Internal Solitary Waves within the Cold Tongue of the Equatorial Pacific Generated by Buoyant Gravity Currents

Abstract

The equatorial cold tongue in the Pacific Ocean has been intensely studied during the last decades as it plays an important role in air-sea interactions and climate issues. Recently, Warner et al. (2018) revealed gravity currents apparently originating in Tropical Instability Waves. Both phenomena have strong dissipation rates, and were considered to play a significant role in cascading energy from the mesoscale to smaller horizontal scales, as well as to vertical scales less than one meter. Here, we present Sentinel-3 satellite observations of Internal Solitary Waves (ISWs) in the Pacific cold tongue near the equator, in a zonal band stretching from 210°E to 265°E, away from any steep bottom topography. Within this band these waves propagate in multiple directions. Some of the waves’ characteristics, such as the distance between wave crests, crest lengths and time scales, are estimated from satellite observations. In total we identify 116 ISW trains during one full year (2020), with typical distances between crests of 1500 m and crest lengths of hundreds of km. These ISW trains appear to be generated by buoyant gravity currents having sharp fronts detectable in thermal infrared satellite images. A 2D numerical model confirms that resonantly generated nonlinear internal waves with amplitudes of O(10) m may be continuously initiated at the fronts of advancing gravity currents.

Nonlinear Internal Tides in a Realistically Forced Global Ocean Simulation

“…However, solitary waves are also detected in regions with relatively low supertidal energy flux, such as over large areas in the eastern Atlantic and Pacific Oceans (rectangles 8 and 9 in Figure 9b) and on the continental shelves. In a recent study, Santos-Ferreira et al (2023) showed that the solitary waves in the eastern Pacific evolve from gravity currents associated with tropical instability waves. The discrepancy between HYCOM and MODIS in rectangles 8 and 9 in Figure 9b may also be explained by weaker surface tides and/or unresolved bathymetric features in the HYCOM simulation, which may cause weaker primary and higher-harmonic internal wave flux beams.…”