Modelling shallow mixed layers in the northeast Atlantic (original) (raw)
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Journal of Geophysical Research, 2000
A 50-year coupled atmosphere-ocean model integration is used to study sea surface temperature (SST) and mixed layer depth (h), and the processes which influence them. The model consists of an atmospheric general circulation model coupled to an ocean mixed layer model in ice-free regions. The midlatitude SST variability is simulated fairly well, although the maximum variance is underestimated and located farther south than observed. The model is clearly deficient in the vicinity of the Gulf Stream and in the eastern tropical Pacific where advective processes are important. The model generally reproduces the observed structure of the mean h in both March and September but underestimates it in the North Atlantic during winter.
Numerical experiments with a wind‐ and buoyancy‐driven two‐and‐a‐half‐layer upper ocean model
Journal of Geophysical Research: Oceans, 1990
We describe numerical experiments with a limited domain (15°–67°N, 65° west to east) coarse‐resolution two‐and‐a‐half‐layer upper ocean model. The model consists of two active variable density layers: a Niiler and Kraus (1977) type mixed layer and a pycnocline layer, which overlays a semipassive deep ocean. The mixed layer is forced with a cosine wind stress and Haney type heat and precipitation‐evaporation fluxes, which were derived from zonally averaged climatological (Levitus, 1982) surface temperatures and salinities for the North Atlantic. The second layer is forced from below with (1) Newtonian cooling to climatological temperatures and salinities at the lower boundary, (2) convective adjustment, which occurs whenever the density of the second layer is unstable with respect to climatology, and (3) mass entrainment in areas of strong upwelling, when the deep ocean ventilates through the bottom surface. The sensitivity of this model to changes in its internal (mixed layer) and e...
An embedded mixed-layer-ocean circulation model
Dynamics of Atmospheres and Oceans, 1981
The rationale and numerical technique of embedding an oceanic bulk mixed layer model with a multi-level primitive equation model is presented. In addition to the usual prognostic variables that exist in a multi-level primitive equation model, the embedded model predicts the depth of the well mixed layer as well as the jumps in temperature and velocity that occur at the base of that layer. The depth of the mixed layer need not coincide with any of the fixed model levels used in the primitive equations calculations. In addition to advective changes, the mixed layer can deepen by entrainment and it can reform at a shallower depth in the absence of entrainment. When the mixed layer reforms at a shallower depth, the vertical profile of temperature below the new, shallower mixed layer is adjusted to fit the fixedlevel structure used in the primitive equations calculations using a method which conserves heat, momentum and potential energy. Finally, a dynamic stability condition, which includes a consideration of both the vertical current shear and the vertical temperature gradient, is introduced in place of the traditional "convective adjustment". A two-dimensional version of the model is used to test the embedded model formulations and to study the response of the ocean to a stationary axisymmetric hurricane. The model results indicate a strong interdependence between vertical turbulent mixing and advection of heat.
Mixed layer transformation for the North Atlantic for 1990–2000
Journal of Geophysical Research, 2004
1] The buoyancy balance between two outcropping isopycnals leads to diagnostics for quantifying water mass formation rates between them due to air-sea buoyancy fluxes . The surface air-sea transformation gets modified by mixed layer processes so that the net formation rate below the winter mixed layer depth is different from that given by surface air-sea fluxes alone. Here we estimate the role of time dependence and mixed layer deepening to quantify the water mass transformation due to mixed layer entrainment fluxes. We focus on the mixed layer transformation in the North Atlantic for 1990-2000 during the World Ocean Circulation Experiment period, using both entrainment parameters and isopycnal geometry. The water mass transformation due to mixed layer entrainment is calculated using a large number of gridded one-dimensional mixed layer models forced by National Centers for Environmental Prediction (NCEP) reanalysis air-sea fluxes (daily/6 hourly) to calculate the local entrainment parameters; Reynolds sea surface temperature and Levitus monthly salinity data determine the isopycnal geometry. To get a closed annual cycle in the mixed layer depth, any net annual heat flux is ascribed to advective processes. These are included in the annual mean, leaving any synoptic forcing in the NCEP forcing unperturbed. In general, the mixed layer transformation opposes air-sea interaction, with amplitudes of (1.3 Sv) in the equatorial region (without equatorial upwelling contribution) and (0.5 Sv) in the overflow region. Mean cross-isopycnal volume fluxes are O(1 Sv), with considerable interannual variability. These estimates of water mass formation due to mixed layer processes are sensitive to synoptic frequencies, but not to climatological mean air-sea fluxes, and are within the imposed noise levels for inverse box models of the North Atlantic.
