A generic framework for evaluating the oceanic mixed layer depth dynamics

Oceanic convection, parameterized through vertical mixing schemes, is still not well captured by ocean general circulation models. A preliminary step necessary to improve these schemes is to evaluate and compare how the models behave for different forcing regimes. Literature often proposes single-case comparison (either on a specific location or a specific time or with a specific metrics). The goal of our work is to propose a more systematic framework allowing evaluations and comparisons over a larger range of forcing regimes. For doing so, we define a parameter space which has been derived thanks to a theoretical 1D model of the mixed-layer depth (MLD) evolution. This parameter space is formed by two dimensionless numbers : λs which describes the relative contribution of the buoyancy flux and the wind in the surface layer, and the Richardson number Rh which characterizes the stability of the water column at the mixed layer base. In this presentation, I will highlight the key features of this parameter space and I will illustrate its physical robustness with an ensemble of 1D simulations. These simulations were conducted by applying a 10 years JRA55-do 1.4.0 atmospheric forcing within a 1D standalone code making use of the NEMO Turbulent Kinetic Energy + Enhanced Vertical Diffusivity (TKE + EVD) scheme. Then, I will present a test case to study the impact of the horizontal resolution on the convection regimes for a TKE + EVD scheme in 1D, 1°, 1/12° and 1/60° realistic NEMO simulations. I will define convective regimes by sorting the values according to the normalized evolution of the mixed layer depth dt MLD / MLD and I will show that these regimes are almost kept in the parameter space between 1D and 1° but become generally less convective / more restratifiying when increasing the resolution, highlighting the restratification processes by lateral fluxes. Moreover, I will show that the dynamics in the Mediterranean is much more affected by the increase of resolution than the Labrador sea, suggesting that it involves more intense lateral restratification processes.

A usually neglected Coriolis term in the turbulent kinetic energy budget of the ocean surface mixed layer is examined for the special case of a steady-state mixed layer depth with no entrainment. In this term, the Reynolds stresses interact with the northward component of planetary rotation to exchange turbulent kinetic energy between horizontal and vertical components. The direction of the exchange depends upon the sign of east-west component of the wind stress. A vertically integrated or bulk theory is derived for a non-entraining mixed layer in equilibrium. The results show that the vertical turbulent kinetic energy and hence the vertical mixing and mixed layer depth will be reduced for westerly winds. Fasterly winds will increase vertical mixing and mixed layer depth.

Seasonal and interannual variability of pan-Arctic surface mixed layer properties from 1979 to 2012 from hydrographic data, and the dominance of stratification for multiyear mixed layer depth shoaling

Subtropical South Indian Ocean salinity maxima region plays an important role in transporting temperature anomalies towards north along the isopycnals following geostrophic pathways. In this study, interannual and decadal changes in temperature and salinity at the base of mixed layer during austral winters are investigated for the Argo era. Winter time deep mixed layer allows these Temperature/Salinity (T/S) changes to penetrate to the permanent pycnocline. Interannual changes in the mixed layer depth (MLD) are mostly driven by convective buoyancy and wind forcing. Contribution of different atmospheric and oceanic forcing to the changes in mixed layer temperature and salinity are shown using mixed layer budget calculation. It is observed that net heat flux term dominates the temperature changes whereas meridional advection plays a important role in driving salinity changes in the mixed layer. Mixed layer T/S changes are subducted to the permanent pycnocline mainly by lateral induction process because of large meridional MLD gradient. Density compensated anomalies also contribute to the T/S changes at the bottom of the mixed layer. Interannual temperature anomalies due to spiciness and heaving are further explored.

A diapycnal diffusion algorithm for isopycnal ocean circulation models with special application to mixed layers

