| NSF Org: |
OCE Division Of Ocean Sciences |
| Recipient: |
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| Initial Amendment Date: | February 28, 2014 |
| Latest Amendment Date: | February 28, 2014 |
| Award Number: | 1355768 |
| 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: | March 1, 2014 |
| End Date: | February 29, 2020 (Estimated) |
| Total Intended Award Amount: | $482,006.00 |
| Total Awarded Amount to Date: | $482,006.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
1500 SW JEFFERSON AVE CORVALLIS OR US 97331-8655 (541)737-4933 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
OR US 97331-5503 |
| 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): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Overview: Thirty years after its discovery, the deep cycle of equatorial turbulence remains unexplained. Climate forecast models are unable to represent it accurately, leading to errors in ocean heat uptake. This project brings together two research groups who have made recent progress on the problem to pursue a unified understanding. Combining existing observations and new large-eddy simulations (LES), they will explore deep cycle physics over the range of seasonal and ENSO variability and in all three equatorial oceans. The previously-unexploited deep cycle property of marginal instability will be used together with numerical simulations to (1) document the history of the deep cycle over the past 25 years, (2) describe the mechanics of the deep cycle, and (3) develop improved parameterizations for use in climate models.
Intellectual merit: The broad variability of mixing in the upper equatorial Pacific is exemplified by the turbulent diapycnal heat flux sampled at the same location and in the same season, but coinciding with different phases of the El Niño/ Southern Oscillation (ENSO) cycle. Early in the 1991 El Niño, currents were slow and the heat flux was relatively weak. In the 2008 cruise, which coincided with La Niña, currents were much more energetic. The resulting heat flux was stronger by an order of magnitude, and heat was transported to 100 m depth. Despite the extreme difference in magnitude, these mixing regimes exhibited a striking commonality: strong turbulence coincided with a distinct layer in which the gradient Richardson number (Ri) remained within a factor of two of the value of a quarter. Persistent clustering of Ri near 1/4 over a range of depths signifies the state of marginal instability. This near-critical state is maintained in the mean by a balance between large-scale forcing (which reduces Ri) and turbulence (which increases it). Deep cycle mixing is crucial to the climate via its effects on both the zonal current system and the sea surface temperature. In this observational data analyses and large eddy simulations, the investigators will explore the physics of the deep cycle with particular attention to the property of marginal instability. Using marginal instability as a proxy, they will document the history and longitudinal dependence of the deep cycle from existing data. This will lead to both a deeper understanding of the phenomenon and an improved ability to parameterize its effects in large-scale models.
Broader Impacts: The causes of long-term Sea Surface Temperature variations will be examined and the results will contribute to ENSO and interdecadal climate prediction. The project will also foster a new collaboration, support a junior researcher, and complement an existing NSF observational project. Two presentations on equatorial oceanography will be developed and presented at high schools and community colleges.
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
Heat exchange between the oceans and the atmosphere is a dominant factor governing Earth's climate. Heat transfer into the ocean is strongest at the equator, especially in the eastern Pacific, where currents bring a steady supply of cool water to be warmed by the tropical sun. The fate of this heat, i.e. whether it is returned quickly to the atmosphere or carried into the deep ocean, depends on how ocean turbulence exchanges heat between the warm surface water and the cold ocean interior.
This heat exchange is modulated by the so-called deep cycle of equatorial turbulence. Discovered in the 1980s, the deep cycle has been a mystery until recently. This project has led to a much better understanding of the interactions of wind and solar heating that maintain the deep cycle. Essential to the process is the state of marginal instability: ocean temperature and currents are nearly in equilibrium with the forces that drive them: the trade winds, the Earth's rotation, and solar heating. Deep cycle turbulence is the process that restores equilibrium when those drivers change. This insight has led us to propose useful methods for predicting turbulence in large-scale ocean and climate models.
Mixing warm, buoyant surface water with cold, dense deep water requires that turbulence do work against gravity. This mixing process must compete with friction, which typically dissipates a large fraction of the energy of the turbulence. It has long been known, in fact, that the efficiency of ocean mixing (the fraction of turbulent energy available to do work against gravity) is remarkably consistent: about 20%. This project has furnished a long-sought explanation for that uniformity: the 20% efficiency is an expected property of marginal instability as described above. It turns out that marginal instability is not just a property of equatorial turbulence but is in fact found throughout the world ocean, thus explaining the ubiquity of 20% mixing efficiency.
The equatorial deep cycle varies from season to season. We investigated that variation using large-eddy simulations of flow regimes characteristic of the four seasons. This allowed us, for example, to correct a previous impression that the deep cycle stops in boreal spring. This misapprehension was due to the limitations of existing measurements. In fact, the simulations indicate that the deep cycle continues in all seasons and remains important to air-sea heat exchange, but in spring it becomes too shallow to be detected by existing moored instruments. Future instrument placements will be guided by this prediction.
While existing, multi-decadal observations of the equatorial oceans do not measure the deep cycle directly, they do allow us to detect marginal instability. We have therefore been able to develop a seasonal climatology of the deep cycle based on three decades of observations. For the future, we are in a position to use large eddy simulations to study the variation of deep cycle turbulence over the El Niño - Southern Oscillation cycle.
Broader Impacts
As noted above, marginal instability is not purely a phenomenon of the equatorial oceans; it appears to be the rule wherever ocean currents are subjected to slow forcing by winds or gravity. This includes estuarine flows, deep-ocean gravity currents and low-frequency internal waves. Similar phenomena occur in the atmosphere in connection with clear-air turbulence, and during night-time or winter cooling. Our new understanding of margial instability will be useful in these other regimes. Marginal instability is also an example of self-organized criticality, a new paradigm in statistics that describes episodic phenomena such as earthquakes, landslides, solar flares and even stock market fluctuations. This insight promises multiple avenues for interdisciplinary research in the future.
The project supported junior scientist Dr. Hieu Pham, who was promoted to the rank of project scientist during the tenure of the grant. His skillset expanded from numerical modeling to include observational techniques and data analysis as a result of working with the OSU PIs Smyth & Moum.
Last Modified: 05/02/2020
Modified by: William D Smyth
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