Examining Modulations of Internal Tides within An Anticyclonic Eddy Using a Wavelet-Coherence Network Approach

Abstract

Interactions between internal tides and mesoscale eddies are an important topic. However, examining modulations of internal tides inside a mesoscale eddy based on observations is difficult due to limited observation duration and inaccurate positioning within the eddy. In order to overcome these two practical limitations, we use the active navigation capability of underwater gliders to conduct measurements inside the targeted eddy and utilize the wavelet approach to investigate modulations of internal tides with diurnal and semidiurnal periods inside the eddy. Based on the wavelet’s frequency–time locality, we construct scale-specific networks via wavelet coherence (WC) from multivariate timeseries with a small sample size. The modulation of internal tides is then examined in terms of temporal evolutionary characteristics of the WC network’s topological structure. Our findings are as follows: (1) the studied eddy is vertically separated into two layers, the upper (<400 m) and lower (>400 m) layers, indicating that the eddy is surface intensified; (2) the eddy is also horizontally divided into two domains, the inner and outer centers, where the modulation of internal tides seems to actively occur in the inner center; and (3) diurnal internal tides are more strongly modulated compared to semidiurnal ones, indicating the influence of spatial scales on the strength of interactions between internal tides and eddies.

Intrinsic Mode-Based Network Approach to Examining Multiscale Characteristics of Sea Surface Temperature Variability

“…In this study, we devised a new approach for examining the multiscale characteristics of oscillatory oceanic properties, such as SSTA variability, that is based on the combination of two well-known methods, such as EMD and complex network theory. Although our wavelet-based network methodology shows a noticeable performance in examining the variability of internal tides inside a mesoscale eddy [50], there is still a shortcoming in eliminating spurious harmonics due to a strong nonstationary behavior. Our new EMDbased approach can avoid spurious harmonics due to an abrupt frequency modulation [33].…”