Seasonal West‐East Seesaw of M₂ Internal Tides From the Luzon Strait

Abstract

Satellite altimetry sea surface height (SSH) measurements from 1993 to 2017 are used to investigate the seasonal variability of mode‐1 M2 internal tides from the Luzon Strait. The 25 years of SSH data are divided into four seasonal subsets, from which four seasonal internal tide models are constructed following the same mapping procedure. Climatological seasonal hydrography in the World Ocean Atlas 2013 is used to calculate two seasonally variable parameters required in the mapping procedure: Wavelength and the transfer function from the SSH amplitude to depth‐integrated energy flux. The M2 internal tides from the Luzon Strait are extracted using propagation direction determined in plane wave analysis. The satellite results show that the westward and eastward M2 internal tides both demonstrate significant seasonal variation. The westward and eastward internal tides seesaw seasonally: The westward internal tides strengthen (weaken) in summer and fall (winter and spring); while the eastward internal tides strengthen (weaken) in winter and spring (summer and fall). We suggest that the seasonal seesaw is mainly determined by ocean stratification and the Kuroshio Current; however, further studies are needed to quantify their relative contributions.

Satellite Evidence for Strengthened M2 Internal Tides in the Past 30 Years

“…Figures 2c and 2d show the resulting model errors. We find that large model errors are mainly associated with strong mesoscale eddies, because the models errors are mainly leaked mesoscale signals (Zhao & Qiu, 2023). Figure 2e shows that the two models both have errors as low as 1 mm, much lower than M 2 internal tides.…”

“…Model errors are estimated using background internal tides following Zhao and Qiu (2023). Background internal tides are mapped using the same altimetry data sets and following the same mapping procedure, but the tidal periods are slightly different.…”

Internal tide variability off Central California: multiple sources, seasonality, and eddying background

“…Following the new mapping technique described in Zhao and Qiu (2023), the regional M 2 internal tidal field is mapped using 27 years Zhao & Qiu, 2023) used the same data, except with a 25-year (1993-2017) altimetry record.…”

“…In addition, interference due to wave-wave interaction and the absence of comprehensive 4-dimensional observations hinder the way to dynamically link these main drivers to internal tide features (M. C. Buijsman et al, 2017), leading to incomplete understanding of temporal variations of internal tides. Here, using the advanced wave decomposition method (Zhao & Qiu, 2023), we focus on investigating the seasonal variations of internal tides in the presence of mesoscale eddies off California, where a complex internal tidal field is seen from observations. This study is also motivated by the availability of moored observation from the Surface Water and Ocean Topography (SWOT) mission pre-launch campaign in 2019 (J.…”

“…Wang et al, 2022), and the latest advanced satellite altimetry model (Zhao, 2022) that is able to derive the seasonality of internal tides. The global climatological seasonality of internal tides was successfully extracted by subsetting altimetry SSH data into four seasons, leveraging a mapping method that incorporates two key techniques: plane wave analysis and spatial band-pass filtering Zhao & Qiu, 2023). Characterized by its global coverage and minimal errors, the latest altimetry model enables a global assessment of seasonal variations of internal tides while offering a meaningful comparison with in situ observations and numerical simulations.…”