Ocean tides can drag the atmosphere and cause tidal winds over broad continental shelves

Ocean tides, primarily generated by the gravitational pull of the Sun and Moon, represent a significant energy source within the world's oceans. Their influence on oceanic dissipation, mixing, and general circulation is substantial, making them a key area of oceanographic study. Satellite altimetry estimates the total work done by tidal forces to be around 3.5 TW, with the M2 tide (12.42 h period) accounting for 2.4 TW. The primary dissipation mechanisms of tidal energy are generally considered to be bottom friction on continental shelves and the dispersion of surface tides into internal waves by ocean topography. Recently, attention has turned to air-sea interactions at the oceanic mesoscale, specifically the Current Feedback (CFB) effect. CFB describes how surface ocean currents influence the overlying atmosphere, leading to two major impacts on ocean circulation. Firstly, at larger scales, CFB reduces mean atmospheric energy input into the ocean, slowing down mean ocean circulation. Secondly, CFB acts as an "eddy killer", reducing submesoscale activity by about 30%. This highlights CFB's role in controlling energetic currents like western boundary currents by influencing the wind-driven current paradigm—the wind's influence isn't solely limited to large-scale energy input, but also involves fine-scale interactions affecting the entire oceanic spectrum. CFB essentially acts as a top drag, affecting various spatial and temporal scales, implying that tides should also be subject to this effect. While the impact of tidal currents on the overlying atmosphere, potentially modifying surface stress and low-level winds, has been largely unexplored, this study investigates the amount of energy dissipated through this atmospheric interaction and its effectiveness in generating tidal winds. Current research primarily focuses on the S2 component of tidal winds in the upper atmosphere, attributed to thermal heating rather than friction effects of oceanic tides. However, observations suggest a diurnal frequency that could be linked to this previously unexplored mechanism.

Existing literature extensively covers the energetics of ocean tides and their dissipation mechanisms, focusing on bottom friction and internal wave generation. Studies using satellite altimetry have quantified the total energy dissipation of tides. The Current Feedback (CFB) effect, detailing the influence of surface ocean currents on the overlying atmosphere, has been a subject of growing research in recent years. Several studies have demonstrated CFB's impact on slowing mean ocean circulation, reducing mesoscale activity, and affecting western boundary currents. The concept of CFB as a "top drag" affecting various scales, including tidal currents, has emerged. However, the interaction of ocean tides with the atmosphere and the generation of tidal winds by this mechanism have remained largely uninvestigated. Existing research on atmospheric tides largely focuses on gravitational and thermal tides, which are significant in the upper atmosphere but weak in the troposphere. This study addresses the gap by exploring the potential for a new category of atmospheric tides induced by ocean tide drag.

To investigate the interaction between ocean tides and the atmosphere, the researchers conducted coupled eddy-rich ocean-atmosphere simulations over the English Channel using the Coastal and Regional Ocean Community model (CROCO) and the Weather Research and Forecast model (WRF). The study employed two twin experiments: a control simulation (CTRL) incorporating tides and CFB, and a simulation without CFB (NOCFB). Comparing these simulations highlighted the top drag effect on tides and demonstrated the existence of tidal winds induced by tidal currents. The CROCO model, a free-surface, terrain-following coordinate model, was configured with a 2.7-4.0 km grid resolution covering the English Channel. Bathymetry data was sourced from the Shuttle Radar Topography Mission (SRTM30 plus) dataset, with smoothing applied to avoid aliasing. 50 vertical levels with enhanced resolution near the surface, bottom, and thermocline were used. The model was initialized using the GLORYS2V4 global reanalysis, spun up as an ocean-only model for a year, then run as a coupled model for 2010 using interannual oceanic boundary forcing from the GLORYS reanalysis and surface forcing from CFSR atmospheric reanalysis. Tidal forcing in CROCO was applied through Flather-type conditions at open boundaries using data from the TPXO global tidal model. Tidal potential was also applied as a body force, including astronomical contributions, solid Earth tides, self-attraction of ocean tides, and load tides. A logarithmic law was used for bottom drag calculation, and the KPP parameterization was used for vertical mixing. The WRF model (version 4.1) was used with a 6 km resolution, employing data from the Climate Forecast System Reanalysis (CFSR) for initialization and boundary forcing. The models were coupled hourly using the OASIS3 coupler, exchanging freshwater, heat, and momentum fluxes. The CTRL simulation included CFB by estimating surface stress using the relative wind (10 m wind relative to the moving ocean surface), while NOCFB used the absolute wind. This allowed the researchers to isolate the effects of CFB and quantify the contribution of tidal currents to surface stress, energy flux, and wind generation. Spectral analysis of sea surface height, surface stress, wind work, and wind speed was conducted to identify tidal signals and quantify energy transfer between ocean and atmosphere. A coupling coefficient (sw) between surface currents and low-level winds was calculated to characterize the wind response to tidal currents, similar to the approach used for mesoscale ocean eddies.

