Spontaneous Generation of Near-Inertial Waves by the Kuroshio Front

Abstract

While near-inertial waves are known to be generated by atmospheric storms, recent observations in the Kuroshio Front find intense near-inertial internal-wave shear along sloping isopycnals, even during calm weather. Recent literature suggests that spontaneous generation of near-inertial waves by frontal instabilities could represent a major sink for the subinertial quasigeostrophic circulation. An unforced three-dimensional 1-km-resolution model, initialized with the observed cross-Kuroshio structure, is used to explore this mechanism. After several weeks, the model exhibits growth of 10-100-km-scale frontal meanders, accompanied by O(10) mW m 22 spontaneous generation of near-inertial waves associated with readjustment of submesoscale fronts forced out of balance by mesoscale confluent flows. These waves have properties resembling those in the observations. However, they are reabsorbed into the model Kuroshio Front with no more than 15% dissipating or radiating away. Thus, spontaneous generation of near-inertial waves represents a redistribution of quasigeostrophic energy rather than a significant sink.

Evidence of enhanced double‐diffusive convection below the main stream of the Kuroshio Extension

“…Irrespective of whether they are spontaneously or wind generated, because near‐inertial internal waves can be trapped and amplified by geostrophic horizontal [ Kunze , ] and vertical shear [ Mooers , ; Whitt and Thomas , ], enhanced turbulent kinetic energy dissipation observed with banded shear can be attributed to breaking of near‐inertial waves under the Kuroshio, although Nagai et al . [] showed numerically that most of the spontaneously generated near‐inertial waves are reabsorbed into subinertial flow, without inducing significant kinetic energy dissipation and turbulent mixing.…”

“…In contrast, recent in‐situ observations during calm summer conditions show very large turbulent kinetic energy dissipation rates in the thermocline under the main stream of the Kuroshio [ Nagai et al ., ]. Repeated observations revealed dissipation rates in the Kuroshio thermocline on average 10–100 times greater than typical thermocline values, which were accompanied by near‐inertial internal‐wave velocity shear along isopycnals [ Nagai et al ., ]. Winds are the primary power source of near‐inertial waves.…”

“…Winds are the primary power source of near-inertial waves. However, recent theoretical, numerical, laboratory and field studies have suggested that frontal instabilities can spontaneously emit near-inertial internal waves by geostrophic adjustment [Snyder et al, 1993;Snyder, 2005, 2007;Williams et al, 2008;Danioux et al, 2012;Alford et al, 2013;Shakespeare and Taylor, 2014;Nagai et al, 2015]. Irrespective of whether they are spontaneously or wind generated, because near-inertial internal waves can be trapped and amplified by geostrophic horizontal [Kunze, 1985] and vertical shear [Mooers, 1975;Whitt and Thomas, 2013], enhanced turbulent kinetic energy dissipation observed with banded shear can be attributed to breaking of near-inertial waves under the Kuroshio, although Nagai et al [2015] showed numerically that most of the spontaneously generated near-inertial waves are reabsorbed into subinertial flow, without inducing significant kinetic energy dissipation and turbulent mixing.…”

Surface Kinetic Energy Distributions in the Global Oceans From a High‐Resolution Numerical Model and Surface Drifter Observations

“…Small‐scale waves, on the other hand, may be sensitive to Doppler frequency shifting by lower‐frequency currents and have thus been expected to exhibit more compact frequency distributions in Lagrangian space and to be more easily identifiable there (Nagai et al, ; Shakespeare & Hogg, ). Interestingly, the Doppler frequency shift equals the wave frequency times UT w / L w , where U is the lower‐frequency current amplitude, T w the wave period, and L w the wave length (note the similarity with α ).…”

The viscous lee wave problem and its implications for ocean modelling

“…St Laurent and Garrett, 2002), geostrophic flows over rough topography on the sea floor (Nikurashin and Ferrari, 2010), and small-scale unbalanced flow at the ocean surface including submesoscale eddies, fronts and filaments (e.g. Danioux et al, 2012;Nagai et al, 2015;Shakespeare and Taylor, 2016).…”