Over the past several decades, there has developed a community-wide appreciation for the importance of mixing at the smallest scales to geophysical fluid dynamics on all scales. This appreciation has ...spawned greater participation in the investigation of ocean mixing and new ways to measure it. These are welcome developments given the tremendous separation in scales between the basins,
) m, and the turbulence,
) m, and the fact that turbulence that leads to thermodynamically irreversible mixing in high-Reynolds-number geophysical flows varies by at least eight orders of magnitude in both space and time. In many cases, it is difficult to separate the dependencies because measurements are sparse, also in both space and time. Comprehensive shipboard turbulence profiling experiments supplemented by Doppler sonar current measurements provide detailed observations of the evolution of the vertical structure of upper-ocean turbulence on timescales of minutes to weeks. Recent technical developments now permit measurements of turbulence in the ocean, at least at a few locations, for extended periods. This review summarizes recent and classic results in the context of our expanding knowledge of the temporal variability of ocean mixing, beginning with a discussion of the timescales of the turbulence itself (seconds to minutes) and how turbulence-enhanced mixing varies over hours, days, tidal cycles, monsoons, seasons, and El Niño-Southern Oscillation timescales (years).
Feedback of Mixing to ENSO Phase Change Warner, Sally J.; Moum, James N.
Geophysical research letters,
16 December 2019, Letnik:
46, Številka:
23
Journal Article
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A decade‐long time series of mixing in the equatorial Pacific cold tongue at 0°, 140°W reveals how mixing changes on El Niño–Southern Oscillation (ENSO) time scales. Separated into phase transitions ...to and from the neutral state, we find that mixing is most intense during the perturbation from the neutral state to peak La Niña when sea surface temperature cools and weakest during the perturbation from the neutral state to peak El Niño when sea surface temperature warms. Intermediate levels of mixing occur during relaxations back to the neutral state. Heating and cooling rates due to the divergence of turbulence heat flux across the mixed layer, where the net surface heat flux is the value of the turbulence heat flux at the sea surface, have the same amplitude and sign as sea surface heating and cooling rates during ENSO phase transitions. We suggest that the basic Bjerknes feedback must include mixing.
Plain Language Summary
Transitions to the peak El Niño state are accompanied by sea surface warming and to the peak La Niña state by sea surface cooling. Since, on a daily averaged basis, the sea surface at the equator is always heated by the atmosphere, thermodynamic cooling is only achieved by mixing of warm surface waters with cooler water from greater depths. Instruments that measure mixing have been deployed on long term equatorial oceanographic moorings for more than a decade and have now captured several El Niños and La Niñas. These show that mixing—or lack thereof—contributes to heating the sea surface during transitions to peak El Niño and cooling the sea surface during transitions to peak La Niña. Mixing therefore acts as a positive feedback mechanism to the development of El Niños and La Niñas.
Key Points
A decade of mixing measurements in the cold tongue of the equatorial Pacific now includes several ENSO events
Heating/cooling rates due to mixing have the same amplitude and sign as observed sea surface heating/cooling rates on ENSO time scales
Bjerknes feedback must include mixing in addition to upwelling
Sea surface temperature (SST) is a critical control on the atmosphere, and numerical models of atmosphere-ocean circulation emphasize its accurate prediction. Yet many models demonstrate large, ...systematic biases in simulated SST in the equatorial 'cold tongues' (expansive regions of net heat uptake from the atmosphere) of the Atlantic and Pacific oceans, particularly with regard to a central but little-understood feature of tropical oceans: a strong seasonal cycle. The biases may be related to the inability of models to constrain turbulent mixing realistically, given that turbulent mixing, combined with seasonal variations in atmospheric heating, determines SST. In temperate oceans, the seasonal SST cycle is clearly related to varying solar heating; in the tropics, however, SSTs vary seasonally in the absence of similar variations in solar inputs. Turbulent mixing has long been a likely explanation, but firm, long-term observational evidence has been absent. Here we show the existence of a distinctive seasonal cycle of subsurface cooling via mixing in the equatorial Pacific cold tongue, using multi-year measurements of turbulence in the ocean. In boreal spring, SST rises by 2 kelvin when heating of the upper ocean by the atmosphere exceeds cooling by mixing from below. In boreal summer, SST decreases because cooling from below exceeds heating from above. When the effects of lateral advection are considered, the magnitude of summer cooling via mixing (4 kelvin per month) is equivalent to that required to counter the heating terms. These results provide quantitative assessment of how mixing varies on timescales longer than a few weeks, clearly showing its controlling influence on seasonal cooling of SST in a critical oceanic regime.
