Near-inertial waves enhance vertical transport at ocean fronts

Abstract

The interactions between near-inertial waves (NIWs) and submesoscale currents in the surface ocean are challenging to deconvolve due to their overlapping temporal and spatial scales. The frequency of NIW is modulated by the relative vorticity, $\zeta$, of submesoscale currents, which varies between positive and negative $\zeta$ of $O(f)$ on spatial scales of 1 -- 10~$km$, particularly across fronts where the horizontal buoyancy gradient, $\nabla_H b$, is intensified. The effective NIW frequency $f_{\scriptstyle{eff}} = f + \zeta/2$ can therefore also vary by $O(f)$ on these scales, causing the waves to be out of phase. This generates periodic convergence and divergence in the surface layer, particularly at fronts. The resulting vertical motion, known as inertial pumping, is traditionally considered to be reversible. However, the strong vertical shear of the horizontal velocity at fronts, $v_z \sim |\nabla_H b|/f$, implies that not all of the water that is pumped downward will return. We examine the effect of this asymmetry on the vertical transport of tracers with an ambient vertical gradient, analogous to biogeochemical tracers, such as oxygen and dissolved organic carbon. Using numerical simulations of an unstable front forced by NIW, we demonstrate that inertial pumping can lead to net vertical tracer transport. Spectral analysis of the vertical tracer flux given by the covariance between tracer and vertical velocity anomalies reveals that the interaction of strong NIW with submesoscale currents enhances the vertical exchange at the front on both the sub-inertial and inertial time scales.