The study's key findings demonstrate that ocean tides exert a significant drag on the overlying atmosphere, generating measurable tidal winds. Analysis of the English Channel revealed a strong anti-correlation (−0.8) between zonal surface currents and surface stress anomalies (the difference between CTRL and NOCFB simulations). The surface stress spectrum showed a clear peak at the M2 tidal frequency in the CTRL simulation, indicating a direct link between tidal currents and surface stress. The wind work (energy flux) analysis showed a substantial energy sink from the ocean to the atmosphere at locations of strong tidal currents, particularly evident at the M2 frequency. This indicates a transfer of tidal energy from the ocean to the atmosphere, a previously undocumented process. The simulations revealed the presence of tidal winds with amplitudes exceeding 1 m s⁻¹, exhibiting a spatial pattern similar to that of surface currents and stress anomalies. The tidal wind effect extended to the top of the atmospheric boundary layer (around 800 m in the example shown), with a high positive correlation ( > 0.7 over a week) between currents and winds within this layer. Above the boundary layer, the correlation diminished. The rapid response of the atmosphere (less than an hour) to changes in ocean tides was also observed. Statistical analysis yielded a coupling coefficient (sw) of 0.32, indicating that the tidal wind amplitude is approximately one-third the amplitude of the surface currents. This is similar to the coupling coefficient observed in mesoscale eddies. This relationship was then used to estimate global M2 tidal winds using a global tidal model (TPXO9-v5). The resulting global map showed that tidal winds are most prominent in coastal areas with wide shelves where tidal currents are strongest, with potential amplitudes up to 1.5 m s⁻¹ in areas like Alaska, Argentina, Norway, and the Amazon. A simple estimate suggests that approximately 0.012 TW of M2 tidal energy might be transferred to the atmosphere through this mechanism.

The discovery of current-induced tidal winds represents a significant advancement in our understanding of air-sea interaction and tidal energy dissipation. The previously unaccounted-for energy transfer from ocean tides to the atmosphere through atmospheric drag has clear implications for climate modeling. Accurate representation of this energy flux is crucial for improving the accuracy of climate models. This necessitates the development and testing of parameterizations specifically for tidal frequencies to represent this effect in ocean-only models. The identified tidal wind regime is not insignificant. In regions with strong tidal currents, these winds could add a considerable amount to the overall wind speed, impacting wind energy production. Considering an average wind speed of 6 ms⁻¹, a 1ms⁻¹ tidal wind could lead to a 10% increase in offshore wind energy, a magnitude comparable to the projected impacts of climate change. This emphasizes the need to account for this phenomenon in the development and planning of offshore wind farms. The study’s findings also suggest that these tidal winds may affect weather forecasting, search and rescue operations, shipping, and responses to environmental events such as oil spills, where accurate wind information is critical.

This research demonstrates, for the first time, the generation of a new class of atmospheric tides induced by the drag of ocean tides. These tidal winds, present in broad continental shelf regions, have amplitudes up to 1.5 m/s and significantly impact tidal energy dissipation, with a substantial portion transferred to the atmosphere. This necessitates adjustments in climate models and improved predictions in areas sensitive to high-frequency wind variability. Future research should focus on refining the parameterizations of tidal wind generation for incorporation into oceanographic and atmospheric models and explore the regional and temporal variations of this phenomenon.

The study focuses primarily on the English Channel. While the global estimate of tidal winds provides a valuable overview, it might underestimate the locally generated tidal currents near smaller bathymetric features not resolved in the global tidal model. Further regional studies are needed to fully assess the extent and impact of this tidal wind generation mechanism across different geographical locations and tidal regimes. The accuracy of the global estimate relies on the quality of the global tidal model used, which has inherent uncertainties and limitations in resolving small-scale bathymetric features.