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DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Oceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy. This method assumes a linear scaling between the Thorpe length scale
L
T
and the ...Ozmidov length scale
L
O
. Historic evidence supporting
L
T
~
L
O
has been shown for relatively weak shear-driven turbulence of the thermocline; however, little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open-ocean standards. This study presents a direct comparison of
L
T
and
L
O
, using vertical profiles of temperature and microstructure shear collected in the Luzon Strait—a site characterized by topographically influenced overturns up to
O
(100) m in scale. The comparison is also done for open-ocean sites in the Brazil basin and North Atlantic where overturns are generally smaller and due to different processes. A key result is that
L
T
/
L
O
increases with overturn size in a fashion similar to that observed in numerical studies of Kelvin–Helmholtz (K–H) instabilities for all sites but is most clear in data from the Luzon Strait. Resultant bias in parameterized dissipation is mitigated by ensemble averaging; however, a positive bias appears when instantaneous observations are depth and time integrated. For a series of profiles taken during a spring tidal period in the Luzon Strait, the integrated value is nearly an order of magnitude larger than that based on the microstructure observations. Physical arguments supporting
L
T
~
L
O
are revisited, and conceptual regimes explaining the relationship between
L
T
/
L
O
and a nondimensional overturn size
are proposed. In a companion paper, Scotti obtains similar conclusions from energetics arguments and simulations.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Kelvin-Helmholtz (KH) instability, characterized by the distinctive finite-amplitude billows it generates, is an important mechanism in the development of turbulence in the stratified interior of the ...ocean. In particular, it is often assumed that the onset of turbulence in internal waves begins in this way. Clear recognition of the importance of KH instability to ocean mixing arises from recent observations of the phenomenon in a broad range of oceanic environments. KH instability is a critical link in the chain of events that leads from internal waves to mixing. After 150 years of research, identifying the prevalence of KH instability in the ocean and defining useful parameterizations that quantify its contribution to ocean mixing in numerical models remain first-order problems.
Abstract
The daily evolution of temperature, stratification, and turbulence in the diurnal warm layer is described from time series measurements at low to moderate winds and strong insolation in the ...equatorial Indian Ocean. At 2.0-m depth, turbulence dissipation rates (
ε
) decreased by two orders of magnitude over 1–2 h immediately after sunrise, initiated by stratification caused by penetrating solar radiation prior to the change in sign of net surface heat flux from cooling to warming. Decaying turbulence preceded a period of rapid growth, in which
ε
increased by two orders of magnitude over a few hours, and following which
ε
approached a daytime period of near-steady state. Decay and growth rates predicted by a simplified turbulence model are consistent with those observed. During the daytime period of near-steady state, asymmetric temperature ramps were associated with enhanced
ε
, supporting the interpretation that this period represents a balance between buoyancy and shear production associated with a shear-driven response to trapping of momentum within the diurnal warm layer.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
The role of turbulent mixing in regulating the ocean’s response to the Madden–Julian oscillation (MJO) is assessed from measurements of surface forcing, acoustic, and microstructure profiles ...during October–early December 2011 at 0°, 80.5°E in the Indian Ocean. During the active phase of the MJO, the surface mixed layer was cooled from above by air–sea fluxes and from below by turbulent mixing, in roughly equal proportions. During the suppressed and disturbed phases, the mixed layer temperature increased, primarily because of the vertical divergence between net surface warming and turbulent cooling. Despite heavy precipitation during the active phase, subsurface mixing was sufficient to increase the mixed layer salinity by entraining salty Arabian Sea Water from the pycnocline. The turbulent salt flux across the mixed layer base was, on average, 2 times as large as the surface salt flux. Wind stress accelerated the Yoshida–Wyrtki jet, while the turbulent stress was primarily responsible for decelerating the jet through the active phase, during which the mean turbulent stress was roughly 65% of the mean surface wind stress. These turbulent processes may account for systematic errors in numerical models of MJO evolution.