| NSF Org: |
OCE Division Of Ocean Sciences |
| Recipient: |
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| Initial Amendment Date: | July 9, 2014 |
| Latest Amendment Date: | July 9, 2014 |
| Award Number: | 1436522 |
| 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: | January 1, 2015 |
| End Date: | December 31, 2018 (Estimated) |
| Total Intended Award Amount: | $201,871.00 |
| Total Awarded Amount to Date: | $201,871.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
9500 GILMAN DR LA JOLLA CA US 92093-0021 (858)534-4896 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
CA US 92093-0411 |
| 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
In contrast to well-studied continental shelves, relatively little is known about cross-shelf circulation on island coastlines typically characterized by steep bathymetry and narrow shelves. Cross-shelf circulation results in the exchange of water masses and controls the distribution of heat, salt, nutrients, contaminants, sediment, and planktonic organisms like larvae or phytoplankton in the coastal ocean. In addition, in the case of coral reefs, the horizontal redistribution of heat by cross-shelf circulation moderates the daily variations in temperature experienced by coral polyps, thus potentially reducing thermal stress. This study will use existing data sets from two locations and high resolution numerical modeling to study the thermally driven exchange flows and how they vary with cross-shore distance and are modulated by swell energy, shelf slope, nearshore morphology, wind regimes and seasonal changes
Observations at two reefs at Eilat, Israel and Oahu, Hawaii have highlighted the role of thermally forced baroclinic exchange in cross-shore transport. At each site, daytime conditions were characterized by offshore flow at the surface in response to increased temperatures in shallower water nearshore. Nighttime cooling resulted in offshore flow near the bottom. Significant differences in flow response at the two sites indicate distinct dynamic regimes, however. Time series data from Oahu further indicates that the exchange provides a first order contribution to the overall cross-shore exchange. This study will examine the dynamical structure of thermally driven flows and establish the extent of their influence and generality for tropical coasts by leveraging existing data sets in a comprehensive historical analysis coupled with high-resolution numerical modeling spanning a wide parameter space ranging from a relatively simple wedge case with generally two-dimensional bathymetry (similar to Eilat) to a complex (more typical) forereef-lagoon environment with significant wave forcing (Kilo Nalu Observatory, Oahu). It will provide fundamental understanding of cross-shore transport processes in reef environments and should reveal the extent to which thermally driven baroclinic exchange contributes to cross-shore circulation for tropical coastlines in general. Improved understanding should lead to the construction of models capable of making accurate predictions of these flows. The results of the proposed research will have particular applicability to ecologically rich and economically important coral reefs and other steep, tropical, coastal environments, such as understanding how reef ecosystems will respond to environmental changes like ocean acidification or rising ocean temperatures, as well as to efforts to protect them like the establishment of marine reserves.
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.
Nearshore cross-shelf circulation influences coastal ecosystems through the transport of heat, salt, nutrients, and planktonic organisms. In the case of tropical coral reefs characterized by steep and narrow shelves with irregular slopes, cross-shore exchange can be a dominant mechanism for circulation. While there are many mechanisms responsible for driving cross-shelf fluxes, such as winds, surface waves, internal waves, tides, alongshore currents and Coriolis, and buoyant plumes, our interest here is with surface heat fluxes over sloping bathymetry, commonly observed over tropical reef systems, motivated by field observations of thermally-driven exchange from Israel and Hawaii. Over sloping bathymetry, the exchange of heat between the atmosphere and ocean (i.e., shortwave radiation and nighttime cooling) cause greater heating in shallower water than deeper water, establishing a horizontal temperature and pressure gradient that drives the exchange circulation. The flow pattern is characterized by surface offshore flow during heating, with a compensating return flow at depth, and bottom offshore flow during cooling, with counteracting return flow at the surface.
The goal of this project was to examine the dynamical structure of thermally driven flows and understand how important they are, in the face of other better studied processes driving cross shore exchange in the coastal ocean. We use a combination of idealized numerical modeling, theoretical approaches, historical data analysis, and statistical models to accomplish our goals.
Results from the idealized modeling work indicate that thermally-driven flow is a significant process driving cross shelf exchange in the nearshore coastal ocean and can flush the entire volume of nearshore water several times per day. The model results show that the influence of thermally-driven exchange is modulated by bathymetry and other coastal forcing (e.g. alongshore flows, winds) and is variable with distance from shore. Analysis of historical observational data combined with theoretical models help to explain some of the observed time variability in thermal exchange results from the combination of flows driven by daily heating and cooling with tidal timescale flows, which can create fortnightly (14-day) variability.
These studies will allow inference of effects of seasonal and climatological variations in forcing on cross-shore heat and mass transfer for reef environments. Additionally, thermal exchange is a mechanism that shapes the near-bed temperature environment on sloping fore reefs. Work supported by this project highlights the important role that diurnal time scale temperature variability plays in setting the heat tolerance of corals worldwide, modifying their bleaching response. Understanding the mechanisms which create thermal gradients on reefs can help managers identify reefs with the greatest potential for resilience against future anomalously high sea surface temperatures.
Besides tropical coastal regions that are subject to high surface heat exchanges and tidal forcing, the theoretical framework developed as part of this project can be useful for analyzing temperature-driven cross-shore exchange in lentic aquatic systems, such as lakes and reservoirs. Additionally, our results show the potential for tidally-forced along-shore flows to drive systematic shifts in the cross-shore exchange on tidal timescales.
Our results have been presented by graduate students, postdoctoral scholars, and the principal investigators at national conferences and published in peer-reviewed journals such as Nature Communications and the Journal of Physical Oceanography. Funding for this project has made it possible to support the development of a graduate student and a postdoctoral scholar from an underrepresented group and their attendance at professional conferences in their field of study. The Ph.D. student, Aryan Safaie, has also had the opportunity to give lectures to undergraduate classes in the field of fluid mechanics and has integrated his research into these lectures.
As part of this project, we hosted a booth at the 2016 & 2017 Children?s Water Education Festival, held at the University of California, Irvine, and created an interactive demonstration which emphasizes the importance of human influence in our coastal waters. UCI has been host to the Children?s Water Education Festival, an annual event put on by the Orange County Water District, Disneyland Resort, and the National Water Research Institute for 17 years. This event is the largest of its kind in the United States and attracts more than 7,000 third, fourth, and fifth grade students and their teachers. Attendees learn about the interdependence of water, soil, plants, trees, animals and humans through more than 70 interactive and fun-filled activities. The Festival teaches students about local, regional and global water issues and empowers them to make wise choices today and in the future.
Last Modified: 03/05/2019
Modified by: Eugene R Pawlak
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