Internal waves are gravity waves within a stably stratified ocean. The frequency of the near inertial internal wave (NIW) is close to the lower bound of the internal wave frequency, and has the general characteristics of the internal wave.
In a new study published in SCIENCE CHINA Earth Sciences, researchers report that NIWs induced by sequential typhoons transmitted with different frequency.
NIWs excited by the large-scale wind stress are commonplace phenomena. While there has been much work on the study of NIWs induced by a single typhoon, NIWs induced by sequential typhoons and the interaction between the resulting NIW have been rarely studied. In the study the basic characteristics and energy propagation processes for the different NIW induced by these typhoons are studied, and the factors leading to the frequency shift of the two induced NIW are quantitatively analyzed.
Both typhoons generated strong NIWs as detected by a moored array, with the near-inertial velocity to the right of the typhoon path significantly larger than to the left. The vertical phase and group velocities of the NIW induced by Typhoon Nalgae are lower than those of Typhoon Nesat, with the corresponding vertical wavelength only one-half that of Nesat. Near-inertial kinetic energy reach water depths of 300 and 200 m for Typhoons Nesat and Nalgae, respectively, illustrating that the Near-inertial kinetic energy induced by Typhoon Nesat dissipated less with depth. The frequency of NIW induced by Typhoon Nesat significantly increases at water depths of 100-150 m because of Doppler shifting, but decreases significantly at water depths of 100-150 m for Nalgae because of the greater influence of the background vorticity during the passage of Typhoon Nalgae.
In order to calculate the Doppler shift resulting from low-frequency fluctuations in the background field, researchers used Butterworth low-pass filter and empirical mode decomposition to extract low-frequency fluctuations current velocity. And used the optimal model to extract the optimal low-frequency fluctuations. They provide a new practical method for calculating the internal wave Doppler shifted by low-frequency fluctuations.
This work was supported by the National Natural Science Foundation of China (Grant Nos. 41676008 & 40876005), the National Key Research and Development Program of China (Grant No. 2016YFC14001403), and the National Program on Global Change and Air-Sea Interaction (Grant No. GASI-IPOVI-04).
See the article: Ma Y, Zhang S, Qi Y, Jing Z. 2019. Upper ocean near-inertial response to the passage of two sequential typhoons in the northwestern South China Sea. Science China Earth Sciences, 62: 863-871, https://doi.org/10.1007/s11430-018-9292-3
Science China Earth Sciences