Local inertial oscillations in the surface ocean generated by time-varying winds

Abstract

A new relationship is presented to give a review study on the evolution of inertial oscillations in the surface ocean locally generated by time-varying wind stress. The inertial oscillation is expressed as the superposition of a previous oscillation and a newly generated oscillation, which depends upon the time-varying wind stress. This relationship is employed to investigate some idealized wind change events. For a wind series varying temporally with different rates, the induced inertial oscillation is dominated by the wind with the greatest variation. The resonant wind, which rotates anticyclonically at the local inertial frequency with time, produces maximal amplitude of inertial oscillations, which grows monotonically. For the wind rotating at non-inertial frequencies, the responses vary periodically, with wind injecting inertial energy when it is in phase with the currents, but removing inertial energy when it is out of phase. The wind rotating anticyclonically with time is much more favorable to generate inertial oscillations than the cyclonic rotating wind. The wind with a frequency closer to the inertial frequency generates stronger inertial oscillations. For a diurnal wind, the induced inertial oscillation is dependent on latitude and is most significant at 30°. This relationship is also applied to examine idealized moving cyclones. The inertial oscillation is much stronger on the right-hand side of the cyclone path than on the left-hand side (in the northern hemisphere). This is due to the wind being anti-cyclonic with time on the right-hand side, but cyclonic on the other side. The inertial oscillation varies with the cyclone translation speed. The optimal translation speed generating the greatest inertial oscillations is 2 m/s at the latitude of 10°and gradually increases to 6 m/s at the latitude of 30°.

Near‐Inertial Waves and Their Underlying Mechanisms Based on the South China Sea Internal Wave Experiment (2010–2011)

“…Based on the best track data from CMA, the translation speed of Meranti around 0000 UTC + 8 9 September was approximately 3.6–4.8 m/s, whereas that of Megi around 0000 UTC + 8 22 October was approximately 2.3–3.1 m/s. According to S. Chen et al (), the intensity of NIWs increases with typhoon translation speed if the translation speed is smaller than a critical value and falloff otherwise. At 21°N, the critical value is approximately 4–5 m/s.…”

“…An interesting phenomenon is found in which the weaker typhoon Meranti generates stronger NIWs than the stronger typhoon Megi. By analyzing the typhoon translation speed using the best track data from CMA, the larger translation speed of Meranti may be a cause of the phenomenon, according to S. Chen et al (). Guan et al () also suggested that Megi could reinforce the D1 energy in the surface layer, which affected the energy imported to the NIWs and may be another possible cause of this phenomenon.…”

Evolution of Oceanic Near-Surface Stratification in Response to an Autumn Storm

“…Generation of turbulence by shear across the TL is particularly efficient when the rate at which near-surface winds rotate due to the motion of a storm, matching the inertial period of the wind-driven currents (Large and Crawford 1995;Dohan and Davis 2011;Chen et al 2015). This resonance condition allows large shears to build at the base of the well-mixed layer, which can lead to the growth of a stratified shear layer below the WML, without significant impact on the thickness of the WML (Dohan and Davis 2011;Johnston et al 2016;Chen et al 2016).…”

Correlation of Near-Inertial Wind Stress in Typhoon and Typhoon-Induced Oceanic Near-Inertial Kinetic Energy in the Upper South China Sea

“…For a specified stratification and fixed latitude, the maximum of the internal wave power always presents an optimal turn with the speed of the hurricane. Chen et al [18] subsequently pointed out that the oceanic near-inertial response arises when the wind speed increases or with a closer frequency to the inertial frequency at a fixed point. Meanwhile, the amplitude of oceanic inertial oscillation increases with increasing translation speed, due to the higher local rotation frequency, but a faster cyclone has shorter influence duration, which may lead to less oceanic near-inertial energy induced in the ocean.…”

“…The rightward bias is usually attributed to the resonance effect via the air-sea interactions, i.e., the wind stress vectors on the right (or left) of the cyclone track turn clockwise (or anticlockwise) and resonate with typhoon-generated oceanic currents, resulting in more (or less) intensive kinetic energy generated in the upper ocean. When the rotation rate of resonant near-inertial oceanic currents matches the local inertial frequency, the maximal amplitude of inertial oscillations is generated [18,24,25].…”