M2 Monthly and annual mode 1 and mode 2 internal tide atlases from altimetry data and MIOST: focus on the Indo-Philippine Archipelago and the region off the Amazon shelf

Abstract. The M2 MIOST24 (Multivariate Inversion of Ocean Surface Topography 2024) internal tide atlases are available in annual (MIOST24a) and monthly (MIOST24m) regional versions for modes 1 and 2 in the Indo-Philippine archipelago and the region off the Amazon shelf. They are derived from 25 years (1993–2017, period 1) of sea level anomalies (SLA) from altimetry observations. Compared to MIOST22, MIOST24 incorporates M2 modes 1 and 2 wavelengths based on monthly stratification profiles from GLORYS12v1 (1993–2020). The differences between MIOST24a and MIOST22a lead to RMSE (Root Mean Square Error) of up to 3 cm, reflecting amplitude changes, while the RMSE between MIOST24a and HRET (High-Resolution Empirical Tide) indicate both amplitude and phase differences. MIOST24m highlights significant monthly variability of M2 internal tides in the Indo-Philippine archipelago and off the Amazon shelf. In the Amazon region, the internal tide propagates far offshore from March to June but is blocked closer to the coast from August to December. For both regions, mode 1 monthly variability is mainly phase-related, while mode 2 is more amplitude-dependent. Variance reductions of SLA show that MIOST24m outperforms MIOST24a, MIOST22a and HRET on period 1 in the two regions of interest, and on the Amazon region in period 2 (2018–2023). Monthly atlases are therefore recommended to correct the internal tides of the SLA used to derive them. All these results support the development of an improved MIOST24 global atlas.