The role of lee waves for the energy loss from the eddy field in the Southern Ocean
Updated: Mar 7, 2019
Are you familiar with this painting (Figure 1)? Do you know that you can find a similar pattern in the ocean? They are eddies, some of them appear as coherent structures (rings) while others are more irregular. Generally, eddies mean deviations from the mean (Vallis 2006).
Figure 1. The Starry Night by Vincent van Gogh.
A video made by Ryan Abernathey showing the circulation simulated by an idealised channel model representing the Southern Ocean. This video nicely visualises the generation of eddies in the Southern Ocean as the large-scale current becomes unstable gradually. Colours represent ocean temperature and white contours are constant density layers (isopycnals).
Generation of eddies
The main generation mechanism of eddies has been long known as the baroclinic instability (e.g., Gill et al. 1974). In the Southern Ocean, the baroclinicity of the Antarctic Circumpolar Current (ACC) is induced and sustained by the westerly wind and buoyancy forcing (e.g., Howard et al. 2015). Recent studies (e.g., Barthel et al. 2017; Youngs et al. 2017) have shown that eddies are actually generated by the mixed barotropic-baroclinic instability.
A video introducing the circulation in the Southern Ocean produced by National Computational Infrastructure (NCI) Australia.
Energy equilibration of eddies
The transient eddy field is a large energy reservoir in the Southern Ocean; the eddy kinetic energy (EKE) dominates the kinetic energy field in the Southern Ocean (e.g., Ferrari and Wunsch 2009). However, the energy budget of the eddy field in the ocean remains unclear (Figure 2, e.g., Wunsch and Ferrari 2004; Ferrari and Wunsch 2009). Eddies in the ocean get energy from the wind and large-scale circulation (e.g., fronts). There are several dissipation mechanisms that have been proposed for the eddy flow:
dissipation in the turbulent bottom boundary layer (TBBL) (e.g., Sen et al. 2008; Arbic et al. 2009),
interactions with the background internal wave field (e.g., Bühler and McIntyre 2005; Polzin 2008),
loss of balance (e.g., McWilliams and Yavneh 1998; Molemaker et al. 2005), and
Kelvin wave hydraulic control at large-scale topography (e.g., Hogg et al. 2011).
Figure 2. Strawman energy budget for the global ocean circulation (source: Wunsch and Ferrari 2004).
Figure 3. The source, flow and fate of energy in the oceanic general circulation (source: McWilliams 2016).
Do lee waves matter in the energy budget of eddies?
Lee waves have been suggested to extract a significant amount of energy from the eddy field. Observations have found elevated turbulent energy dissipation rates and inferred enhanced diffusivity with respect to their background values over rough topography in the Southern Ocean (Figure 4, e.g., Polzin et al. 1997; Naveira Garabato et al. 2004; St. Laurent et al. 2012; Meyer et al. 2015). Enhanced turbulent dissipation and mixing are modulated by the strength of the eddy field downstream of rough topographic features (e.g., Liang and Thurnherr 2012, Sheen et al. 2014). This modulation is likely through the generation and breaking of lee waves. The hypothesis that breaking lee waves being a major driver for vigorous mixing is supported by observations from the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) and the Southern Ocean Finestructure project (SOFine). In agreement with observations, idealised simulations also highlight the importance of lee waves in the energy pathway from eddies to turbulence (Figure 5, Nikurashin et al. 2013). However, the energy loss from eddies due to lee wave generation remains poorly estimated.
Figure 4. The spatial distribution of mixing in the Southern Ocean is exemplified by a section along the rim of the Scotia Sea (source: Naveira Garabato et al. 2004). Enhanced mixing rates (represented by light blue to red colours) are strikingly associated with strong flow and rough topography. [SAF, Subantarctic Front, the ACC’s northern boundary; PF, Polar Front and its southern boundary (SB) are marked in the upper axis. The frontal regions are where the flow is strong. These strong flows tend to be barotropic in the Southern Ocean and extend to the seafloor.]
Figure 5. Zonal sections of (a) speed and (b) energy dissipation rate (source: Nikurashin et al. 2013). These are snapshots from a super-high resolution simulation with rough bathymetry. Energy dissipation rate near the bottom is clearly elevated (indicated by red colour in b) where the flow is strong (indicated by the light blue colour in a).
In a part of my PhD work, we quantify the impacts of lee wave generation on the transient eddy flow in the Southern Ocean (Figure 6). While the eddy flow in the Southern Ocean can be dissipated by the generation of lee waves at rough topography and turbulence in the TBBL, among others, the dissipation rate due to the lee wave generation (0.12 TW) exceeds that due to TBBL processes (0.05 TW).
Figure 6. Energy conversion from (a) time-mean and (b) eddy flow to lee waves using topographic parameters from Goff (2010), (c) energy dissipation of the eddy flow in the TBBL (adapted from Yang et al., 2018).
Our results provide a quantitative evidence that lee waves effectively weaken the eddy flow in the Southern Ocean. The implication is that, the effects of (unresolved) lee waves on the (resolved) eddy flow should be included in eddy-resolving ocean models in a self-regulating way to study the sensitivity of the Southern Ocean circulation to changes in wind.
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