| NSF Org: |
OCE Division Of Ocean Sciences |
| Recipient: |
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| Initial Amendment Date: | September 7, 2017 |
| Latest Amendment Date: | July 20, 2022 |
| Award Number: | 1734160 |
| 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 15, 2017 |
| End Date: | August 31, 2023 (Estimated) |
| Total Intended Award Amount: | $1,177,691.00 |
| Total Awarded Amount to Date: | $1,177,691.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
4333 BROOKLYN AVE NE SEATTLE WA US 98195-1016 (206)543-4043 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
1013 NE 40th Street Seattle WA US 98105-6698 |
| 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
Lateral stirring is among the primary processes, along with advection and diapycnal mixing, that determines the distribution and fate of water-mass properties, nutrients and dissolved gases in the upper ocean. Ocean general circulation models resolve stirring on scales larger than thirty kilometers, and regional models on scales close to one kilometer, but isopycnal stirring by unresolved submesoscale processes must still be parameterized. Novel high-resolution observations in the sub-tropical Pacific Ocean will provide insight into the underlying physics and how these processes operate in the upper ocean in order to improve such parameterizations for eddy-resolving ocean circulation models. Results should be applicable to the submesoscale in other rotating stratified fluids such as the atmosphere of Earth, stars and gas giants that occupy the same parameter space. A post-doctoral fellow will be trained in interpretation of data in the rapidly emerging field of oceanic submesoscales. Two graduate students will have an opportunity to participate in sea-going research.
Order one Rossby number and gradient Richardson number sub-inertial flows have emerged as an area of ocean parameter space that is incompletely understood but becoming accessible to observations, modelling and theory. The kinematics and dynamics of isopycnal stirring on length scales shorter than the Rossby deformation length of order 100 kilometer and longer than outer turbulence scales of order one meter do not have a well-established theoretical underpinning because of the paucity of observational constraints. At scales shorter than the Rossby length scale, quasigeostrophic (QG) theory predicts that tracers should have a spectrum of slope of minus one [corresponding to tracer-gradient spectra with slope plus one]. This contrasts with numerous observations spanning the horizontal horizontal wavelengths 30 m to 500 km which report significantly redder and often almost flat tracer-gradient spectra over this wavenumber band. Similar dynamical comparisons are in short supply. High resolution sections of the upper 150 meter of the ocean will be sampled using a towed chain with tightly spaced temperature, conductivity and pressure (CTD) sensors and the shipboard Acoustic Doppler Current Profiler (ADCP). This project will characterize the horizontal wavenumber spectra of passive tracers (water-mass, spice, salinity anomalies) on isopycnals to higher horizontal wavenumbers than previously accomplished, and resolve the sub-inertial O(1 km) horizontal strain tensor thought responsible for stirring on these scales, as well as determine strain timescales. These measurements will provide key observational constraints on order one Rossby number and Richardson number flows.
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.
The energy pathways from forcing at ocean-basin lengthscales to dissipation and mixing at small lengthscales in the ocean are not fully understood, especially in terms of contributions from different dynamics, energy cascade rates, energy exchange rates, or dominant instability mechanisms leading to turbulence production occurring on the horizontal submesoscale O(1-100 km) and vertical finescale ~O(1-50 m). These scales are challenging because they represent a transition from linear to nonlinear dynamics, and there are few instruments designed to measure them.
To better characterize these scales, two-dimensional density and velocity measurements were collected off Baja California using a towed chain of temperature, salinity and pressure sensors and two shipboard acoustic Doppler current profilers, supplemented with profile time-series from 6 electromagnetic (EM-APEX) profiling floats.
The towed chain data were used to compute 2-D (horizontal, vertical) wavenumber spectra for isopycnal slope, vertical strain and isopycnal salinity-gradient across the finescale transition between internal waves and a spectrally distinct waveband between internal waves and isotropic turbulence. Spectral slopes compared favorably with internal-wave and anisotropic stratified turbulence spectral models. Parameterizations for the energy cascade rate to dissipation were consistent between the two wavebands, suggesting a continuous cascade through internal waves and anisotropic stratified turbulence. This has implications for the energy cascade to small scales in other rotating stratified fluids such as the atmosphere. The +1 spectral slope for isopycnal salinity-gradient is consistent with nonlocal stirring.
Combined with shipboard ADCP velocity data, two decomposition methods were used to segregate kinetic and potential energies into quasigeostrophy (vortical mode) and internal waves. While these wavebands are traditionally assumed to be dominated by internal waves, quasigeostrophy was found to be significant at all vertical and horizontal wavenumbers. These results are consistent with recent numerical simulations which suggest broadband energy exchanges between quasigeostrophy and internal waves. Energy ratios depend on the dynamical aspect ratio or Burger number which may be universal. Inferences were also made about horizontal dispersion, horizontal and vertical shear instability and potential vorticity.
These measurements have shed light on poorly understood submeso- and finescale dynamics over a broad horizontal and vertical wavenumber range in the stratified ocean interior, pointing to a role for anisotropic stratified turbulence in the spectrally distinct vertical wavenumber band between internal waves and isotropic turbulence, and finding significant quasigeostrophic vortical motions in wavebands traditionally attributed to internal waves. These results have implications for isopycnal stirring, the energy cascade to small lengthscales, shear instability, turbulence production, and the energy budgets for these two dynamics. The data can provide a reference for future numerical modeling in this parameter space and guide future observational programs.
Last Modified: 10/10/2023
Modified by: Ren-Chieh Lien
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