Forcing of the overturning circulation across a circumpolar channel by internal wave breaking

Abstract

The hypothesis that the impingement of mesoscale eddy flows on small-scale topography regulates diapycnal mixing and meridional overturning across the deep Southern Ocean is assessed in an idealized model. The model simulates an eddying circumpolar current coupled to a double-celled meridional overturning with properties broadly resembling those of the Southern Ocean circulation and represents lee wave-induced diapycnal mixing using an online formulation grounded on wave radiation theory. The diapycnal mixing generated by the simulated eddy field is found to play a major role in sustaining the lower overturning cell in the model, and to underpin a significant sensitivity of this cell to wind forcing. The vertical structure of lower overturning is set by mesoscale eddies, which propagate the effects of near-bottom diapycnal mixing by displacing isopycnals vertically.

The Energetics of Southern Ocean Upwelling

“…The amplification of this abyssal cell is seen in both the latitude-density and depth-density streamfunctions, but it is a transient effect due to the breakdown of the polynya after year 520. However, it has also been proposed that the abyssal cell may be sensitive to the strength of abyssal mixing, induced by the wind-driven Southern Ocean mesoscale eddy field (Stanley and Saenko 2014;Broadbridge et al 2016). This mechanism would act to sustain the thermally direct abyssal cell by enhancing the water mass transformation that is required to close the magenta cell in Fig.…”

The Impact of Representations of Realistic Topography on Parameterized Oceanic Lee Wave Energy Flux

“…Implementations of lee wave mixing parameterizations in global models have generally employed a static map of lee wave energy flux (Melet et al., 2014; Nikurashin & Ferrari, 2013), rather than using online bottom velocities and stratification to update the energy flux and inferred dissipation and mixing as the model runs. However, there is evidence from idealized models that feedbacks between bottom velocities, stratification, and lee waves can significantly alter the ocean state (Broadbridge et al., 2016; Yang et al., 2021), motivating the need for online parameterizations. Both the spectral and peaks methods considered here would be computationally expensive if implemented directly at run‐time in a coarse global ocean model.…”

“…Global estimates based on linear theory have estimated that the global energy flux into lee waves is between 0.15 and 0.75 TW, with over half occurring in the SO (Nikurashin & Ferrari, 2011; Nikurashin et al., 2014; Scott et al., 2011; Trossman et al., 2013; Wright et al., 2014). Parameterizations of lee wave driven mixing have been applied to ocean models using these estimated maps of wave generation by assuming that wave energy decays in the bottom few hundred meters of the ocean and inferring a corresponding turbulent diffusivity, showing that lee wave driven mixing has a significant impact on the ocean state and MOC through deep water mass transformation (Broadbridge et al., 2016; Melet et al., 2014; Nikurashin & Ferrari, 2013).…”

Mixing by Oceanic Lee Waves

“…The idealized eddy-resolving channel model of Broadbridge et al (2016), a representation of the Southern Ocean, does include a temporally and spatially varying parameterization of lee-wave driven mixing. The diapycnal mixing is calculated using eqn 50, with q L = 1, a linear function for F (z), and a time-dependent estimate of E L (x, y, t), using a (Goff & Arbic, 2010) spectral representation of topography and the instantaneous model near-bottom velocities.…”

“…: While some large-scale circulation models have connected the mesoscale eddy energy loss to mixing, and examined the impact on the large-scale flow (Stanley & Saenko, 2014), these models have not incorporated lee-wave theory to tie this energy transfer to topography. Lee-wave parameterizations have only been implemented in static form in global circulation models (Melet et al, 2014), or in a fully time-evolving form in an idealized model (Broadbridge et al, 2016), and the associated bottom drag has not been included. We have not yet fully explored the influence of lee wave-driven mixing in a realistic global model where the energy conversion varies spatially and temporally as a function of changing mesoscale eddy velocities and stratification.…”