Session 2

Tuesday, 17 May 2016, 11:00 AM to 12:30 PM

20 minutes per talk, 10 minutes for questions and speaker change

From	To	Name	Affiliation	Title
11:00am	11:30am	Tom Solomon	Bucknell University	Coherent structures in reactive front propagation and bacterial swimming in fluid flows
11:30am	12:00pm	Amala Mahadevan	Woods Hole Oceanographic Institution	Coherent pathways for vertical transport of carbon and oxygen from the ocean surface to depth
12:00pm	10:30pm	Larry Pratt	Woods Hole Oceanographic Institution	Computing and Visualizing Residual Property Fluxes

 

Session 2 Abstracts

Coherent structures in reactive front propagation and bacterial swimming in fluid flows

Tom Solomon (Bucknell University)
We present experiments on the behavior of propagating reaction fronts and swimming bacteria in laminar
fluid flows. Two- and three-dimensional magnetohydrodynamically-driven flows are used for the reaction
experiments, and a microfluidic flow in a T-channel is used for the bacteria studies. Reaction fronts are
produced by the excitable Belousov-Zhabotinsky chemical reaction, and the bacteria studied are a smoothswimming mutation of bacillus subtilis. All of these experiments are analyzed in terms of burning invariant
manifolds (BIMs) and swimming invariant manifolds (SWiMs) that act as local, one-way barriers that
impede the motion of the reaction front or of the swimming bacteria. We have completed several studies
showing the importance of BIMs for understanding reaction propagation in 2D flows. More recently, we
have extended these ideas to 3D flows in which the BIMs take the form of tubes or sheets, which are also
one-way in their blocking behavior. The bacteria experiments have only recently been initiated, but
preliminary experiments have shown evidence of one-way barriers consistent with predictions of SWiMs.

Coherent pathways for vertical transport of carbon and oxygen from the ocean surface to depth

Amala Mahadevan (Woods Hole Oceanographic Institution)
Transport within the oceanic eddy field is largely horizontal; vertical velocities are several orders of
magnitude smaller than horizontal velocities at scales of 1-100 km. But, the vertical transport of water
and biogeochemical properties is crucial for the oceanic biological pump. Here, I describe how water from
the surface mixed layer of the ocean is conveyed to depth in coherent features formed along the edges of
eddies. Using glider observations of oxygen and particulate organic carbon from the North Atlantic Bloom
experiment, we identify tongue-shaped filaments of carbon- and oxygen-rich waters that are subducted
from the surface. Process modeling experiments show that eddies and fronts provide downwelling
pathways along sloping isopycnal surfaces. We derive a scaling relationship to quantify the eddy-driven
subduction rate of phytoplankton carbon from the oceanic mixed layer, and infer that this mechanism
contributes to significant export of carbon and oxygen from the surface ocean to depth.

Computing and Visualizing Residual Property Fluxes

Larry Pratt (Woods Hole Oceanographic Institution)
Dynamical systems analysis provides for the mapping and visualization of fluid transport pathways and
barriers. Some techniques such as lobe analysis or elliptical boundary identification can lead to
quantification of volume transport between different regions of a flow field. Although a knowledge of
volume transport may be valuable, one is often more interested in the fluxes and budgets of scalar
properties such as heat, vorticity or nutrients. In addition, the presence of small-scale structures in the
flow field, particularly when sub-mesoscales are resolved, may render techniques such as lobe analysis
impractical. I will describe a new approach designed to address these difficulties.