Importance of the mesoscale circulation in the energetic of internal tides in two contrasted areas of the North Atlantic.

Ocean internal tides are both ubiquitous and important for the transport of tracers and the meridional overturning circulation, because of the mixing in the deep ocean they cause when they break. They are specific kind of internal waves generated when the astronomical tide encounters topographic features, which can propagate over more than a thousand kilometres. This long range propagation leaves opportunities for these waves to interact with mesoscale flows and eddies, which can be of comparable length scale. These interactions are of importance since they impact both the wavelength and the phase of internal tides, making them difficult to map using satellite altimeter once they have lost their coherency with the astronomical forcing. These interactions may also impact the energy budget of the internal tide and the cascade of energy from the astronomical (barotropic) tide to shorter internal waves (down to scales under 1 km), down to 3D turbulence and dissipative scale. 

In this presentation, we will describe the energy life cycle of internal tides in the North Atlantic basin using outputs from the numerical simulation eNATL60. This simulation has an horizontal resolution of around 2 kilometres and 300 vertical levels. Using a vertical mode decomposition, we investigate the energy budget of the semi-diurnal internal tide and more precisely the exchanges of energy between modes triggered by the topography, the mesoscale flow and the variations of the ambient density field, as well as their time variability.

We will focus on two contrasted areas of the North Atlantic: the Azores Islands and the North mid Atlantic ridge with a weak mesoscale activity and strong topographic features, such as seamounts and a ridge, and the Gulf Stream area featuring a strong western boundary current as well as a continental shelf break.
In the vicinity of the Azores, topographic induced couplings are of leading order for mode 0 and 1 and induce a substantial transfer of energy from low to high modes. The fraction of energy transferred toward high modes by the topography is almost 100% of the total transfer, but advection by the balanced flow becomes significant in the energy budget for modes 2 or higher.
In contrast, in the Gulf stream region, interactions with the mesoscale balanced flow accounts for more than 35% of the energy transfer from low baroclinic modes to high modes, and the mesoscale plays an important role in the energy budget for all baroclinic modes. The most prominent contribution it is the advection by the mean flow: it accounts for 26% of this transfer toward high modes and dominate over topographic scattering for modes 1 to 10.

Advection of the internal tides is the dominant contribution of the interaction between internal tides and the mesoscale balanced flow in the two areas. We find that the internal tide only weakly extracts energy from the mesoscale flow and associated buoyancy field in the two areas.