Impacts of Mesoscale Currents on the Diurnal Critical Latitude Dependence of Internal Tides: A Numerical Experiment Based on Barcoo Seamount

Abstract

Critical latitudes are a significant area of tidal dissipation. Generally, critical latitudes are taken to be the exact latitude where the tidal frequency equals the inertial frequency. However, the key is really where the tidal frequency equals the combination of planetary vorticity and relative vorticity from background currents. Although the influence of background currents on critical latitude effects and nonlinear interactions have been noted for many years, their exact impacts are not well known. The latitude dependence of critical latitude impacts on the tides, internal tides, and internal waves in the presence of background currents was investigated using the Regional Ocean Modeling System by shifting a small domain including a seamount from 20.6° to 38.6°S and comparing simulations with and without background currents. The diurnal kinetic energy with mesoscale currents was relatively unchanged for most latitudes, except for a slight decrease 1–4° poleward of the critical latitude. However, the semidiurnal and high‐frequency (≥3 cycles per day) kinetic energy increased with the presence of mesoscale currents, especially within the diurnal critical latitude range. Spectral and nonlinear analyses indicated mesoscale currents broadened the range of critical latitude effects and enhanced energy transferring from diurnal frequencies to semidiurnal and high frequencies and from low to high mode waves. Local diffusivities increased, roughly an order of magnitude, when mesoscale currents were present. The impacts of mesoscale currents on the broadening of the critical latitude range and enhancement of nonlinear interactions were attributed to the additional relative vorticity and near‐inertial internal waves generated by mesoscale currents.

Parametric Subharmonic Instability of the Semidiurnal Internal Tides at the East China Sea Shelf Slope

“…It is numerically predicted that the relative vorticity induced by the spatially varying background geostrophic flow can change the PSI efficiency by changing the local effective Coriolis frequency f eff . This is related to the fact that the propagation of PSI-generated subharmonic wave is controlled by f eff rather than the local Coriolis frequency f. More recently, using 3D simulations, Dong et al (2019) achieved similar results to Yang et al (2018), that the presence of the background geostrophic flow can significantly change the PSI efficiency.…”

“…Recently, it has been found that some realistic oceanic background conditions being ignored in most previous idealized simulations, such as the presence of background geostrophic flow (Richet et al 2017;Yang et al 2018;Dong et al 2019) and nonuniform stratification (Hazewinkel and Winters 2011;Onuki and Hibiya 2018), can significantly influence the PSI behavior. Richet et al (2017) showed that the Doppler effect of the mean current could shift the frequencies of primary ITs, hence, further changed the position of PSI critical latitudes.…”

Enhanced internal tidal mixing in the Philippine Sea mesoscale environment

“…The existence of mesoscale features changed the vertical structure of internal tides and transferred the internal tides energy from low modes to higher modes. It was more likely to cause internal tide breaking (Dunphy and Lamb, 2014). The enhancement by mesoscale motions in tidal mixing was more significant for M 2 internal tides.…”

“…Numerical simulation results support these observations (Kerry et al, 2014), indicating that the patterns of internal tides are largely modulated by the position of eddies. An idealized numerical experiment shows that the energy of internal tides shows bundled beams after passing through an eddy (Dunphy and Lamb, 2014). And the mode-1 internal tidal interactions with eddies will trigger higher-mode signals.…”

“…Similarly, regional studies indicate that mesoscale features modulate the generation and propagation of internal tides. Mesoscale currents can also broaden the range undergoing internal tide critical latitude effects and enhance the energy transfer from diurnal frequen-cies to semidiurnal or high frequencies (Dong et al, 2019). Mesoscale eddies are found to modulate internal tide propagation (Rainville and Pinkel, 2006;Park and Watts, 2006;Zhao et al, 2010) and enable the internal tide to lose its coherence (Nash et al, 2012;Kerry et al, 2016;Ponte and Klein, 2015).…”