An intercomparison of four mixed layer models in a shallow inland sea
Journal of Geophysical Research-Oceans
Four mixed layer (ML) models after Denman [1973] (KLD), Garwood [1977] (RWG), McCormick and Scavia [1981] (K), and Thompson [1976] (RT) were compared against an extensive water temperature data set collected in the central basin of Lake Erie during the summer of ...
Observations and scaling of the upper mixed layer in the North Atlantic
Journal of Geophysical Research, 2005
1] The dependence of the mixed layer depth h D on the sea surface fluxes is analyzed based on measurements taken along a cross-Atlantic section 53°N. A linear function h D % 0.44L f , where L f = u * /f is the Ekman scale, well represents the influence of the wind stress u * and rotation f on the mixed-layer deepening, thus indicating that the influence of convective mixing in late spring at this latitude is of a lesser importance. Also, data showed reasonable correlation of h D with the stratified Ekman scale L fN = u * / ffiffiffiffiffiffiffiffi fN pc p , where N pc is the buoyancy frequency in the pycnocline, according to h D % 1.9L fN . In both cases the highest correlation between h D and the corresponding lengthscales is achieved when u * values taken 12 hours in advance of the mixed layer measurements were used, which may signify the adjustment time of inertial oscillations to produce critical shear at the base of the mixed layer. The vertical profiles of the dissipation rate e(z) are parameterized by two formulae that are based on the law of the wall scaling e s (z) = u * 3 /0.4z and the buoyancy flux J b : e 1 (z) = 2.6e s (z) + 0.6J b and e 2 (z) = e s (z) e s (z) + 3.7J b . The first parameterization is used to calculate the integrated dissipationẽ int over the mixing layer, which was found to be $3-7% (5% on the average) of the wind work E 10 . The positive correlation between h D andẽ int /E 10 suggests that in deeper quasi-homogeneous layers a larger portion of the wind work is consumed by viscous dissipation vis-à-vis that is used for entrainment. As such, the mixing efficiency, which is based on integral quantities, is expected to decrease with the growth of the mixed layer.
Atmosphere, 2020
This study investigates the physical processes controlling the mixed layer buoyancy using a regional configuration of an ocean general circulation model. Processes are quantified by using a linearized equation of state, a mixed-layer heat, and a salt budget. Model results correctly reproduce the observed seasonal near-surface density tendencies. The results indicate that the heat flux is located poleward of 10° of latitude, which is at least three times greater than the freshwater flux that mainly controls mixed layer buoyancy. During boreal spring-summer of each hemisphere, the freshwater flux partly compensates the heat flux in terms of buoyancy loss while, during the fall-winter, they act together. Under the seasonal march of the Inter-tropical Convergence Zone and in coastal areas affected by the river, the contribution of ocean processes on the upper density becomes important. Along the north Brazilian coast and the Gulf of Guinea, horizontal and vertical processes involving sa...
Comparison of the simulated upper-ocean vertical structure using 1-dimensional mixed-layer models
Ocean Science Discussions, 2016
Atmospheric fluxes influence the momentum and scalar properties in the upper-cean. Buoyancy fluxes result in a diurnal variability in the sea-surface temperature (SST), whereas the wind stress forms near-inertial currents in the mixed layer (ML). In this study, we investigate the contrasts between the simulated SST and the vertical structure of the temperature and shear by three different mixing models: the PWP bulk mixed-layer model, the KPP non-local boundary layer model and the κ−ϵ local mixing model. We choose two upper-ocean datasets for our studies, namely the SWAPP (1990) and the MLML (1991). The SWAPP dataset shows the presence of strong near-inertial shear below the ML and negligible near-inertial shear within the ML. The MLML dataset shows a negligible rise in the SST during the first 22 day mixing phase, which is followed by a steep rise by 6 °C during the subsequent 75 day restratification phase. <br><br> Comparison with the SWAPP d...