Internal Tide Impacts on Ocean Circulation

Douglas S. Luther, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, HI, USA. Extracted from OOI Science Plan, 2021.

Internal tides (ITs) provide over half of the ~2 TW of power needed to maintain the deep ocean’s stratification via mixing of upper warm water with deep cold water. Accordingly, they have critical roles in determining the meridional overturning circulation and oceanic heat budget (e.g., Wunsch and Ferrari, 2004; Waterhouse et al., 2014). Generated by the surface tide flowing over topography, ITs propagate throughout the ocean interior (e.g., Morozov, 2018). Unfortunately, the great uncertainties of how and where tidal energy flows and transforms through the ITs from their globally distributed sources to their equally well-dispersed sinks, significantly hinders understanding of how the structures of the abyssal stratification and the global ocean thermohaline circulation are produced (e.g., Garrett and Kunze, 2007; Ferrari and Wunsch, 2009; Melet et al., 2016; Oka and Niwa, 2018; Vic et al., 2019).

The OOI profiling current meter and CTD data now extend to six years of high temporal and vertical resolution observations at many sites, especially within the Cabled and Endurance Arrays. These data are an incredible novelty for internal tide studies, enabling the delineation of the relative contributions of many processes that provide pathways for energy through the ITs and on to dissipation and mixing. The long duration enables discrimination of processes in frequency space that have very similar frequencies. The high vertical resolution enables the differentiation of reversible (i.e., vertical advection) and irreversible (i.e., diapycnal mixing) processes via the definition of a semi-Lagrangian coordinate system, based on tidal isopycnal displacements. The long duration also enables calculation of the statistics of the impacts of intermittent inertial waves, long period currents (e.g., eddies; upwelling), and seasonal stratification changes on the shear, strain, and turbulent mixing associated with the ITs. We know these interactions occur, but over a long period of time how important is each one?

The value of long duration, high-vertical- resolution observations for studying ITs can be discerned from Figure to the right. It shows the horizontal velocity power spectral density (PSD; m2/(rad/s)) of the semidiurnal ITs, as a function of depth for six months, obtained via a mooring at Kaena Ridge in Hawaii in 2002 (Carter et al., 2008). The spectra show a “beam” of semidiurnal IT energy peaked at roughly 600 m, that we now know is propagating southwestward from its origin on the north edge of the Ridge. The beam’s vertical structure varies strongly in time, as does its spring-neap tidal cycle; longer-period variability is due in part to an eddy (within the dark blue contour) interfering with the beam (e.g., Chavanne et al., 2010). Clearly, these six months of data are too brief to reliably disentangle the probable processes revealed in the figure. [N.b., the solid white curves at the bottom indicate the local amplitude variations of the barotropic, semidiurnal tidal sea level based on TPXO 6.2 (Egbert, 1997; Egbert and Erofeeva, 2002). Shaded regions are where the data quality dropped below an arbitrary threshold.