The spatial and monthly variability of the climatological mixed layer depth (MLD) for the global ocean is examined using the recently developed Naval Research Laboratory (NRL) Ocean Mixed Layer Depth (NMLD) climatologies. The MLD fields are constructed using the subsurface temperature and salinity data from the World Ocean Atlas 1994 [Levitus et al., 1994; Levitus and Boyer, 1994]. To minimize the limitations of these global data in the MLD determination, a simple mixing scheme is introduced to form a stable water column. Using these new data sets, global MLD characteristics are produced on the basis of an optimal definition that employs a density‐based criterion having a fixed temperature difference of ΔT = 0.8°C and variable salinity. Strong seasonality of MLD is found in the subtropical Pacific Ocean and at high latitudes, as well as a very deep mixed layer in the North Atlantic Ocean in winter and a very shallow mixed layer in the Antarctic in all months. Using the climatological monthly MLD and isothermal layer depth (ILD) fields from the NMLD climatologies, an annual mean ΔT field is presented, providing criteria for determining an ILD that is approximately equivalent to the optimal MLD. This enables MLD to be determined in cases where salinity data are not available. The validity of the correspondence between ILD and MLD is demonstrated using daily averaged subsurface temperature and salinity from two moorings: a Tropical Atmosphere Ocean array mooring in the western equatorial Pacific warm pool, where salinity stratification is important, and a Woods Hole Oceanographic Institute (WHOI) mooring in the Arabian Sea, where strongly reversing seasonal monsoon winds prevail. In the western equatorial Pacific warm pool the use of ILD criterion with an annual mean ΔT value of 0.3°C yields comparable results with the optimal MLD, while large ΔT values yield an overestimated MLD. An analysis of ILD and MLD in the WHOI mooring show that use of an incorrect ΔT criterion for the ILD may underestimate or overestimate the optimal MLD. Finally, use of the spatial annual mean ΔT values constructed from the NMLD climatologies can be used to estimate the optimal MLD from only subsurface temperature data via an equivalent ILD for any location over the global ocean.

We have recently developed a three‐dimensional semi‐analytical mixed layer remote sensing model which includes the horizontal advection and diffusion processes in our previous one dimensional model (Yan et al., 1991a). The three‐dimensional (3‐D) model can be used as a supplement to the one‐dimensional (1‐D) model in the area where advection is important and for a finer resolution grid prediction. The 3‐D thermal inertia model is based on the 3‐D thermal energy conservation equation. Using sine transformation and inverse sine transformation methods, we obtained the solution for mixed layer thermal inertia which, in turn, can be used to calculate mixed layer depth using a similar method to that of Yan et al. (1991a). In the solution, the thermal inertia (which is proportional to the mixed layer depth) is expressed as functions of sea surface temperature changes (δSST), heat flux (Q) and surface layer velocity (u, v, and w). The vertical entrainment and wind effects are accounted for in the model by eddy diffusivity while changes of δSST result from entrainment, e.g., if vertical entrainment is larger, δSST will be smaller and vice versa. The advantages of this model are that it can be easily forced by remotely sensed data and that it is much simpler to compute than numerical 3‐D mixed layer turbulence closure models. The model was tested using the data from the advanced very high resolution radiometer/multichannel sea surface temperature data set and the Cooperative Ocean Atmosphere Data Set for the North Pacific Ocean. The model‐predicted mixed layer depths compared favorably with the mixed layer depths calculated from 14 years of Volunteer Observing Ship/expendable bathythermograph observation data. The model/data comparison also exhibited a number of mesoscale features of seasonal changes and anomalies of the mixed layer depth.

Formation and circulation of mode waters in the subtropical North Pacific are investigated by performing a numerical simulation with a high-resolution general circulation model. In the model, important features for forming mode waters, such as separation of western boundary currents, frontal systems, mixed layer processes, and eddy activities, are well reproduced. Annual subduction rates, calculated by following the mixed layer base, have major peaks in three density ranges and are related to the deep winter mixed layer at the south of the fronts in the western North Pacific, that is, the Kuroshio Extension with a winter surface density of 25.5 σθ, the Kuroshio bifurcation front (26.2 σθ), and the subarctic front (26.5 σθ). The lightest peak corresponds to the Subtropical Mode Water (STMW). It is formed at the south of the Kuroshio Extension and enters the main pycnocline as it crosses the mixed layer depth (MLD) front to the south. The remaining two peaks correspond to the Central Mode Water (CM...

