WHOI PO

Title: Effect of near-inertial pumping on subduction at an ocean front

Abstract: "The relative vorticity of the flow field modifies the effective frequency of near-inertial waves (NIWs). Submesoscale currents have significant variations in their relative vorticity on scales of 0.1-10 $km$, leading to spatial variations in the NIW frequency and phase differences in inertial motion. This results in convergences and divergences of inertial currents in the surface layer. Although such inertial pumping reverses over the inertial period, the induced vertical transport becomes coupled with the horizontal motion within vertically sheared submesoscale currents. As a result, the vertical transport of the tracers from the mixed layer to the pycnocline is modified by wind-generated NIWs in a submesoscale flow field. Using a numerical process study model and observations made during the CALYPSO field campaign in the Balearic Sea, we examine the effect of such near-inertial pumping on the vertical transport of tracers. The particular model set-up has an oceanic front (with a strong horizontal density gradient and flow field of relative vorticity $\zeta = O(f)$), which is forced by winds for four inertial periods and then allowed to evolve freely. Passive tracer initialized with linear vertical distribution after wind forcing is turned off. The model output reveals that the vertical transport of tracer is enhanced when near-inertial waves (NIWs) are superimposed on the front (F) compared to the case with only the front (F) and no wind forcing. The vertical velocity for F with NIWs is enhanced compared to only F in subinertial frequency, with the emergence of inertial frequency extending to higher order harmonics via non-linear resonance. A modest fraction of the passive tracer (which could represent pollutants, dissolved carbon, or larvae) is drawn down from the surface, some of which even goes below the surface mixed layer. The tracer flux $w'c'$ given by the covariance of the anomalies (from the horizontal mean) in vertical velocity, $w'$, and tracer concentration, $c'$, is used to examine vertical transport. Integrating the covariance spectra in wavenumber-frequency space shows that the interaction of NIW with the front enhances the vertical tracer flux at both the subinertial and inertial frequencies, revealing a mode of vertical transport from the coupling of near-inertial waves and submesoscale currents."