Comment on egusphere-2022-1029

Abstract

Satellite altimetry provides a unique technique for observing the sea surface height (SSH) signature of internal tides from space. Previous studies have constructed empirical internal tide models for major tidal constituents M2, S2, K1, and O1. Minor tidal constituents are difficult to observe, due to their weak SSH signals. The advances in mapping technique, combined with the accumulation of satellite altimetry data, make it possible to construct empirical models for minor internal tidal constituents. In this paper, the global mode-1 N2 internal tides (the fifth largest oceanic tidal constituent) are observed using 100 satellite-years of SSH data from 1993 through 2019. This paper employs a newly-developed mapping procedure that includes two rounds of plane wave analysis and a two-dimensional bandpass filter in between. Thanks to the large SSH data set and the new mapping procedure, we can observe mode-1 N2 internal tides with O (1 mm) SSH amplitudes. The results are confirmed using independent satellite altimetry data in 2020 and 2021. The satellite observations show that N2 and M2 internal tides have similar spatial patterns, and that the N2 amplitudes are about 20 % of the M2 amplitudes. Both features mimic their barotropic counterparts. The satellite observations also show that N2 and M2 internal tides can propagate hundreds to thousands of kilometers in the open ocean, but at different phase speeds as predicted by theory.