Observed near-inertial waves and shears east of Luzon during Typhoons Mangkhut and Yutu in 2018

Abstract

Based on acoustic Doppler current profiler observations from three moorings along the 18° N latitude line east of Luzon (at 122.7° E, 123° E, and 123.3° E), the characteristics, vertical propagation, modes, and shear properties of the near-inertial waves (NIWs) induced by Typhoon Mangkhut and Typhoon Yutu in 2018 were investigated. Energetic NIW events were generated that dominated ocean currents after the passage of the typhoons. The wavenumber–frequency spectra and downward/upward current decomposition demonstrated that the near-inertial currents rotated clockwise and mainly propagated downward energy in the Northern Hemisphere. The estimated phase velocities and vertical wavelengths were 4.9–9.4 m/h and 155–376 m, respectively. The NIWs at the three moorings showed red-shifted frequencies in the upper ocean but blue-shifted frequencies in the deep ocean, which were affected by background vorticity. Vertical mode analysis showed that the near-inertial kinetic energy (NIKE) was dominated by the third, fourth, and fifth higher modes above 400 m and by the first, second, and third lower modes below 400 m. The translation speeds of Typhoons Mangkhut (29 km/h) and Yutu (24 km/h) exceeded the critical value (14.4 km/h). Since the intensity of NIWs decreases with an increase in typhoon translation speed, the maximum NIKE caused by Typhoon Yutu was higher than that caused by Typhoon Mangkhut because of the lower translation speed of Typhoon Yutu. However, the NIKE caused by Typhoon Mangkhut propagated vertically into a deeper layer than that caused by Typhoon Yutu. The strong anticyclonic eddy near the moorings during the period of Typhoon Mangkhut played a more important role in the vertical propagation of NIKE than the weaker anticyclonic eddy during the period of Typhoon Yutu. This study shows the significant contribution of NIWs to shear. NIW-induced near-inertial shear was significantly enhanced after the two typhoons passed, with contributions of more than 50%. Although NIW-induced shear appeared to be stronger during the typhoons, no strong shear instabilities or turbulent mixing were observed.