Energy Loss from Transient Eddies due to Lee Wave Generation in the Southern Ocean

Observations suggest that enhanced turbulent dissipation and mixing over rough topography are modulated by the transient eddy field through the generation and breaking of lee waves in the Southern Ocean. Idealized simulations also suggest that lee waves are important in the energy pathway from eddies to turbulence. However, the energy loss from eddies due to lee wave generation remains poorly estimated. This study quantifies the relative energy loss from the time-mean and transient eddy flow in the Southern Ocean due to lee wave generation using an eddy-resolving global ocean model and three independent topographic datasets. The authors find that the energy loss from the transient eddy flow (0.12 TW; 1 TW = 1012 W) is larger than that from the time-mean flow (0.04 TW) due to lee wave generation; lee wave generation makes a larger contribution (0.12 TW) to the energy loss from the transient eddy flow than the dissipation in turbulent bottom boundary layer (0.05 TW). This study also shows that the energy loss from the time-mean flow is regulated by the transient eddy flow, and energy loss from the transient eddy flow is sensitive to the representation of anisotropy in small-scale topography. It is implied that lee waves should be parameterized in eddy-resolving global ocean models to improve the energetics of resolved flow.This research was undertaken on the NCI National Facility in Canberra, Australia, which is supported by the Australian Government. LY was supported by the joint CSIRO–UTAS QMS program. MN was supported by the Australian Research Council (ARC) Discovery Early Career Research Award (DECRA) Fellowship (DE150100937)

Revisiting the seasonal cycle of the Timor throughflow: impacts of winds, waves and eddies

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Peña‐Molino, B., Sloyan, B., Nikurashin, M., Richet, O., & Wijffels, S. Revisiting the seasonal cycle of the Timor throughflow: impacts of winds, waves and eddies. Journal of Geophysical Research: Oceans, 127, (2022): e2021JC018133, https://doi.org/10.1029/2021jc018133.The tropical Pacific and Indian Oceans are connected via a complex system of currents known as the Indonesian Throughflow (ITF). More than 30% of the variability in the ITF is linked to the seasonal cycle, influenced by the Monsoon winds. Despite previous efforts, a detailed knowledge of the ITF response to the components of the seasonal forcing is still lacking. Here, we describe the seasonal cycle of the ITF based on new observations of velocity and properties in Timor Passage, satellite altimetry and a high-resolution regional model. These new observations reveal a complex mean and seasonally varying flow field. The amplitude of the seasonal cycle in volume transport is approximately 6 Sv. The timing of the seasonal cycle, with semi-annual maxima (minima) in May and December (February and September), is controlled by the flow below 600 m associated with semi-annual Kelvin waves. The transport of thermocline waters (<300 m) is less variable than the deep flow but larger in magnitude. This top layer is modulated remotely by cycles of divergence in the Banda Sea, and locally through Ekman transport, coastal upwelling, and non-linearities of the flow. The latter manifests through the formation of eddies that reduce the throughflow during the Southeast Monsoon, when is expected to be maximum. While the reduction in transport associated with the eddies is small, its impact on heat transport is large. These non-linear dynamics develop over small scales (<10 km), and without high enough resolution, both observations and models will fail to capture them adequately.B. Peña-Molino, B. M. Sloyan, M. Nikurashin, and O. Richet were supported by the Centre for Southern Hemisphere Oceans Research (CSHOR). CSHOR is a joint research Centre for Southern Hemisphere Ocean Research between QNLM and CSIRO. S. E. Wijffels was supported by the US National Science Foundation Grant No. OCE-1851333

Temporal Variability of Diapycnal Mixing in Shag Rocks Passage

Diapycnal mixing rates in the oceans have been shown to have a great deal of spatial variability, but the temporal variability has been little studied. Here we present results from a method developed to calculate diapycnal diffusivity from moored Acoustic Doppler Current Profiler (ADCP) velocity shear profiles. An 18-month time series of diffusivity is presented from data taken by a LongRanger ADCP moored at 2400 m depth, 600 m above the sea floor, in Shag Rocks Passage, a deep passage in the North Scotia Ridge (Southern Ocean). The Polar Front is constrained to pass through this passage, and the strong currents and complex topography are expected to result in enhanced mixing. The spatial distribution of diffusivity in Shag Rocks Passage deduced from lowered ADCP shear is consistent with published values for similar regions, with diffusivity possibly as large as 90 × 10-4 m2 s-1 near the sea floor, decreasing to the expected background level of ~ 0.1 × 10-4 m2 s-1 in areas away from topography. The moored ADCP profiles spanned a depth range of 2400 to 1800 m; thus the moored time series was obtained from a region of moderately enhanced diffusivity. The diffusivity time series has a median of 3.3 × 10-4 m2 s-1 and a range of 0.5 × 10-4 m2 s-1 to 57 × 10-4 m2 s-1. There is no significant signal at annual or semiannual periods, but there is evidence of signals at periods of approximately fourteen days (likely due to the spring-neaps tidal cycle), and at periods of 3.8 and 2.6 days most likely due to topographically-trapped waves propagating around the local seamount. Using the observed stratification and an axisymmetric seamount, of similar dimensions to the one west of the mooring, in a model of baroclinic topographically-trapped waves, produces periods of 3.8 and 2.6 days, in agreement with the signals observed. The diffusivity is anti-correlated with the rotary coefficient (indicating that stronger mixing occurs during times of upward energy propagation), which suggests that mixing occurs due to the breaking of internal waves generated at topography

Efficiency of turbulent mixing in the abyssal ocean circulation

Turbulent mixing produced by breaking of internal waves plays an important role in setting the patterns of downwelling and upwelling of deep dense waters and thereby helps sustain the global deep ocean overturning circulation. A key parameter used to characterize turbulent mixing is its efficiency, defined here as the fraction of the energy available to turbulence that is invested in mixing. Efficiency is conventionally approximated by a constant value near one sixth. Here we show that efficiency varies significantly in the abyssal ocean and can be as large as approximately one third in density stratified regions near topographic features. Our results indicate that variations in efficiency exert a first-order control over the rate of overturning of the lower branch of the meridional overturning circulation.EPSRC Programme grant EP/K034529/1 entitled “Mathematical Underpinnings of Stratified Turbulence.