Propagation of near-inertial waves beneath atmospheric storm tracks on the non-traditional β-plane

Propagation of near-inertial waves beneath atmospheric storm tracks on the non-traditional β -plane Marine Tort and Kraig B. Winters Scripps Institution of Oceanography, University of California San Diego mtort@ucsd.edu Abstract We present results of 2d, in the meridional-depth plane, and 3d numerical simulations of near-inertial waves (NIWs) propagating in a stratified, turbulent mid-latitude ocean on the non-traditional (NT) β-plane. Here NT refers to the retention of the horizontal component of the Coriolis force. Continuous surface wind forcing, containing both high- and low-frequency components produces propagating NIWs and a baroclinic unstable zonal jet, sustaining an active field of mesoscale eddies in 3d. In 2d, most of the NIWs propagate equatorward, with poleward propagating waves being reflected back toward the equator at a nearby turning latitude. With NT effects included, waveguides exist that permit weak sub-inertial wave poleward propagation. In 3d, a poleward energy flux, carried by super-inertial internal waves, is found to be as strong as the equatorward flux. These results suggest that, while the inclusion of NT effects permits sub-inertial wave propagation, fluctuating zonal winds over an eddying ocean do not excite these motions very strongly. Introduction NIWs are rapidly oscillating waves generated by wind bursts at the ocean surface that significantly contribute to the interior ocean mixing required to maintain the global merid- ional overturning circulation (Garrett and Munk, 1972). These waves transfer energy from large to small scales, where the energy is converted into heat through dissipation and potential energy through diapycnal mixing. It is unclear however what fraction of the energy input at the sea surface is actually available for ocean mixing and how the cascade to smaller scales and mixing occurs. NIWs may spread both horizontally and vertically into the interior, but only when their horizontal scale is much smaller than the wind scales. Two mechanisms significantly reduce the initially large scales of these wind- driven motions: (1) the β-effect (e.g. D’Asaro 1989) which can reduce infinite scales to R d = O(100) km in about t = 2π/(βR d ) = 40 days and (2) the relative vorticity gradient ∇ζ (Kunze, 1985) which may be a much more efficient mechanism to generate small scales than (1) if ∇ζ β. Those two effects also alter the propagation of the NIWs. (1) Within the traditional approximation (TA), when the near-inertial (NI) energy has been drained from the surface to the interior, all energy radiated from the source eventu- ally propagates towards the equator, with the initial poleward propagation being reflected at the inertial latitude due to the β-dispersion effect (e.g. Anderson and Gill 1979; Gar- rett 2001). While relaxing the TA, the allowed frequency range is enlarged and the NIWs can propagate sub-inertially beyond the inertial latitude. The combined effects of the NT Coriolis force and weak stratification leads to the existence of waveguides for sub-inertial waves, which get trapped and propagate poleward (Gerkema and Shrira, 2005b,a; Winters et al., 2011). VIII th Int. Symp. on Stratified Flows, San Diego, USA, Aug. 29 - Sept. 1, 2016