Archive for turbulence

Turbulent Mixing: A Dominant Source of Oxygen in the Upper Equatorial Pacific

Posted by mmaheigan 
· Tuesday, March 12th, 2024 

What balances oxygen removal in the equatorial Pacific? For a long time, oxygen in the eastern and central tropical Pacific was assumed to be mainly supplied by the large-scale advection of remotely ventilated waters via the equatorial current system and meridional circulation. A recent study used an eddy-resolving simulation of a global ocean model to show that turbulent mixing and its regulation by mesoscale eddies play a critical role in balancing oxygen removal (by consumption and upwelling) in the upper thermocline. Deeper in the water column, mean advection by the zonal currents and meridional circulation dominates. This mixing is tightly regulated by tropical instability waves, which intensify the shear between the equatorial currents and enhance the downward turbulent mixing flux of oxygen into the thermocline. Mesoscale phenomena thus play an indirect yet critical role as a local pathway of ventilation in this region. Testing these model-based hypotheses in the real ocean through dedicated field studies and long-term observations is needed to advance our understanding of the observed expansion of the oxygen deficient zones (ODZs) and model their future trajectory in a warmer and more stratified ocean.

Figure: The main processes that set the mean structure of oxygen in the equatorial Pacific are assessed in an eddy resolving simulation of the Community Earth System Model (CESM). Panel a shows the climatological oxygen distribution on the 26.25 isopycnal in CESM. Panels b-e show the contribution of advection by mean circulation and eddies, vertical mixing, and production and consumption. These processes are illustrated in panel f). Figure adapted from Eddebbar et al (2024).

Authors
Yassir A. Eddebbar (Scripps Institution of Oceanography)
Daniel B. Whitt (NASA Ames)
Ariane Verdy, (Scripps Institution of Oceanography)
Matthew R. Mazloff (Scripps Institution of Oceanography)
Aneesh C. Subramanian (CU Boulder)
Matthew C. Long, (National Center for Atmospheric Research)

Phytoplankton can actively diversify their migration strategy in response to turbulent cues

Posted by mmaheigan 
· Thursday, August 17th, 2017 

Turbulence is known to be a primary determinant of plankton fitness and succession. However, open questions remain about whether phytoplankton can actively respond to turbulence and, if so, how rapidly they can adapt to it. Recent experiments have revealed that phytoplankton can behaviorally respond to turbulent cues with a rapid change in shape, and this response occurs over a few minutes. This challenges a fundamental paradigm in oceanography that phytoplankton are passively at the mercy of turbulence.

Phytoplankton are photosynthetic microorganisms that form the base of most aquatic food webs, impact global biogeochemical cycles, and produce half of the world’s oxygen. Many species of phytoplankton are motile and migrate in response to gravity and light levels: Upward toward light during the day to photosynthesize and downward at night toward higher nutrient concentrations. Disruption of this diurnal migratory strategy is an important contributor to the succession between motile and non-motile species when conditions become more turbulent. However, this classical view neglects the possibility that motile species can actively respond in an effort to avoid layers of strong turbulence. A recent study by Sengupta, Carrara and Stocker, published in Nature has shown that some raphidophyte and dinoflagellate phytoplankton can actively diversify their migratory strategy in response to hydrodynamic cues characteristic of overturning by the smallest turbulent eddies in the ocean. Laboratory experiments in which cells experienced repeated overturning with timescales and statistics representative of ocean turbulence revealed that over timescales as short as ten minutes, an upward-swimming population split into two subpopulations, one swimming upward and one swimming downward. Quantitative morphological analysis of the harmful algal bloom-forming raphidophyte Heterosigma akashiwo revealed that this behavior was accompanied by a change in cell shape, wherein the cells that changed their swimming direction did so by going from an asymmetric pear shape to a more symmetric egg shape. A model of cell mechanics showed that the magnitude of this shift was minute, yet sufficient to invert the cells’ preferential swimming direction. The results highlight the advanced level of control that phytoplankton have on their migratory behavior.

Understanding how fluctuations in the oceans’ turbulence landscape impacts phytoplankton is of fundamental importance, especially for predicting species succession and community structure given projected climate-driven changes in temperature, winds, and upper ocean structure.

An upward-swimming phytoplankton population splits into upward- and downward-swimming sub-populations when exposed to turbulent eddies, due to a subtle change in cell shape. Illustration by: A. Sengupta, G. Gorick, F. Carrara and R. Stocker

 

This work was co-funded by a Human Frontier Science Program Cross Disciplinary Fellowship (LT000993/2014-C to A.S.), a Swiss National Science Foundation Early Postdoc Mobility Fellowship (to F.C.), and a Gordon and Betty Moore Marine Microbial Initiative Investigator Award (GBMF 3783 to R.S.)

 

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