gradually. Typical eddy scale and its westward propagation speed are ∼100 km and ∼0.6 km day⁻¹, respectively, which are consistent with the observations by satellites. The westward propagation can be explained partly due to nonlinear effect of self-offshore advection and partly due to the β-effect. With the inclusion of the upper ocean restoring, the dominance of anticyclonic eddy, extending from surface to a depth of ∼1200 m, can be reproduced.     

New buoy observation system for tsunami and crustal deformation

We have developed a new system for real-time observation of tsunamis and crustal deformation using a seafloor pressure sensor, an array of seafloor transponders and a Precise Point Positioning (PPP ) system on a buoy. The seafloor pressure sensor and the PPP system detect tsunamis, and the pressure sensor and the transponder array measure crustal deformation. The system is designed to be capable of detecting tsunami and vertical crustal deformation of ±8 m with a resolution of less than 5 mm. A noteworthy innovation in our system is its resistance to disturbance by strong ocean currents. Seismogenic zones near Japan lie in areas of strong currents like the Kuroshio, which reaches speeds of approximately 5.5 kt (2.8 m/s) around the Nankai Trough. Our techniques include slack mooring and new acoustic transmission methods using double pulses for sending tsunami data. The slack ratio can be specified for the environment of the deployment location. We can adjust slack ratios, rope lengths, anchor weights and buoy sizes to control the ability of the buoy system to maintain freeboard. The measured pressure data is converted to time difference of a double pulse and this simple method is effective to save battery to transmit data. The time difference of the double pulse has error due to move of the buoy and fluctuation of the seawater environment. We set a wire-end station 1,000 m beneath the buoy to minimize the error. The crustal deformation data is measured by acoustic ranging between the buoy and six transponders on the seafloor. All pressure and crustal deformation data are sent to land station in real-time using iridium communication.     

Two records of patchy occurrences of the oceanic shrimpSergestes similis Hansen off the east coast of Honshu,Japan

Two recent records of the occurrences of the pelagic shrimp,Sergestes similis Hansen, are reported. They provide evidence of dense swarms ofS. similis in midwater off the east coast of Honshu, Japan, in spring. On one occasion,S. similis was found in a stomach of the basking shark,Cetorhinus maximus (Gunnerus). The shark, 8.11 m in total length, was accidentally trapped in a set net off Izu-oshima Island on April 18, 1974, and its foregut was filled only withS. similis. Feeding and swimming behavior of the basking shark, condition of the gut contents, and temperature profiles of the sea revealed thatS. similis was preyed upon by the shark near the island when the shrimp swarmed below 100 m depth at night. On another occasion,S. similis was fished by a commercial ground trawler 31 miles off Cape Shioyazaki on April 4 and 5 and off Hitachi on April 11, 1976. The trawlings were made around 500 m depth at daytime, and a total of 3.8 tons ofS. similis was caught and marketed. The size-frequency distribution and sex ratio ofS. similis in both occurrences from off Izu-oshima Island and off Cape Shioyazaki coincided very well. The samples consisted of identical population of probably 2nd year class having a body length mode between 46.0 and 50.0 mm. The sex ratio was skewed greatly toward females in both samples, that is 38∶2 and 100∶3 respectively. These two phenomena are discussed in relation to the life history ofS. similis.