An Idealized Model Study of Eddy Energetics in the Western Boundary “Graveyard”

Abstract

Recent studies show that the western boundary acts as a ‘graveyard’ for westward-propagating ocean eddies. However, how the eddy energy incident on the western boundary is dissipated remains unclear. Here we investigate the energetics of eddy-western boundary interaction using an idealised MIT ocean circulation model with a spatially variable grid resolution. Four types of model experiments are conducted: (1) single eddy cases, (2) a sea of random eddies, (3) with a smooth topography and (4) with a rough topography. We find significant dissipation of incident eddy energy at the western boundary, regardless of whether the model topography at the western boundary is smooth or rough. However, in the presence of rough topography, not only the eddy energy dissipation rate is enhanced, but more importantly, the leading process for removing eddy energy in the model switches from bottom frictional drag as in the case of smooth topography to viscous dissipation in the ocean interior above the rough topography. Further analysis shows that the enhanced eddy energy dissipation in the experiment with rough topography is associated with greater anticyclonic-ageostrophic instability (AAI), possibly as a result of lee wave generation and non-propagating form drag effect.

Can mesoscale eddy kinetic energy sources and sinks be inferred from sea surface height in the Agulhas Current ?

“…Our study supports the WB region of the Agulhas Current as a hotspot of mesoscale eddy generation, whose energy is then exported (EKE flux divergence > 0; Figures 2b and 4b). It is in contrast with the paradigm of the decay of remotely-generated mesoscale eddies at western boundaries (EKE flux divergence < 0), likely due to direct EKE routes channeled by topography (Zhai et al, 2010;Chelton et al, 2011;Evans et al, 2020;Z. Yang et al, 2021;Evans et al, 2022).…”

“…Western boundaries have been suggested to be mesoscale eddy kinetic energy (EKE) sinks (Zhai et al, 2010). This suggestion closes the following paradigm of mesoscale eddy lifecycle: mesoscale eddies originate nearly everywhere in the ocean, propagate westward at about the speed of long baroclinic Rossby waves and decay upon western boundaries, likely due to direct energy routes, down to dissipation, channeled by topography (Gill et al, 1974;Zhai et al, 2010;Chelton et al, 2011;Evans et al, 2020;Z. Yang et al, 2021;Evans et al, 2022).…”

“…In the same way, a study simulating an idealized western boundary, free of a mean current, showed a mesoscale eddies decay due to topographically-channeled turbulence in the presence of rough topography (Z. Yang et al, 2021). The different studies suggest that in the presence of an intense mean current, the local generation of EKE may overcome the local decay, while in the absence of intense generation processes, the local EKE decay will likely dominate.…”

“…Yang et al, 2021;Evans et al, 2022). This scenario has been confirmed, using in situ measurements and idealized numerical simulations, in regions free of western boundary current (Evans et al, 2020;Z. Yang et al, 2021;Evans et al, 2022).…”

Remote energy sources for mixing in the Indonesian Seas

“…On the other hand, the eddy energy flux convergence (negative, i.e., a sink) is concentrated in narrow regions along topography and within the straits, suggesting that the eddy energy is likely dissipated through eddy (wave)-topography interaction. Idealized model simulations for western boundary regions of ocean basin 30 show that significant eddy energy dissipation occurs at the western boundary regardless of whether the model topography is smooth or rough, with rough topography leading to enhanced energy dissipation. The magnitude of the eddy energy flux convergence is 10–30 mW m −2 along topography and reaches 100 mW m −2 within the straits.…”