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Turbulence and Mixing

The Ocean plays a major role in the Earth's climate system, as the major converter of short wave solar radiation into heat, as a transporter of heat around the planet and as a major sink of atmospheric carbon dioxide. Fluxes across key interfaces within the marine system are therefore exert a key control on global climate variability.

The aim of the Turbulence and Mixing group is the identification, quantification The aim of the Turbulence and Mixing group is the identification, quantification and parameterisation of the key physical processes which drive fluxes across the critical interfaces within the marine system thus contributing to the improved predictive capacity of ocean and climate models.

Wind Driven "Shear Spikes"

We have recently identified wind – shear alignment as a key mechanism by which wind driven inertial oscillations drive mixing across the seasonal thermocline in seasonally stratified and polar shelf seas (Burchard and Rippeth, 2009; Lenn et al., 2011) and the open ocean (Brannigan et al, 2013), and also in surface mixed layer deepening (Lincoln et al., 2016), leading to a wind driven nutrient pump (Williams et al., 2013; Rumyantseva et al., 2015). This work has been further developed to look at the impact of mixing across the seasonal thermocline on the air-sea CO2 flux in seasonally stratified shelf seas (Rippeth et al, 2015).

The Internal Tide and tidal drag

Internal tides are also implicated as a key energy source in sustaining the climate controlling oceanic meridional overturning circulation. Our work here has focused on improving our understanding of how the internal tide is generated and dissipated (eg. Stephenson et al, 2015; Rippeth et al., 2017; Vic et al., 2017), improved parameterisations (eg. Green and Nycander, 2013), how it may change over long time scales, along with tidal dissipation in general (Green et al., 2009; Wilmes and Green, 2014; Green and Huber 2013; Green et al., 2017), and what impact it has on the large scale ocean circulation (Green et al., 2009; Green and Bigg, 2011; Green and Huber 2013; Scmittner et al., 2015).

Internal Hydraulic Jumps

Our work has identified unsteady lee-waves and internal hydraulic jumps as key mechanisms for the transfer of energy from the tides and geostrophic flow to turbulence and mixing in the ocean. In particular we have demonstrated that unsteady lee-wave formation over sloping topography poleward of the critical latitude is a key mechanism for driving mixing poleward of the critical latitude (Rippeth et al., 2017). We have also demonstrated the key role of hydraulic jumps in driving mixing in the deep ocean (Thorpe et al., 2017).

Technical developments

The group's work is strongly underpinned through technical developments in our ability to measure turbulence in the ocean. The group has led in the development and implementation of novel techniques for the measurement of turbulent fluxes within the marine environment for over two decades. Recent advances have included the extension of the ADCP structure function method originally proposed by Wiles et al. (2006) to low energy environments from a fixed platform suspended from a mooring (Lucas et al, 2014) and in the presence of waves (Scannell et al., 2017).

Associated publications

Brannigan, L; Y-D Lenn, Y-D; TP Rippeth; E McDonagh, TK Chereskin and J Sprintall (2013). Shear at the Base of the Oceanic Mixed Layer Generated by Wind Shear Alignment. Journal of Physical Oceanography, 43(8), 1798-1810 DOI: 10.1175/JPO-D-12-0104.1

Burchard, H and Rippeth, TP (2009). Generation of bulk shear spikes in shallow stratified tidal seas. Journal of Physical Oceanography 39, 969-985.

Green, J. A. M. and Bigg, G. R. (2011). Impacts on the global ocean circulation from vertical mixing and a collapsing Ice Sheet. Journal of Marine Research, 69, 221-244.

Green, J. A. M. and Huber, M. (2013). Tidal dissipation in the early Eocene and implications for ocean mixing. Geophysical Research Letters, 40, doi:10.1002/grl.50510.

Green, J.A.M. and Nycander, J. (2013). A comparison of internal wave-drag parameterizations for tidal models. Journal of Physical Oceanography, 43, 104-119.

Green, J.A.M., Green, C.L., Bigg, G.R., Rippeth, T.P., Scourse, J.D. and Uehara, K. (2009). Tidal mixing and the strength of the Meridional Overturning Circulation from the Last Glacial Maximum. Geophysical Research Letters 36, L15603.

Green J. A. M., M.Huber D.Waltham, J.Buzan, and M.Wells (2017): Explicitly modeled deep-time tidal dissipation and its implication for Lunar history. Earth and Planetary Science Letters, 461, 46–53.

Lenn Y-D, TP Rippeth, CP Old, S Bacon, I Polyakov, V Ivanov, J Hölemann (2011). Intermittent Intense Turbulent Mixing under Ice in the Laptev Sea Continental Shelf. Journal of Physical Oceanography, 41(3), 531-547

Lucas, NS, JH Simpson, TP Rippeth and CP Old (2014). Measuring turbulent dissipation using a tethered ADCP. Journal of Atmospheric and Oceanic Technology. 31 (8). pp. 1826-1837.

Rippeth, TP; Lincoln, BJ; Kennedy, HA; Palmer, MR; Sharples, J; Williams, CAJ (2014). Impact of vertical mixing on sea surface pCO2 in temperate seasonally stratified shelf seas. Journal of Geophysical Research: Oceans 06/2014; DOI: 10.1002/2014JC01008.

Rippeth, TP; Vlasenko, V; Stashchuk, N; Scannell, B; Green, JAM; Lincoln, B; Bacon, S (2017).Tidal conversion and mixing poleward of the critical latitude (an Arctic case study). Geophysical Research Letters, Vol. 44, No. 24, 10.1002/2017GL075310, 28.12.2017, p. 12349-12357

Rumyantseva A., Lucas N., Rippeth T., Henson S., Martin A. and Painter S., Boyd, TJ and Henson, S (2015). Ocean nutrient pathways associated with passage of a storm. Global Biogeochemical Cycles, 29, doi:10.1002/2015GB005097.

Schmittner, A., J. A. M. Green, and S-B. Wilmes (2015): Large acceleration of the ocean’s meridional overturning circulation during the Last Glacial Maximum due to enhanced tidal mixing. Geophysical Research Letters, GL063561

Stephenson, G., J. A. M. Green, and M. E. Inall (2016): Quantifying the bias in estimates of the baroclinic energy flux in shelf seas. Journal of Physical Oceanography, 56, 2851–2862.

Thorpe, S; Malarkey, J; Voet, G; Alford, M; Girton and Carter, G (2017). Application of a model of internal hydraulic jumps. Journal of Fluid Mechanics, 834, 17.11.2017, 125-148.

Vic, C., A. Naveira-Garabato, J. A. M. Green, C. Spingys, A. Forryan, Z. Zhao, and J. Sharples (2018): Internal tides and mixing over the northern Mid-Atlantic Ridge. Journal of Physical Oceanography, 48, 61–80

Williams, C., Sharples, J., Mahaffey, C. and Rippeth, T. (2013). Wind driven nutrient pulses to the subsurface chlorophyll maximum in seasonally stratified shelf seas Geophysical Research Letters, DOI: 10.1002/2013GL058171

Wilmes, S-B., and J. A. M. Green (2014): The evolution of tides and tidally driven mixing over 21,000 years. Journal of Geophysical Research, 119, doi:10.1002/2013JC009605.