Vertical structural variability of diurnal internal tides inside a mesoscale anticyclonic eddy based on single virtual-moored Slocum glider observations

Abstract

The vertical structural variability of the diurnal internal tide (DIT) with a mode-1 wavelength of ~420 km inside a mesoscale baroclinic anticyclonic eddy was examined based on observations by a single virtual-moored (VM) Slocum glider. During the glider observational period from 10 to 19 September 2018, the eddy traveled northward at approximately 50 km, allowing the glider to scan a cross section of 50 km wide and 800 m deep inside the eddy. VM observations showed that DIT experienced a noticeable vertical structural variability near the eddy center. In a range of 30 km horizontally from the eddy center (inner center), DIT’s vertical displacements were significantly intensified in the 400–800-m depth below the eddy. In the range of 30–50 km from the eddy center (outer center), DIT was almost uniformly distributed from the surface to 800-m depth. Owing to the spatiotemporally restricted dataset by the glider, the significance of DIT’s modulation observed inside the eddy can be questionable. As a result of comparing DIT’s vertical structural variability in two domains in terms of available potential energy (APE) and horizontal kinetic energy (HKE) using CTDs inside the eddy and ADCPs outside the eddy, DIT’s vertical structure inside the eddy was significantly distinguished from that outside the eddy. The relative vorticity inside the eddy was estimated based on the satellite dataset; it was negatively great in the inner center (approximately 0.35 – 0.25f) and small in the outer center (approximately 0.2 – 0.1f). These observational behaviors indicate a close relationship between them; the vorticity-dependent modulation of the DIT seems to be observationally confirmed inside the eddy. Further, in order to examine the energy transferring behavior in low vertical modes, a wavenumber spectral analysis was performed on the DIT displacements and the lowest four wavenumbers, Kz (1) through Kz (4), showed a similar behavior to those observed in DIT’s vertical structural variability inside of the eddy; the relative power of the sum of Kz (2) ~ Kz (4) with respect to Kz (1) was strong in the inner center and weakened in the outer center. These results seem to support that the wave–eddy interaction is non-uniform inside the eddy and partially depends on the relative vorticity.

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

“…The distinct behaviors of SIT and DIT networks within the mesoscale eddy, in terms of network metrics, such as mean degree and mean clustering coefficients, are consistent with observational and numerical facts [7,45] that the horizontal scale of the eddy is comparable to that of the lowest mode of DIT. This distinction is also confirmed by the total weights of MST and modularity-based structural entropy.…”

“…In relation to the global wavelet spectra (GWS) behaviors depicted in Figure 5, these results demonstrate that internal tide modulations within the mesoscale eddy are primarily driven by interactions, such as subharmonics and cascading, between the diurnal internal tide (DIT) and the eddy. It is worth noting that our analysis findings are well consistent with the results of Lim and Park [45], in which a traditional verticalmode decomposition method was used to reach the same conclusion.…”

“…However, our new approach has an advantage in obtaining the same results solely from the glider's observations. By exploiting a wavelet approach optimized for the time-frequency locality, we found the spatial dependency of modulations of internal tides inside the mesoscale anticyclonic eddy, strongly indicating that the eddy is intensified in the surface layer (less than 400 m) and exhibits a varying strength of interaction depending on the distance from the eddy's center point; as reported in Lim and Park [45], interactions are more actively occurring in the inner center (within 30 km of the eddy's center point) due to its high relative vorticity.…”

“…Our glider dataset has a spatiotemporal limitation; its spatial coverage is very limited to the upper surface layer (up to 800 m depth) compared to the full depth of 6000 m, and its temporal span is less than 10 days (225 h). Due to these restrictions, the previous analysis by Lim and Park [45] required additional datasets, such as ADCP from the research vessel ISABU and satellite-based measurements. However, our new approach has an advantage in obtaining the same results solely from the glider's observations.…”