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A low-power (<10 mW), physically small (15.6 cm long 3.2 cm diameter), lightweight (600 g Cu; alternatively, 200 g Al), robust, and simply calibrated pitot-static tube to measure mean speed and ...turbulence dissipation is described and evaluated. The measurement of speed is derived from differential pressure via Bernoulli's principle. The differential pressure sensor employed here has relatively small, but significant, adverse sensitivities to static pressure, temperature, and acceleration, which are characterized in tests in the college's laboratory. Results from field tests on moorings indicate acceptable agreement in pitot-static speed measurements with independent acoustic Doppler current profiler speeds, characterized as linear fits with slope = 1 (95% confidence), plus or minus 0.02 m s super(-1) bias, and root-mean-square error of residuals (observed minus fitted values) = 0.055 m s super(-1). Direct estimates of are derived from fits of velocity spectra to a theoretical turbulence inertial subrange. From near-bottom measurements, these estimates are interpreted as seafloor friction velocities, which yield drag coefficients consistent with expected values. Noise levels for , based on 40-min spectral fits, are <10 super(-9) m super(2) s super(-3). In comparison to the airfoil (or shear) probe, the pitot-static tube provides the full spectrum of velocity, not just the dissipation range of the spectrum. In comparison to acoustic measurements of velocity, the pitot-static tube does not require acoustic scatters in the measurement volume. This makes the sensor a candidate for use in the deep ocean, for example, where acoustic scatterers are weak.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Abstract
The daily formation of near-surface ocean stratification caused by penetrating solar radiation modifies heat fluxes through the air–sea interface, turbulence dissipation in the mixed layer, ...and the vertical profile of lateral transport. The transport is altered because momentum from wind is trapped in a thin near-surface layer, the diurnal warm layer. We investigate the dynamics of this layer, with particular attention to the vertical shear of horizontal velocity. We first develop a quantitative link between the near-surface shear components that relates the crosswind component to the inertial turning of the along-wind component. Three days of high-resolution velocity observations confirm this relation. Clear colocation of shear and stratification with Richardson numbers near 0.25 indicate marginal instability. Idealized numerical modeling is then invoked to extrapolate below the observed wind speeds. This modeling, together with a simple energetic scaling analysis, provides a rule of thumb that the diurnal shear evolves differently above and below a 2 m s
−1
wind speed, with limited sensitivity of this threshold to latitude and mean net surface heat flux. Only above this wind speed is the energy input sufficient to overcome the stabilizing buoyancy flux and thereby induce marginal instability. The differing shear regimes explain differences in the timing and magnitude of diurnal sea surface temperature anomalies.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
The inner shelf is a region inshore of that part of the shelf that roughly obeys Ekman dynamics and offshore of the surf zone. Importantly, this is where surface and bottom boundary layers ...are in close proximity, overlap, and interact. The internal tide carries a substantial amount of energy into the inner shelf region were it eventually dissipates and contributes to mixing. A part of this energy transformation is due to a complex interaction with the bottom, where distinctions between nonlinear internal waves of depression and elevation are blurred, indeed, where polarity reversals of incoming waves take place. From an intensive set of measurements over the inner shelf off central California, we identify salient differences between onshore pulses from waves with properties of elevation waves and offshore pulses from shallowing depression waves. While the velocity structures and amplitudes of on/offshore pulses 1 m above the seafloor are not detectably different, onshore pulses are both more energetically turbulent and carry more sediments than offshore pulses. Their turbulence is also oppositely skewed: onshore pulses slightly to the leading edges, offshore pulses to the trailing edges of the pulses. We consider in turn three independent mechanisms that may contribute to the observed asymmetry: propagation in adverse pressure gradients and the resultant inflection point instability, residence time of a fluid parcel in the pulse, and turbulence suppression by stratification. The first mechanism may largely explain higher turbulence in the trailing edge of offshore pulses. The extended residence time may be responsible for the high and more uniform turbulence distribution across onshore compared to offshore pulses. Stratification does not play a leading role in turbulence modification inside of the pulses 1 m above the bed.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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