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  • 1
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    Near-inertial kinetic energy budget of the mixed layer and shear evolution in the transition layer in the Arabian Sea during the monsoons (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 6492–6507, doi:10.1002/2014JC010198.
    Description: We present the horizontal kinetic energy (KE) balance of near-inertial currents in the mixed layer and explain shear evolution in the transition layer using observations from a mooring at 15.26° N in the Arabian Sea during the southwest monsoon. The highly sheared and stratified transition layer at the mixed-layer base varies between 5 m and 35 m and correlates negatively with the wind stress. Results from the mixed layer near-inertial KE (NIKE) balance suggest that wind energy at times can energize the transition layer and at other times is fully utilized within the mixed layer. A simple two layer model is utilized to study the shear evolution in the transition layer and shown to match well with observations. The shear production in this model arises from alignment of wind stress and shear. Although the winds are unidirectional during the monsoon, the shear in the transition layer is predominantly near-inertial. The near-inertial shear bursts in the observations show the same phasing and magnitude at near-inertial frequencies as the wind-shear alignment term.
    Description: NASA Grant Number: NNX12AD47G, NSF Grant Number: 0928138, ONR Grant Numbers: N00014-11-1-0429 and N00014-10-1-0273, NSF Grant Number: OCE-0745508
    Description: 2016-03-26
    Keywords: Near inertial energy ; Transition layer ; Near inertial shear
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/pdf
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  • 2
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    Enhancement in vertical fluxes at a front by mesoscale-submesoscale coupling (2015)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 8495–8511, doi:10.1002/2014JC010211.
    Description: Oceanic frontal instabilities are of importance for the vertical exchange of properties in the ocean. Submesoscale, O(1) Rossby number, dynamics are particularly relevant for inducing the vertical (and lateral) flux of buoyancy and tracers in the mixed layer, but how these couple with the stratified pycnocline is less clear. Observations show surface fronts often persist beneath the mixed layer. Here we use idealized, three-dimensional model simulations to show how surface fronts that extend deeper into the pycnocline invoke enhanced vertical fluxes through the coupling of submesoscale and mesoscale instabilities. We contrast simulations in which the front is restricted to the mixed layer with those in which it extends deeper. For the deeper fronts, we examine the effect of density stratification on the vertical coupling. Our results show deep fronts can dynamically couple the mixed layer and pycnocline on time scales that increase with the peak stratification beneath the mixed layer. Eddies in the interior generate skew fluxes of buoyancy and tracer oriented along isopycnals, thus providing an adiabatic pathway for the interior to interact with the mixed layer at fronts. The vertical enhancement of tracer fluxes through the mesoscale-submesoscale coupling described here is thus relevant to the vertical supply of nutrients for phytoplankton in the ocean. A further implication for wind-forced fronts is that the vertical structure of the stream function characterizing the exchange between the interior and the mixed layer exhibits significant qualitative differences compared to a linear combination of existing parameterizations of submesoscale eddies in the mixed layer and mesoscale eddies in the interior. The discrepancies are most severe within the mixed layer suggesting a potential role for Ekman-layer dynamics absent in existing submesoscale parameterizations.
    Description: S.R. and A.T. acknowledge financial support from the National Science Foundation (NSF OCE-0928138) and the Office of Naval Research (ONR N00014-09-1-0196, ONR N00014-12-1-0101). A.M. acknowledges funding from the National Science Foundation (NSF OCE-0928617) and the Office of Naval Research (ONR N00014-12-1-0101).
    Description: 2015-06-11
    Keywords: Submesoscale ; Mixed layer ; Meso-submeso coupling ; Deep fronts
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/pdf
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  • 3
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    Sustenance of phytoplankton in the subpolar North Atlantic during winter (2018)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of [publisher] for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 123 (2018): 6531-6548, doi:10.1029/2017JC013639.
    Description: We consider two factors that affect the mixed layer depth (MLD) and potentially contribute to phytoplankton sustenance over winter—variability of air‐sea fluxes and three‐dimensional processes arising from horizontal density gradients (fronts). The role of these two factors is addressed using several three‐dimensional idealized numerical simulations in a process study ocean model forced with air‐sea fluxes at different temporal averaging frequencies. Results show that in winter, when the average mixed layer is much deeper than the euphotic layer and the period of daylight is short, phytoplankton production is relatively insensitive to high‐frequency variability in air‐sea fluxes. Short‐lived stratification events during light‐limited conditions have very little impact on phytoplankton production. On the other hand, the slumping of fronts shallows the mixed layer in a patchy manner and the associated restratification persists considerably longer than that caused by changes in air‐sea fluxes. Simulations with fronts show that in winter, the average MLD is about 600 m shallower than simulations without fronts. Prior to spring warming, the depth‐integrated phytoplankton concentration in the model with fronts is about twice as large as the case without fronts. Hence, even in winter, restratification by fronts is important for setting the MLD; it increases the residence time of phytoplankton in the euphotic layer and contributes to phytoplankton growth, thereby sustaining phytoplankton populations in winter. Higher model resolution intensifies submesoscale dynamics, leading to stronger restratification, shallower mixed layers, greater variability in the MLD, and more production of phytoplankton.
    Description: National Science Foundation Grant Numbers: OCE-1434512, OCE-1434788
    Description: 2019-03-14
    Repository Name: Woods Hole Open Access Server
    Type: Article
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