This page, and its subpages, contains (or will contain) brief, accessible descriptions of the range of projects that I am currently working on or have completed. Please follow the links for more detailed subpages and/or the published articles.
Diathermal Heat Transport in a Global Ocean Model
With Jan Zika, Matthew England and a number of other collaborators national and international collaborators I am looking at how heat is transported between different temperature layers within a series of global ocean models. We have developed a novel set of heat budget diagnostics within the MOM5 global ocean model in which we can robustly diagnose heat transport across isotherms and in three-dimensional space and its various contributing processes.
The first part of this project focused on the temperature and seasonal structure of the various mixing processes that contribute to the ocean’s globally-integrated diathermal heat transport (Holmes et al. 2019, Diathermal heat transport in a global ocean model, J. Phys. Oceanogr.).
In collaboration with Raffaele Ferrari (MIT), Andrew Thompson (CalTech) and Emily Newsom (Oxford), we have published a follow-up study that shows the critical role that turbulent mixing and air-sea heat fluxes in the tropical Pacific ocean play for the global ocean heat transport (Holmes et al. 2019, Atlantic ocean heat transport enabled by Indo-Pacific heat uptake and mixing, Geophysical Research Letters). Specifically, this study showed that mixing drives heat out of the shallow wind-driven circulation in the Indo-Pacific basins to the deep cold overturning circulation in the Atlantic allowing heat to be transfered northward in the Atlantic.
Finally, this project has also given us a method in which to accurately and robustly quantify the spatial and temporal structure of numerical mixing. Numerical `spurious’ mixing arrises from the model’s discretization of the tracer advection scheme. We are currently applying this method to quantify and compare the levels of numerical mixing across a suite of global ACCESS-OM2 simulations with varying horizontal and vertical resolutions, and varying explicit mixing parameterizations.
Tracer Transport near Abyssal Sloping Topography
With Casimir de Lavergne and Trevor McDougall I have studied the behavoir of tracers released near the abyssal seafloor where turbulent mixing is bottom intensified (Holmes et al. 2019, Tracer transport within abyssal mixing layers, J. Phys Oceanogr.). Using one-dimensional boundary layer theory and the statisical moment method we have shown that the presence of the bottom boundary can in many cases reduce the rate at which tracers are transported across isopycnals. This work should help in the analysis of future tracer release experiments performed in the field (such as the planned Rockall Trough experiment).
Contribution of Stochastic Oceanic Variability to ENSO
Holmes R. M., S. McGregor, A. Santoso and M.H. England (2018) Contribution of Tropical Instability Waves to ENSO Irregularity, Climate Dynamics
The Influence of Topography on Dianeutral Transport in the Abyssal Ocean
Holmes R. M., C. de Lavergne and T. J. McDougall (2018) Ridges, Seamounts, Troughs and Bowls: Topographic Control of the Dianeutral Circulation in the Abyssal Ocean, Journal of Physical Oceanography, 48, 861–882. (Description coming…)
de Lavergne, C., G. Madec, F. Roquet, R. M. Holmes and T. J. McDougall (2017) Abyssal ocean overturning shaped by seafloor distribution, Nature, 551,181–186.
Coastal Antarctic subsurface warming and ice sheet melt
Spence, P., R. M. Holmes, A. McC. Hogg, S. M. Griffies, K. D. Stewart, and M. H. England (2017), Localized rapid warming of West Antarctic subsurface waters by remote winds, Nature Climate Change, 7 (8), 595-603.
Webb, D., Holmes R. M., Spence, P. and England, M.H. (2019): Barotropic Kelvin wave-induced bottom boundary layer warming along the West Antarctic Peninsula. Journal of Geophysical Research, 124 (3), 1595-1615.
The unique dynamics of the equatorial oceans play an important role in the El Nino - Southern Oscillation (ENSO) and the ocean’s meridional overturning circulation (MOC), both of which are critical processes that drive global climate variability on a range of time-scales. The character of ENSO depends on the detailed upper ocean processes that influence the sea surface temperature (SST) budget and the equatorial waves that help the upper Pacific ocean to adjust to perturbations in atmospheric forcing. Inverse models suggest that the abyssal cell of the MOC is closed through diapycnal upwelling mostly in the tropical oceans and thus depends on abyssal mixing there that has not been well observed. My research focuses on understanding equatorial ocean processes on a range of scales, how they interact and how they influence large scale climate variability such as the ENSO cycle and the MOC.
The equatorial oceans are a unique region of the ocean where the normal ‘rules of ocean circulation’ do not apply. Large scale processes (mesoscale and above) in the mid- and high- latitude oceans are heavily constrained by rotation (through the Coriolis frequency f) and stratification (through the buoyancy frequency N). These balanced flows obey the the geostrophic and hydrostatic force balances, and are approximately 2D. However, at the equator the Coriolis frequency goes to zero, and thus geostrophic balance can break down. This change in dynamics can have important consequences including an enhanced role for nonlinear and frictional processes at large scales and the unique character of equatorial internal waves.
