Southern Ocean Dynamics and the Climate

Southern Ocean Dynamics and the Climate

The approximate domain of my focus in the Southern Ocean within which I have studied (I) lateral (along density or isopycnal) stirring and mixing across the Antarctic Circumpolar Current (ACC) fronts (SAF, PF and SB marked here) both upstream and downstream of the Drake Passage, (II) vertical mixing and its inter-connection with lateral stirring in the Drake Passage and Scotia Sea region and (III) internal wave generation off rough topography and the stratified turbulence that ensues.

Turbulent mixing across the Antarctic Circumpolar Current (ACC) as well as mixing in the vertical go hand in hand in playing important roles in (I) exchange of buoyancy and momentum between the ocean and atmosphere, (II) oceanic uptake of atmospheric CO_2, (III) regulating the Southern Ocean branch of the Meridional Overturning Circulation(MOC), (IV) regulation of the role of the upper branch of MOC in transferring the global warming signal `felt' by the upper ocean and in the Northern Hemisphere to the Southern Ocean, and (V) redistribution of such heat around Antarctica.

Tracer mixing in the Southern Ocean

Tracer mixing in the Southern Ocean

Numerical modeling of lateral stirring and vertical mixing of an anthropogenic tracer released in the Southern Ocean in the Drake Passage (DP) and Scotia Sea(SS) region. DP and SS host rough topographic features in form of ridge systems and seamounts. Hence, they play a critical role in mixing across the ACC and linkage the horizontal mixing with vertical mixing over mixing hotspots.

The simulation, based on the MIT general circulation model,  is forced by observations at the boundaries. The tracer is also closely modeled after a real tracer injected in the Southern Ocean as a part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES).   [from Mashayek et al. Nature Comm. 2017 In Press

Upwelling Along Rough Topography

Upwelling Along Rough Topography

Upwelling of abyssal carbon rich waters along rough topographic slopes in the Southern Ocean (Drake Passage and Western Scotia Sea). Red represents cross-density upwelling while blue represents weak broad interior sinking of water. This work is being written up and connects to the story of role of along-boundary upwelling in closure of the global meridional overturning circulation. The global connection is discussed in another one of images in this series. The combined works are highlighted here:

http://www.nature.com/news/role-of-chaos-in-deep-ocean-turned-upside-down-1.19455

[see Mashayek et al. JPO 2015, Ferrari, Mashayek et al. JPO 2016, Mashayek et al. Nature Comm. 2017-In Press]

Turbulence Energetics of Global Ocean

Turbulence Energetics of Global Ocean

Insofar as deep ocean shear-induced turbulent mixing is concerned, energy is provided to turbulence through the internal wave field radiated at the bottom (by tidal and geostrophic motions over rough topography) as well as the downward propagating waves generated in the upper ocean. It is commonly assumed that from the energy available to turbulence, ~20% goes to cross-density mixing (i.e. net downward diffusion of buoyancy which leads to a net rise in potential energy; buoyancy decays with depth) with the rest of the energy simply dissipating to heat. Recent developments in fluid mechanical understanding of turbulence induced by internal waves has allowed us to take a first stab at relaxing this assumption. This image shows the global distribution of the fraction of energy that goes to mixing. The spatial variations is significant and has a leading order influence on the rate of meridional overturning circulation of the ocean. [from Mashayek et al. Submitted to GRL, 2017]

Lee Wave Generation

Lee Wave Generation

Generation of lee waves over the Phoenix Ridge in the Southern Ocean. ACC flows from left to right. This is a preliminary plot from a very recent high resolution simulation (ongoing work) which will allow me connect the larger scale influence of turbulence to micro-scale physics of wave-induced turbulence. 

Mega Turbulence in Samoan Passage

Mega Turbulence in Samoan Passage

Turbulence induced by giant waves formed over rough topography in the Samoan Passage in the equatorial Pacific Ocean. Left panel shows mixing of dense waters of Antarctic origin (blue) with overlying lighter waters, leading to lighter waters in the deep North Pacific ocean (red). The choke-point is believed to be responsible for ~6 SV [1 SV=10^6 m^3/s] of transformation of dense waters. The right panel shows flow over the sills along one of the channels of the passage. The right-top panel shows dense (blue) waters mixing intensely with lighter overlying waters (red) after crossing the second sill. The lower panel shows rate of dissipation of kinetic energy (marking regions of enhanced turbulent mixing) zoomed over the second sill.  Hydraulic overturns formed in the lee of the sills are as deep as 500m and shear instability wave trains forming in the lee of the hydraulic jumps can be few hundred meters deep. This is an exquisite example that directly connects phenomenology of turbulence to large scale ocean basin mixing and circulation.

I have just begun work on this project in collaboration with Gunnar Voet and Matthew Alford at SIO, and others involved in a recently funded project. The image is made by Gunnar. [see Voet et al. JPO 2015,2016, Alford et al. 2013]

Density Stratified Turbulence

Density Stratified Turbulence

Study of the turbulence induced by breaking of waves is a classic topic in Fluid Mechanics. Recently it has been re-energized (the arrogant me likes to think that my PhD thesis had was partially responsible for it) thanks to high performance computing. 

The picture shows a Kelvin Helmholtz billow in transition to turbulence. The transition is facilitated through a cascade of instabilities. A nice example of self-similar structures. The color shows interwinding of heavy (blue) and light (red) waters. Once turbulence saturates, mixing leads to water of intermediate density.

Studying such structures is at the core of understanding of wave-induced turbulence on a global scale and its influence on global circulation, as discussed in various images in this gallery. Just as cloud physics is a micro-scale process with global climatic implications, micro-scale mixing is of leading order importance for ocean circulation and thereby its role in the climate system.

[see Mashayek and Peltier JFM 2011a,b,2013; Salehipour et al. JFM 2015, Mashayek et al JFM 2013, under review] 

Mixing and Closure of MOC

Mixing and Closure of MOC

An Idealized attempt to link deep ocean mixing to closure of the Meridional Overturning Circulation (MOC). The bowl-shaped domain with a re-entrant periodic channel represents and idealized Pacific Ocean. The model is forced by prescribing a bottom enhanced buoyancy flux (motivated by a wealth of localized measurements of turbulent across the global ocean). While the flux increases with depth (mimicking bottom enhanced mixing due to breaking of internal waves), it ultimately goes to zero when approaching the bottom boundary over a narrow boundary layer.  Here we showed that such a variation in the flux, which stands in a temporal-spatial averaged sense, implies upwelling along sloped boundaries and sinking within the interior, which is somewhat counter to common assumption that internal wave-induced mixing leads to global upwelling. The realistic work we've done in the Drake Passage (discussed in one of the images in this series) shows that such upwelling/downwelling pattern holds in the Southern Ocean. Global extension is focus of ongoing research, as is the physics of turbulence in the boundary layers within which upwelling occurs.

[see Mashayek & Ferrari et al. and Ferrari & Mashayek et al.  JPO 2015,2016]