Energy Flux Observations in an Internal Tide Beam in the Eastern North Atlantic

Abstract

Low‐mode internal waves propagate over large distances and provide energy for turbulent mixing when they break far from their generation sites. A realistic representation of the oceanic energy cycle in ocean and climate models requires a consistent implementation of their generation, propagation, and dissipation. Here we combine the long‐term mean energy flux from satellite altimetry with results from a 1/10° global ocean general circulation model that resolves the low modes of internal waves and in situ observations of stratification and horizontal currents to study energy flux and dissipation along a 1000 km internal tide beam in the eastern North Atlantic. Internal wave fluxes were estimated from twelve 36‐ to 48‐hr stations in along‐ and across‐beam direction to resolve both the inertial period and tidal cycle. The observed internal tide energy fluxes range from 5.9 kW m−1 near the generation sites to 0.5 kW m−1 at distant stations. Estimates of energy dissipation come from both finestructure and upper ocean microstructure profiles and range, vertically integrated, from 0.5 to 3.3 mW m−2 along the beam. Overall, the in situ observations confirm the internal tide pattern derived from satellite altimetry, but the in situ energy fluxes are more variable and decrease less monotonically along the beam. Internal tides in the model propagate over shorter distances compared to results from altimetry and in situ measurements, but more spatial details close the main generation sites are resolved.

Observations of the Low‐Mode Internal Tide and Its Interaction With Mesoscale Flow South of the Azores

“…These distances in combination with the mean direction of the energy flux point toward the seamount chain south of the Azores as generation site for the observed internal tides, confirming the earlier study by Köhler et al. (2019).…”

“…The significantly weaker energy flux in the STORMTIDE2 model (in the total semidiurnal band, as well as in the M 2 coherent case) in comparison to the energy flux from the measurements is most likely caused by an unrealistically high (numerical) damping in the model (Köhler et al., 2019; Müller et al., 2012). Another possible explanation for the lower energy flux in STORMTIDE2 could be an insufficient barotropic to baroclinic energy conversion rate at the seamount chain where the observed internal tides are generated.…”

“…(2018), we do not find a high directional variability of internal tides which they linked to multiple sources around their mooring. This is likely due to the position of the mooring, where the interference patterns of different wave trains from different sources form a beamlike structure with a distinct direction (Köhler et al., 2019). The directional variability on short time scales might be attributed to reflection, scattering, and interferences between waves from different sources (Zaron & Egbert, 2014).…”

“…The phase and group speed of the internal waves decrease with increasing mode number. Low‐mode motions contain appreciable energy depending on the spectral characteristics of the generating topography (e.g., de Lavergne et al., 2019) and quickly propagate away laterally up to thousands of kilometers from their generation sites, for example the Hawaiian Ridge (Alford, 2003; Cummins et al., 2001; Dushaw et al., 1995; Nash et al., 2004; Zhao et al., 2016) or the seamount chain south of the Azores (Köhler et al., 2019), our study region (Figure 1). Higher modes contain much of the vertical shear in the ocean (Simmons & Alford, 2012) and dissipate close to their generation region, also owing to their slower group velocities.…”

“…We calculated energy flux in the semidiurnal frequency (M 2 ) from our mooring data as well as in the model STORMTIDE2 following the method described in Alford and Zhao (2007) and Nash et al (2005). The seamount chain south of the Azores, which has been earlier identified by Köhler et al (2019) as the generation site for the internal tides, is considered to be one of the main generation sites for internal tides in the North Atlantic (Müller et al, 2012;Zhao et al, 2016). We look at the interactions of one surface and one subsurface intensified eddy present in our observational record with the semidiurnal internal tide energy flux calculated from mooring measurements.…”