My PhD research was centered around both vertical and lateral mixing at the equator. I worked on a number of projects involving the dynamics of tropical instability waves (TIWs), vertical and lateral mixing at the equator, abyssal mixing in the equatorial oceans and equatorial Kelvin waves.
Chapter 1: Dynamics of Tropical Instability Vortices
Due to their location near the equator where the Coriolis parameter is small, submesoscale-like dynamics appear at a larger spatial scale than in the mid-latitudes. In an article published in the Journal of Physical Oceanography (Potential Vorticity Dynamics of Tropical Instability Vortices), I have shown that the formation of TIV core water shares some similarities with the formation of submesoscale intrathermocline eddies in the midlatitudes. (see subsection for more information)
Chapter 1b: TIW Fronts
Part of the R/V Oceanus research cruise in 2014 that I participated in (see chapter 4 below) was aimed at studying the dynamics of strong TIW fronts. These fronts have a large influence on ocean ecosystems and may also constitute a significant energy loss mechanism for the TIWs. In collaboration with Sally Warner and Jim Moum at Oregon State University I investigated these fronts from a modelling pospective, and compared them to several real fronts observed using ADCP, CTD and microstructure turbulence instruments on two separate research cruises in the equatorial Pacific. This work has been published in the Journal of Physical Oceanography
Chapter 2: The influence of Tropical Instability Waves on Vertical Mixing in the EUC
Recent observations have shown that turbulent mixing within the EUC is modulated by TIWs, with implications for the role of TIWs in the heat budget of the upper equatorial Pacific. In an article published in the Journal of Physical Oceanography (The Modulation of Equatorial Turbulence by Tropical Instability Waves in a Regional Ocean Model), I showed that large-scale TIW strain can modulate the shear of the EUC through horizontal vortex stretching. This modulation of the EUC shear drives a subsequent modulation of the Richardson number and thus shear-driven vertical mixing. Furthermore, using a simple 1D mixing model of the EUC driven by periodic TIW strain, I was able to show that this modulation drove an overall increase in the turbulent heat flux, with the amount of increase dependent on the parameterization for shear-driven vertical mixing included in the mixing model. Thus, while TIWs are thought to warm the equatorial cold tongue through lateral mixing, their influence on vertical mixing results in a net surface cooling that offsets the lateral heating.
Chapter 3: The Interaction of TIWs and Kelvin waves
For the final part of my PhD I looked at the interaction of TIWs with equatorial Kelvin waves in an idealized ocean model. This work was motivated by observations made during the R/V Oceanus research cruise where an interseasonal Kelvin wave propagating through the eastern Pacific induced large changes in the intensity of the TIW field. Using an ensemble set of ROMS simulations we showed that downwelling Kelvin waves can drive drop of up to 40% in the intensity of TIWs, as measured with their kinetic energy (TIWKE). In contrast, upwelling Kelvin waves drove an intensification in TIWKE of a similar magnitude. These TIWKE changes occured because of Kelvin wave induced modifications to the background equatorial circulation from which TIWs gain energy. This work also revealed the importance of wave radiation in the TIWKE budget. The results of this study were published in the Journal of Physical Oceanography Holmes and Thomas (2016) Modulation of Tropical Instability Wave Intensity by Equatorial Kelvin Waves.
Chapter 4: Abyssal Mixing at the Equator
Inverse models of the meridional overturning circulation (MOC) of the global ocean suggest that much of the diapycnal mixing required to sustain the upwelling of deep waters as part of the abyssal cell occurs in the tropical oceans. However, there are very few direct measurements of small-scale turbulence in the abyssal equatorial oceans. In November and December 2014 I participated in a research cruise aboard the R/V Oceanus organised by Jim Moum from Oregon State University who’s aims included measuring abyssal mixing in the eastern equatorial Pacific. Our measurements, published in Geophysical Research Letters (Holmes et. al. (2016) Evidence for Seafloor-Intensified Mixing by Surface-Generated Equatorial Waves), revealed intense mixing located near the seafloor over a region of smooth topography near the equator. These measurements are unique because previously most intense mixing in the deep ocean was thought to occur only over rough topographic features. Instead, we suggest that the mixing that we observed was generated by downward propagating equatorial waves generated at the surface. This constitutes a unique energy pathway through which wind energy can mix the deep ocean, and highlights the need for more observations and research work looking at deep mixing near the equator.
Also see the Visualizations page for an animation of deep equatorial waves under the traditional and non-traditional approximation.