Storms modify baroclinic energy fluxes in a seasonally stratified shelf sea: Inertial‐tidal interaction

Abstract

Observations made near the Celtic Sea shelf edge are used to investigate the interaction between wind-generated near-inertial oscillations and the semidiurnal internal tide. Linear, baroclinic energy fluxes within the near-inertial (f) and semidiurnal (M 2 ) wave bands are calculated from measurements of velocity and density structure at two moorings located 40 km from the internal tidal generation zone. Over the 2 week deployment period, the semidiurnal tide drove 28-48 W m 21 of energy directly onshelf. Little spring-neap variability could be detected. Horizontal near-inertial energy fluxes were an order of magnitude weaker, but nonlinear interaction between the vertical shear of inertial oscillations and the vertical velocity associated with the semidiurnal internal tide led to a 25-43% increase in positive on-shelf energy flux. The phase relationship between f and M 2 determines whether this nonlinear interaction enhances or dampens the linear tidal component of the flux, and introduces a 2 day counter-clockwise beating to the energy transport. Two very clear contrasting regimes of (a) tidally and (b) inertially driven shear and energy flux are captured in the observations.

Sea surface cooling in the Northern South China Sea observed using Chinese sea-wing underwater glider measurements

“…It indicates that vertical mixing induced by eddies or internal wave rather than Ekman pumping played more important role in ML cooing, which is consistent with the upper ocean response to Typhoon (e.g., Guan et al, 2014) or storm (e.g., Hopkins et al, 2014). Previous studies have generally ignored the contribution of MLD deepening and have only considered Ekman pumping, due to the difficulty in making the corresponding observations (e.g., Li et al, 2006;Qiu et al, 2014).…”

Baroclinic energy flux at the continental shelf edge modified by wind‐mixing

“…The data were used to calculate M 2 baroclinic energy fluxes from the correlation between semidiurnal perturbations in velocity and pressure (see, e.g., Kunze et al [] for details). Power spectra of the baroclinic currents [see Hopkins et al , , Figure 4] show distinct, well‐separated peaks at inertial and M 2 tidal frequencies, so in computing the M 2 energy fluxes, a 58 h window was used, sufficient to resolve the difference between inertial (∼16 h period) and M 2 (12.4 h period) signals.…”

“…At ST5, an RDI 150 kHz ADCP with 2 min sampling intervals and 2.5 m vertical bins measured from ∼8 m above the bottom to ∼21 m below the surface. Full details of the instrumentation and data processing are described in Hopkins et al [] and Vlasenko et al []. The data were used to calculate M 2 baroclinic energy fluxes from the correlation between semidiurnal perturbations in velocity and pressure (see, e.g., Kunze et al [] for details).…”

“…The IT field at a location consists of a locally generated component and a remotely generated, difficult to predict, signal with distorted phases [Nash et al, 2012]. Other complications in predicting and analyzing the ITs come from mesoscale variability changing the vorticity balance along a shelf slope [Hosegood and van Haren, 2006], interference and nonlinear interaction by waves from multiple sources [e.g., Lien et al, 2005;Vlasenko et al, 2014], or interactions between the semidiurnal tide and near-inertial waves [Shroyer et al, 2011;Hopkins et al, 2014]. Hopkins et al [2014] found that nonlinear interactions between wind-driven near-inertial oscillations and the internal tide alter tidal energy fluxes and produce a regular, ∼2 day beat frequency in onshore and offshore fluxes.…”

Kilometric Scale Modeling of the North West European Shelf Seas: Exploring the Spatial and Temporal Variability of Internal Tides

“…The thermocline depth at the Celtic Sea shelf break varies between 40 and 60 m, with colder waters at the bottom (12.5°C at the shelf break, and 11.5°C on the shelf), while surface temperatures vary from 13 to 14°C. There is a cooling of the surface layer and deepening of the thermocline on the 15 June, correlated with a storm event, followed by a progressive return to a shallower warmer surface layer (Hopkins et al, ; Stephenson et al, ). This is observed at the four locations, but one can notice more mixing of the bottom layer at the shelf break (St1 and St2).…”