Two decadal mode-1 M2 internal tide models

It is not surprising that ocean stratification increases with global ocean warming, because the upper ocean stores more extra heat and becomes lighter. Given that ocean stratification becomes stronger, it is naturally expected that internal tides should strengthen with global ocean warming as well. However, it is extremely difficult to quantify internal tide strengthening on a global scale. Here we show for the first time that mode-1 M2 internal tides strengthened in the past 30 years using satellite altimetry data from 1993 to 2022. We further show that the internal tide strengthening is spatially inhomogeneous, which has important implications on the downward heat transport. The strengthened internal tides and eventual ocean mixing may facilitate the vertical transport of tracers and nutrients, compensating partly for the reduction caused by strengthened ocean stratification. The results suggest that the long-term changes should be taken into account in making internal tide correction for SWOT.Satellite altimetry sea surface height (SSH) measurements from 1993 to 2022 are used to show the strengthened mode-1 M2 internal tides in the past 30 years. Two mode-1 M2 internal tide models M9509 and M1019 are constructed using the data in 1995-2009 and 2010-2019, respectively. The results show that the global mean M2 internal tides strengthened by 6% in energy. However, the internal tide strengthening is spatially inhomogeneous. Significantly strengthened internal tides are observed in a number of regions including the Aleutian Ridge and the Madagascar-Mascarene region. Weakened internal tides are observed in the central Pacific. On global average, M1019 leads M9509 by about 10o (20 minutes in time), suggesting that the propagation speed of M2 internal tides increased. M9509 and M1019 are evaluated using independent altimetry data. The results show that M9509 and M1019 perform better for the data in 1993-1994 and 2020-2022, respectively.