Estimation of Propagation Speed and Direction of Nonlinear Internal Waves from Underway and Moored Measurements

Seung-Woo Lee,Sunghyun Nam

doi:10.3390/jmse9101089

Abstract

Propagation speed and direction of nonlinear internal waves (NLIWs) are important parameters for understanding the generation and propagation of waves, and ultimately clarifying regional ocean circulation. However, these parameters cannot be directly measured from in-situ instruments, but can only be estimated from post-processing in situ data. Herein, we present two methods and an optimal approach to estimate the propagation speed and direction of waves using underway and moored observations. The Doppler shift method estimates these parameters from apparent observations concerning a moving ship using the Doppler shift induced by the changing relative distance of the NLIWs from the moving ship. The time lag method estimates the parameters using the distance between two locations of the NLIW observed at different times and the time lag. To optimize the speed and direction of NLIWs, the difference in the propagation direction independently estimated by the two methods needs to be minimized concerning the optimal propagation speed to yield the optimal propagation direction. The methods were applied to two cases observed in the northern East China Sea in May 2015 and August 2018. This study has practical significance for better estimating the propagation speed and direction of NILWs particularly over a broad continental shelf.

Highlights

Nonlinear internal waves (NLIWs) are ubiquitous in stratified seas and are accompanied by isopycnal fluctuations with a sharp vertical density gradient
We present a novel method to estimate the propagation speed and direction of NLIWs using widely collected underway and moored observations, and the results of applying the method to two cases of NLIWs observed in May 2015 (SAVEX15) and August 2018 (IORS18)
The propagation direction of NLIWs was estimated with respect to a moving ship using the Doppler shift relationship

Summary

Introduction

Nonlinear internal waves (NLIWs) are ubiquitous in stratified seas and are accompanied by isopycnal fluctuations with a sharp vertical density gradient. They play an important role in underwater acoustics, regional circulation, local biogeochemistry, and energetics, mostly via vertical mixing in the stages of generation, propagation, evolution, and dissipation. Estimating the propagation speed and direction can be important for assessing regional ocean circulation, biogeochemical cycles, energetics, underwater acoustics, and the dynamics of NLIWs. Methods to estimate the propagation speed and direction have been suggested but are mostly limited by sampling strategies that have not yet been validated. Methods to estimate the propagation speed and direction have been suggested but are mostly limited by sampling strategies that have not yet been validated. Eng. 2021, 9, 1089 the method to two cases of NLIWs observed in the northern ECS in May 2015 and August 2018 (Figure 1)

Data and Processing

Methods

Time Lag Method

Estimation of Propagation Speed and Direction

Estimation of Propagation Direction Using Satellite Images

Findings

Concluding Remarks