Mixed layer depth front and subduction of low potential vorticity water in an idealized ocean GCM

2007 ◽  
Vol 63 (1) ◽  
pp. 125-134 ◽  
Author(s):  
Shiro Nishikawa ◽  
Atsushi Kubokawa
Keyword(s):  
Mixed Layer ◽  
Potential Vorticity ◽  
Mixed Layer Depth ◽  
Layer Depth ◽  
Mixed Layer Depth Front ◽  
Low Potential Vorticity Water

scholarly journals Observations of the Transition Layer

2009 ◽  
Vol 39 (3) ◽  
pp. 780-797 ◽  
Author(s):  
T. M. Shaun Johnston ◽  
Daniel L. Rudnick
Keyword(s):  
Layer Thickness ◽  
Mixed Layer ◽  
Potential Vorticity ◽  
Transition Layer ◽  
Mixed Layer Depth ◽  
Density Ratio ◽  
Horizontal Resolution ◽  
Surface Mixed Layer ◽  
Layer Depth ◽  
Vertical Displacements

Abstract The transition layer is the poorly understood interface between the stratified, weakly turbulent interior and the strongly turbulent surface mixed layer. The transition layer displays elevated thermohaline variance compared to the interior and maxima in current shear, vertical stratification, and potential vorticity. A database of 91 916 km or 25 426 vertical profiles of temperature and salinity from SeaSoar, a towed vehicle, is used to define the transition layer thickness. Acoustic Doppler current measurements are also used, when available. Statistics of the transition layer thickness are compared for 232 straight SeaSoar sections, which range in length from 65 to 1129 km with typical horizontal resolution of ∼4 km and vertical resolution of 8 m. Transition layer thicknesses are calculated in three groups from 1) vertical displacements of the mixed layer base and of interior isopycnals into the mixed layer; 2) the depths below the mixed layer depth of peaks in shear, stratification, and potential vorticity and their widths; and 3) the depths below or above the mixed layer depth of extrema in thermohaline variance, density ratio, and isopycnal slope. From each SeaSoar section, the authors compile either a single value or a median value for each of the above measures. Each definition yields a median transition layer thickness from 8 to 24 m below the mixed layer depth. The only exception is the median depth of the maximum isopycnal slope, which is 37 m above the mixed layer base, but its mode is 15–25 m above the mixed layer base. Although the depths of the stratification, shear, and potential vorticity peaks below the mixed layer are not correlated with the mixed layer depth, the widths of the shear and potential vorticity peaks are. Transition layer thicknesses from displacements and the full width at half maximum of the shear and potential vorticity peak give transition layer thicknesses from 0.11× to 0.22× the mean depth of the mixed layer. From individual profiles, the depth of the shear peak below the stratification peak has a median value of 6 m, which shows that momentum fluxes penetrate farther than buoyancy fluxes. A typical horizontal scale of 5–10 km for the transition layer comes from the product of the isopycnal slope and a transition layer thickness suggesting the importance of submesoscale processes in forming the transition layer. Two possible parameterizations for transition layer thickness are 1) a constant of 11–24 m below the mixed layer depth as found for the shear, stratification, potential vorticity, and thermohaline variance maxima and the density ratio extrema; and 2) a linear function of mixed layer depth as found for isopycnal displacements and the widths of the shear and potential vorticity peaks.

scholarly journals Langmuir–Submesoscale Interactions: Descriptive Analysis of Multiscale Frontal Spindown Simulations

2014 ◽  
Vol 44 (9) ◽  
pp. 2249-2272 ◽  
Author(s):  
Peter E. Hamlington ◽  
Luke P. Van Roekel ◽  
Baylor Fox-Kemper ◽  
Keith Julien ◽  
Gregory P. Chini
Keyword(s):  
Surface Wave ◽  
Mixed Layer ◽  
Potential Vorticity ◽  
Mixed Layer Depth ◽  
Descriptive Analysis ◽  
Boussinesq Equations ◽  
Stokes Drift ◽  
Small Scale ◽  
Langmuir Turbulence ◽  
Layer Depth

