| NSF Org: |
OCE Division Of Ocean Sciences |
| Recipient: |
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| Initial Amendment Date: | July 11, 2014 |
| Latest Amendment Date: | July 11, 2014 |
| Award Number: | 1434512 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Baris Uz
bmuz@nsf.gov (703)292-4557 OCE Division Of Ocean Sciences GEO Directorate for Geosciences |
| Start Date: | September 1, 2014 |
| End Date: | August 31, 2019 (Estimated) |
| Total Intended Award Amount: | $324,615.00 |
| Total Awarded Amount to Date: | $324,615.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
285 OLD WESTPORT RD NORTH DARTMOUTH MA US 02747-2356 (508)999-8953 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
285 Old Westport Rd North Dartmouth MA US 02747-2300 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | PHYSICAL OCEANOGRAPHY |
| Primary Program Source: |
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| Program Reference Code(s): | |
| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
This study contributes to the understanding of a biologically highly relevant region of the ocean, the subpolar North Atlantic, which acts as a persistent sink for atmospheric carbon. It assesses the biological importance of submesoscale processes, which are not represented in global carbon cycle models. Mixed layer eddies, along with the turbulence induced by air-sea fluxes, modulate both light and nutrients for phytoplankton productivity. This study explores the role of mixed layer eddies on phytoplankton productivity in the subpolar gyres during three distinct phases of the annual cycle - winter, spring, and summer. By modeling the interaction of mixed layer eddies with mesoscale eddies, the study will provide insight on how their coupling affects vertical exchange between the pycnocline and the mixed layer. The project will support a postdoctoral scientist at University of Massachusetts Dartmouth and a Ph.D. student in the Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program for four years. The investigators will contribute to outreach efforts including teacher training, and ocean literacy workshops through the Ocean Academy at the Ocean Explorium at New Bedford Seaport by demonstrating ocean dynamics using numerical and tabletop experiments.
This study will be amongst the first to include vertical and lateral processes (namely turbulent mixing and 1-10 km scale mixed layer eddies) in understanding the upper ocean's response to temporally varying air-sea fluxes on short time scales (hours-days). By studying the mechanisms generating stratification of deep winter mixed layers, the researchers will assess the effect of eddies and air-sea fluxes on phytoplankton primary productivity during times when a scarcity of light limits photosynthesis (in winter and early spring). By studying the coupling between subsurface mesoscale eddies and the surface submesoscale field subject to intermittent air-sea fluxes, they will examine the advective vertical nutrient supply during periods of strong surface stratification (summer). These process studies will help to address the following important questions about phytoplankton productivity in the subpolar oceans: (i) How is a seed phytoplankton population sustained over the winter? (ii) Is the mechanism for spring stratification, be it eddies or thermal effects, relevant to the net primary productivity over the season? (iii) How are nutrients supplied to the surface layer during times of strong stratification in the summer? This investigation will extend current understanding of mixed layer eddies, which is based on studies in idealized mixed layer settings, by considering their interaction with mesoscale eddies in the pycnocline beneath, and with surface turbulence in response to temporally fluctuating air-sea fluxes.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Intellectual Merit
Each Spring, the subpolar North Atlantic comes to life, initiated by the blooming of phytoplankton that results from the shoaling of the deep, nutrient-replete, winter mixed layer and the availability of light for photosynthesis. In previous work, the PIs had shown that restratification of the deep winter mixed layer by mixed layer eddies (MLE) can initiate the subpolar Spring phytoplankton bloom prior to the onset of positive heat flux in Spring (Mahadevan et al., 2012). But, the role of MLE on winter phytoplankton productivity had not been explored. Here, the researchers use process study ocean model experiments to examine how MLE contribute to the sustenance of phytoplankton during the winter months, when sunlight is scant and the sea surface is strongly forced by heat loss and winds. It is found that even in winter, when the mixed layer is deep, the tendency of MLE to shoal the mixed layer reduces the mixed layer depth, increases the solar exposure of phytoplankton contributing to their growth and sustaining their population in the winter. In the absence of lateral density gradients, baroclinic instability, and MLE, the mixed layer is about 600 m deeper in model simulations, and phytoplankton productivity is much lower. However, changes in mixed layer depth due to interruptions in wind and cooling effects are more transient, and have little effect on phytoplankton productivity because of the limited light in winter.
In other work, the research group has explored scales and mechanisms for vertical transport of phytoplankton nutrients in oligotrophic regimes. A decomposition of the vertical velocity in the pycnocline into an along-isopycnal component, and a component related to the uplift or subsidence of an isopycnal surface, enables attribution of nutrient supply to different mechanisms that are scale-dependent. The along-isopycnal component of the vertical velocity is dependent on the isopycnal slope and aspect ratio of the flow, and is more sensitive to model resolution. It may hence be termed as ?submesoscale?. Its contribution to vertical transport is found to increase at fronts, and is more dependent on model resolution than the uplift/subsidence of the isopycnal surface.
Further, a modeling study helped quantify the source depth of upwelling waters in a coastal upwelling system. It is dependent not only on the wind stress, but varies inversely with the strength of density stratification of the water column.
Finally, the field sampling and analysis of phytoplankton community structure across an upper ocean front revealed that upper ocean phytoplankton communities can be subducted along isopycnal surfaces into the stratified pycnocline.
Broader Impacts
Phytoplankton plays an important role in carbon cycle and its seasonal productivity in the subpolar oceans is strongly modulated by mixed layer depth. The conventional paradigm for the evolution of mixed layer depth and blooming of phytoplankton relies on a one-dimensional approximation of turbulent depth based on surface forcing. This work shows the importance of three-dimensional eddying processes in controlling mixed layer depth and light availability for phytoplankton even in the winter. The patchy shoaling of the mixed layer by MLE formed at fronts explains the growth and sustenance of phytoplankton during the winter. The phytoplankton that over-winters, acts as the seed population for the spring bloom.
A scientific controversy concerning the role of mesoscale vs. submesoscale vertical transport mechanisms for nutrient supply in subtropical gyres is resolved through a mechanistic decomposition of the vertical velocity field and an examination of its scale dependence.
While the strength of coastal upwelling is known to be related to the alongshore wind stress, the source depth of upwelling waters had not been quantified previously. It is shown to be dependent on both the wind stress and the density stratification. This has implications for estimation of nutrient supply and new production rates in coastal upwelling regions.
This project has supported three PhD students and 3 summer guest students at WHOI, and three undergraduates and a postdoc at UMass Dartmouth, including three women scientists.
Last Modified: 11/21/2019
Modified by: Amit Tandon
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