The 18.6‐Year Lunar Nodal Cycle in Internal Tides

Abstract

Both the 18.6‐year lunar nodal cycle (LNC) and long‐term quasi‐linear trends in oceanic internal tides are detected using 30 years of satellite altimeter data (1993–2022). The data are divided into 28 overlapping equal‐sized subsets, from which 28 sets of global internal tide fields are constructed for mode‐1 , , and constituents. Their global‐mean amplitude and phase time series demonstrate that the detected LNC is consistent with the equilibrium theory. The detected quasi‐linear trends are evidently the anticipated consequence of the increasingly stratified ocean caused by global warming. The global distribution of internal tidal energy is significantly modulated by the 18.6‐year LNC, because (a) and are 180 out of phase with in the cycle, and (b) the diurnal and semidiurnal constituents have different spatial patterns. We suggest that the nodal modulation of internal tides may affect internal tide‐driven ocean mixing and thus large‐scale ocean circulations.