A crucial controlling factor on the stability of the two-layered water column in the Weddell Sea is its sea ice cover, which acts through a negative feedback mechanism (NFM) due to the thermal barrier at the mixed layer base. The present study revisits the problem of how the NFM affects the stability and overturning of the water column using a fine-resolution nonhydrostatic model to run experiments employing various background mixing coefficients (viscosity $$\nu$$ and diffusivity $$\kappa$$ ) and Earth rotation (Coriolis parameter f) with and without the sea ice cover. In the ice-free case without the NFM, destabilization and overturning of the water column are accelerated by lower $$\nu$$ ( $$\kappa$$ ) and $$f=0$$ , since convection is enhanced in the mixed layer. Vigorous convection increases the upward salt flux from the deep layer and densifies the mixed layer against the concurrent upward heat flux. In the ice-covered case with the NFM, to the contrary, the destabilization and overturning are decelerated by lower $$\nu$$ ( $$\kappa$$ ) and $$f=0$$ . The upward heat flux due to mixed layer convection delays destabilization by reducing ice formation through the NFM. On the other hand, lower $$\kappa$$ decelerates the overturning, suppressing cabbeling instability at the mixed layer base. As a result, the entire process of destabilization and overturning needs more than two months at the maximum. Additional experiments, together with observational data, suggest that such a slow process may have occurred in the Weddell Sea.

A critical step in evaluating and selecting contaminated sediment remediation options is the simulation of a long-term future trend of exposure concentrations that will occur under natural conditions without active in situ remediation. This ‘natural attenuation’ forecast provides a baseline reference against which to compare alternative sediment management options. Model development and application at a number of contaminated sediment sites has indicated that one of the most important parameters for accurately simulating natural attenuation is the depth of the upper mixed (i.e., ‘active’) sediment layer. The upper mixed sediment layer essentially acts as a biologically available contaminant reservoir, determining risks to ecological and human health. Contaminant concentrations in the upper mixed sediment layer are dynamic and determined by: the sediment inventory created by historical loadings to the sediment; a balance of fate and transport processes both into and out of the mixed layer at the sediment water interface; and burial of contaminants from the mixed layer into deeply buried ‘inactive’ sediments. Model sensitivity analyses at one of our sediment management sites, the Lower Fox River, has demonstrated that the rate at which polychlorinated biphenyl exposure decreases with time under natural processes is inversely proportional and very sensitive to the depth of mixing in the surficial sediments of the system. These studies have shown that increasing the assumed mixed layer depth in the model from 10 cm to 30 cm increases the simulated recovery half-time—as measured by the rate of change of the upper layer sediment polychlorinated biphenyl concentration—from about 20 years to more than 100 years. In the case of the Lower Fox River, radioisotope and polychlorinated biphenyl concentration depth profiling supports a 10 cm mixed depth layer. Implementing a 30 cm mixed depth in the model smears the existing polychlorinated biphenyl profile, artificially mixing higher concentrations in the 10 to 30 cm layer with lower concentrations in the 0 to 10 cm layer, resulting in the increased simulated recovery times.

The paper extends Deardorff's general structure parameterization for a shear-free convective boundary layer. The model suggested employs the mixed layer hypothesis that the buoyancy (which is defined as b = g(ρ0 − &rho)/ρ0, where ρ is the density, ρ0 is the reference density, and g is the acceleration due to gravity) is constant with height within the mixed layer. The buoyancy flux zero-crossing height is taken as the mixed layer depth. The vertical buoyancy profile within the capping inversion, where the buoyancy flux is negative due to entrainment, is made dimensionless, using the buoyancy difference across the inversion and its thickness as appropriate scales. The approach was first suggested by Kitaigorodskii and Miropolsky for the oceanic seasonal thermocline. The authors examine the idea against the data from atmospheric measurements, laboratory experiments with buoyancy-agitated turbulence, and large-eddy simulations. The rate equations for the mixed layer and inversion layer depths are derived using the turbulent kinetic energy equation and Deardorff's scaling hypothesis refined to account for the inversion layer structure. The constants of the model are evaluated from the data of atmospheric, oceanic, and laboratory measurements, and large-eddy simulations. The causes of divergence of the estimates based on data of different origin are discussed. The model is applied to simulate convective entrainment in laboratory experiments. A reasonable explanation for ambiguous behavior of the entrainment zone in the experiments with a two-layer fluid is suggested. The model is found to simulate transition regimes of convective entrainment in multilayer fluid strongly affected by the nonstationarity of the entrainment zone.

