Meeting Documents

Mapping internal tides using LLC4320 and SWOT measurements in the Southern Ocean

Abstract

Throughout the global ocean, internal tides play a key role in driving ocean mixing, influencing the ocean general circulation and climate. The past decades have seen an increased interest and success in mapping global internal tides with satellite and mooring observations. However, mapping internal tides in the energetic Southern Ocean (poleward of 35^oS) has been problematic due to signal contamination by Antarctic Circumpolar Current (ACC) jets and eddies. In this study, we use both LLC4320 and SWOT to track the internal tides at the scale of eddy interactions. LLC4320 is a global eddy and tide-resolving high-resolution (horizontal resolution of 1/48^o and 90 vertical levels), high-frequency (hourly output) model simulation with atmospheric and tidal forcing. SWOT measures sea surface height (SSH) with 2.5 km-scale spatial resolution along a swath, providing across-track information for the first time, and thus an enhanced opportunity to track the internal tides relative to what is possible with conventional nadir altimetry.

A two-dimensional plane wave fit method is implemented to extract the surface amplitude and phase of M2 internal tides, first evaluated in LLC4320, and then tested in SWOT datasets. An inverse Fourier transform using least-squares is used to increase the computational efficiency to process the 1/48^o hourly LLC4320 output. We first present the results in the Tasman Sea in the southwest Pacific, a region characterized by a complex bathymetry and therefore a hot spot of internal-tide generation and wave-wave interactions.

To better track the internal wave energetics, we quantify the spatial distribution of shoreward internal-wave-band energy fluxes through the Tasman Sea. In LLC4320, the internal-wave-induced energy flux is directly computed as a correlation between baroclinic pressure and velocity. However, due to the high computational cost, we limit the computation across the four selected latitudes 35^oS, 45^oS, 55^oS, and 65^oS. In the Tasman Sea, we estimate a net meridional internal wave energy flux of 0.18 GW (equatorward), 2.7 GW (equatorward), and -0.33 GW (poleward), across 35^oS, 45^oS, and 55^oS, respectively. In SWOT, the depth-integrated-flux will be inferred via transfer functions based on modal decomposition developed as in LLC4320.