PAOC Spotlights

John Taylor, who came to PAOC as an NSF funded Mathematical Sciences Postdoctoral Research Fellow, is moving to Cambridge University in the U.K. at the end of the month, to take up a faculty position as a University Lecturer in the Department of Applied Mathematics and Theoretical Physics.

During his time at MIT John has been studying the impact of fronts and small eddies on mixing in the upper ocean and most recently how this mixing affects phytoplankton. One of the key themes of his research has been to better understand small-scale processes that are not captured in climate-scale models. It is becoming increasingly apparent, he says, that small scale physical processes have a profound impact on the ocean's biology, and he will continue to study this connection.

In the context of seeking to understand how fronts and small eddies impact upper ocean mixing, the movie below illustrates how symmetric instability can develop in the ocean and atmosphere when the potential vorticity is less than zero. In this large-eddy simulation, a density front in the upper ocean was forced by cooling the surface of the ocean with a constant heat flux, which also acts to decrease the potential vorticity. As symmetric instability develops, the flow (vectors) becomes nearly aligned with the isopycnal surfaces (color). Eventually, the vertical shear associated with symmetric instability becomes unstable to a secondary Kelvin-Helmholtz instability which can be clearly seen when the green isopycnal sheet rolls up into multiple billows. Taylor and Ferrari (2009) showed that this secondary shear instability is responsible for equilibrating symmetric instability, and Taylor and Ferrari (2010) examined the role of forced symmetric instability in generating turbulence at upper ocean fronts.

A large-eddy simulation of symmetric instability - animation credit: John Taylor, Ferrari Group, MIT.

In work with Roman Stocker (CEE, MIT), John explored how ocean turbulence affects the recycling of nutrients by swimming bacteria through a process known as chemotaxis. (Taylor and Stocker, 2011, in preparation). In the following movie, from a direct numerical simulation, a patch of dissolved nutrients was injected into fully-developed homogeneous, isotropic turbulence. The red iso-surface indicates where the nutrient concentration is 10% of its initial maximum value. Bacteria start out uniformly distributed throughout the computational volume, but quickly begin to cluster around the nutrient filaments. The color volume rendering shows where the bacteria concentration is larger than the initial value. As reported in Taylor and Stocker (2010), the optimal advantage of chemotactic bacteria occurs at intermediate turbulence levels.

A direct numerical simulation of the evolution of a patch of dissolved nutrients within a field of fully-developed homogeneous, isotropic turbulence - animation credit: John Taylor, Ferrari Group, MIT.

Between now and July 31 when he flys out, John and his wife plan to take a week or so to drive around the Northeast one last time. We wish them both well in that other Cambridge where it looks like John is going to fit in to his new surroundings "to a T"!

John's advisor was Prof. Raffaele Ferrari.

Manuscripts In Preparation:

Taylor, J.R. and R. Ferrari (2011) The role of surface heat fluxes in the onset of phytoplankton blooms. Accepted in Limnology and Oceanograph. [pdf]
Thomas, L.N, J.R. Taylor, R. Ferrari, T.M. Joyce, Symmetric Instability in the Gulfstream. Submitted to Deep Sea Research.
Taylor, J.R., R. Ferrari, Ocean fronts trigger high latitude phytoplankton blooms. In prep, 2011.
Taylor, J.R, S. Smith, R. Ferrari, Tracer and energy cascades in rotating stratified turbulence. In prep, 2011.
Taylor, J.R. R. Stocker. Trade-offs of chemotactic foraging in turbulent waters. In prep, 2011.

References:

Taylor, J.R. and R. Ferrari (2010) Buoyancy and wind-driven convection at mixed layer density frontsJournal of Physical Oceanography 40, 1222-1242.
Taylor J.R. and R. Ferrari (2009)On the equilibration of a symmetrically unstable front via a secondary shear instability Journal of Fluid Mechanics 622, 103-113