Upper-ocean heat and mass budgets are examined from three snapshots of data acquired during and after the passage of Hurricane Gilbert in the western Gulf of Mexico. Measurements prior to storm passage indicated a warm core eddy in the region with velocities of O(1) m s21. Based upon conservation of heat and mass, the threedimensional mixed layer processes are quantified from the data. During and subsequent to hurricane passage, horizontal advection due to geostrophic velocities is significant in the eddy regime, suggesting that prestorm oceanic variability is important when background flows have the same magnitude as the mixed layer current response. Storm-induced near-inertial currents lead to large vertical advection magnitudes as they diverge from and converge toward the storm track. Surface fluxes, estimated by reducing flight-level winds to 10 m, indicate a maximum wind stress of 4.2 N m22 and a heat flux of 1200 W m 22 in the directly forced region. The upward heat flux after the passage of the storm has a maximum of 200 W m22 corresponding to a less than 7 m s21 wind speed. Entrainment mixing across the mixed layer base is estimated using three bulk entrainment closure schemes that differ in their physical basis of parameterization. Entrainment remains the dominant mechanism in controlling the heat and mass budgets irrespective of the scheme. Depending on the magnitudes of friction velocity, surface fluxes and/or shear across the mixed layer base, the pattern and location of maximum entrainment rates differ in the directly forced region. While the general area of maximum entrainment is in the right-rear quadrant of the storm, shear-induced entrainment scheme predicts a narrow region of cooling compared to the the stressinduced mixing scheme and observed SST decreases. After the storm passage, the maximum contribution to the mixed layer dynamics is associated with shear-induced entrainment mixing forced by near-inertial motions up to the third day as indicated by bulk Richardson numbers that remained below criticality. Thus, entrainment based on a combination of surface fluxes, friction velocity and shear across the entrainment zone may be more relevant for three-dimensional ocean response studies.

We report on novel observations of deep-reaching submesoscale eddy (SME). Open-ocean SMEs typically appear as mixed layer eddies (MLEs), with signatures rapidly decaying beneath the mixed layer depth. The observed eddy is, like MLEs, top-intensified, but its signature reaches to 300 m, over 10 times the mixed layer depth. Moreover, the eddy was detected in summer in shallow (20 m) mixed layer conditions, i.e., where relatively weak mixed layer instability is expected. Despite this, the eddy Rossby number and Richardson number are measured as O(1), and the eddy radius is 7 km, all in accordance with submesoscale flows. The vortex was detected in the East Mediterranean Sea at the foot of the continental slope offshore of Israel and was monitored via multiple in-situ platforms and via remote sensing for its full lifetime of three weeks. Based on satellite imagery, we attribute the anomalous deep-signature of the eddy to a different formation mechanism than mixed layer instability, namely formation from a meander in the regional boundary current, which has a similar depth signature. Finally, we note multiple similar events in the satellite record and discuss the border implications, including on cross-shelf material transport.

Glacial variations of mineral dust in comprehensive climate model simulations

This study takes advantage of the newly established observational network of Argo floats to investigate the variability of the oceanic surface mixed layer (ML) in response to typhoons occurring over the period 2000–08 in the western North Pacific Ocean. After removing the background variability due to the seasonal cycle, the regionally averaged ML response is statistically analyzed as a function of the distance from the typhoon centre, the time after typhoon passage, the geographic location, the translation speed of the typhoon and the pre-existing patterns of oceanic circulation. Based on an unprecedented amount of new observational data, our analysis reveals some notable differences between ML temperature and ML depth changes induced by a typhoon, including the delayed response of ML temperature relative to ML depth, the longer restoring time of ML temperature, and the tendency of pre-existing cold-core features to favour ML cooling while warm-core features favour ML deepening. RÉSUMÉ [Traduit par la rédaction] La présente étude tire profit du nouveau réseau de balises d'observation Argo qui étudie la variabilité de la couche de mélange de la surface océanique en réponse aux typhons survenus durant la période 2000–2008 dans l'ouest du Pacifique Nord. Après avoir éliminé la variabilité de fond due au cycle saisonnier, nous faisons une analyse statistique de la réponse de la couche de mélange, moyennée dans une région, en fonction de la distance du centre du typhon, du temps écoulé depuis le passage du typhon, du lieu géographique, de la vitesse de déplacement du typhon et des configurations de circulation océanique préexistantes. En se basant sur une quantité sans précédent de nouvelles données d'observation, notre analyse révèle des différences notables entre les changements de température et de profondeur de la couche de mélange produits par le typhon, y compris la réponse retardée de la température de la couche de mélange par rapport à la profondeur de la couche de mélange, le temps de rétablissement plus long de la température de la couche de mélange et la tendance des caractéristiques de noyaux froids préexistantes à favoriser le refroidissement de la couche de mélange pendant que les caractéristiques de noyaux chauds favorisent l'approfondissement de la couche de mélange.