Internal tides off the Amazon shelf – Part 2: temperature variability at tidal frequencies

Abstract. The northern Brazilian region constitutes one of the most energetic tidal environments of the tropical Atlantic, where distinct mixing regimes coexist over short spatial scales. While barotropic tidal motions exert a dominant control on turbulent mixing across the shallow continental shelf, energy dissipation associated with internal tides (ITs) governs the intensity and distribution of mixing at the shelf-break and in offshore waters. As demonstrated in the Part 1 companion study (Assene et al., 2024), these contrasting processes strongly influence upper-ocean thermal structure. Yet, the expression of tidal forcing in temperature variability at tidal timescales—particularly at semidiurnal (principal solar: M₂ and principal lunar: S₂) and fortnightly (lunisolar synodic: MSf) frequencies—remains poorly documented in this region. In this study, we investigate the role of tides, with a focus on ITs, in shaping temperature variability throughout the NBR by combining long-term satellite sea surface temperature (SST) records with high-resolution three-dimensional numerical simulations operated with and without tidal forcing.

The main findings are as follows:

1. At semidiurnal frequencies, temperature variability at the sea surface is very weak offshore and remains modest over the continental shelf, consistent with the prevalence of barotropic mixing that acts largely as a depth-integrated process in shallow waters. In contrast, pronounced temperature variability emerges at thermocline depths, with mean amplitudes reaching approximately 0.6 °C for S₂ and exceeding 2 °C for M₂. The spatial structure of these subsurface signals aligns closely with simulated mode-1 and mode-2 IT wavelengths, propagation pathways, and dissipation hot-spots, underscoring the central role of ITs in driving semidiurnal thermal variability below the surface mixed layer;
2. Fortnightly (MSf) variability contrasts sharply with the semidiurnal response. Both satellite observations (MUR, TMI) and tidal simulations reveal low amplitudes on the order of 0.15 °C, with maximums confined to the northwestern shelf where Spring–Neap modulation of barotropic tidal currents is the dominant tidal process. Composite analyzes contrasting Spring versus Neap conditions further suggest that this MSf variability manifests primarily as a net cooling with the same amplitude. At the surface, neither model nor satellite observations exhibit a significant SST expression at MSf frequency along internal tide propagation pathways. This may reflect a rapid atmospheric heat flux adjustment that counteracts internal tide–induced cooling and/or the inherently incoherent nature of internal tide dynamics that disperses energy across frequencies, preventing harmonic methods from capturing a clear MSf signature. At subsurface depths (~120 m), MSf temperature variability becomes more pronounced along IT pathways, particularly near the shelf break and downstream of generation sites where dissipation is strongest.
3. The vertical penetration depth of tidally driven temperature variability decreases systematically with increasing tidal period, from penetration depths approaching 2500 m for M₂, to 800–1000 m for S₂ and 600–800 m for MSf. These contrasts indicate that the capacity of tidal motions to influence the water column depends strongly on the available energy at each frequency and points to a frequency-dependent control of deep-ocean mixing and heat redistribution.

Together, these findings provide the first regional quantification of temperature variability at tidal frequencies in the northern Brazilian region and demonstrate that internal tides constitute a major driver of subsurface thermal structure across this dynamically energetic margin. This improved characterization is essential for understanding heat redistribution, interpreting coastal and open-ocean temperature variability, and ultimately constraining the representation of tidal processes in ocean and climate models.