Abstract The interactions between boundary layer turbulence, including Langmuir turbulence, and submesoscale processes in the oceanic mixed layer are described using large-eddy simulations of the spindown of a temperature front in the presence of submesoscale eddies, winds, and waves. The simulations solve the surface-wave-averaged Boussinesq equations with Stokes drift wave forcing at a resolution that is sufficiently fine to capture small-scale Langmuir turbulence. A simulation without Stokes drift forcing is also performed for comparison. Spatial and spectral properties of temperature, velocity, and vorticity fields are described, and these fields are scale decomposed in order to examine multiscale fluxes of momentum and buoyancy. Buoyancy flux results indicate that Langmuir turbulence counters the restratifying effects of submesoscale eddies, leading to small-scale vertical transport and mixing that is 4 times greater than in the simulations without Stokes drift forcing. The observed fluxes are also shown to be in good agreement with results from an asymptotic analysis of the surface-wave-averaged, or Craik–Leibovich, equations. Regions of potential instability in the flow are identified using Richardson and Rossby numbers, and it is found that mixed gravitational/symmetric instabilities are nearly twice as prevalent when Langmuir turbulence is present, in contrast to simulations without Stokes drift forcing, which are dominated by symmetric instabilities. Mixed layer depth calculations based on potential vorticity and temperature show that the mixed layer is up to 2 times deeper in the presence of Langmuir turbulence. Differences between measures of the mixed layer depth based on potential vorticity and temperature are smaller in the simulations with Stokes drift forcing, indicating a reduced incidence of symmetric instabilities in the presence of Langmuir turbulence.

scholarly journals The Annual Cycle of Upper-Ocean Potential Vorticity and its Relationship to Submesoscale Instabilities

2020 ◽  
Author(s):  
Xiaolong Yu ◽  
Alberto C. Naveira Garabato ◽  
Adrian P. Martin ◽  
David P. Marshall
Keyword(s):  
Mixed Layer ◽  
Potential Vorticity ◽  
Gravitational Instability ◽  
Mixed Layer Depth ◽  
Upper Ocean ◽  
Physical Processes ◽  
Layer Depth ◽  
Order Of Magnitude ◽  
Mixed Layers

AbstractThe evolution of upper-ocean potential vorticity (PV) over a full year in a typical mid-ocean area of the Northeast Atlantic is examined using submesoscale- and mesoscale-resolving hydrographic and velocity measurements from a mooring array. A PV budget framework is applied to quantitatively document the competing physical processes responsible for deepening and shoaling the mixed layer. The observations reveal a distinct seasonal cycle in upper-ocean PV, characterized by frequent occurrences of negative PV within deep (up to about 350 m) mixed layers in winter to mid spring, and positive PV beneath shallow (mostly less than 50 m) mixed layers during the remainder of the year. The cumulative positive and negative subinertial changes in the mixed layer depth, which are largely unaccounted for by advective contributions, exceed the deepest mixed layer by one order of magnitude, suggesting that mixed layer depth is shaped by the competing effects of de-stratifying and re-stratifying processes. Deep mixed layers are attributed to persistent atmospheric cooling in winter to mid spring, which triggers gravitational instability leading to mixed layer deepening. However, on shorter time scales of days, conditions favourable to symmetric instability often occur as winds intermittently align with transient frontal flows. The ensuing submesoscale frontal instabilities are found to fundamentally alter upper-ocean turbulent convection, and limit the deepening of the mixed layer in the winter-to-mid-spring period. These results emphasize the key role of submesoscale frontal instabilities in determining the seasonal evolution of the mixed layer in the open ocean.

Interannual variability of the Tropical Indian Ocean mixed layer depth

2012 ◽  
Vol 40 (3-4) ◽  
pp. 743-759 ◽  
Author(s):  
M. G. Keerthi ◽  
M. Lengaigne ◽  
J. Vialard ◽  
C. de Boyer Montégut ◽  
P. M. Muraleedharan
Keyword(s):  
Indian Ocean ◽  
Interannual Variability ◽  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Tropical Indian Ocean ◽  
Ocean Mixed Layer ◽  
Layer Depth ◽  
Ocean Mixed Layer Depth

scholarly journals Summertime increases in upper-ocean stratification and mixed-layer depth

Nature ◽  
2021 ◽  
Vol 591 (7851) ◽  
pp. 592-598
Author(s):  
Jean-Baptiste Sallée ◽  
Violaine Pellichero ◽  
Camille Akhoudas ◽  
Etienne Pauthenet ◽  
Lucie Vignes ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Upper Ocean ◽  
Layer Depth ◽  
Ocean Stratification

A numerical study on the role of atmospheric forcing on mixed layer depth variability in the Bay of Bengal using a regional ocean model

2021 ◽  
Author(s):  
Sumit Dandapat ◽  
Arun Chakraborty ◽  
Jayanarayanan Kuttippurath ◽  
Chirantan Bhagawati ◽  
Radharani Sen
Keyword(s):  
Mixed Layer ◽  
Bay Of Bengal ◽  
Numerical Study ◽  
Mixed Layer Depth ◽  
Ocean Model ◽  
Atmospheric Forcing ◽  
Layer Depth ◽  
Regional Ocean Model
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